JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 5
PAGES 885^900
2007
doi:10.1093/petrology/egm005
Nature of Sub-volcanic Magma Chambers,
Deccan Province, India: Evidence from
Quantitative Textural Analysis of Plagioclase
Megacrysts in the Giant Plagioclase Basalts
MICHAEL D. HIGGINS1* AND D. CHANDRASEKHARAM2
SCIENCES DE LA TERRE, UNIVERSITE¤ DU QUE¤BEC A' CHICOUTIMI, CHICOUTIMI, QUE. G7H 2B1, CANADA
1
2
DEPARTMENT OF EARTH SCIENCES, INDIAN INSTITUTE OF TECHNOLOGY, MUMBAI 400076, INDIA
RECEIVED SEPTEMBER 27, 2006; ACCEPTED FEBRUARY 5, 2007
ADVANCE ACCESS PUBLICATION MARCH 8, 2007
Sub-volcanic magma chambers might be a widespread component of
flood basalt provinces, and their presence can be revealed in some cases
by plagioclase megacrystic basalts. In at least four levels within the
Deccan flood basalt sequence the generally low abundance of small plagioclase crystals increases to 5^25%, with some as large as 30 mm
long. These Giant Plagioclase Basalt (GPB) flows were formed by
mixing of megacryst-rich and megacryst-poor magmas. The crystal
size distributions (CSD) of these megacrysts mostly plot as almost
straight lines on a classic CSD diagram. For a plagioclase growth rate
of 1010 mm/s steady-state magma chamber models and simple continuous growth suggest residence times of 500^1500 years. However, the
lack of crystals smaller than 2 mm suggests that coarsening may have
been involved and crystal shape can help define the environment where
this happened. Plagioclase megacrysts are very tabular and commonly
form clusters of sub-parallel crystals, characteristics that are also found
in the plagioclase of anorthosites formed by flotation at the top of
shallow magma chambers and crystallization in a high Peclet number
regime (e.g. Skaergaard; Sept I“les). A possible history is as follows. (1)
Plagioclase megacrysts crystallize in a convecting magma chamber just
below the lava pile. (2) Currents sweep the crystals to the top of the
chamber, where they accumulate as a result of their buoyancy.The crystals coarsen in response to the continuous supply of hot magma. (3) New
magma sweeps through the plagioclase mush, mingles and mixes, then
erupts to form the GPBs.The residence time recorded by the megacrysts
in the GPBs is that of the magma chamber where the megacrysts
formed, not that of the magmas that make up the megacryst-poor part
of the GPBs or the other megacryst-poor lavas. Lavas with
megacrysts similar to the GPBs are uncommon but widespread
*Corresponding author. E-mail: [email protected]
(Galapagos, Surtsey, etc.), and suggest the presence of sub-volcanic
magma chambers elsewhere.
KEY WORDS: texture; microstructure; continental basalt; megacryst;
plagioclase; crystal shape
I N T RO D U C T I O N
The Deccan traps are one of the most important continental flood basalt provinces (Sen, 2001; Chandrasekharam,
2003; Jerram & Widdowson, 2005). They erupted about 65
Myr ago, synchronous with the K^T boundary, and it has
been suggested that they might have caused or contributed
to the major mass extinction event at that time (Duncan &
Pyle, 1988). However, the effect of magmatism on climate is
critically dependent on eruption rate. It is not easy to
determine the eruption age of the Deccan Traps, as most
of the rocks are hydrothermally altered. Hofmann et al.
(2000) considered that the best ages are from 40Ar/39Ar
dating of mineral separates, mostly plagioclase. They
determined the age of lavas at the top of the Western
Ghats pile to be 654 07 Ma and that of the base as
652 04 Ma. Hence, the main eruptive period was probably less than 1 Myr. Nevertheless, magmatism may have
occurred in a much shorter period or episodically, neither
of which can be resolved by current isotopic techniques.
It has been suggested that the duration of magmatism of
the Deccan group can also be determined from rock
ß The Author 2007. Published by Oxford University Press. All rights
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VOLUME 48
22°
100 km
Mhow
Girnar
Toranmal
ada-rtif
Narm
Jabalpur
Tapi-rift
Shahada
Shirpur
Dhul e
Buldana
Nasik
Lonar
20°
Mumbai
(Bombay)
Area of
interest
Mahabaleswar
Ambol i
16°
Bay of Bengal
Arabian Sea
74°
70°
D EC C A N G EOLO GY A N D T H E
G I A N T P L AG I O C L A S E B A S A LT S
Introduction
MAY 2007
y-rift
Camba
textures (Sen, 2001, 2002). Most formations in the lower
part of the Deccan group terminate in an evolved magma
that contains plagioclase megacrysts up to 50 mm long
(Giant Plagioclase Basalts; GPBs). If these crystals grew
continuously in the magma storage zone and their growth
rate could be estimated then the duration of each magmatic
cycle could be determined. If there were no major hiatuses
between the magmatic cycles then the total duration could
be determined, or at least a minimum value. Using this
method, Sen et al. (2006) proposed that the duration of magmatism may have been as little as 22 800 years. Obviously,
such a rapid rate of magmatism would have perturbed the
climate considerably and could have influenced biotic evolution. In this paper we will examine quantitatively the
textures (¼ microstructure) of plagioclase in the GPB flows
and propose a model for their formation.
NUMBER 5
78°
Fig. 1. The Deccan volcanic province. The samples are from the
region east of Mumbai, where the basalt section is thickest (circled).
The basalts of the Deccan province were erupted from
many centres, but the most important is a shield-volcanolike structure in the Western Ghats, near Mumbai (Fig. 1)
(Subbarao et al., 1994). Rifting and erosion have exposed a
17 km thick section of flows, which is the focus of this
study. Aphyric or almost aphyric tholeiitic basalts dominate the section. It has been divided into sub-groups using
field geology, petrography and geochemistry (Beane et al.,
1986). The lowest sub-group, the Kalsubai, contains a
number of horizons of lavas with abundant, large tabular
plagioclase crystals termed megacrysts: these are the
GPBs (Beane et al., 1986; Hooper et al., 1988). Megacrysts
are crystals that are strikingly larger than other crystals
in the rock, but there is no genetic implication as to
whether such crystals are phenocrysts or xenocrysts.
Similar GPB flows have also been reported from the northern Deccan province but are not considered here
(Chandrasekharam et al., 1999). In the western Deccan,
GPBs cap each formation, have the highest total Fe and Ti
contents, and are considered to be the most chemically
evolved.‘Plagioclase-phyric’ flows are similar to the GPBs,
but with less and/or smaller plagioclase megacrysts; they
are combined in this study with the GPBs. The tops of
some flows are covered by ‘red boles’ça fine-grained
material rich in haematite thought to have formed by
weathering of the basalts and hence indicating a period of
volcanic repose (Fig. 2).
Giant Plagioclase Basalt flows
Individual GPB flows are 5^10 m thick and occur at distinct horizons within the eruptive sequence (Beane et al.,
1986). However, the extent of individual flows is unclear
and hence it is impossible to say if the same flow occurs at
Deccan-Western Ghats section
1.5
Tunnel Five GPB
1.0
Height (km)
JOURNAL OF PETROLOGY
Kashele GPB
0.5
Thalghat GPB
‘Red boles’
0
0
10
20
Phenocrysts (%)
Fig. 2. Part of the Western Ghats section of the Deccan series. The
phenocrysts are dominated by plagioclase. GPB, Giant Plagioclase
Basalt. ‘Red boles’ are red fine-grained material rich in haematite
thought to represent periods of erosion. Modified from Sen (2001).
different places or if it is just another flow of the same package. GPB flows were mostly sampled in road cuts and quarries, where the flows are relatively well exposed (Table 1).
At these locations it is clear that the GPBs are not homogeneous, but are composed of at least two magmatic components: one aphyric or sparsely phyric and another rich in
plagioclase megacrysts (Fig. 3). The fabric defined by
886
HIGGINS & CHANDRASEKHARAM
SUB-VOLCANIC MAGMA CHAMBERS
Table 1: Sample locations
Sample no.
Latitude (N)
Longitude (E)
Altitude (m)
Location
Notes
MH-04-04
1984155
7382981
422
Nasik road
Thalghat
MH-04-05A
1985645
7384385
697
Pandavlena quarry
Kashele
MH-04-05B
1985643
7384383
730
Pandavlena quarry
Kashele
MH-04-05C
1985643
7384373
725
Pandavlena quarry
Kashele
MH-04-06
1985630
7384387
676
Near Pandavlena quarry
Kashele
MH-04-07
1985668
7384356
675
Near Pandavlena quarry
Kashele
MH-04-10
1985840
7382657
425
Trimbak–Jawhar road
Thalghat
MH-04-12
1985901
7382668
503
Trimbak–Jawhar road
Thalghat
MH-04-14
1981161
7385136
868
Shivaneri Fort
Manchar
MH-04-16
1982500
7381500
775
Bote hill
Manchar
5 cm
Megacryst-poor
component
Megacryst-rich
component
Fig. 3. Variations in the plagioclase megacryst content within a single
GPB flow. The field of view is 40 cm.
plagioclase crystals within the megacryst-rich component
is extremely variable in direction and intensity. The spatial
arrangement of these components and their fabric suggests
that they originated by incomplete mixing (mingling) of
two components. This may have occurred by simultaneous
eruption of two magmas into the flow, or it could also have
happened at depth in the feeder and been preserved during
transport. This effect is seen in rivers, where water from a
tributary may be clearly distinguished far from the junction of the streams. All GPB flows examined in this study
had similar inhomogeneous structures.
Petrography
The GPB lavas consist of plagioclase megacrysts in a finegrained matrix. The plagioclase megacrysts are 2^50 mm
long. They have compositions ranging from An61 to An64
(Hooper et al., 1988) and are generally weakly zoned,
except for a narrow rim. Many crystals appear to have a
narrow hollow centre, which is filled with fine-grained
material similar to the matrix. This is assumed to be
trapped melt. Plagioclase in the matrix is much smaller
than the smallest megacrystsçtypically 025^1mm long.
Hence, it is generally easy to distinguish megacrystic and
matrix plagioclase.
The texture of the plagioclase megacrysts was examined
both qualitatively and quantitatively (see below). In both
cases the first step was the creation of a binary image of
plagioclase distribution. Rock samples 10^20 cm square
were slabbed normal to the foliation, if present, and
polished. In most samples the plagioclase was sufficiently
altered that it stood out clearly from the matrix. However,
in some very fresh samples it was not possible to distinguish clearly the plagioclase megacrysts. In these samples
the polished surface was painted with hydrofluoric acid,
which generally accentuated the contrast. Slabs were then
scanned using a conventional document scanner and
the images transferred to a vector drafting program
(CorelDraw). There the crystals were outlined on the
screen using a mouse. Crystals that intersect in the plane
of the section, but are clearly two separate crystals were
outlined and translated to separate them. The crystal outlines were then filled and exported as tiff files for further
processing (Fig. 4). Large thin sections (50 mm by 75 mm)
from the same samples were also cut, scanned and digitized in the same way.
Most plagioclase megacrysts are not distributed
homogeneously, but are clustered in touching groups
(Fig. 4). An important qualitative aspect of this texture is
the angular relationship of crystals within these clusters.
In some of the samples with the coarsest textures plagioclase crystals occur as clusters of sub-parallel crystals
(Fig. 4). In samples with smaller plagioclase crystals some
of the clusters may have a radiating texture. We have
not yet found a way to quantify this aspect of textural
variation.
887
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NUMBER 5
MAY 2007
MH-04-06 Kashele
MH-04-14 Manchar
20 mm
20 mm
20 mm
MH-04-05a Kashele
MH-04-07 Kashele
MH-04-04
Thalghat
MH-04-16 Manchar
20 mm
20 mm 20 mm
MH-04-05b
MH-04-10 Thalghat
Kashele
Kashel e
Plagioclase
megacrysts
in GPB
20 mm 20 mm
MH-04-05c Kashele
MH-04-12 Thalghat
20 mm
20 mm
Fig. 4. Textural variations of plagioclase megacrysts in GPBs. Samples MH-04-05a, 05b and 05c are from the same flow, a few metres apart.
Other samples are from other GPB flows. MH-04-12 is from a plagioclase-phyric flow just above the Thalghat GPB. Images were produced by
manual digitizing of crystal outlines in polished slabs.
888
HIGGINS & CHANDRASEKHARAM
SUB-VOLCANIC MAGMA CHAMBERS
Table 2: Number of crystals per unit area
Sample:
MH-04-04
MH-04-05A
MH-04-05B
MH-04-05C
MH-04-06
MH-04-07
MH-04-10
MH-04-12
MH-04-14
MH-04-16
Area (mm2):
17090
39000
27000
17200
33800
10900
23100
11860
10100
20130
Size bin range (mm)
398–251
5
0
0
0
1
1
0
0
1
0
251–158
33
0
11
0
5
7
0
4
4
17
158–100
39
13
35
8
18
19
23
21
35
31
100–631
55
25
77
34
27
32
49
62
55
49
631–398
63
43
83
60
41
25
52
99
77
48
398–251
35
51
64
59
39
33
39
119
76
59
251–158
32
43
20
76
51
34
29
94
61
47
158–100
12
25
9
38
28
29
11
90
52
38
100–0631
9
6
2
16
24
9
1
27
16
16
0631–0398
1
4
2
5
9
2
1
11
5
6
0398–0251
0
5
0
2
1
0
0
2
1
1
Q UA N T I TAT I V E T E X T U R A L
MEASUREMENTS
Introduction
The binary plagioclase images described above were quantified using the program ImageJ, a java version of the wellknown program NIHImage. This program calculates
dimensions of a best-fit ellipse to the crystal outlines and
its orientation and position. Intersection size data were
converted to true crystal size distributions (CSDs) using
CSDCorrections 1.3 (Higgins, 2000, 2002). All CSDs were
calculated for a shape of 1:5:5 (see below) and a roundness
parameter of 02 (close to parallelepiped). CSD calculations are not very sensitive to the weak fabrics observed
here, especially for sections cut orthogonal to the foliation;
hence a massive fabric was used. Intervals with fewer than
two crystals were eliminated from the diagrams, as they
are not precise.
Crystal size distribution
CSDs were determined in 10 samples from three
GPB groups (Table 2). Three samples from a single flow of
the Kashele GPB had CSDs that were almost straight on a
classic CSD diagram [S-type CSD of Higgins (2006b)],
but lacked small crystals (Fig. 5a). This was a real effect,
and not a measurement artefact, as size data were combined from one or more slabs and large thin sections for
each sample. Two samples, MH-04-05a and MH-04-05c,
were sub-parallel, but separated by 15 ln(population
density)(mm4) units. The mean size of the largest interval
whose population density could be measured was 12 mm,
and there were no plagioclase megacrysts in intervals
smaller than 2 mm. Much smaller plagioclase crystals are
present in the matrix of these samples, but were not
measured in this study. The third sample, MH-04-05b,
was also straight, but extended to larger crystals, in the
20 mm interval. There were no crystals smaller than
35 mm. The slope of this sample was significantly less
than that of the other samples. Samples MH-04-06 and
MH-04-07 were taken from the same flow as sample MH04-05, about 1km away. They both have CSDs that are
similar to those of samples MH-04-05a, MH-04-05b and
MH-04-05c. However, sample MH-04-06 continues to
larger crystals than samples MH-04-05a and MH-04-05c.
The Thalghat GPB was sampled at two locations.
Sample MH-04-04, from the Nasik road section, also has
an almost straight CSD, the largest crystals of any sample
studied and the shallowest slope. An attempt was made to
quantify plagioclase megacrysts in the field at this location
by measuring crystals on fresh surfaces with a ruler.
The CSD for larger crystals followed that determined
from the slabs of sample MH-04-04, but the lower cutoff
size in the field was only 10 mm, compared with 2 mm in
the slabs. Because of the importance of crystals smaller
than 10 mm this method was not pursued further.
Samples MH-04-10 and MH-04-12 were taken from the
Trimbak^Jawhar section. There is a considerable difference
in elevation for these two samples, but both are considered
to be part of the Thalghat GPB unit. The CSD of sample
MH-04-12 was straight and steep, whereas MH-04-10 was
collinear with MH-04-04, but did not extend to such large
crystals.
The Manchar GPB was sampled at two locations.
Both CSDs are approximately straight and not dissimilar
to CSDs from the other GPBs. The two CSDs are parallel
except for the largest size interval, which is not well
determined.
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−4
NUMBER 5
MAY 2007
(b) −4
Kashele GPB
MH-04-05c
−8
−10
MH-04-05a
−12
MH-04-07
MH-04-05b
−14
0
(c)
ln (population density)(mm−4)
ln (population density)(mm−4)
−6
Thalghat GPB
5
10
15
20
MH-04-06
25
MH-04-12
−8
−10
MH-04-10
MH-04-04
−12
−14
30
35
0
Size (mm)
−4
−6
5
10
15
20
25
30
35
Size (mm)
ln (population density)(mm−4)
Manchar GPB
−6
Data point
−8
Termination of CSD
(no smaller or larger crystals)
MH-04-14
−10
MH-04-16
−12
−14
0
5
10
15
20
25
30
35
Size (mm)
Fig. 5. Crystal size distributions in the Kashele (a), Thalghat (b) and Manchar (c) Giant Plagioclase Basalts plotted following the conventions
of Higgins (2006a). All CSDs terminate to the left; that is, there are no megacrysts smaller than 2 mm. However, there are very small plagioclase
crystals in the matrix that were not measured in this study.
Because all the megacrysts have CSDs close to S-type
(Higgins, 2006b) they can be reduced to a characteristic
length (Cl ¼ 1/slope) and intercept. Higgins (2002)
showed that closure produces a correlation between
intercept and characteristic length, even if the volumetric
phase proportion is variable. Hence, the easiest way
to summarize the CSD data is with a graph of characteristic length against modal plagioclase (Fig. 6a).
The characteristic length of a CSD says nothing about the
largest crystal in the rock. This can be determined precisely
only by 3D methods, but can be estimated from the largest
intersection (Fig. 6b). In general, there is a good correlation
between the largest intersection and the total amount of
plagioclase, with the exception of sample MH-04-06.
Crystal shape
The relationship between the shape of crystals in rocks and
their petrogenesis has been examined for a very long time
(e.g. Kostov & Kostov, 1999; Higgins, 2006a). Shape is best
examined in three dimensions, but for regular crystals
with more or less uniform shapes intersection data can be
used to estimate overall shape. Shape is generally
expressed in terms of the ratio of short:intermediate:long
(S:I:L) for a bounding parallelepiped or best-fit ellipsoid.
For parallelepipeds (Higgins, 1994) and triaxial ellipsoids
(Higgins, 2006a) the mode of intersection width/intersection length (2D aspect ratio) is equal to the ratio S/I.
Euhedral plagioclase crystals do not fit either of these
models but are close enough for this simple criterion to be
used to estimate accurately this aspect of their overall
shape.
It is not easy to determine precisely the ratio I/L
from intersection data of randomly oriented crystals
(Higgins, 2006a). I/L has been estimated from the statistical parameters of the intersection length/width
890
HIGGINS & CHANDRASEKHARAM
6
Characteristic length (mm)
5
4
1200
16
10
3
05b
07
14
06
2
800
12
05c
05a
(110)
0
5
10
15
Plagioclase (%)
(b) 30
ratio will be used to characterize the shape of the
plagioclase crystals.
Although plagioclase in all samples is strongly tabular
the I/S ratio is variable, from 4 to 13 (Fig. 8). All Kashele
and Manchar GPB samples have I/S of 4^7. Two of the
Thalghat samples also have similar values of I/S, but
sample MH-04-10 stands out with I/S ¼13. There is a
weak correlation between the I/S ratio and the characteristic length of the CSDs.
For the purposes of calculating the CSDs an aspect ratio
of S:I:L ¼1:5:5 was used for all samples so that they could
be readily compared. It should be remembered that
changes in the ratio I/L will change the size scale of the
CSD, but different values of S/I will change only the tailing corrections (Higgins, 2000).
1000
04
07
16
14
05b
12
20
10
15
500
05c
05a
10
Residence time (yrs)
06
25
5
0
(010)
Fig. 7. (a) Ideal shape of a plagioclase crystal. Here, the growth rate
of {010} is 02 times that of the other faces. This figure was produced
using the program WinXMorph (Kaminsky, 2005). (b) Broken surface
of a GPB sample with a plagioclase crystal showing the (010) face. The
crystal is 20 mm wide.
0
25
20
(110)
(001)
400
0
(101)
(001)
(101)
1600
04
1
Largest intersection (mm)
(b) ‘Giant’ phenocryst
(a) Model plagioclase tablet
Residence time (yrs)
(a)
SUB-VOLCANIC MAGMA CHAMBERS
0
0
5
10
15
20
Crystal orientation
25
Plagioclase (%)
Fig. 6. (a) Characteristic length vs plagioclase abundance. (b)
Maximum intersection size vs plagioclase abundance. Residence
times were calculated for a growth rate of 1010 mm/s.
distribution (Higgins, 1994; Garrido et al., 2001) and by
modelling (Morgan & Jerram, 2006). However, none of
these methods give precise values, especially for tabular
crystals (I/L close to one). Morgan & Jerram (2006)
developed a method where the whole intersection length/
width distribution is fitted to model distributions, thus
they can obtain S/I and I/L. However, the method does
not take into account the much better precision with
which the S/I values can be determined, compared with
I/L. In this study a more pragmatic approach is used
with sections of crystals with special orientations
(Higgins, 2006a): plagioclase tablets are commonly
flattened along {010} (Fig. 7a). A good cleavage parallel
to (010) ensures that the tabular face of plagioclase crystals
is sometimes exposed on broken surfaces (Fig. 7b).
This can be used with the observation that crystals
viewed in the (010) plane are equant to establish
that I ¼ L. Hence only the S/I (or more conveniently I/S)
The orientations of crystal outlines were also measured.
Orthogonal sections were not measured, hence the true 3D
orientation cannot be determined. The sections were cut
parallel to the foliation, therefore the only useful parameter
is the dispersion of orientations, which is a measure of the
‘quality of foliation’. Here, we used the normalized length
of the direction vector as the quality parameter (Higgins,
2006a): for perfectly aligned crystals it has a maximum
value of one; massive rocks give a value of zero. The orientation quality of the 10 samples studied here does not appear
to correlate significantly with crystal shape (Fig. 9) or any
other textural parameter; however, the sample with the
lowest quality foliation, MH-04-10, also has one of the
lowest plagioclase contents and the most tabular crystals.
DISCUSSION
Equilibrium crystallization of phases
The composition of the magma from which the plagioclase
megacrysts of the GPB crystallized is of fundamental interest for the calculation of equilibrium crystallization.
Ideally, this composition could be determined from melt
inclusions in the plagioclase megacrysts; however, this is
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NUMBER 5
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10
Thalghat GPB
6
05c
Frequency density
Frequency density
04
07
3
06
2
4
2
12
1
05a
0
0
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
(c)
(d)
Manchar GPB
16
Blocks-0.2 rounding
Frequency density
Frequency density
1
1:12:12
25
3
0.8
2D aspect ratio
2D aspect ratio
4
0.6
14
2
1
20
15
1:07:07
10
1:04:04
5
0
0
0.2
0.4
0.6
0.8
0
1
0
0.2
2D aspect ratio
0.4
0.6
0.8
1
2D aspect ratio
(e) 14
Summary
10
Shape I/S
12
10
8
06
6
05c
05b
14
4
07
05a
1
04
16
12
2
3
4
5
Characteristic length (mm)
Fig. 8. Intersection shape distributions for samples from (a) Kashele GPB, (b) Thalghat GPB and (c) Manchar GPB. The frequency density is
the number of crystals in an interval of 2D aspect ratios divided by the width of the interval and the total number of crystals (see Appendix).
(d) Intersection shape distributions of parallelepiped tablets with a rounding factor of 02. (e) Summary of I/S shape ratio vs characteristic
length. These values were determined from the mode of the intersection shape distributions. Schematic cross-sections of crystals with I/S ¼ 4
and 12 are shown at the left of the diagram.
beyond the scope of this paper. Sano et al. (2001) examined
the compositions of melt inclusions in olivine phenocrysts
from a regular Deccan basalt that lacked megacrysts.
They were able to determine all the more abundant
elements, as well as the water content of the magma,
which is essential for modelling crystallization. We
used the mean composition of glass inclusions in their
sample IC15 to model crystallization using the PELE
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HIGGINS & CHANDRASEKHARAM
SUB-VOLCANIC MAGMA CHAMBERS
Foliated
0.4
05b
6
16
05a
0.3
14
4
12
0.2
05c
0.1
7
Massive
Orientation Dispersion (quality)
0.5
10
0.0
4
6
8
10
12
14
Shape I/S
Fig. 9. Orientation dispersion (quality of foliation) vs crystal shape
(I/S). Orientation dispersion varies from zero for massive rocks
to 10 for parallel crystals. Schematic cross-sections of crystals with
I/S ¼ 4 and 12 are shown at the bottom of the diagram.
program (Boudreau, 1999), which is a version of MELTS
(Ghiorso & Sack, 1995). Plagioclase was the first phase to
crystallize when oxygen fugacity was buffered at FMQ
(fayalite^magnetite^quartz). The density of this plagioclase, 2670 kg/m3, is considerably less than that of the
magma, 2730 kg/m3, confirming that plagioclase floats
and hence can accumulate at the top of a magma chamber.
It is likely that the original magma from which the GPB
megacrysts grew was similar in composition to IC15 melt
inclusions and hence plagioclase also crystallized first and
floated in the GPB magma.
Crystal size
In igneous petrology we are concerned commonly with the
balance between kinetic and equilibrium processes
(Higgins, 2006a). This is expressed by the kinetic processes
of nucleation and growth, which are controlled by the
degree of undercooling (or supersaturation) of the
magma. If the crystallization driving force is reduced
then the texture of the magma or rock will adjust to minimize the overall energy of the system. This can be achieved
by coarsening, also known as Ostwald ripening or textural
equilibrium. Mechanical processes can also further modify
both kinetic and equilibrium textures. Finally, increases in
undercooling may rejuvenate kinetic processes. It is not
always easy to distinguish kinetic and equilibrium textures,
especially as perfect equilibrium is never achieved.
We will start with a simple kinetic model for crystal size
development.
A simple kinetic model for the GPB is crystallization
from a single magma in a chamber that is continuously
filled and drained. Marsh (1988) showed that in such a
steady-state system the CSDs are S-type and that the
mean residence time of a crystal ¼ characteristic length/
growth rate. The choice of growth rate is clearly very
important. Cashman’s (1993) compilation of data for basaltic systems suggested a growth rate of 109 mm/s for a cooling time of 3 yearsçperhaps typical of thicker lava flows.
For a cooling time of 300 years she suggested a growth rate
of 1010 mm/s. This is similar to what has been estimated
for the Palisades sill (Cashman, 1993). Growth rates for
plagioclase in the Makaopuhi lava lake were slightly
greater at (35^65) 1010 mm/s [data from Cashman &
Marsh (1988), recalculated by Higgins (2006a)].
Crystallization at greater depths would have occurred at
lower degrees of undercooling and hence slow growth
rates: if the relationship between cooling time and growth
rate continues to be linear then a cooling time of 30 000
years would give a growth rate of 1011 mm/s. A plagioclase growth rate of 1010 mm/s is chosen here as cooling
must have been beneath a lava pile, at the very least, and
hence slower than the Makaopuhi lava lake. This should
probably be considered a maximum value, as crystallization may have occurred at greater depths. It should be
noted that Sen et al. (2006) used a growth rate of
109 mm/s in their study of the Deccan GPB.
If plagioclase megacrysts crystallized in a steady-state
system with a growth rate of 1010 mm/s, then residence
times vary from 600 to 1500 years (Fig. 6a). Another
kinetic model involves simple, continuous growth of a crystal. For the same growth rate the largest crystal, as estimated from the largest intersection, would grow in
500^1000 years (Fig. 6b). The two methods correlate
loosely, except for two points. The CSD method is more
robust as it depends on the whole population of crystals,
whereas the second method depends only on one crystal,
the largest.
The volume of the magma chamber where the megacrysts grew can also be loosely estimated: it equals the residence time multiplied by the refilling rate. If all Deccan
magmas passed through the magma chamber at this time
then the refilling rate equals the eruption rate. The eruption rate of a volcanic province equals the volume divided
by the duration of volcanism. In the Deccan both
these parameters are poorly known. The most recent compilation of eruption rates (White et al., 2006) proposed
a value of 09 km3/year for flood basalt provinces, which
would give a chamber volume of 450^1350 km3.
These values would be proportionally lower if magma
bypassed the megacryst-bearing magma chamber. The
lower limit is only about twice that of the Skaergaard
intrusion, Greenland, recently estimated at 280 km3
(Nielsen, 2004). This intrusion was emplaced beneath
flood basalts and has a roof with plagioclase crystals
similar to those seen in the GPB (Naslund, 1984).
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VOLUME 48
NUMBER 5
MAY 2007
crystals dissolve at the same time as larger crystals are
growing, so that the overall surface energy is minimized
(Voorhees, 1992). It occurs when the temperature of the
system is maintained close to the mineral liquidus. Under
these conditions the nucleation rate is zero, but the growth
rate is significant. Higgins (1998) showed that coarsening
following the Communicating Neighbours model of
Dehoff (1991) will lead to increases in characteristic
length. If the system is allowed to be open (i.e. material is
added from circulating fluids), then the volumetric phase
proportion can also increase. The phase proportion can
also increase if enthalpy is withdrawn from the system at
low degrees of undercooling. Either process will give a
diagonal vector on a diagram of characteristic length vs
volumetric phase abundance. The distribution of data
points in Fig. 6 suggests that we are seeing the combined
effect of coarsening of a megacryst-rich magma and
mixing with an aphyric magma. We will now consider the
shape of the megacrysts and what it can tell us about the
crystallization environment.
Crystal shape
The shape of plagioclase crystals has been determined
quantitatively in a number of studies, as an estimate of
shape is necessary for the calculation of CSDs from intersection lengths. Higgins (2006a) has summarized these
studies and concluded that tabular crystals occur in two
environments: as microlites in lavas and as much larger
crystals in laminated cumulate rocks such as anorthosites
and troctolites. He proposed that tabular growth occurs
(001)
(001)
(010)
(010)
Larger volume chambers are also possible: Higgins (2005)
has proposed that the Sept I“les intrusion, Canada, with a
volume of 35 000 km3 was emplaced beneath flood basalts.
It, too, has a roof that contains large, tabular plagioclase
crystals similar to the GPB megacrysts. However, such a
large intrusion would probably produce a distinctive gravity and magnetic anomaly, which is not observed in the
Deccan. Sen et al. (2006) have proposed that the eruption
duration was only 23 000 years, which would give an eruption rate of 87 km3/year. The chamber volume would then
expand to 50 000^130 000 km3, which seems to be unrealistically large and would certainly be evident from geophysical measurements.
The steady-state and simple growth models presented
above produce a single CSD. Clearly, the different GPBs
have different CSDs and hence the situation must be more
complex. There are a number of processes that can change
the CSD. Compaction of a crystal mush involves loss of
intercrystal fluid, whereas mixing with an aphyric
magma (dilution) increases the amount of intercrystal
fluid; hence the two processes are texturally similar. In
neither case is Cl changed, hence the analysis discussed
above is unchanged. On a plot of Cl vs volumetric phase
proportion compaction will displace the points to the
right, whereas dilution will produce a vector towards the
left. Within each packet of GPB flows, or even within a
single GPB flow, the variation of Cl exceeds analytical
error, hence dilution and compaction cannot account for
all the textural variation. Clearly, another mechanism
must be invoked that can change the Cl. If the overall crystal growth rate increases, then the Cl will also increase.
However, we would then expect to see a correlation
between Cl and the volumetric abundance of plagioclase,
which is not observed. Hence, pure kinetic steady-state
models do not seem to be able to account for textural
observations. Before we leave pure kinetic models the
approach of Sen et al. (2006) must be discussed.
Crystals grow not only in chambers, but also in conduits.
Sen et al. (2006) proposed that plagioclase megacrysts grew
in a ‘chicken-wire’ mesh across a conduit, bathed by a
quasi-continuous flow of magma. They applied the
Johnson^Mehl^Avrami equation and found a value of
1580 years, for a growth rate of 109 mm/s. If they had
used a growth rate of 1010 mm/s, as above, then they
would have found a growth time of 15 800 years. It seems
unlikely that a dyke would have been continuously active
for such a long period of time. Also, this model does not
fit the idea of Sen (2001) that the GPBs develop during
times of magmatic repose. Finally, it will be shown that
the textures of the megacrysts do not seem to support such
a model.
The lack of small megacrysts in the GPB magmas suggests that the system is returning towards an equilibrium
texture by coarsening. This is the process by which small
Face growth rate
JOURNAL OF PETROLOGY
Rate{001}
Rate{010}
Chemical potential gradient
Fig. 10. Schematic illustration of the response of plagioclase crystal
faces to the chemical potential gradient around the growing crystal.
It is assumed that the growth rates of the {110}, {101} and {1 10}
faces are the same as that of the {001} faces. The overall size of the
illustrated crystal reflects both growth rates and crystallization time.
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HIGGINS & CHANDRASEKHARAM
SUB-VOLCANIC MAGMA CHAMBERS
in situations where there is a high chemical potential gradient around the growing crystal (Fig. 10). This is because
the growth rate of the {010} faces responds less than the
other faces to increases in the chemical potential gradient.
It should be noted that experimental studies of plagioclase
morphology vs growth have been primarily concerned
with growth forms at large degrees of undercooling, such
as the transitions from laths to skeletal forms to spherulites
(Lofgren, 1974). Here we are concerned with variation of
the shape of euhedral crystals, which form at much lower
degrees of undercooling where time constraints limit
experiments.
The chemical potential gradient around a growing crystal is controlled by the addition of crystal nutrients and the
removal of unwanted components. These occur in response
to diffusion and advection (relative movement between the
crystal and the growing medium). The ratio of the time
scales of mass transfer by advection and diffusion is
called the mass transfer Peclet number (PeMT) and is
defined as
PeMT ¼
PeT ¼
vlc
k
where v and l are defined as before, is the density, c is the
specific heat and k is the thermal conductivity. If the thermal Peclet number is high then latent heat will be removed
by advection rather than conduction. As before, the thermal Peclet number can be estimated for a basaltic magma
under typical conditions where the density is 2730 kg/m3
(from calculations using PELE; see section 41), the thermal conductivity is 14 W/m per K (Horai & Susaki, 1989)
and the specific heat is 840 J/kg per K (Clarke, 1966).
A crystal size of 10 mm and an advection velocity of
1mm/s (as above) would give a thermal Peclet number
of 16. Hence, under these conditions heat is removed by
advection and not conduction.
The shape of the crystals can therefore give an idea of
the environment of crystallization. The most tabular
plagioclase crystals must have grown in an environment
with the strongest advection, that is shearing or stirring.
Crystal orientation and clustering
vl
D
where v is the velocity of the growing medium with respect
to the crystal, l is the length scale (size of crystal), and D is
the chemical diffusivity (diffusion coefficient). If the Peclet
number is high then advection will renew the supply of
nutrients around the crystal and the chemical potential
gradient will be greater than that which is possible by diffusion alone. In the case of microlites a high growth rate
ensures a high Peclet number because the ends of the crystal outpace the growth of the depleted zone around the
crystal. Mechanical movement of magma (‘stirring’) by
convection currents can also ensure a high Peclet number
and has been proposed as an explanation for tabular crystals (Kouchi et al., 1986; Higgins, 1991). Such conditions
may occur near the margins of magma chambers, where
there is a significant velocity gradient.
The mass transfer Peclet number in a magma chamber
can be estimated for a basaltic magma under typical conditions. Chemical diffusivity varies with magma composition, temperature and element (Chakraborty, 1995). Order
of magnitude values for a mafic magma at 13008C are
1011 m2/s for network-modifiers such as Ca and Mg,
109 m2/s for Na and K, and 1012 m2/s for network formers
such as Si. If we take a typical value of D of 1011 m2/s, a
length scale equal to the length of a typical crystal, 10 mm,
then a convection velocity of 1mm/s would give a Peclet
number of 106. Hence, advective transport is much more
important than diffusive transport where crystals are
exposed to convection currents.
The thermal Peclet number (PeT) can also be used to
determine if transport of latent heat away from a growing
crystal is controlled by advection or diffusion. Here the
equation is
If the crystal mush was transported from the magma
chamber by laminar flow then the relative orientation of
the crystals could be preserved. We would then expect
that well-foliated samples formed in a strongly sheared
environment and hence would have the most tabular
crystals. The absence of a significant correlation between
crystal shape and quality of foliation suggests that
transport was turbulent.
Although quantitative measures of the overall orientation of crystals do not appear to be very helpful, qualitative
observations of short-range order may help clarify the petrogenesis of the GPB. In many samples tabular plagioclase
occurs as clusters of sub-parallel crystals. This structure
has been termed synneusis (‘swimming together’) and is
considered to form during flow (Schwindinger, 1999).
The driving force is the minimization of the surface
energy of the crystal aggregate (Ikeda et al., 2002), as in
the processes of coarsening, evidenced here by the shape
of the CSDs.
Some samples have radiating clusters of crystals, rather
than sub-parallel aggregates (e.g. MH-04-12; Fig. 4). It
could be significant that these samples are poorly foliated
and have CSDs with the lowest slopes. This suggests that in
some samples a later phase of crystal growth may have
occurred rapidly in a static environment. We will now discuss the occurrence of plagioclase megacrysts in other
igneous rocks, to allow the creation of a plausible model
for the origin of GPB.
Plagioclase megacrysts in other volcanic
and plutonic rocks
Basaltic lavas with plagioclase megacrysts similar to the
GPBs are uncommon but widespread. They occur in
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JOURNAL OF PETROLOGY
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oceanic basalts in a wide variety of settings: spreading
ridges, intraplate hotspots, aseismic ridges and fracture
zones (Cullen et al., 1989). Plagioclase megacrysts occur in
certain phases of the Surtsey volcano and sporadically in
the Eldfel lavas (Furman et al., 1991). They are also well
documented on the northern Galapagos Islands (Cullen
et al., 1989). Here, the plagioclase megacrysts are heterogeneously distributed and have a texture very similar to the
GPBs: plagioclase crystals are tabular and occur in subparallel glomerocrysts. The cores of the megacrysts are
more calcic than the rims, which are in equilibrium with
the host magma, as in the GPBs.
Many mafic intrusions contain layers or zones of rocks
rich in plagioclase, parts of which may be laminated. For
instance, the upper border zone of the Skaergaard intrusion, Greenland, is partly composed of well-laminated
leucogabbro (Naslund, 1984). The position of this unit
necessitates that it formed by accumulation of plagioclase
by flotation. This reflects the greater density of evolved,
iron-rich, magmas with respect to plagioclase (Scoates,
2000). The upper part of the Sept I“les intrusive suite,
Canada, is dominated by anorthosite, some of which is
well laminated with tabular crystals (Higgins, 1991).
Recently, Higgins (2005) has proposed that these rocks
were also formed by flotation of plagioclase. Partial disruption of these plagioclase cumulates by granitic fractionates
shows that such accumulations remained very loose and
poorly cemented, even at high crystal concentrations.
In both cases convection currents would have brought
plagioclase primocrysts up from lower levels and their low
density would have allowed them to remain in the upper
border zone. These currents would have produced a local
environment with high Peclet numbers and hence the plagioclase crystals would have tended to grow with a tabular
habit. It is now possible to integrate field data, quantitative
textural measurements and the occurrence of megacrysts
in other environments in the form of an emplacement
model for the GPBs.
Emplacement model of GPB flows
We propose that a cycle of magmatism started with the
eruption of ‘normal’ basalts, with few or no megacrysts
(Fig. 11a). Such magmas were derived from the mantle,
but in most cases must have been stored deep in the crust,
where there was differentiation and possibly contamination
by the lower crust. No large magma chambers are envisaged, but more localized swellings in the conduits.
Magmas rise to the surface along faults, probably related
to rifting (Hooper, 1990). Such a cycle may end when the
magma is unable to penetrate the lava pile or perhaps
when the production rate of magma wanes. At this point
erosion of fresh lavas may produce a regolith, now
preserved as the ‘red boles’.
The next stage of the cycle starts with plutonism and
may continue directly without a pause. Magmatism
NUMBER 5
MAY 2007
continues, perhaps at a lower rate, but the magma now
starts to accumulate at depth. The most likely place for
this is along the interface between the lava pile and the
basement, where there is likely to be a density contrast
(Fig. 11b). The magma may wedge its way out between the
base of the lava pile and the basement (right side of
Fig. 11b), or tectonic forces may produce a rift (central
part of Fig. 11b). The chamber will be filled gradually
with hot, new magma, ensuring that vigorous convection
occurs. Crystallization of plagioclase and mafic minerals
produces dense, iron-rich magmas and basal cumulates.
Plagioclase will float if its density is less than that of the
magma, but the density difference is not usually large.
Plagioclase crystals may nucleate and start to grow at
depth and be wafted by convective currents up to the roof,
where some crystals will tend to remain, or they may
nucleate and grow at the top of the chamber. Passage of
convection currents will keep the crystals bathed in hot
magma, such that they remain close to their liquidus temperature and coarsen. Such currents will also ensure a high
Peclet number environment conducive to the crystallization of tabular crystals. The upper border zone of the
magma chamber will comprise a loose crystal mush that
is easily remobilized. Mafic minerals will be denser than
the magma and will continue to accumulate along the
base of the magma chamber. This situation could continue
until the magma has solidified completely as a pluton, or is
disrupted by tectonic forces.
The next stage is the transport of the megacryst-bearing
magma to the surface. Reopening of a channel from the
partially solidified magma chamber to the surface might
result from rejuvenation of existing faults, or by the initiation of new faults (Fig. 11c). Once a conduit has formed
magma will be drained from the chamber and feed flows
of GPB basalts. The magma drawn from the uppermost
layer will be rich in megacrysts and may mix turbulently
with more evolved magmas devoid of crystals drawn from
lower levels of the magma chamber. Some magma may
continue to crystallize in higher-level static staging areas,
producing radiating clusters of crystals. Once magma is
drained from the chamber the cycle of normal basalt
flows may recommence.
Sen et al. (2006) have proposed two models for the origin
of the GPB, the first of which somewhat resembles the model
proposed above. In their first model the plagioclase megacrysts form statically on the walls of a magma chamber in
the advancing solidification front (Marsh, 1996). However,
we have suggested above that the plagioclase megacrysts
preserve evidence of accumulation by flotation and crystallization in a dynamic regime, neither of which processes is
included in the model of Sen et al. Their second model,
which they developed quantitatively, proposes that the
megacrysts grew in a magma conduit, where they formed a
‘chicken-wire’ network. Magma flowing along the conduit
896
HIGGINS & CHANDRASEKHARAM
SUB-VOLCANIC MAGMA CHAMBERS
(a) Eruption of ‘normal’ flood basalts
Magma
conduit
Faults
(b) Surface weathering and sub-volcanic plutonism
Red bole
Magma chamber
Plagioclase
Coarsening
flotation
cumulates
Magma
conduit
Hot, rising
magma
(c) Draining of magma chamber and eruption of Giant Phenocryst Basalts
Magma chamber
Plagioclase
flotation
cumulates
Mafic cumulates
Magma
conduit
Fig. 11. Model for the formation of Giant Plagioclase Basalt flows, Deccan.
fed the crystals, accounting for their great size. Subsequent
flow disrupted the network and incorporated the megacrysts
into a low-crystallinity magma. This model has the advantage that the crystals grew in a dynamic environment, but
cannot produce the observed textures: there is no reason
why plagioclase should occur as groups of aligned crystals.
It also seems unlikely that a chicken-wire network could
exist for the 1538 years that Sen et al. (2006) calculated.
Finally, the theory does not account for the presence of
similar plagioclase crystals at the top of sub-volcanic
magma chambers such as Skaergaard.
CONC LUSIONS
Plagioclase megacrystic basalts are a widespread, but variable component of flood basalt provinces. They should be
viewed as evidence of magma storage in the crust.
Plagioclase is the most common mineral to occur as
megacrysts in these rocks because it is the only mineral
that can float in evolved basaltic magmas. Hence, the
presence of plagioclase megacrysts indicates that
sub-volcanic magma chambers were an important component of the magmatism. The Skaergaard intrusion,
Greenland, is probably a good model for the magma
897
JOURNAL OF PETROLOGY
VOLUME 48
chamber in which the megacrysts of the GPB grew. Larger
sub-volcanic intrusions, such as Sept I“les, Canada, could
produce the megacrysts, but would produce a significant
geophysical anomaly, which is not observed.
Quantitative studies of plagioclase megacrysts can provide information on the duration of crystallization and
possibly the size of such magma chambers, but cannot
give an idea of the total eruption duration, as has been
proposed by Sen et al. (2006). This is because there is
no evidence that all magmas have passed through the
same magma chamber. It is likely that sub-volcanic
magma chambers existed intermittently, just before the
GPBs formed. At other times magma passed almost
unchanged from the lower crust or the mantle source to
the surface.
AC K N O W L E D G E M E N T S
This research was partly funded by operating grants from
the Natural Science and Engineering Research Council of
Canada to M.D.H. We would like to thank Hazel Jenkins,
Melroy Borges, Gautam Sen and Ayaz Alam. The
manuscript was improved by the insightful reviews of
Dougal Jerram, James Scoates and Dick Naslund.
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898
HIGGINS & CHANDRASEKHARAM
SUB-VOLCANIC MAGMA CHAMBERS
A P P E N D I X : C A L C U L AT I O N O F
F R EQU E NC Y DE NSI T Y
The modal value of the 2D aspect ratio is not easy to determine precisely for strongly skewed distributions, as we have
here. The conventional approach is to use bins of fixed
width and compile a simple frequency diagram
(e.g. Higgins, 1994). If too many bins are used then the frequency diagram is very irregular, whereas too few do not
make it possible to define exactly the modal value. A new
diagram of frequency density is proposed here, which is
analogous to simple population density diagrams for
CSDs (Higgins, 2006a). The 2D aspect ratios of the intersections ( values) are sorted into ascending order and
divided into a number of bins, each with approximately
the same number of intersections. Ten bins were used here,
so each bin had 20^40 intersections. The frequency density
is the number of intersections in each bin divided by the
difference between the upper and lower 2D aspect ratio
bounds and the total number of intersections. This diagram has the advantage that the 2D aspect ratio bins are
narrowest around the modal value and hence can define it
precisely (Fig. A1). An example of the calculation is shown
in Table A1.
6
Fixed number bins
5
Frequency density
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4
3
2
1
Fixed width bins
0
0
0.2
0.4
0.6
0.8
1
2D aspect ratio
Fig. A1. Comparison of 2D aspect ratio diagrams compiled with
fixed width and fixed number bins. The peak is defined by two intervals for fixed width bins but six for fixed number bins.
899
JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 5
MAY 2007
Table A1: Calculation of frequency density for sample MH-04-04
values of crystal intersections
00371, 00427, 00566, 00594,
mean
range
Number of
Frequency
intersections
density
00737
00883 00371 ¼ 00512
28
2186
01025
01100 00892 ¼ 00208
28
4568
...
...
...
...
00631, 00653, 00664, 00676,
00687, 00732, 00736, 00745,
00745, 00761, 00765, 00779,
00794, 00795, 00801, 00822,
00831, 00842, 00844, 00862,
00872, 00877, 00880, 00883
00892, 00914, 00942, 00954,
00973, 00982, 00987, 00992,
01005, 01010, 01014, 01015,
01020, 01029, 01034, 01042,
01042, 01050, 01051, 01056,
01059, 01064, 01077, 01087,
01097, 01105, 01107, 01100
...
is the 2D aspect ratio of measured crystal intersections. Ten bins of 28 or 29
intersections made the total of 284 intersections.
900