JOURNAL OF PETROLOGY
VOLUME 37
NUMBER •
PAGES ffll-636
1996
ANTHONY R. PHILPOTTS*, MAUREEN CARROLL and JAMES M. HILL
DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF CONNECTICUT, STORRS, CT 06269, USA
Crystal-Mush Compaction and the Origin
of Pegmatitic Segregation Sheets in a
Thick Flood-Basalt Flow in the Mesozoic
Hartford Basin, Connecticut
Where the Holyokeflood-basaltflowin the Mesozoic Hartford & Horter, 1993; Wheelock & Marsh, 1994). Similar
Basin in Connecticut is thick and contains coarse-grained, hori-sheets are found in the Hawaiian lava lakes (Richter
zontal segregation sheets in its central part, the lower part of the & Moore, 1966; Moore & Evans, 1967; Wright &
flow is strongly depleted in incompatible elements; where the Okamura, 1977; Helz, 1980) and in many basaltic
sills and shallow intrusions (Carman, 1994; Larsen &
flow is thin and contains no segregation sheets it is homogeneous
throughout. This chemical variation can be explained only Brooks, 1994; Marsh & Wheelock, 1994).
through compaction of the partly crystallized basalt. The com- Two distinct types of segregation sheet are recogposition of the segregation sheets shows that they separatedfrom nized, a more abundant coarse-grained basaltic type,
the basalt following only 33% crystallization. The segregation and a fine-grained granophyric (granitic) variety.
sheets, however, are clearly intrusive into the basalt, which must The composition of the basaltic type corresponds to
therefore have alreadyformed a crystal mush withfinitestrengthliquids that would form after as little as ~25%
at this low degree of crystallinity. The incompatible element crystallization of the host basalt, and the granoconcentrations indicate that the partly crystallized basalt phyric type, following ~75% crystallization. The
underwent as much as 28% compaction in the lowest 60 m of segregation of the liquids is thought normally to
the flow. Between 60 and 130 m above the base of theflow,the have resulted from some type of filter pressing.
crystal mush became dilated, and eventually ruptured with for- Although this might be expected in the case of latemation of the segregation sheets. No segregation sheet has a forming granophyric liquid, separation of the
composition indicating separation after more than 33% crys- basaltic fraction from a still largely molten parent is
tallization of the basalt. This is interpreted to indicate that more difficult to envision. Clearly the rheological
compaction ceased at this stage because of the increasing properties of partly crystallized basalt must play an
strength of the mush and the increasing density of the fraction- important role in the formation of these sheets.
ating interstitial liquid
Detailed sampling through a thick flood basalt,
which contains many segregation sheets, in the
Basin of Connecticut has allowed
where in the flow the segregation
liquid was derived and the degree of solidification of
the host basalt at the time of formation of the segregation sheets. The data indicate that the basalt,
when only one-third crystallized, already formed a
rigid, but elastic, framework, which was capable of
being fractured but through which residual liquid
was able to move by porous flow. The generation of
these fractionated rocks in the relatively simple
KEY WORDS ciystal-mush compaction; segrtgation shtets; flood
Mesozoic Hartford
basalt; tholeiitie; Connecticut
us to identify from
INTRODUCTION
The central parts of thick tholeiitieflood-basaltflows
commonly contain thin sheets of rock that apparently have formed from fractionated residual liquids
(Cornwall, 1951; Lindsley et al., 1971; Dostal &
Greenough, 1992; Greenough & Dostal, 1992; Puffer
*Corre»ponding author.
e-mail; [email protected]
I Oxford Univerrity Preu 1996
JOURNAL OF PETROLOGY
VOLUME S7
environment of a crystallizing flood basalt provides
insight into mechanisms that also must operate in
magma chambers at depth, where interpretation is
complicated by other factors, such as magma
replenishment and mixing, and wallrock assimilation.
FIELD RELATIONS
The Hartford Basin in Connecticut (Fig. 1) contains
three flood-basalt units, which are of early Jurassic
age (Puffer et al., 1981). All three contain segregation sheets, but the Holyoke Basalt, which was the
second to erupt, contains the thickest and most
abundant sheets. This basalt is also the thickest of
the three flood-basalt units, typically being ~100 m
thick. Its original lateral extent is uncertain but, in
addition to occurring in the Hartford Basin, it is
found to the west in the Southbury Basin, and the
Deerfield Basalt in the Deerfield Basin to the north
(Fig. 1) is stratigraphically correlative (Luttrell,
1989). The flow must therefore have extended for
> 150 km in a north-south direction and at least 50
km east—west.
Deerfield Basin
Holyoke
Basalt
Southbury
Basin
0
tO
20
30 40
Fig. 1. Outline map of the Mesozoic Hartford Basin and the
smaller Deerfield and Southbury buini. The eutward-dipping
sedimentary rocks of the hatinj are interrupted near the Triassic—
Jurauic boundary by three flood basalts, the middle and thickest
of which is the Holyoke Basalt. Sills (diagonal rule) related to the
first flood basalt occur near the base of the Mesozoic succession.
Feeder dikes to the flood basalts are shown by heavy lines.
NUMBER 4
AUGUST 1996
The Holyoke Basalt, like all of the rocks in the
Hartford Basin, dips gently eastward as a result of
the greater down-drop of the basin rocks on the
eastern border fault than on the western one. The
basalt consequently forms a north-south-trending
ridge through the basin. At Tariffville, the Farmington River cuts through the ridge to form a prominent gorge, which provides a complete section
through the flow.
The continuous exposures of basalt in the gorge
and in road cuts on both sides of the river were
mapped using an electronic total station, which
measures horizontal and vertical angles relative to a
given reference frame (e.g. north and horizontal),
and it measures distances by determining the travel
time of a laser beam bounced off a prism held at the
point to be surveyed. From these measurements, the
instrument calculates internally the x (east), y
(north), and z (vertical) coordinates of the surveyed
point to millimeter accuracy. These values can be
down-loaded directly into a portable computer in
the field, thus providing a rapid and accurate way of
collecting three-dimensional field data. Thus, despite
the length of the exposure through the gently
dipping (23°) flow, we were able to calculate the
original stratigraphic positions of all contacts and
sample locations in the flow with considerable
accuracy.
The section of Holyoke Basalt exposed in the
Tariffville gorge is 174 m thick. The flow is
underlain and overlain by lacustrine sediments,
many of which contain salt casts, indicative of a
playa lake environment. The great thickness of the
basalt at this locality cannot, therefore, be attributed
to the filling of an erosional channel. Instead, the
locality may have been near the center of the
Hartford basin, and thus it experienced the deepest
ponding of the basalt.
The flow can be divided into upper and lower
parts based on the nature of fractures. The upper
part is characterized by irregularly oriented fractures, which give the outcrop a blocky appearance,
whereas the lower part is characterized by vertical
fractures which commonly break the rock into
meter-long slender prisms. In places, colonnade-type
joints with horizontal striations are present in the
lower part of the flow, but most of these columns
appear to have broken into the vertical splinter-like
joints. The boundary between the upper and lower
parts of the flow in the Tariffville section occurs
97 m above the base, that is, 55% of the way up
through the flow. In contrast, the boundary between
colonnade and entablature style jointing in most
thick flows is ~ 20-40% of the height of the flow
(Long & Wood, 1986; Marsh, 1988). The upward
812
PHILPOTTS et d.
CRYSTAL-MUSH COMPACTION
displacement of this boundary in the Tariffville
section probably results from a redistribution of heat
within the flow during the formation of the segregation sheets (see, e.g. Moore & Evans, 1967), as
described below.
The surface of the flow was weathered in the
Mesozoic before being buried beneath sediments. No
erosion took place, however, as indicated by the
preservation of flow-top features, such as scoria,
rafted crustal slabs, and even ropy structures. The
section therefore still preserves the original flow
thickness. During weathering, the primary minerals
in the upper part of the flow were completely altered
(to clay and carbonate minerals), which makes it
impossible to obtain reliable whole-rock chemical
analyses from the upper third of the flow. From textures, however, the types and abundances of the
phenocrysts and the nature of the groundmass can
still be determined.
Although the flow top and lower chilled margin
contain abundant vesicles which are now filled with
chlorite, carbonate, quartz, and zeolites, the rest of
the flow is completely devoid of them, except near
the tops of segregation sheets. The main part of the
flow appears to have purged itself of gas bubbles
early in the cooling history, but gas that was
exsolved during the final stages of crystallization did
produce a dictytaxitic texture, which occurs
throughout the basalt and the segregation sheets.
These late-forming gas cavities are typically filled
with chlorophaeite.
The basaltic segregation sheets are distinguished
from the host basalt by their coarser grain size and
dark brown color on the weathered surface; the
basalt weathers a rusty orange. In addition, the
sheets are not as closely jointed as the basalt, and
their fracture surfaces are much rougher. The strike
and dip of the sheets is precisely the same as that of
the flow itself, and therefore they must originally
have been horizontal.
The sheets occur in a central zone extending from
70 to 85 m above the base of the flow (Fig. 2). The
zone contains 15 sheets which are spaced ~ 1 m
apart. Most sheets are a few decimeters thick, but
the lowest one is 2 m thick. Their cumulative
thickness is 3-9 m, which is 26% of this central zone.
They are laterally extensive, with the thicker sheets
being traceable from one side of the gorge to the
other, a distance of at least 400 m. Some are seen to
thin and pinch out, but in such cases their stratigraphic position in the flow is marked by a horizontal fracture. Others are seen to bifurcate (Fig. 3)
or to connect with underlying or overlying sheets via
short dikes (Fig. 4). The matching of irregularities in
the contacts on opposing sides of such dike—sheet
813
intersections indicates that room for the segregation
liquid was created by dilation of fractures in the host
basalt. However, no angular corners are found on
the contacts; instead, they are all rounded. Similar
plastic, intrusive relations have been described from
the prehistoric Makaopuhi lava lake in Hawaii
(Moore & Evans, 1967). The contacts on all but the
lowest of the segregation sheets at Tariflville are
sharp. The upper contact on the lowest sheet is also
sharp, but the lower contact is gradational over a
few decimeters.
The fine-gTained granophyric segregation sheets,
which are typically ~ 1 cm thick, occur toward the
tops of basaltic segregation sheets, in the fine-grained
basalt immediately overlying these sheets, and as
extensions from the tapering ends of these sheets.
For comparison, we have also studied the Holyoke
Basalt in a section from the Southbury Basin (Fig.
1), where the flow is thinner and contains no segregation sheets. A series of cuestas in this highly faulted
basin provide exposures through the eastwarddipping flow. The 57-m-thick flow has a welldeveloped colonnade in the lower 15 m, which is
overlain by an entablature and a scoriaceous flow
top. Thus the boundary between the colonnade and
entablature in this exposure of the Holyoke Basalt is
more typical in being at 37% of the flow's height.
The basalt at this locality has well-preserved immiscible glasses in its mesostasis (Philpotts, 1979; Philpotts & Doyle, 1983).
PETROGRAPHY
At Tariffville, the upper part of the flow that is
characterized by irregular fractures, and the lower
chilled margin contain a small percentage of millimeter-size
phenocrysts
of plagioclase and
microphenocrysts of totally altered olivine in a
groundmass of plagioclase, augite, and pigeonite
crystals, with a mesostatis characterized by dendritic
magnetite crystals and micron-size droplets of ironrich immiscible liquid (now opaque spheres).
Throughout the rest of the flow, plagioclase (An73_63)
is the only phenocrystic mineral ( < 5 % ) , olivine
having been reacted out to form pigeonite. The
groundmass consists of a fine-grained subophitic
intergrowth
of plagioclase
(An<63),
augite
(\Vo34En52-36Fsi6_3o), and pigeonite (WoiiEn 5 ,_4 3
Fs38^t6) (Fig. 5), with equant, subhedral grains of
magnetite in a felsic mesostasis. The grain size of the
basalt remains fine throughout, despite the great
thickness of the flow.
The basaltic segregation sheets, however, are
coarse grained, with centimeter-long bladed crystals
of plagioclase (An €0 _ w ) and pyroxene forming a
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER •
AUGUST 1996
E T
-•-65
Fig. 2. Vertical section through the central part of the Holyoke Basalt exposed in the road-cut (outlined) on the north side of the Farmington River gorge at Tariflville, Connecticut. The section has been rotated 23° to restore the lava to its original horizontal position.
The zone between 70 and 85 m above the base of the flow contains coane-grained basaltic segregation sheets (black) which transgress
the fine-grained host basalt. Boxes indicate areas shown in more detail in Figs 3 and 4.
Fig. 3. Bifurcation in a segregation sheet (see center of Fig. 2). (Note the bullet-nosed termination on the finger of basalt, which suggests
plastic behavior at the time of intrusion of the segregation sheet.)
subophitic texture with a mesostasis of a feathery to
granophyric intergrowth of alkali feldspar and tridymitc, and skeletal crystals of titanifcrous magnetite. Most pyroxene laths consist of a core of
pigeonite (altered) rimmed by augite (\V033_37En52_21
FS15-42) (Fig. 5), both of which contain exsolution
lamellae of the other. These lamellae typically form
a herring-bone pattern because of the presence of a
814
PHILPOTTS ttd.
CRYSTAL-MUSH COMPACTION
Fig. 4. Intrusive relationj of the tegregation theett in the bauJt (fourth theet up in Fig. 2).
DtoptW*
htadwitwrgit*
Augit*
w O,lv1n,
;"°n»'
,
„
+./W
o
io
Bilt (lit*
Fig. 5. Pyroxene composition! in the batalt ( • ) , the coarsegrained tegregation iheeU ( • ) , and the granophyre theeti ( + )
in the Holyoke Basalt at TariflVille, plotted following the method
of Iindiley (1983). Pigeonite occun in the tegregation theets but u
too altered for analysu. Olivinc it alto totally altered in the rockt.
central twin in the pyroxene lath. Some reddish
brown interstitial patches of alteration material were
initially fayalitic olivine crystals, but most of them
are dictytaxitic cavity fillings, as evidenced by their
concentric banding. Plagioclase and pyroxene laths
are locally up to 10 cm long (Emerson, 1905). Many
of them are curved, commonly through angles of
>90° (Fig. 6a). This apparently resulted from
deformation during growth, and in extreme cases the
crystals were actually broken (Fig. 6b). In parts of
the thicker sheets, the febic mesostasis is segregated
into centimeter-size blobs, which commonly have a
quartz and carbonate amygdale at their center. The
amygdales are more abundant toward the tops of
thick sheets, or where the sheets are thin.
The granophyre sheets are fine grained and consist
of a cloudy alkali feldspar, tridymite needles,
brownish green ferrohedenbergite (Wosg.jEnj.jFsss),
altered fayalitic olivine, magnetite, and minute
needles of apatite. Amygdales filled with quartz,
bladed albite crystals, and carbonate are common.
In thin section, the lower sides of granophyre sheets
are seen to have small, irregular veinlets cutting the
underlying basalt (Fig. 7). These could have been
the conduits through which the granophyre liquid
entered the horizontal sheets.
The Holyoke Basalt in the Southbury exposure is
much fresher than that in the Hartford Basin, and
even has unaltered glass preserved in its mesostasis.
The basalt is remarkably constant in appearance
throughout most of the flow and consists of scattered
phenocrysts of plagioclase set in a fine-grained, subophitic groundmass of plagioclase and pyroxene
crystals, with a dark mesostasis consisting of immiscible iron- and silica-rich glasses. Many of the Ferich globules have crystallized to form spheres of
pyroxene and magnetite. The mesostasis constitutes
32% of this rock (Philpotts, 1982). The rock's
texture changes significantly across the lowest 9 m
even though its composition remains essentially the
same (Philpotts & Doyle, 1983). The abundance of
Fe-rich globules decreases toward the base of the
815
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER •
AUGUST 1996
Fig. 6. (a) Bent crystal of pyroxene in segregation sheet (width of Geld is 8 mm), (b) Bent and broken cryital of plagiodase in segregation iheet (width of field is 2 mm).
flow and none are present below 5-6 m. Instead, the and under controlled oxygen fugacities. The results
mesostasis consists of a clear Si-rich glass that con- of these studies have been compiled in Fig. 8.
tains equant-shaped magnetite crystals. Analyses
Philpotts & Reichenbach (1985) studied the nearshow that the flow becomes more oxidized toward liquidus phase relations in representative samples of
the base, and this caused magnetite to crystallize each of the three basalt units in the Hartford Basin
earlier there and prevent the iron enrichment to determine their possible eruption temperatures.
necessary to fractionate the residual liquid into the ' The sample of Holyoke Basalt was from the base of
immiscibility field (Philpotts & Doyle, 1983). This the flow in the Tariffville gorge. The experiments
oxidation probably resulted from interaction of the show that this basalt is saturated with both olivine
ponded lava with the underlying red beds.
and plagiodase on its liquidus and that augite and
pigeonite appear only 20°G below this. With falling
temperature, olivine decreases and pigeonite
increases in abundance, as would be expected from a
PREVIOUS EXPERIMENTAL
reaction relation, but at the lowest temperature
investigated (1125°C), olivine was still not comSTUDIES
Three different experimental studies have been pletely eliminated. Based on previous lower-temcarried out on the Holyoke Basalt at low pressure perature experiments (Philpotts & Doyle, 1983)
816
PHILPOTTS tt at
CRYSTAL-MUSH COMPACTION
Fig. 7. Base of a fine-grained 1-cm-thick granophyre sheet near the top of a coarse-grained basaltic segregation sheet. Small granophyre
veinlets connect with the lower side of the granophyre sheet (see to the right of the photograph). The base of the granophyre sheet is
mounded up around the veinlet in a manner similar to the chimneys produced in laboratory experiments by channelized buoyant liquid
rising from a crystallizing mush (Tait * Jaupart, 1992). Width of photomicrograph is 8 mm.
olivinc is known to be absent at 1100°C, so a postulated olivine-out line has been inserted as a dashed
line in Fig. 8 at ~1120°C.
In the earlier study, phase relations just above the
solidus of the Holyoke Basalt were investigated to
determine whether the immiscible liquid droplets
found in the mesostasis of this rock were the products
of stable or metastable immiscibility (Philpotts &
Doyle, 1983). The results show that the residual
liquid in the basalt enters a stable two-liquid field on
cooling below 1020°C at oxygen fugacities near the
quartz-fayalite-magnetite (QFM) buffer. However,
at the higher oxygen fugacities of the nickel—nickel
oxide (NNO) buffer, no two-liquid field is encountered, because of the earlier crystallization of magnetite at these higher oxygen fugacities (Fig. 8). On
the low-temperature side of the two-liquid field,
crystallization of magnetite consumes the iron-rich
immiscible liquid and only the silica-rich liquid
remains, which eventually crystallizes at ~980°C.
The experiments demonstrate that oxygen fugacity
plays a crucial role in determining whether immiscible liquids form. Although the sample of basalt
1180
1160
1140
1120
° 1100
3 1080
e
L + PI + 2P> + Mt
8.1060
'
L/+ PI + 2Py 4 Mt + Tr
£ 1040
1020
1000
13
12
11
10
980
Fig. 8. Experimentally determined phase relations in the Holyoke
Basalt from Philpotu (1979), Fhilpotts & Doyle (1983), and Philpotts & Reichenbach (1985). The heavy dashed line marks the
experimentally measured intrinsic oxygen fiigacity of the rock.
817
JOURNAL OF PETROLOGY
VOLUME 37
used in this study is from the Southbury Basin, its
composition is almost identical to that used in the
higher-temperature study from the base of the flow
at Tariffville (see Tables 1 and 2).
In a still earlier study of the basalt from the
Southbury Basin, Philpotts (1979) measured the
intrinsic oxygen fugacity of this rock with an
yttrium-doped zirconia cell. The measurements
indicate that, at 1150°C, the oxygen fugacity was
near that of the magnetite—wiistite (MW) buffer but,
by 1000°C, it had risen to almost the QFM buffer
(Fig. 8).
These experimental results provide a framework
with which to interpret the crystallization history of
the Holyoke Basalt and the fractionation that led to
the development of the segregation sheets. Because
the chilled margins of the flow contain only a few
percent phenocrysts of plagioclase and olivine, the
basalt must have been erupted near its liquidus
temperature at ~1160cC and under oxygen fugacities between those of the MW and QFM buffers.
Despite a brief period during which only plagioclase
and olivine crystallized, the Holyoke Basalt must
have been multiply saturated with plagioclase,
augite, and pigeonite throughout most of its crystallization history, with the early crystallizing olivine
reacting out to form pigeonite. When a tholeiitic
basalt is multiply saturated, its rate of solidification
changes almost linearly with temperature (Ryerson
et al., 1988). Thus, if olivine reacted out by 1120°C,
the basalt would have been ~25% crystallized at
this stage. At lower temperatures, better estimates of
the fraction crystallized can be made using the nonlinear relation of McKenzie & Bickle (1988) which is
based on many careful experimental studies, but the
differences from the linear extrapolation are only a
few percent. According to this relation, plagioclase,
augite, and pigeonite would have been joined by
magnetite after ~ 6 3 % crystallization of the basalt,
and after 71% crystallization, the residual liquid
would have entered the two-liquid field, with the
generation of silica-rich and iron-rich immiscible
liquids.
CHEMISTRY OF THE FLOW
To obtain an accurate picture of the compositional
variation through the flow at Tariffville, samples
were collected at ~5-m intervals, except in the zone
of the segregation sheets where the interval was
decreased so that any migration of residual liquids
towards the sheets might be detected. The location
of each sample in the flow was determined with the
electronic total station. Obtaining representative
analyses of the coarse-grained segregation sheets was
NUMBER 4
AUGUST 1996
difficult because of their coarse grain size and
blotchy distribution of felsic mesostasis. Samples for
analysis of a sheet were prepared by crushing sawn
slabs that passed through the entire sheet. Care was
taken to avoid amygdales, but dictytaxitic cavity
fillings could not be eliminated. Despite these precautions, analyses of the sheets appear slightly
variable.
Whole-rock X-ray fluorescence (XRF) analyses
for the major elements were done on glass disks prepared by fusing two parts lithium tetraborate with
one part rock, whereas the trace elements were done
on pressed-powder pellets. The analyses were performed on a Kevex Delta analyzer, equipped with a
thin-windowed energy-dispersive detector and a
multiple X-ray target head, which allows the excitation radiation to be selected so as to maximize the
peak to background counts for any given group of
elements. Five different targets were used for each
major-element analysis and for each trace-element
analysis. Analyses were calibrated against US Geological Survey standards BCR-1 and W-l, which
were run with each batch of samples. Estimated
relative errors, based on replicate analyses of W-l,
are < 1 % for the major elements except for Na
(5%), Mg (1-7%), K (1-5%), and P (10%). Estimated relative errors for the trace elements are 5%
for V, Cr, Ni, Cu, Zn, Sr, and Zr, and 10% for Rb,
Y, Nb, and Ba. Quantification was through the
Toolbox software. Ferrous iron was determined in
each sample by the vanadate method (Wilson,
1955). Mineral analyses were obtained using an
M.A.C. 5 electron microprobe equipped with a
Kevex Quantum energy-dispersive detector. The
electron beam was operated at 15 kV and 800 pA.
Spectra were processed and quantified with the
Quantex software, which is based on the MAGIC V
ZAF correction procedure. The analytical results,
CIPW norms, and stratigraphic heights of the
Tariffville samples are given in Table 1 and those of
the Southbury samples in Table 2. Analyses of
samples from the highly altered upper part of the
Tariffville section are not included in Table 1, but
analyses 26, 27, and 28 already show signs of this
alteration. Their lack of normative quartz and elevated Sr are probably a result of alteration. These
analyses have been included, however, because elements such as Ti, Zr, Nb, P, and Y have been shown
to be relatively immobile during the low-grade
zeolite type of metamorphism to which these rocks
were exposed (Wood et al., 1976). With these elements we can compare the upper part of the flow
with the lower part, which is essentially unaltered.
Despite the homogeneous appearance of the basalt
in the Tariffville section, the analyses reveal a
818
CRYSTAL-MUSH COMPACTION
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PHILPOTTS it aL
CRYSIAL-MUSH COMPACTION
systematic variation in composition with height.
Figure 9 shows profiles through the flow for several
major and minor elements. The concentration of Ti,
for example, steadily decreases with height until
reaching a minimum at 47 m. It then steadily
increases, and at 60 m passes the concentration it
had at the base of the flow. It then reaches a
maximum at 63 m and decreases to the initial concentration again by 130 m. Zirconium and ferric
iron show similar profiles, but ferrous iron remains
relatively constant throughout the flow. The MgO
profile, on the other hand, is a mirror image of the
Ti profile, with MgO being elevated where Ti is low,
and vice versa.
The analyses of the coarse-grained segregation
sheets (Nos. 30-33) are somewhat variable, probably
as a result of the inhomogeneity of this pegmatitic
rock. These three analyses, consequently, have been
averaged (Table 3) to obtain a more representative
composition for this rock type. These segregation
sheets are similar to the basalt but are slightly
depleted in MgO and GaO and enriched in ferric
iron, Ti, and Zr. The segregation sheets have a lower
ferrous to ferric iron ratio than the basalt. The
granophyre sheet (Analysis 34) has rather low alkalis
and relatively calcic normative plagioclase (Aoto)
compared with normal granites. It consequently
resembles more the plagiogranites associated with
ophiolites and mid-ocean ridge basalts (e.g. Dixon &
Rutherford, 1979; Gerlach ct aL, 1981). The nor-
mative corundum in this rock probably results from
the cloudy alteration products in the feldspar.
In contrast to the variable composition of the
Holyoke Basalt at Tariffville, the flow at Southbury
(Table 2) is remarkably constant throughout most of
the section (Fig. 10). Only in the bottom few meters
does the composition vary significantly, and here it is
probably contaminated with the underlying sediments (Philpotts & Doyle, 1983). The Southbury
analyses are almost identical to those of the basalt
from the bottom of the flow at Tariffville, except
that the Southbury samples contain ~0-5% less
alumina. A small but significant increase in incompatible elements (Ti and Zr in Fig. 10) and decrease
in MgO is evident with increasing height in the flow
at Southbury. The profiles do not, however, have
sigmoid shapes and therefore probably reflect
primary variations in the composition of the erupted
lava rather than post-emplacement differentiation.
The compositional variation in the Tariffville
section is clearly the result of crystal—liquid fractionation. Rocks in the lower zone, extending from
10 to 60 m above the base, are depleted in the
incompatible elements that would be expected to
enter the liquid (e.g. Ti, Zr) and enriched in the
compatible elements (Mg, Cr). Between 60 and 130
m, however, the basalt is slightly enriched in incompatible elements and depleted in compatible ones.
This zone contains the segregation sheets, which are
significantly enriched in incompatible elements. The
Flow Top
1801
O)
4000 6000
8000 60 80 100 120 4
Ti (ppm)
Zr (ppm)
5
6
MgO(wt%)
7
9
10
11
1 2
FeO(wt%)
3
4
5
Fe2O3(wt%)
Fig. 9. Chemical profiles through the flow at Tariffville. • , basalt; • , segregation sheeU. Dashed lines indicate the calculated initial
magma composition. Cross-hatches indicate upper zone of irregular cooling fractures.
823
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 4
AUGUST 1996
Table 3: Analyses ofminerals from sample 2 and the calculated bulk composition of the early
crystallizing solids, and the average of the segregation sheets and the calculated initial magma
composition (see text for explanation)
1
2
3
4
5
Plagiodasa
Auglte
Plgeonite
Bulk
Average
Initial
55%
21%
24%
solids
segregation
magma
53-38
6
SIO2
50-09
62-77
52-33
51-19
54-08
TiO2
0-00
0-20
0-13
007
1-43
0-97
30-97
1-17
0-55
17-41
13-18
14-60
AljOj
Fe,O3
1-33
O-OO
000
0-72
2 93
2-13
FeO
000
9-36
20-96
6-99
11-62
9-76
MnO
0-00
0-35
0-63
0-22
0-24
0-20
MgO
0-00
1908
20-46
8-92
4-61
6-94
13-85
16-61
4-39
12-16
788
9-35
3-42
0-16
008
1-93
2-94
2-60
CaO
NajO
K20
0-36
002
002
0-21
0-94
0-73
P2O,
0-00
000
000
000
0-10
0-05
Total
100-00
99-71
99-56
99-83
99-84
99-71
Flow Top
E 60 ,
~
4000 6000 800060 80 100 120 4
Ti (ppm)
Zr (ppm)
MgO(wt%)
Fe2O3 (wt
Fig. 10. Chemical profiles through the flow at Southbury. Cross-hatches indicate upper zone of irregular cooling fractures.
question then is: what were the minerals involved in
this fractionation and when did the fractionation
occur?
The effect of crystal fractionation on the composition of the liquid should be evident in a plot of Zr vs
Ti (Fig. 11), because these elements are essentially
incompatible and should therefore be enriched in the
liquid while maintaining the same relative proportions during crystallization. Figure 11 shows that,
indeed, all of the analyses, except for that of the
granophyre (which is not plotted), cluster along a
straight line having a Ti:Zr ratio of 66 (regression
line, R = 0-95). The granophyre does not plot on this
line because titaniferous magnetite had begun to
crystallize (estimated to begin after 63% crystallization) by the time this late-stage liquid formed,
and therefore Ti was no longer incompatible.
However, the coarse-grained basaltic segregations do
lie on this line, and therefore they can be interpreted
as fractionated liquids that formed before crystallization of magnetite from an initial magma that
probably had a composition similar to that of the
basalt near the base of the flow (Table 1, No. 1),
which is shown by a A in Fig. 11. The analyses of
samples from the 10-60-m zone (x in Fig. 11) also
lie on the same linear trend but at lower concentrations of Ti and Zr. Thus their compositions probably
resulted from either the addition of early crystallizing minerals or the subtraction of a liquid fraction
having a Ti:Zr ratio of 66. The samples of basalt
from above the depleted zone (O in Fig. 11) are
slightly enriched in Ti and Zr and therefore must be
enriched in the liquid fraction.
The linear trend in Fig. 11 is to be expected if Zr
and Ti are incompatible. No zircon crystals are
found in any of the rocks, and the amount of zir-
824
PHILPOTTS etd.
CRYSTCAL-MUSH COMPACTION
the segregation samples, and for AI2O3, total iron
(FeO1), MgO, CaO, and K2O the regression lines
agree well with the lines joining the two extreme
100
compositions. The variation in these elements is
«£• 80
therefore satisfactorily explained by the model.
Unlike
the total iron trend, the FeO and Fe2O3
& 60
trends (not shown in Fig. 12) deviate slightly from
Bulk Solids
the line joining the extreme compositions; the segreM 40
gation liquid is too poor in FeO and too rich in
20
Fc2C>3. The most probable explanation for this discrepancy is that before crystallization of the coarse0
2000
4000
6000
grained segregation sheets, the residual magma was
8000
slightly oxidized, perhaps by water expelled from the
Ti (ppm)
underlying playa lake sediments (compare the
Fig. 11. Plot of Zr v» Ti for Tariflville rocki. Symbols arc ai
ferrous/ferric
ratios in the segregation sheets and in
followi: coarse-grained segregations (O); basalt from above 60 m
the basalts).
(O); basalt from below 60 m ( x ) , except for lowest sample (A);
calculated initial magma ( + ); daihed regression line through
The SiC>2 in the samples of basalt and coarsedata; and Rayleigh fractionation of initial magma is shown as
grained segregations arc similar, which is consistent
continuous line with numbers indicating the per cent crystallized
(see text for explanation).
with fractionation of plagioclase, augite, and
pigeonite because these minerals all have approxiconium that entered the early crystallizing minerals mately the same concentrations of SiC>2 as the basalt.
was so small that this clement can be considered Had early crystallizing olivine been involved, the
totally incompatible. Small amounts of Ti, however, SiC>2 in the residual liquid would have increased.
did enter the early crystallizing pyroxenes. The There is a slight, but significant increase in SiC>2
amount of Ti removed from the liquid by the early toward the base of the flow, with the lowest sample
crystallizing minerals can be estimated from electron having the highest SiC>2 content of all the basalt
microprobe and modal analyses of the minerals. For samples. This increased silica is largely responsible
example, the modal weight percentages of these for the difference between the regression line and the
minerals in the sample at 19-9 m above the base, line joining the bulk solids and the average of the
where the first depletion in incompatible elements is coarse-grained segregations. The higher silica
evident, is 55% plagioclasc, 21% augite, and 24% probably resulted from a small degree of conpigeonite (no olivine is present). Based on typical tamination by the underlying sediments, which are
analyses of these minerals from this rock (Table 3), now composed largely of albite, quartz, and minor
the bulk composition of the early crystallizing sphene. The slightly elevated TiO 2 of the lowest
minerals would have had zero Zr and 444 p.p.m. Ti. sample may also be due to contamination. The
Precisely the same composition is indicated by the lowest sample from the flow at Southbury also has a
intercept on the Ti axis of the regression line slightly elevated TiO2 content (Table 2), and as
(R = 0-95) through the data in Fig. 11. Thus, at least stated above, this rock was probably contaminated
in terms of Ti and Zr, the compositional variation in by the underlying sediments (Philpotts & Doyle,
all of the rocks, except the granophyre, can be 1983).
attributed to fractional crystallization of plagioclase,
The most puzzling variation is that of sodium.
augite, and pigeonite from the initial magma.
Whereas the basalt samples in general have Na2O
If valid, this interpretation must account for the contents between those of the coarse-grained segrevariation in the other elements through the flow. In gations and the calculated bulk composition of the
Fig. 12, the major-element abundances in the early crystallizing minerals, the Na2O values themsamples from the lowest 60 m (i.e. the lower chilled selves are negatively correlated with the Ti values.
margin and the zone of depletion) have been plotted This is surprising because both elements would be
against the Ti content of the rocks. The calculated expected to concentrate in the liquid, so that as the
bulk composition of the early crystallizing minerals Ti content increased so would the Na2O content.
and the average composition of the coarse-grained The negative correlation is evident only in the
segregation sheets are also plotted and joined by samples between 20 and 60 m (Fig. 13a); samples
dashed lines. Linear regression lines through the from above this show a random spread. Not only is
analyzed rocks are shown as continuous lines. In all the negative correlation pronounced (the R value of
cases, the basalt samples have compositions inter- the regression line in Fig. 12 is 0-91), but the change
mediate between the early crystallizing minerals and is systematic with height. This is shown by the line
120
825
JOURNAL OF PETROLOGY
VOLUME 37
2000
NUMBER 4
4000
Ti (ppm)
6000
AUGUST 1996
8000
Fig. 12. Plot of major oxides vs Ti in lamples from the lowest 60 m of the flow. Dashed lines join the compositions of the average of the
coarse-grained segregations and the calculated bulk composition of the early crystallizing minerals (Table 3). Continuous lines are
regression lines drawn through the analyzed rocks.
T. JU
70
"
8
4.00-
o
o
3.50(M
Zro
2.502.004500
o
V
5500
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O
° o
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x x
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o 3.00 •
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^
o
o
30'
20 i6500
45
Ti (ppm)
o
o
•+•
-+•
50
55
60
% Total normative feldspar
Fig. 13. (a) Variation in NajO vs Ti in basalt samples from Tariffville. The systematic change in the composition with height of samples
from below 60 m ( x ) is shown by the arrowed line connecting successively higher samples starting with the lower chilled margin (A)The long arrow shows the trend that these rocks would have followed if compaction had involved simply expulsion of interstitial liquid.
(b) The same samples are plotted in terms of their total normative feldspar content vs their normative anorthite content.
joining the samples starting with the lower chilled
margin (A in Fig. 13a). With increasing height, the
Ti decreases and the Na2O increases until the sample
at 47 m, after which the trend is reversed. This
leaves little doubt that the Na2O content of these
rocks is related to the process that caused the
depletion in titanium. As argued above, this zone has
the greatest concentration of early crystallizing
minerals and would therefore be expected to have a
greater concentration of more calcic plagioclase and
a lower Ti content. Whereas the zone as a whole has
826
slightly higher anorthite contents than the basalt
from above 60 m, the rocks that contain the largest
amount of feldspar contain the least anorthite-rich
plagioclase. Indeed, as shown in Fig. 13b, the higher
the plagioclase content of the rock the lower is the
anorthite content of its normative plagioclase.
Also of interest is the magnitude of the change in
the concentration of Na2O relative to that of Ti
across this zone, the NajO changing by 33% and the
Ti by only 20%. The cause of this large change in
is uncertain. In the upper part of the flow,
PHILPOTTS«a£
CRYSTAL-MUSH COMPACTION
where the basalt is altered, high NajO can be
attributed to albitization of the primary plagioclase,
but in this lower zone the plagioclase is unaltered.
The sodium could have been derived from the
underlying playa-lake sediments by some process of
vapor transport into the bottom of the still molten
lava, but this would not explain the negative correlation with Ti. As the lava cooled and a solidification
front moved upward from the base, exsolved volatiles also could have transported sodium upward, but
why this should then have been concentrated in
rocks with low Ti contents is not obvious. The simplest explanation is that the sodium content of the
plagioclase was increased during recrystallization
that accompanied compaction of the crystal mush.
We will return to this problem after further discussion of the mechanism of fractionation in the flow.
Except for the sodium data, the compositional
variation in the basalt and the coarse-grained segregation sheets is satisfactorily accounted for by the
fractional crystallization of plagioclase, augite, and
pigeonite from a magma that initially would have
had a composition similar to that of the basalt near
the base of the flow. However, as argued above, the
lowest analyzed sample (Table 1, No. 1) appears to
be slightly contaminated with SiOj and Ti from the
underlying sediment. However, because of the linear
arrays produced by most of the elements in Fig. 11,
the regression lines can be used to extrapolate
through the samples from the lowest 60 m of the flow
to the composition that the basalt probably had
before contamination. This method of determining
the composition of the initial magma has the added
advantage of making use of multiple analyses of different rocks rather than basing the determination on
one analysis of one rock. To make the calculation, a
reasonable value for the initial Ti content of the
magma must be selected. A value of 5800 p.p.m. was
chosen from the basalt analyses from just above the
zone of segregation sheets. This value is only slightly
lower than that of the possibly contaminated sample
from the bottom of the flow, and it allows a reasonable mass balance calculation to be made of the
various rocks in the flow (see below). Because of the
disparate sodium trend, the initial NajO was calculated from the linear extrapolation between the
average of the segregations and the early crystallizing bulk solids. Analysis 6 in Table 3 gives the
results of this calculation.
Using this calculated original magma composition
and the analyses of the early crystallizing minerals
(Table 3), we can calculate the composition of successive liquids, produced by any given fractionation
scheme (Rayleigh or equilibrium, in the extreme
cases). If this is done for Zr and Ti, the specific
827
fractionation scheme makes little difference to the
results, because both of these elements are essentially
incompatible. Figure 11 shows the composition of
liquids formed by fractionation of the assumed initial
magma. The calculated trend is seen to pass through
the plotted positions of the basaltic segregation
sheets after ~ 3 3 % crystallization of the initial
magma. This value is similar to the degree of crystallization that has been estimated for segregation
sheets in other basalt flows—26% in the Lolo flow of
the Columbia River Group and 28% in the
Watchung flows of New Jersey (Puffer & Horter,
1993), and, based on TiO 2 analyses, 26-33% in the
Keweenawan basalts of Michigan (Cornwall, 1951).
Thus, ignoring for the moment the question of how
the fractionated liquids were able to segregate, their
composition corresponds to that of a residual liquid
that would form after no more than one-third crystallization of the basalt. Similarly, based on the
enrichment in zirconium, the granophyric sheets in
the Holyoke Basalt would have formed after 75%
fractional crystallization of the original magma.
The elements that are concentrated in the coarsegrained segregation sheets almost certainly came
from the zone between 20 and 60 m above the base
of the flow where the incompatible elements are most
strongly depleted. They were not, for example,
derived from the basalt surrounding the segregation
sheets, for this rock is actually slighdy enriched in
these components. The zone of depletion has two
possible origins. Early crystallizing minerals could
have accumulated there by settling through the
magma. The fractionated liquid remaining above
then would have formed the segregation sheets. On
the other hand, the depletion could have resulted by
compaction of the partly crystallized basalt, with the
expelled liquid rising to form the segregation sheets.
Plagioclase is the only mineral that forms phenocrysts throughout the flow, and their abundance
remains relatively constant ( < 5 % ) , even at 47 m,
where the basalt has the lowest concentration of
incompatible elements. Furthermore, plagioclase is
most unlikely to have settled, because its density
would have been almost identical to that of the
magma (p m = 2-64 Mg/m 3 ). The chemical variation
through the lower zone would have required accumulation, not only of plagioclase, but also of pyroxenes. However, pyroxenes never form phenocrysts
in this basalt, nor is there any macroscopic field
evidence for their accumulation, such as mineral
layering. The steadily decreasing Ti content through
this zone also is difficult to explain by crystal settling, because it would have required the rate of
accumulation of early crystallizing minerals to have
steadily increased while the relative proportions of
JOURNAL OF PETROLOGY
VOLUME 37
these minerals remained the same. The only simple
way of concentrating all of the early crystallizing
minerals (primocrysts) in a fixed proportion is
through compaction of the partly crystallized basalt.
The flow could have maintained a homogeneous
composition until it was one-third crystallized, and
then compaction of the crystal mush, which would
have contained plagioclase, augite, and pigeonite by
this stage of crystallization, would have expelled the
residual liquid to higher levels in the pile, where
some of it would have formed the segregation sheets.
The steady variation through the 10—60-m zone
would then simply reflect different degrees of compaction.
Two important conclusions can therefore be drawn
from the analyses. First, the coarse-grained segregations have compositions that are consistent with their
having been derived from the original basalt following ~ 33 wt % crystallization of the three-phase
assemblage plagioclase + augite + pigeonite in the
proportions that these minerals coprecipitated from
the basaltic magma. Second, the chemical variation
through the flow indicates that this segregation
liquid, which forms the sheets between 70 and 85 m
above the base of the flow, was derived from the zone
between 10 and 60 m, probably as a result of compaction of the crystal mush.
PREVIOUS MODELS FOR
THE GENERATION OF
SEGREGATION SHEETS
Several mechanisms have been proposed for the formation of the coarse-grained basaltic- and the finegrained granophyric-segregation sheets in flood
basalts, but none satisfactorily explains all of the
features seen in the Tariflville occurrence. Puffer &
Horter (1993) concluded that the basaltic segregation liquid is generated on the lower solidification
front of the flow, from where it rises, buoyed up by
bubbles, to accumulate beneath the downwardgTOwing upper crust. Helz (1980) suggested a similar
origin for the segregation sheets in the Kilauea Iki
lava lake. Segregation liquids formed in this way
would have to rise through the less fractionated
central part of the flow without mixing with it. This
mechanism also does not account for the clear
intrusive and branching nature of many of the
sheets, not only in the Holyoke Basalt, but in the
Columbia River basalts (Lindsley et al., 1971), in the
Kcweenawan basalts (Cornwall, 1951), and in the
Hawaiian lava lakes (Moore & Evans, 1967).
Cornwall (1951) believed the segregation sheets in
the Keweenawan basalts formed from cool, dense,
NUMBER 4
AUGUST 1996
volatile-rich crystal mush that periodically fell from
the roof and accumulated on the floor of the flow,
where it was covered and trapped beneath accumulations of plagioclase, olivine, and pyroxene which
settled as individual crystals to form the host basalt
between the sheets. Such an origin would not
account for the compositional variation seen in the
Holyoke Basalt. Wright & Okamura (1977) suggested that residual liquids in the Makaopuhi lava
lake, Hawaii, were injected into horizontal fractures
that formed when the upper crust became supported
by the walls of the lava lake and thus was not free to
subside with the cooling and shrinking lens of liquid
in the lake. Although this is feasible in a lava lake, it
is not likely to have happened in the Holyoke flow,
where the 'shoreline' of the lava lake (or sea) may
have been many tens of kilometers away from the
Tariffville locality.
PROPOSED MODEL FOR
THE GENERATION OF
SEGREGATION SHEETS
A successful model for the origin of the coarsegrained segregation sheets must account for the following features:
(1) Segregation sheets form only in thickflowsor
lava lakes. The minimum thickness probably
depends on the type of lava but is of the order of
70 m.
(2) The sheets, which are commonly regularly
spaced (~1 m), form in a central zone of the flow,
where they constitute from 10 to 26% of that zone.
(3) Their grain size is at least an order of magnitude greater than that of the host basalt and in
places is pegmatitic. They commonly contain amygdales or are dictytaxitic. Many of their long bladed
crystals of plagioclase and pyroxene are bent.
(4) Contacts with the basalt are normally sharp
but some are gradational, as in the case of the
bottom contact on the lowest sheet at Tarifrville.
(5) The sheets are essentially horizontal, but they
can bifurcate or have small dikes connecting them
with sheets above or below them, which clearly
demonstrates their intrusive nature.
(6) The host basalt was capable of being fractured, but the rounding of contacts suggests that it
was also plastic at the time the sheets were intruded.
(7) The composition of the segregations corresponds to liquids that can form by as little as 25%
crystallization of the initial basalt.
(8) This fractionated liquid is extracted from the
basalt in the lower part of the flow beneath the zone
of segregation sheets.
828
PHILPOTTS et aL
CRYSTAL-MUSH COMPACTION
(9) Whereas thin granophyric segregation sheets,
which form after > 75% crystallization of the initial
basalt, can also be present, segregation sheets with
compositions intermediate between the granophyres
and the coarse-grained basalt do not occur.
(10) Granophyre segregation sheets occur either in
the coarse-grained segregation sheets or in the basalt
immediately overlying such sheets.
The restriction of segregation sheets to thick, slowly
cooled flows and lava lakes is probably attributable
to kinetic factors and process that are restricted to
thick piles of crystal mush. The Holyoke Basalt, and
other similarly thick flows, would have taken many
tens of years to solidify. Consequently, volatiles that
were exsolved during eruption or were derived from
underlying sediments had adequate time to rise and
accumulate in the early formed upper crust. The
lower and central parts of the flow consequently are
completely devoid of vesicles. The early convection
of magma brought about by these rising bubbles
eventually would have been replaced by thermal
convection once the magma had purged itself of
bubbles (Worster et al., 1993). The result of this
convection was to keep the magma thoroughly
mixed and, as it cooled, crystal nuclei would have
been distributed throughout the main part of the
flow. This accounts for the fine grain size of the
basalt throughout the flow.
Convection eventually would have stopped when
the crystallizing magma developed a yield strength
that could oppose the convective forces. This must
have occurred before the Holyoke Basalt was
intruded by the segregation sheets, whose composition indicates that the initial basalt had undergone
33 w t % (31 vol. %) crystallization by that time
(Fig. 11). In other flows this fraction can be as low
as 25 vol. %. Laboratory measurements on a
Kilauea Iki picrite show a marked non-Newtonian
behavior when the crystal fraction goes above 25%,
and the yield strength increases rapidly with
increasing crystal content (Ryerson et al., 1988).
Convection therefore probably stopped in the
Holyoke Basalt by the time it was one-quarter crystallized.
Once convection had stopped, the lower and
central parts of the flow would have consisted of a
delicate crystal mush that was at least two-thirds
liquid. The more abundant plagioclase, with its lathshaped crystals, probably played a dominant role in
forming this network. As cooling and crystallization
continued, the network would have developed more
strength, and by the time the segregation sheets
formed, the mush was capable of being fractured,
albeit in a plastic manner. Before this, however,
there was a redistribution of the melt within the
mush, as indicated by variation in the abundance of
incompatible elements, with melt migrating upward
from the zone between 10 and 60 m above the base
of the flow into the zone between 60 and 130 m. The
mush must therefore have undergone compaction
below and dilation above. The bulk density of the
primocrysts is calculated to have been 3 0 Mg/m 3 ,
whereas the magmatic liquid changed its density
from an initial value of 266 Mg/m at 1160°C to
2-68 Mg/m 3 at 1100°C when the liquid had the
composition of the segregation sheets [calculated
from data of Lange & Carmichael (1987) and
Warren (1995)]. Thus the residual liquid would
have been buoyant relative to the crystal mush
during this entire period of crystallization.
The degree of compaction or dilation of the crystal
mush can be estimated from the concentration of the
incompatible elements. The rock analyses can be
-'interpreted in terms of two components, a liquid
fraction and a bulk solids fraction that would have
existed at the time of compaction. Given that the
liquid that formed the segregation sheets was
expelled from the crystal mush, its composition
should have been the same as that of the liquid that
remained in the mush (Table 3, No. 5), and the bulk
composition of the solids is known from the electron
microprobe and modal analyses of the early crystallizing minerals (Table 3, No. 4). The concentration
of Ti in the liquid fraction would have been 8533
p.p.m. and in the solids, 444 p.p.m. If the concentration of Ti in a rock is Ti r , the weight percentage of solids at the time of compaction would
have been 100 x (8533- Ti r )/(8533- 444). For
example, the calculated initial magma composition,
with 5800 p.p.m. Ti, would have consisted of
34 wt % solids at this time. This can be taken to be
the fraction of solids in the uncompacted mush.
Where the concentration of Ti reaches its lowest
value at 47 m and the degree of compaction was
presumably greatest, the percentage of solids would
have been 47 wt %, and at 68 m, where Ti reaches
its highest concentration and the mush would have
been most dilated, the percentage of solids decreased
to only 27 wt%. From these numbers we can calculate the percentage of compaction, or dilation, as a
function of height in the flow (Fig. 14).
Interpreted in this light, the analyses point to a
zone between 10 and 60 m where the degree of
compaction steadily increased to a maximum of 28%
at a height of 47 m and then steadily decreased to
zero. Just above 60 m the mush was dilated by
~ 2 0 % , but this decreased to zero at 90 m. Between
90 and 130 m, the mush must also have been slightly
dilated. Analyses of the altered basalt above 130 m
829
JOURNAL OF PETROLOGY
-30 -20 -10
0
10
20
VOLUME 37
30
Percent compaction
Fig. 14. Per cent compaction (positive valua) or dilation (negative values) in the crystal mush as a function of height in the
flow, based on the Ti content of the rocks (lee text for explanation).
indicate essentially constant Ti values, which implies
there was no dilation or compaction above 130 m.
The smooth variation in degree of compaction in the
lower 60 m contrasts dramatically with the erratic
variation in the degree of dilation between 60 and
130 m. The smooth variation in the lower part is
consistent with the proposed model in that the
degree of compaction would be expected to increase
steadily downward until prevented from doing so by
the rising solidification front. The erratic variation
between 60 and 130 m is probably related to the
intrusion of the segregation sheets between 70 and
85 m, which probably produced local variations in
the amount of dilation.
The variation shown in Fig. 14 is interpreted to
indicate that the crystal mush that existed beneath
the solid crust of the Holyoke flow became compacted in its lower part while becoming dilated in its
upper part, with the maximum dilation occurring
between 63 and 75 m above the base of the flow.
This was probably where the mush was hottest and
weakest. The lowest and thickest of the segregation
sheets occurs at 70 m. If this sheet originally was
composed entirely of liquid, the continuity of the
crystal network across this level must have been
broken at this time. Therefore, it is concluded that,
as gravitational forces redistributed the melt through
the mush, the increased pore pressure in the zone of
dilation finally exceeded the tensile strength of the
mush and caused it to fail. Once ruptured, the mush
below the fracture would have been free to continue
compacting, with the expelled liquid rising through
the pores to accumulate in the sheet at 70 m. The
amount of liquid that accumulated at this level
therefore would have been dependent on the amount
of compaction that went on after the rupture.
NUMBER 4
AUGUST 1996
Because the liquid accumulating at this level was
expelled from the underlying compacting crystal
mush by porous flow, it was relatively free of crystal
nuclei, and therefore it formed a coarse-grained rock
on crystallizing.
Once residual liquid began to accumulate in segregation sheets, the dense mush forming the roof to
this sheet would have become unstable and spalled
off into the underlying liquid. The density contrast
between the solids and the liquid (~320 kg/m3)
would have resulted in a stress gradient in the roof of
slighdy more than 3000 Pa/m. The minimum
thickness of the spalled sheet would have been
determined by the tensile strength of the crystal
mush. Because most of the sheets are spaced ~ 1 m
apart, this tensile strength must have been ~3000
Pa, that is, the sheet had to be at least 1 m thick to
generate stresses great enough to exceed the tensile
strength of the roof.
As melt migrated up and around the spalled roof
slab, a second segregation sheet would have formed.
As this thickened, its roof would have become
unstable, and further spalling would have occurred.
In this way, multiple segregation sheets were formed.
During the upward passage of the liquid through the
sheets, fractionation could have occurred if the liquid
had been crystallizing at the time. This does not
appear to have been die case in the Holyoke Basalt,
but it could explain die vertical variation in the segregation sheets in the Keweenawan basalts described
by Cornwall (1951). The concentration of vesicular
segregation material toward the distal ends of sheets
in the Holyoke Basalt probably indicates that a more
volatile-rich fraction was able to migrate into the
fractures before die main body of segregation liquid
entered.
Because the segregation sheets do not have chilled
margins, the temperatures throughout the part of the
flow involved in the compaction and the intrusion of
segregation sheets could not have varied greatly.
Moreover, following the transfer of liquid from the
crystal mush to the segregation sheets, this variation
would have been still less. The interstitial liquid in
the zone of compaction, being closer to the bottom of
the flow, may have been slightly cooler than die
central part of dieflow.This liquid, on rising to form
the first segregation sheet at 70 m, therefore may
have cooled the central part of the flow slighdy. As
the segregation liquid rose to form die higher segregation sheets it would have transferred heat to
higher levels in the flow. This probably explains why
the boundary between the colonnade and entablature in thick flows containing segregation sheets is
higher dian in flows that do not contain segregation
sheets. The result of die redistribution of the segre-
830
PHILPOTTS et d.
CRYSTIAL-MUSH COMPACTION
gation liquid was to produce a region where the
temperature was very nearly constant at ~1100°C
and the basalt would have been ~ 3 5 % crystallized.
If compaction continued during crystallization,
the composition of the expelled liquid should have
changed progressively with time. The basaltic segregation sheets in the Holyoke, however, do not have
compositions indicating segregation after more than
35% crystallization. Basaltic segregation sheets in
other flood basalts show similar limits to the degree
of fractionation, despite considerable variation in the
thickness of the flows, and hence in their cooling
times (Puffer & Horter, 1993). In Hawaiian lava
lakes, the limit may be closer to 50% (Helz, 1980).
Compaction, therefore, appears to occur only during
a small fraction of the total crystallization period of
a flow, and appears to be mainly independent of the
total crystallization time. A number of factors would
contribute to stopping compaction. As crystals
become more tightly packed, the amount of strain
that can result from rotation of plagioclase laths
decreases, and the strain rate would approach the
diffusion rate. With the increase in percentage of
solids, the mush also becomes stronger. And finally,
as crystallization continues, the residual liquid
becomes more iron rich and denser, which would
decrease the driving force for compaction. These
factors are believed to have combined to terminate
compaction in the Holyoke Basalt.
At the time compaction ended, the segregation
sheets were still essentially liquid and at a temperature of 1100°C. With cooling, however, plagioclase and pyroxene would have crystallized from this
liquid. No compaction of crystals appears to have
occurred within the sheets, probably because the
thickness of mush was not great enough and the
density of the residual liquid was still increasing.
However, it was probably during this stage of crystallization of the segregation sheets that the long
plagioclase and pyroxene crystals were bent, when
roof slabs of the basalt fell into the partly crystallized
segregation liquid. At 1055°C, when magnetite
began to crystallize (Fig. 8), the segregation sheets
would have been ~ 3 7 % crystallized, but the surrounding basalt was already 63% crystallized.
When the temperature reached 1020°C, the
residual liquid entered the two-liquid field (Fig. 8)
and split into iron-rich and silica-rich liquids. With
the appearance of the silica-rich fraction, a residual
liquid was formed for the first time in the flow that
was significantly less dense (2*4 Mg/m 3 ) than all the
other phases present. These residual liquids would
have constituted 33% of the segregation sheets at
this temperature, and where the interstitial patches
of liquids were connected, buoyancy was able to
831
cause the silica-rich liquid to rise as blobs toward the
top of the sheets. In addition, by this late stage of
crystallization, volatiles were again being exsolved
from the residual liquids. Gas bubbles in the silicarich liquid would have increased the buoyancy of
this fraction still more. Toward the top of some
sheets, the silica-rich liquid was able to segregate
and form continuous thin sheets of liquid that eventually crystallized to form granophyre. The iron-rich
liquid, with a density of 3-24 Mg/m 3 (Philpotts &
Doyle, 1983) would have remained with the denser
minerals and eventually crystallized as pyroxene,
magnetite, and apatite.
The sequence of events envisaged for the crystallization of a thick flood basalt, or lava lake, are
shown schematically in Fig. 15, where time is represented on the horizontal axis in terms of the degree
of crystallization in the center of the flow. Following
an early period of degassing, the magma enters a
period of thermal convection, during which crystal
nuclei become distributed throughout the flow, thus
ensuring, eventually, a fine grain size to the basalt.
Convection ceases following 25% crystallization, and
by 33% crystallization the crystal mush develops an
interconnected network which undergoes compaction in its lower part and dilation in its upper
part. A horizontal rupture in the dilated mush fills
with liquid expelled from the compacting mush
below and, because it is free of nuclei, the rock
eventually formed from it is coarse grained. Overlying sheets arc formed as slabs spall from the roof of
the first sheet. Finally, when the basalt is > 7 1 %
crystallized, the residual liquid splits into immiscible
fractions, with the low-density silica-rich liquid
rising toward the tops of the earlier segregation
sheets to form thin granophyre sheets.
DISCUSSION AND
CONCLUSIONS
Perhaps the most controversial aspect of the proposed model is that a tholeiitic basalt, when only
one-third crystallized, can form a crystal mush that
is capable of being fractured and from which the
interstitial liquid can be removed. Is it reasonable to
expect a basaltic crystal mush to behave in this way
with such a low fraction of solids? Direct evidence of
the degree of crystallization needed to produce a
rigid framework comes from measurements in drill
holes in Hawaiian lava lakes. At the base of the
crust, where a drill can be pushed by hand into the
underlying liquid, the degree of crystallization was
found to be 65% at Kilauea Iki (Helz, 1980) and
55% at Makaopuhi (Wright & Okamura, 1977).
These are much higher degrees of crystallization
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 4
AUGUST 1996
yyyyyxxxxx
"^V'XA
.2
.3
xxx
.4
.5
.6
.7
.8
.9
1.0
Fraction Crystallized in Central Part
Fig. 15. Schematic representation of events in the solidification of the Holyoke Basalt. Time is represented by the fraction of melt crystallized in the central part of the flow (see text for discussion).
than are proposed in the model, but the stresses and entiation could begin. The fact that this occurred
strain rates produced by a drill are large compared when the magma was only one-third crystallized is
with those resulting from compaction. However, important for two reasons. First, the high porosity
Bruce Marsh (personal communication, 1995) would have provided abundant channels through
reports that 3-D networks form in the Makaopuhi which the liquid could be expelled from the mush.
lava lake at crystallinities as low as 10%. Based on Second, only small amounts of recrystallization
geochemical arguments, Irvine (1980) concluded would have been necessary to bring about large
that the olivine cumulates in the Muskox Intrusion amounts of compaction.
initially contained only 42% crystals. Campbell ttal.
The rate of compaction is difficult to quantify.
(1978) produced olivine cumulates from basaltic The fluxes of the descending solids and rising liquids
melt in high-temperature centrifuge experiments must have been equal, and because there was
that contained as low as 40% crystals. However, the approximately twice as much liquid as solid, the
crystal mush in the Holyoke Basalt would have dif- solids would have had to move twice as fast as the
fered from these cumulates in that it was dominated liquid. The rate of compaction must, therefore, have
by thin laths of plagioclase, and even the pyroxene been controlled by the rate of deformation of the
grains are elongate. It is well known that elongate solids and not by the rate of porous flow of the
particles can form highly porous aggregates, as is liquid. The rate law describing such deformation is
evident from examining the contents of a box of corn uncertain (Ashby & Verrall, 1977). Fluid-phase
flakes. Crystals with an aspect ratio of 10:1, for transfer was probably an important factor, because
example, can form an interlocking network with the minerals were in contact with liquid from which
only 20% solids (Burgers, 1938). We conclude, they had just crystallized, but power-law creep may
therefore, that clusters of plagioclase laths and pyr- also have been involved as the mush became more
oxene grains were primarily responsible for pro- compacted and began to solidify. The relation
ducing the highly porous framework.
between the diffusion rate of components through
Before the formation of the framework, differ- the melt and the strain rates in the crystal pile is
entiation of the magma could have been effected complicated and would have changed with time as
only by settling of individual mineral grains, but the geometry of the solids changed (e.g. were the
there is no evidence that this occurred in the plagioclase laths able to rotate?). The solids
Holyoke Basalt. Once the interconnected crystal probably behaved as a non-Newtonian viscous liquid
mush was formed, however, compaction and differ- with a significant yield strength. This yield strength
832
PHILPOTTS tt aL
CRYSTAL-MUSH COMPACTION
may be what prevents compaction from occurring in
thinner flows.
Because the crystal mush contained > 50% plagioclase, compaction of the crystal mush probably
involved recrystallization or dissolution and redeposition of the plagioclase. Where this occurred,
the plagioclase would have re-equilibrated with the
residual liquid and become more sodic. This may
provide an explanation for the unexpected negative
correlation between the NajO and Ti contents of the
rocks in the zone of compaction. The long arrow in
Fig. 13a shows the trend that these rocks would have
followed if compaction had simply involved
expulsion of the interstitial liquid, that is, the Na2O
and Ti would both have decreased. At the lowest Ti
values, however, the Na2O content is 1-5% higher
than predicted by this trend. The alumina content of
these Ti-poor rocks is elevated (Fig. 12), as would be
expected from the compaction of anorthitic plagioclase, but up to 30% of this alumina must have been
transferred from the anorthite to the albite component of the plagioclase during the recrystallization
that accompanied compaction. The fact that sodium
is the only element that shows this unusual trend
suggests that plagioclase recrystallization controlled
the compaction of the crystal mush, which is not
surprising, given its abundance.
According to the model, the total thickness of the
segregation veins depends on the amount of compaction that occurred after the dilated part of the
crystal mush ruptured. In addition, all of the liquid
that was displaced from the compacted zone should
have entered the dilated crystal mush or the segregation sheets themselves. Using the chemical profiles
through the flow as indicators of the distribution of
this liquid in the compacted and dilated parts of the
crystal mush, we can calculate the amount of segregation material needed to effect a mass balance.
As described above, the deviation of a rock's Ti
content from that of the assumed initial magma
(5800 p.p.m.) is a measure of its liquid content at
the time of compaction. Based on a graphical integration of the Ti profile, a total of 4-4 m of segregation sheets, having an average Ti content of 8533
p.p.m., should be present in the flow. This is
slightly more than the 3-9 m that was actually
recorded in the field, but this measurement is only
of the stratigraphic thickness of the sheets and does
not take account of any of the segregation material
that is in dikes connecting the sheets. The calculated and observed amounts are therefore in reasonable agreement.
We are unaware of compaction having been proposed as a mechanism of differentiation in thick lava
flows, although upward migration of volatile ele-
ments and filter pressing in general have been
invoked [see, e.g. Hart et al. (1971) and Helz
(1980)]. Compaction of cumulates on the floor of
plutonic bodies, however, has been carefully documented (Irvine, 1980; Shirley, 1987) and the
mechanics of the process have been investigated by
McKenzie (1984), Richter & McKenzie (1984), and
Shirley (1986). Some workers question whether piles
of cumulates are ever thick enough to generate the
pressure necessary to cause compaction (Morse,
1986). Sparks et al. (1985), in evaluating the role of
compaction as a postcumulus process in layered
mafic intrusions, calculated typical compaction rates
for olivine cumulates. Their calculated time and
length scales, however, are considerably greater than
those of the Holyoke Basalt. Even in a 1-km-thick
sill, their calculated freezing rate is faster than that
of compaction, so negligible compaction would
occur. If the Holyoke mush did undergo compaction,
and we believe the evidence strongly suggests that it
did, the high porosity and low viscosity of this mush
must have greatly reduced the time and length
scales, because compaction would have occurred in
only tens of years in < 100 m of mush. If this is possible in a thick lava flow, compaction must be a
viable mechanism in plutonic bodies, at least where
the cumulates are dominated by the same minerals
as in the Holyoke Basalt (i.e. plagioclase and pyroxene). Recent geophysical data suggest that magma
chambers beneath ocean ridges consist mainly of
crystal mush, with lenses of liquid forming only in
the upper parts of those beneath fast spreading axes
(Sinton & Detrick, 1992; Barth et al., 1994). These
lenses of liquid may form in exactly the same way as
those in the Holyoke Basalt.
Compaction has also been invoked as a segregation mechanism for basaltic magmas in their
source regions in the upper mantle (Walker et al.,
1978; Stolper et al., 1981; McKenzie, 1984, 1985).
Whereas this involves the migration of small fractions of melt through a largely crystalline peridotite,
the physical arrangement of segregation sheets
relative to the zone of compaction in the Holyoke
Basalt may provide a model for the structure in such
source regions. In the Holyoke Basalt, the transition
from compacting crystal mush to segregation liquid
is sharp, and only on the lower side of the lowest
sheet (i.e. the top of the compacting crystal mush) is
the boundary gradational over a few decimeters.
The transition from a mush to a crystal-free liquid is
therefore rapid. If crystals from the mush had been
carried along in the segregation liquid, the liquid
would not have crystallized to such a coarse-grained
rock. By analogy, primary basaltic magma in its
source region could be relatively free of restite
833
JOURNAL OF PETROLOGY
VOLUME 37
AUGUST 1996
ruptures the crystal mush in the zone of dilation to
form a horizontal sheet of liquid.
(5) As the underlying crystal mush continues to
compact, the sheet of liquid thickens. Because the
liquid enters the sheet by porous flow, it is essentially
free of crystal nuclei, and consequently crystallizes
eventually to a coarse-grained basaltic segregation
sheet.
(6) Because the dilated basalt above the sheet of
liquid is denser than the liquid, the roof becomes
unstable, and thin slabs sink into the liquid. In this
way, multiple thin sills of segregation liquid are
produced at ~l-m spacings above the initial segregation sheet.
(7) Compaction is arrested by the increasing
strength of the crystal mush and the increasing
density of the residual liquid. In quartz tholeiites the
compaction process is terminated by the time the
basalt becomes ~35% crystallized.
(8) A second generation of segregation sheets is
formed near the top of the earlier sheets when the
original basalt is >71% crystallized. These are
granophyric in composition, and are probably
formed from a late-stage, silica-rich immiscible
liquid which, because of its low density, collects near
the top of the earlier segregation sheets.
crystals and thus have a narrow compositional
range, which would depend only on the degree of
partial melting. The focus of such segregation
depends on where the buildup of melt and pore
pressure becomes great enough to break the continuity of the framework of the partly molten crystal
mush (Stolper et al., 1981). Once a rupture forms, it
is likely to propagate in the plane normal to the
minimum stress direction and form a laterally
extensive horizontal sheet, as it did in the Holyoke
Basalt. By analogy, it is also likely that a series of
interconnected sheets would form above the primary
segregation sheet, but here magma movement would
be by channeled flow rather than by porous flow.
Just as the rigid crust on the Holyoke Basalt arrested
the upward movement of the segregation liquid, so
does the lithosphere provide a cap for the melts from
the upper mantle. However, if the lithosphere is
fractured, the sheets of primary magma at its base
would provide a laterally extensive source of homogeneous magma that could rise in regional dikes to
form flood basalts. On the other hand, diapiric
structures could also develop from these sheets to
give rise to central complexes. Even in the Holyoke
Basalt such diapirs may have formed in places on the
segregation sheets, for at Totocket Mountain at the
southeastern end of the Hartford Basin, the segregation material, rather than forming thin sheets, has
thickened into lenses that are many tens of meters
thick.
ACKNOWLEDGEMENTS
We are grateful to Jim Garabedian and Dan Zeidler
for their assistance with the surveying and sampling
of the flow under very chilly winter conditions. We
are also grateful to Richard Tollo and Bruce Marsh
for their thoughtful reviews of the manuscript.
The following are the main conclusions that can
be drawn from this study:
(1) Quartz tholeiitic magmas with compositions
similar to that of the Holyoke Basalt, when only onethird crystallized, can form an interconnected
network of crystals that has a finite strength (it can
be fractured and intruded by segregation liquid). We
caution against assuming that all basalts would
behave similarly at this degree of crystallization.
Hawaiian olivine tholeiites, for example, appear to
develop a strength at ~50% crystallization.
(2) The strength of the mush is not great enough
to prevent compaction from taking place in flows
that are sufficiently thick. The minimum thickness
probably depends, again, on the composition of the
basalt, but in the case of quartz tholeiites is probably
~70m.
(3) The buoyant rise of the residual liquid through
the crystal mush results in compaction below and
dilation above. This produces a sigmoid profile in
the abundance of incompatible elements through
these zones.
(4) If the flow is thick enough, the pore pressure
developed by the buoyantly rising liquid eventually
NUMBER 4
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