Geochemistry
Geophysics
Geosystems
Article
3
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Volume 1
May 30, 2000
Paper number 1999GC000017
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Published by AGU and the Geochemical Society
ISSN: 1525-2027
Role of a Cl-bearing flux in the origin of depleted
ocean floor magmas
Roger L. Nielsen and Rachel E. Sours-Page
College of Oceanic and Atmospheric Sciences, 104 Oceanic Administration, Oregon State University, Corvallis,
Oregon 97331-5503 ([email protected])
Karen S. Harpp
Department of Geology, Colgate University, 13 Oak Drive, Hamilton, New York 13346
.
[1]5 Abstract: Plagioclase-hosted melt inclusions from ocean floor lavas are characterized by great
diversity in their minor and trace element compositions. The incompatible element contents of the
inclusions range from enriched to ultradepleted. Ultradepleted inclusions in lavas sampled from areas
such as the Galapagos Platform and the Endeavour Segment of the Juan de Fuca Ridge have Ti/Zr
values as high as 3000. Such high Ti/Zr (low Ti and much lower Zr, as little as >2 ppm) suggest a
source material that was melted past the point of the exhaustion of clinopyroxene (harzburgite melting).
Nevertheless, these same inclusions have (La/Sm)n>0.25 (although with low La and Sm
concentrations), low K2O (0.01 ±0.04 wt%), and high Cl (up to 1600 ppm). In addition, examination
of melt inclusion data from over 30 locations worldwide shows that there is a correlation between the
level of enrichment of the host lava and the trend in K2O versus Cl described by the inclusion
population. Melt inclusions from enriched lavas are characterized by relatively low Cl contents at a
given K2O and no high Cl inclusions. A wide range of Cl contents and uniformly low K2O characterizes
inclusions from depleted lavas. Transitional lavas exhibit either intermediate slopes or two separate
trends. In contrast, other incompatible elements, such as P, Ti, and the high field strength elements
(HFSE), behave coherently. The paradox presented by high Cl and (La/Sm)n together with low HFSE
and K2O in melt inclusion populations from depleted lavas, coupled with uniformly high incompatible
element contents in enriched lava inclusions, is inconsistent with their derivation by variable degrees of
melting. Correlation of Ti/Zr with Cl in the ultradepleted inclusions, plus the absence of high Cl
contents in inclusions from enriched lavas supports the contention that this signal is a mantle
phenomenon and not the result of alteration at or near the seafloor. The close association of
ultradepleted and normal mid-ocean ridge basalt (NMORB) melt inclusions, sometimes in the same
phenocryst, together with the existence of a complete array of melt compositions (i.e., not two distinct
populations), demonstrates that the magma types were part of the same magma production episode.
This combination of characteristics exhibited by melt inclusions may be produced by the interaction of
a harzburgite source with a Cl, light rare earth element ± bearing fluid derived from deep hydrothermal
circulation. Fluctuation of magma supply may allow the upper mantle to be periodically cooled and
altered by hydrothermal fluids, then undergo melting during resurgence of the magma supply. The
ultradepleted component produced by that fluxed harzburgite may comprise as much as 5± 10% of the
array of magmas that make up NMORB magmas. If this is true, it has important implications for the
thermal and mass budget and perhaps the rheology of the upper mantle and lower crust.
Keywords: Melt inclusions, MORB, ultradepleted.
Index terms: Igneous petrology; minor and trace element composition; magma migration; hydrothermal systems.
C o p y r i g h t 2000 by the American Geophysical Union
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
1999GC000017
Received October 1, 1999; Revised January 3, 2000; Accepted January 10, 2000; Published May 30, 2000.
Nielsen, R. L., R. E. Sours-Page, and K. S. Harpp, 2000. Role of a Cl-bearing flux in the origin of depleted ocean floor
magmas, Geochem. Geophys. Geosyst., vol. 1, Paper number 1999GC000017 [10,919 words, 6 figures, 3 tables].
May 30, 2000.
1. Introduction
The diversity of primary magmas for a
specific suite of lavas is a critical, if rarely
obtained, piece of information. Nevertheless, it
is generally agreed that such information is
necessary to fully understand mantle processes.
One of the sources for information on parent
magma diversity that has been increasingly
utilized in the past decade has been melt inclusions hosted in compositionally primitive phenocrysts [Sobolev and Shimizu, 1992; Sobolev
et al., 1992; Sobolev and Shimizu, 1993; Sobolev and Shimizu, 1994; Shimizu, 1994; Sobolev
et al., 1994; Nielsen et al., 1995a; Sours-Page et
al., 1999]. These inclusions trap melts that have
undergone significantly less mixing and fractionation than the host lava suite and therefore
exhibit more of the original chemical signal
imparted on them by source diversity and melting processes.
[2]5
Much of the existing work on melt inclusions describes their diversity and relationship
to the host suites. Examination of such data
shows that although more diverse, the vast
majority fall within the range of compositions
characteristic of their host magmas [Nielsen et
al., 1995a; Sours-Page et al., 1999]. However,
melt inclusions are generally more primitive
than their host lavas, consistent with the primitive character of their host phenocrysts.
[3]5
To date, relatively little has been done to
incorporate melt inclusion data into our concepts of how mantle processes create the observed diversity in erupted lava suites. Part of
[4]5
the reason for this is that many characteristics
of the variation exhibited by melt inclusion data
fall within our existing models of mantle processes. Therefore, in many cases they have
been used to provide additional details of a
picture whose outlines were already drawn.
One specific characteristic where this is not
the case is the independent behavior of K, the
light rare earth elements (LREE), and the high
field strength elements (HFSE) as described by
Sours-Page et al. [1999] for melt inclusion data
from the Juan de Fuca Ridge. The purpose of
this paper is to pursue this topic in detail and to
add evidence in the form of Cl concentrations
for the inclusions and their host lavas and to
further describe the behavior of these element
groups based on an extensive, globally distributed collection of melt inclusion data.
On the basis of these new data we propose a
model for the origin of a significant mid-ocean
ridge basalt (MORB) component, the ultradepleted melts [Sobolev and Shimizu, 1992; Shimizu, 1994; Nielsen et al., 1995a; Sours-Page
et al., 1999], and their relationship to the more
enriched parts of the array of magmas that
constitute the parental components of ocean
floor magmas.
[5]5
2. Geologic Setting and Samples
The majority of the inclusion compositions
described in this paper were derived from
plagioclase ultraphyric ocean floor lavas. This
strongly porphyritic rock type is widespread in
the mid-ocean ridge and some hot spot environments but generally low in abundance in any
[6]5
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
particular suite (AMAR [Frey et al., 1993],
SEIR [Christie et al., 1995], Amsterdam/St.
Paul [Douglas, 1998], FAMOUS [Langmuir
et al., 1977], East Pacific Rise, [Hekinian and
Walker, 1987; Batiza et al., 1989], Gorda Ridge
[Davis and Clague, 1987; Nielsen et al.,
1995a], and Juan de Fuca Ridge [e.g., Karsten
et al., 1986, 1990; Sours-Page et al., 1999].
In this paper we report on data we have
collected on melt inclusions from lavas
sampled from over 30 locations worldwide.
We use data from two of those areas, the
Endeavour Segment of the Juan de Fuca Ridge
and the northeastern Galapagos Platform, to
focus on the detailed systematics exhibited by
melt inclusion compositions. Samples from
these two locations are similar in that they
contain abundant (10 ± 30%), large (2 ± 20
mm) plagioclase crystals, with relatively minor
olivine and chromite. The plagioclase phenocrysts exhibit a range of An85 ± 94.
[7]
[8] The Galapagos Platform samples were
dredged from a small seamount on the northeastern edge of a plateau produced by volcanism from the Galapagos Hotspot in the eastern
Pacific Ocean (PL02-13-12 [Sinton et al.,
1993]). As with many of the plagioclase
ultraphyric lavas and most of the lavas erupted
on that part of the platform, PL13-12 is depleted
and relatively primitive (MgO, 9.2 wt%).
The Endeavour Segment is the northernmost segment of the Juan de Fuca Ridge in
the northeast Pacific Ocean. It has a slow to
moderate spreading rate with an average
spreading rate of 29 mm/yr, half rate [Karsten
et al., 1986]. The specific sample used for this
study is a depleted lava (O-2), dredged from an
abyssal hill adjacent to the South Endeavour
Valley [Karsten et al., 1990].
[9]
This study focuses primarily on trapped
melts hosted in anorthitic plagioclase. This is
[10]
1999GC000017
largely due to the many advantages of working
with plagioclase versus olivine or pyroxenehosted inclusions when looking for the spectrum of primary magmas. First, the presence of
large numbers of melt inclusions in plagioclase
allows us to evaluate the diversity of melt
inclusion compositions and volumetric proportion of the parental melts [Nielsen et al., 1995a;
Sours-Page et al., 1999]. Second, we can
constrain the melt inclusion entrapment temperature using the temperature at which olivine
daughter crystals melt back into the glass.
Because the inclusions are hosted in plagioclase and are primitive in composition, they
may be assumed to be multiply saturated.
Third, plagioclase has slow reaction rates relative to olivine, therefore reducing the likelihood that the phenocryst would reequilibrate
with the surrounding magma during subsequent
transport [Gaetani and Watson, 1999]. Fourth,
anorthitic feldspar can be produced from normal ocean floor magmas only at low pressure
[Sinton et al., 1993; Fram and Longhi, 1992;
Nielsen et al., 1995a; Panjasawatwong et al.,
1995]. Therefore we can constrain the depth of
entrapment to the uppermost mantle or lower
crust. Finally, because plagioclase has such low
concentrations of the rare earth and high field
strength elements, it is unlikely that interaction
with the host crystal would contaminate the
melt inclusion.
[11] In spite of the several advantages noted
above, there are limitations to using plagioclase-hosted melt inclusions: (1) plagioclasephyric samples only tell the history of the
magma beginning with plagioclase saturation;
(2) plagioclase-phyric lavas are only a subset of
the magmas erupted and therefore must be
treated with caution when compared to aphyric
MORB. Their anomalously plagioclase-rich
mineralogy, plus the range of melt inclusion
compositions found in each lava, suggests that
these plagioclase ultraphyric lavas represent
merely one set of many processes that produce
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1999GC000017
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
oceanic basalts. Nevertheless, the relationships
among such melt inclusions, their host lavas,
and abyssal peridotites demonstrate that the
inclusions represent a significant component
of the spectrum of processes that generates
mantle melts.
3. Experimental and Analytical
Procedure
3.1. Rehomogenization Technique
The crystals used for this study were
removed from the sample after a coarse crushing. Rehomogenization of the melt inclusions
is performed by suspending individual crystals
by 0.003-inch-thick Pt wire or a Pt boat in a 1atm gas mixing vertical quench furnace. Crystals are held at 10008C for 20 min, then heated
to the rehomogenization temperature for between 0.5 and 2 hours. Basic phase equilibria
and our experience have demonstrated that the
entrapment temperature is generally correlated
with the anorthite content of the feldspar host.
The specific temperature is constrained by
running a set of incremental heating experiments at 108 intervals in the range of 12008±
1270 8C. Attainment of the appropriate rehomogenization temperature can be confirmed
using phase equilibria modeling [Johnson et
al., 1996]. If the calculations indicate multiple
saturation with olivine and plagioclase, we
assume that the temperature used for rehomogenization was close to the entrapment temperature.
18.5
18.0
17.0
16.5
16.0
melt inclusions
PL13 lavas
15.5
a
15.0
1.0
0.8
0.6
0.4
[12]
[13] The veracity of rehomogenized inclusion
compositions can be evaluated using a number of independent criteria: (1) the phase
equilibria of the inclusion relative to those
of the host suite, (2) the morphology of the
inclusion, (3) the Mn content of the inclusion relative to the host suite, and (4) the S
content of the rehomogenized inclusion
[Nielsen et al., 1998].
Galapagos Platform
17.5
Al2O3
3
TiO2
Geochemistry
Geophysics
Geosystems
0.2
b
0.0
450
400
Cl (ppm)
350
300
250
200
150
100
50
0
8.0
c
8.5
9.0
9.5
10.0
10.5
11.0
MgO
Figure 1. Electron microprobe analyses of rehomogenized melt inclusions and volcanic glasses
from dredge 13 on the Galapagos Platform. (a)
Al2O3 versus MgO. Note the coherence of the
inclusion and lava trends and the small range of
Al2O3 at any given MgO. (b) TiO2 versus MgO.
Note the greater diversity of TiO2 in the inclusion
population, compared to the lavas from the PL02-13
dredge. (c) Cl versus MgO. Note the wide range of
Cl contents in both the lavas and the inclusion
populations. All melt inclusions are from phenocrysts extracted from a single sample (PL02-13-12,
MgO = 9.52). The standard deviation of replicate
analyses is approximately twice the size of the
symbols.
[14] If the inclusions are genetically related to
the host suite, their compositions should lie on
or near the major element trends described by
that suite. This assumes that if the inclusions
are properly rehomogenized, the olivine +
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
1999GC000017
plagioclase cotectic for the inclusions should be
near that of the host lava suite. In the case of
data from the Galapagos Platform (Figure 1a)
the lavas and inclusions form a continuous
trend in MgO and Al2O3, with the inclusions
exhibiting more primitive compositions than
most of the host suite from that dredge. If the
inclusions are contaminated or if they have
been under- or overheated, their compositions
will lie off the trend.
wich et al., 1980]. Cl peak positions were
calibrated on tugtapite (Astigmex Inc. standard). USNM 113498/1 (Makapuhi Lava) was
used as an internal standard (200 ppm Cl).
Sodium was counted first because of its susceptibility to beam damage [Nielsen et al.,
1995b]. Major elements were counted for 10±
20 s, while elements in low concentrations
required longer counting times (P, 300 s; K,
100 s; Cl, 1000 s).
[15] Inclusions that have been compromised or
contaminated are also characterized by irregular morphologies, anomalously high Mn contents, and, most important, low sulfur contents.
In an investigation of physically and chemically compromised inclusions, Nielsen et al.
[1998] reported that all rehomogenized inclusions from MORB that are not physically
breached or chemically compromised fall on a
line in S-FeO space describing sulfide saturation. Sulfur is lost from compromised inclusions during rehomogenization owing to its
relatively high volatility. Any small pathway
that allows contaminants into the inclusion
provides a pathway for S to vent from the
inclusion. All inclusions used in this study
contained over 500 ppm S, indicating that they
have not been breached.
3.3. Ion Microprobe
3.2. Electron Microprobe
[16] Major element analyses were performed
using the CAMECA SX-50 Electron Microprobe at Oregon State University to determine
the range, distribution, and frequency of melt
compositions. Analyses were carried out using
a beam current of 30 nA, an accelerating
voltage of 15 kV, and a defocused (5-mm)
beam. Smithsonian standards, including USNM
113498/1 (Makapuhi Lava) for Si, Al, Fe, Ca,
and Ti, USNM 133868 (Kakanuhi Anorthoclase) for Na, USNM 143966 (Microcline) for
K, and USNM 122142 (Kakanuhi Augite) for
Mg were used for glass calibrations [Jarose-
On the basis of the range of compositions
determined by electron microprobe, a subset of
large (>35-mm) inclusions was selected for
subsequent trace element analysis by secondary
ion mass spectrometry (SIMS). These measurements were conducted at the University of New
Mexico/Sandia National Laboratories using the
CAMECA ims 4f instrument. For these measurements, a filtered 16O primary beam was
accelerated through a 12.5-kV potential. A
typical 40-nA beam was focused to a 10- to
35-mm-diameter spot. The secondary ions produced from the bombardment of the sample
were accelerated through a nominal potential of
4.5 kV, to which a 60±90 V energy offset had
been applied. The energy acceptance window
was set at 30 ± 50 V full width. The mass
spectrometer was operated at low mass resolution (M/M = 320) in peak stepping mode
which included 18 mass stations (18.75background, 30Si, 47Ti, 88Sr, 89Y, 90Zr, 137Ba,
139
La, 140 Ce, 142 Pr, 146 Nd, 147 Sm, 151 Eu,
153
Eu, 163Dy, 167Er, 174Yb, 175Lu). Secondary
ions were detected using an electron multiplier
operated in pulse-counting mode. Magnetic
peak positions were calibrated with volcanic
glass standards. Absolute elemental concentrations were calculated by comparing the observed metal+/30Si+ ratios in the target to the
same ratio in a basaltic glass standard (working
curves). These standards were analyzed several
times each day. The external precision based on
[17]
Geochemistry
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
observations of the standard were 3% for Ti, Sr,
and Zr, and 10% for the REE except for Eu
(20%).
3.4. Inductively Coupled Plasma±Mass
Spectrometry (ICP-MS)
[18] For the purpose of host glass analysis,
clean glass was handpicked from the bulk
sample following a coarse crushing to 1±2
mm. All further handling and chemical procedures were conducted in HEPA-filtered Plexiglas clean boxes at Colgate University. All
reagents were pre-purified. Water was first
passed through sediment and carbon filters,
followed by a Millipore Elix reverse osmosis
and electrodeionization system. This feedwater
was processed through a Millipore Milli-Q
gradient system for final polishing. Trace metal
grade nitric and hydrofluoric acids were twice
distilled in perfluoroalkoxy (PFA) Teflon subboiling two-bottle stills.
Samples were handpicked to eliminate
glass fragments with visible alteration. Approximately 250 mg of clean glass chips were
digested in a closed PFA Teflon container with
4-mL HNO3/50 mg sample and 1-mL HF/50
mg sample for 40 hours [e.g., Harpp, 1995;
Longerich, 1993]. Where sample amount was
limited, smaller masses of glass were used;
reagent volumes were adjusted accordingly.
The solution was then evaporated to dryness.
The solid residue was dissolved in 2.5-mL
HNO3, transferred via repeated rinses with
purified water to 250-mL polyethylene bottles,
and diluted for a total solution mass of 1000
times the original sample mass. The final solution was a 1000-fold dilution of the original
sample in a 1% HNO3 solution.
[19]
[20] Concentrations of the trace elements were
determined on a Hewlett-Packard HP4500 ICPMS. Measurements were made using an on-line
internal standard correction consisting of a 1:20
1999GC000017
dilution of a 1 ppm 133Cs solution [e.g.,
Doherty, 1989; Eggins et al., 1997]. Contributions from polybaric oxide and doubly charged
interferences were consistently below 1%. At
least three replicate analyses of each solution
were carried out; precision is represented by
multiple analyses of the U.S. Geological Survey standard DNC-1 run repeatedly as an
unknown (Table 1).
4. Results
[21] For the majority of cases we have studied,
plagioclase-hosted melt inclusions are generally more primitive than their host lavas (Tables
2 and 3). Along with the lavas, the melt
inclusions describe a trend representative of
the low-pressure olivine + plagioclase cotectic
(Figures 1a and 1b [Sinton et al., 1993; SoursPage et al., 1999]. In contrast to the tightness
of the trend exhibited by the major elements the
minor and trace elements exhibit a significantly
greater range in concentration at any given
MgO concentration (Figure 1c). On the basis
of these observations most investigators have
concluded that melt inclusion populations are
genetically related to the host lava suite but
have not undergone the same degree of mixing
and fractionation.
[22] In addition to the greater observed diversity
in the melt inclusion populations compared to
the host lava suite, most minor and trace elements trends lead to extremely low concentrations. More important the data exhibit minor
and trace element diversity that is not coherent
between elements that are generally considered
to have similar geochemical characteristics. For
example, if we examine data from the samples
from the Endeavour Segment and the Galapagos Platform (Figures 1±3), we see that the
incompatible elements P, Ti, Zr, and the heavy
rare earth elements (HREE) exhibit a high
degree of correlation with one another. In
contrast, the degree and pattern of correlation
Geochemistry
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1999GC000017
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
Table 1. Inductively Coupled Plasma ±Mass Spectrometry DNC1 Precision Determination, With Standard
Run As an Unknown
Element5
Sc5
Cr5
Ni5
Cu5
Rb5
Sr5
Y5
Zr5
Nb5
Ba5
La5
Ce5
Pr5
Nd5
Sm5
Eu5
Gd5
Tb5
Dy5
Ho5
Er5
Tm5
Yb5
Lu5
Hf5
Ta5
Th5
U5
Mass5
Mean5
45
535
605
635
85
865
895
905
935
1375
1395
1405
1415
1465
1475
1535
1575
1595
1635
165
1665
1695
1725
175
1785
1815
2325
2385
28.125
262.925
245.065
85.725
3.535
145.315
17.75
35.695
1.545
102.765
3.695
8.095
1.095
4.905
1.435
0.585
1.965
0.395
2.725
0.635
1.935
0.305
1.925
0.295
0.95
0.095
0.245
0.05
Standard Deviation5
1.565
12.65
11.895
3.115
0.035
3.505
0.15
0.35
0.025
1.165
0.05
0.115
0.025
0.075
0.035
0.015
0.045
0.015
0.095
0.025
0.075
0.015
0.085
0.015
0.065
0.015
0.005
0.015
RSD%5
Count
5.535
4.815
4.85
3.625
0.875
2.415
0.85
0.995
1.285
1.125
1.395
1.365
1.535
1.535
2.045
2.505
2.145
3.345
3.265
3.585
3.435
3.685
3.975
4.25
5.85
5.745
1.075
12.845
6
6
6
6
6
8
6
6
6
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
6
6
All concentrations are in ppm.
between K, Cl, Ba, and the light rare earth
elements (LREE) with each other and with the
group mentioned above are much more complex than one could predict based on their
uniformly low partition coefficients.
In most cases, melt inclusions sampled
from within a single phenocryst or within a
single band of melt inclusions in a phenocryst
exhibit relatively little variation. However, it is
not uncommon for different bands of inclusions
in a single phenocryst to exhibit significantly
different trace element compositions, occasionally ranging from ultradepleted to NMORB.
Ultradepleted inclusions have never been found
in association with enriched mid-ocean ridge
basalt (EMORB) inclusions.
[23]
Fractional crystallization of most basaltic
magma suites will normally generate a derived
suite characterized by a high degree of correlation among the incompatible elements. Therefore, to better evaluate the range of compositions
represented in the primary magma components,
we have calculated a fractionation correction
for the all melt inclusion data reported here
(note ``fc'' subscript on figures with values so
corrected). This correction is based on an
evaluation of the amount of fractionation separating the inclusion composition from its primary composition. In this case we assumed that
the primary magmas were in equilibrium with
Fo92 olivine. Such a magma would have an
Mg# (atomic Mg/(Mg + Fe)) of ~75. The
COMAGMAT 3.5 [Ariskin et al., 1999] differ-
[24]
94-31
xtl2.2
94-41
xtl5.2
94-41
xtl10.1
94-41
xtl10.2
94-41
xtl10.3
94-41
xtl15.2
94-58
xtl5.2
94-58
xtl5.3
94-74
xtl8.1
94-74
xtl9.1
94-74
xtl10.2
94-74
xtl10.3
94-74
xtl10.4
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
Cr2O3
S
Total
48.63
0.64
17.09
8.81
0.11
9.85
13.02
1.76
0.02
0.06
0.04
0.08
100.11
49.33
0.36
17.65
7.37
0.11
10.61
13.14
1.55
0.02
0.02
0.04
0.06
100.36
48.44
0.44
18.13
7.49
0.16
10.69
12.98
1.65
0.01
0.01
0.04
0.08
100.11
49.26
0.67
16.90
8.70
0.12
9.51
12.74
2.02
0.02
0.04
0.04
0.09
100.12
48.71
0.50
17.88
7.55
0.13
10.37
12.93
1.88
0.02
0.02
0.04
0.07
100.09
48.68
0.49
17.73
7.71
0.13
10.31
12.97
1.92
0.03
0.02
0.03
0.07
100.09
48.01
0.78
17.92
7.72
0.11
10.62
12.96
1.78
0.01
0.07
0.03
0.07
100.08
49.81
0.49
16.61
8.91
0.14
9.31
12.91
1.73
0.03
0.03
0.04
0.08
100.10
49.14
0.75
17.18
8.31
0.16
9.79
12.91
1.72
0.02
0.02
0.04
0.09
100.13
48.99
0.82
17.18
8.42
0.14
9.80
12.80
1.77
0.02
0.04
0.04
0.08
100.10
48.63
0.58
17.22
8.52
0.13
10.03
13.02
1.79
0.02
0.05
0.03
0.10
100.13
48.88
0.47
17.62
8.15
0.14
10.19
12.95
1.55
0.03
0.02
0.04
0.09
100.13
48.96
0.28
17.63
7.95
0.13
10.21
12.88
1.93
0.02
0.01
0.03
0.08
100.11
48.82
0.31
17.60
7.70
0.12
10.36
13.14
1.93
0.02
0.01
0.03
0.07
100.11
48.91
0.18
17.75
7.60
0.11
10.43
13.18
1.79
0.02
0.02
0.03
0.06
100.08
Cl
Sr
Y
Zr
Ba
Lan
Cen
Ndn*
Smn
Eun
Gdn*
Dyn
Ern*
Ybn
56
146
11.0
27.2
4.8
2.6
3.9
5.8
8.7
13.4
8.4
8.1
7.0
6.1
32
150
11.5
4.6
3.1
2.7
3.7
5.0
6.9
11.9
8.2
9.8
8.0
6.5
44
119
11.7
8.1
3.1
2.8
4.2
5.9
8.1
13.1
8.6
9.1
8.5
7.9
81
152
12.5
25.4
4.8
2.8
3.7
5.3
7.6
16.0
8.2
8.9
9.1
9.3
47
121
14.8
18.8
2.9
3.1
4.4
6.2
8.7
14.1
10.0
11.6
10.9
10.2
51
121
13.9
15.9
3.3
3.1
4.3
6.6
10.2
12.0
10.2
10.2
8.8
7.7
47
118
17.1
38.6
3.7
3.9
5.4
7.8
11.4
11.9
12.8
14.4
13.1
11.9
73
122
14.8
2.2
3.1
2.3
3.6
5.7
9.2
7.3
10.4
11.8
10.6
9.6
40
120
7.8
11.7
3.6
2.1
3.4
5.0
7.2
9.4
6.4
5.7
4.8
4.1
295
141
20.4
33.0
3.2
4.2
5.8
7.5
9.7
11.4
12.0
14.8
14.0
13.3
85
120
11.1
21.8
2.5
2.7
3.3
5.4
8.8
11.6
8.5
8.2
7.3
6.4
118
127
11.0
6.3
3.0
3.1
4.2
5.5
7.3
10.6
7.4
7.5
6.9
6.4
380
121
2.8
0.5
2.7
1.5
2.4
2.8
3.2
11.8
3.2
3.1
1.9
1.1
429
116
3.0
0.8
3.1
1.8
2.1
2.6
3.3
8.9
2.8
2.5
2.1
1.8
410
113
3.0
0.3
2.8
1.6
2.2
2.8
3.5
12.6
2.9
2.4
1.9
1.5
1999GC000017
Major elements, S and Cl were analyzed by electron microprobe. Trace elements were analyzed by ion microprobe. FeO* is the total Fe. REE are chondrite-normalized. Nd, Gd, and Er
are interpolated values. Label designations represent the experiment number (e.g., 94-31) followed by a the crystal number and the analysis number for that crystal. Here 94-41 xtl10.1
and 94-41 xtl10.2 are two inclusions in the same crystal.
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
94-31
xtl2.1
G3
94-31
xtl1.1
Geochemistry
Geophysics
Geosystems
Table 2. Major and Trace Element Composition of Plagioclase-Hosted Melt Inclusions From PL02-13 (Galapagos Platform)
PL02-1327.A.1
PL02-1327.B.1_1
PL02-1327.B.1_2
PL02-1325.A.1
PL02-1325.C.1
PL02-1320.B.1
PL02-1320.C.1
PL02-1322.A.1
PL02-1322.B.1
PL02-1322.C.1
PL02-1329.A.1
PL02-1329.B.1
PL02-1329.C.1
47.94
0.73
17.15
8.36
0.11
9.99
12.91
2.17
0.02
0.04
0.08
0.08
99.58
48.98
0.72
15.92
9.35
0.15
8.56
12.77
2.32
0.02
0.05
0.06
0.10
98.99
49.22
0.69
15.92
9.53
0.13
8.53
12.66
2.41
0.02
0.04
0.05
0.10
99.28
49.14
0.73
15.75
9.71
0.27
8.43
12.77
2.35
0.02
0.04
0.06
0.10
99.37
48.64
0.70
15.53
9.50
0.23
8.30
12.83
2.32
0.02
0.06
0.06
0.10
98.30
48.68
0.70
15.47
9.40
0.14
8.25
12.99
2.38
0.02
0.04
0.05
0.10
98.22
48.57
0.70
15.78
9.38
0.18
8.43
12.49
2.40
0.02
0.04
0.04
0.11
98.14
49.11
0.70
15.95
9.40
0.13
8.56
12.39
2.20
0.02
0.04
0.06
0.10
98.66
48.73
0.74
15.38
9.38
0.16
8.44
12.81
2.27
0.02
0.05
0.06
0.10
98.13
48.81
0.75
15.49
9.29
0.16
8.38
12.83
2.31
0.02
0.04
0.06
0.11
98.25
48.61
0.74
15.75
9.36
0.12
8.68
12.73
2.23
0.02
0.05
0.05
0.10
98.43
48.74
0.72
15.82
9.42
0.18
8.69
12.69
2.32
0.02
0.04
0.05
0.10
98.80
48.74
0.72
15.57
9.15
0.14
8.42
12.78
2.34
0.02
0.05
0.06
0.10
98.08
48.79
0.71
15.86
8.96
0.15
8.53
12.66
2.33
0.02
0.05
0.05
0.09
98.20
48.62
0.72
15.83
8.94
0.14
8.54
12.68
2.34
0.02
0.05
0.05
0.10
98.02
48.83
0.73
15.98
8.98
0.18
8.68
12.62
2.33
0.02
0.04
0.06
0.10
98.55
144
42.9
439.7
96.3
7.9
376.3
26.7
60.3
0.97
27.5
4.5
5.0
7.0
8.7
11.6
12.1
12.4
13.6
13.1
13.0
12.9
12.6
12.3
11.8
1.58
2.49
0.07
0.75
117
42.9
439.7
96.3
7.9
376.3
26.7
60.3
0.97
27.5
4.5
5.0
7.0
8.7
11.6
12.1
12.4
13.6
13.1
13.0
12.9
12.6
12.3
11.8
1.58
2.49
0.07
0.75
200
28.6
254.3
96.4
1.7
127.4
19.1
30.0
0.53
10.2
3.2
2.9
4.4
5.6
7.5
8.5
8.1
8.8
8.5
8.5
8.4
8.2
7.9
7.7
0.87
2.75
0.04
0.69
248
248
139
146
125
231
241
268
243
246
271
267
260
1999GC000017
PL02-1322.B.1
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
Cl
Sc
Cr
Ni
Rb
Sr
Y
Zr
Nb
Ba
Lan
Cen
Prn
Ndn
Smn
Eun
Gdn
Tbn
Dyn
Hon
Ern
Tmn
Ybn
Lun
Hf
Pb
Th
U
PL02-1322.A.1
G3
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
Cr2O3
S
Total
PL0213-12
Geochemistry
Geophysics
Geosystems
Table 3. Major and Trace Element Composition of Lavas Dredged From the NE Galapagos Platform
Table 3. (continued)
PL02-1330.B.1
PL02-1330.C.1
PL02-1318.A.1
PL02-1318.B.1
PL02-1324.A.1
PL02-1324.B.1
PL02-1324.C.1
PL02-1326.A.1_1
PL02-1326.A.1_2
PL02-1326.A.1_3
PL02-1328.A.1_1
PL02-1328.A.1_2
48.63
0.77
16.02
9.06
0.18
8.77
12.70
2.34
0.03
0.04
0.06
0.10
98.68
48.08
0.75
15.78
9.11
0.10
8.76
12.90
2.36
0.02
0.05
0.04
0.11
98.07
48.77
0.75
15.85
9.31
0.18
8.81
12.84
2.38
0.02
0.05
0.06
0.09
99.11
48.31
0.71
16.95
8.62
0.16
9.28
12.86
2.24
0.02
0.05
0.05
0.09
99.34
48.56
0.73
17.09
8.53
0.13
9.52
12.96
2.24
0.01
0.05
0.06
0.10
99.99
47.74
0.72
16.77
8.54
0.09
9.27
13.01
2.14
0.02
0.05
0.05
0.09
98.50
48.82
0.72
16.16
9.38
0.18
8.75
12.73
2.38
0.03
0.05
0.05
0.10
99.36
49.57
0.73
16.30
9.32
0.16
8.64
12.58
2.44
0.02
0.05
0.04
0.11
99.95
48.68
0.77
16.07
9.10
0.20
8.73
12.89
2.36
0.02
0.05
0.05
0.10
99.01
49.07
0.74
16.00
9.33
0.13
8.75
12.60
2.40
0.02
0.05
0.05
0.10
99.25
48.46
0.76
15.98
9.20
0.16
8.75
12.78
2.44
0.02
0.05
0.05
0.11
98.76
49.26
0.70
15.45
9.78
0.16
8.08
12.86
2.35
0.02
0.04
0.05
0.11
98.85
49.38
0.70
15.52
9.81
0.19
8.16
12.79
2.44
0.02
0.04
0.05
0.11
99.20
49.12
0.68
15.52
9.97
0.17
8.20
12.72
2.57
0.01
0.05
0.05
0.10
99.17
48.53
0.74
15.65
9.39
0.19
8.50
12.62
2.30
0.03
0.05
0.06
0.11
98.16
48.66
0.75
15.68
9.44
0.22
8.48
12.75
2.33
0.02
0.05
0.05
0.11
98.54
57
76
85
122
37.7
436.7
81.0
18.7
162.4
22.5
59.8
1.38
14.8
4.6
4.3
6.2
7.7
9.9
10.2
10.4
11.5
11.2
11.1
11.0
10.8
10.5
10.2
1.51
13.71
0.08
0.45
118
37.7
436.7
81.0
18.7
162.4
22.5
59.8
1.38
14.8
4.6
4.3
6.2
7.7
9.9
10.2
10.4
11.5
11.2
11.1
11.0
10.8
10.5
10.2
1.51
13.71
0.08
0.45
120
37.7
436.7
81.0
18.7
162.4
22.5
59.8
1.38
14.8
4.6
4.3
6.2
7.7
9.9
10.2
10.4
11.5
11.2
11.1
11.0
10.8
10.5
10.2
1.51
13.71
0.08
0.45
120
126
149
286
295
425
408
422
135
131
Major elements and Cl were performed by electron microprobe. Trace element analyses were performed by ICP-MS. REE are chondrite normalized. FeO* is total FeO. Note that there
is more than one electron microprobe analysis for many of the samples. This was done to investigate the local variation of Cl contents in the lava samples. However, only one ICP-MS
analysis was performed on each sample. Therefore some samples have multiple microprobe analyses, but the same ICP-MS analysis.
1999GC000017
PL02-1330.A.1
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
PL02-1319.C.1
G3
Cl
Sc
Cr
Ni
Rb
Sr
Y
Zr
Nb
Ba
Lan
Cen
Prn
Ndn
Smn
Eun
Gdn
Tbn
Dyn
Hon
Ern
Tmn
Ybn
Lun
Hf
Pb
Th
U
PL02-1319.B.1
Geochemistry
Geophysics
Geosystems
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
Cr2O3
S
Total
PL02-1319.A.1
3
G
1999GC000017
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
entiation model was used to calculate the
amount of crystallization that represents the
compositional gap between the inclusion and
a hypothetical primary magma. The amount
of fractionation is then based on that value.
For all the data reported here the calculated
extent of crystallization and the appropriate
correction ranged from 0 to 50%.
It is important to note here that making
such a correction makes the assumption that
simple fractional crystallization was the dominant process acting on the suite subsequent to
melt extraction from the source. This is
obviously an oversimplification. However, it
is useful as a method to see through the late
stage evolution of the composite magmas.
4000
Galapagos Platform PL02-13
3500
3000 PL02-13-12 melt inclusions
2500 PL02-13 lavas
2000
1500
Ti/Zr
Geochemistry
Geophysics
Geosystems
1000
500
0
0.0
[25]
Figure 2. Minor and trace element analyses of
melt inclusions and host lavas from dredge PL02-13
from the Galapagos Platform. Melt inclusion Zr, La,
and Sm were analyzed by ion probe and P, Cl, and
Ti were analyzed by electron microprobe. All values
are fractionation compensated (fc subscript) (see
text). (La/Sm)n is chondrite normalized. Trace
element content of the lava was determined by
inductively coupled plasma ± mass spectrometry
(ICP-MS).
0.2
0.3
0.4
0.5
0.6
(La/Sm)n
(La/Sm)n
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0
b
50
100 150 200 250 300 350
Tifc
Clfc
4500
4000
3500
3000
2500
2000
1500
1000
500
0
c
0
50
100 150 200 250 300 350
Clfc
Ti/Zr
[26] As with many inclusion populations and
host lava data, some of the incompatible
elements from the Galapagos Platform lavas
exhibit relatively simple, linear internal correlations (for example, TiO2 and P2O5, Figure
2). Other components of the trace element
signature are either less coherent or in conflict
with predicted patterns based on our understanding of the effects of igneous processes.
As a case in point, inclusion data from the
Endeavour and the Galapagos show no correlation between (La/Sm)n and Ti/Zr even
though the data are characterized by a wide
range of both ratios. In addition, in both cases
the inclusions with the highest (La/Sm)n have
the lowest La and Sm contents and the highest
0.1
a
4500
4000
3500
3000
2500
2000
1500
1000
500
0
d
0
50
100 150 200 250 300 350
Clfc
Geochemistry
Geophysics
Geosystems
3
G
1400
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
Endeavour Segment N-MORB (0-2)
a
1200
Ti/Zr
1000
800
600
400
200
0
0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
(La/Sm)n
1400
b
1200
Ti/Zr
1000
800
600
400
200
0
0
200
400
600
800
1000 1200 1400 1600
Clfc
0.65
c
0.60
(La/Sm)n
0.55
0.50
0.40
0.35
0.30
partial melting trends [Sours-Page et al.,
1999]. This is not true of olivine-hosted inclusions sampled to date [Sobolev and Shimizu, 1993, 1994; Sobolev et al., 1994; A. V.
Sobolev, personal communication, 1999].
[27] Melt inclusion Cl concentrations exhibit
even more variable behavior than the other
incompatible elements. In both cases, melt
inclusion Cl contents exhibit a rough positive
correlation with Ti/Zr, a negative correlation
with Ti, Zr, La, and Sm concentrations but,
anomalously, a slight positive correlation with
(La/Sm)n (Figures 2 and 3). In addition, as
with the other incompatible elements, the Cl
contents exhibited by the inclusions exceed
the range exhibited by the host lava suite
(Figures 1 and 2). Cl is only roughly correlated with K2O for the Endeavour sample and
not at all correlated for the Galapagos Platform sample (Figure 4). Specifically, the Endeavour sample exhibits two separate trends,
while the Galapagos Platform sample is characterized by a narrow range of K2O and a
wide range of Cl.
In an attempt to further describe the
general characteristics of the correlation of
incompatible elements in melt inclusion populations, we compiled melt inclusion data
from 31 sample locations (Figure 5). These
samples were taken from locations along 10
different ridges or hotspot regions distributed
worldwide, with most from the mid-ocean
ridge system. On the basis of this data, we
can make several generalizations. First, the
slope of the correlation of K2O and Cl is a
function of the average K2O content of the
inclusions. More enriched samples have a
shallow slope, and the most K2O depleted
samples have a nearly vertical slope. Second,
intermediate samples, many from transitional
mid-ocean ridge basalt (TMORB) lavas, are
either characterized by an intermediate slope,
a scattered distribution, or more than one
[28]
0.45
0
200
400
600
800
1000 1200 1400 1600
Clfc
Figure 3. Minor and trace element analyses of
melt inclusions and host lavas from sample O-2
from the Endeavour Segment of the Juan de Fuca
Ridge. Zr, La, and Sm were analyzed by ion probe.
P, Cl, and Ti were analyzed by electron microprobe.
(La/Sm)n is chondrite normalized. All values are
fractionation compensated (see text).
Ti/Zr (rough positive correlation of Ti/Zr with
(La/Sm)n, Figures 2a and 3a), opposite of
what one would predict based on calculated
1999GC000017
Geochemistry
Geophysics
Geosystems
3
G
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
1999GC000017
1600
Gorda D9-1
JDF 0-2
1400
PL02-13-12
SEIR
1200
Clfc
1000
800
D145
600
400
200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
K2Ofc
Figure 4. K2O ±Cl correlations for fractionation-corrected melt inclusion analyses from a range of host lava
compositions. Note that the populations of inclusions fans out as a function of the extent of host depletion (as
measured by mean K2O content). PL02-13-12, Galapagos Platform [Sinton et al., 1993]; O-2 Endeavour
Segment of the Juan de Fuca Ridge [Karsten et al., 1990; Sours-Page et al., 1999]; D9-1, Gorda Ridge
[Davis and Clague, 1987; Nielsen et al., 1995a]; D145, dredge 145 from the Southeast Indian Ridge
[Christie et al., 1995].
trend. Third, in contrast to Cl and K2O, other
incompatible elements such as P2O5 and TiO2
are much more coherently correlated with one
another. Fourth, taken as a group, the
averages of melt inclusion compositions for
each sample describe a shallow trajectory,
while the trend lines for individual samples
fan out from those averages (Figure 5).
Finally, inclusions from EMORB lavas do
not contain high Cl, low K2O inclusions.
5. Discussion
[29] These samples present us with a paradox.
Under conditions appropriate for melting of a
dry, depleted spinel or plagioclase lherzolite,
Cl, K, P, the REE, Zr, and Ti are all
incompatible. In addition, Zr is more incompatible than Ti, and La is more incompatible
than Sm [Green, 1994; Hack et al., 1994].
Fractional melting under these conditions
should generate magmas characterized by a
positive correlation of (La/Sm)n with Ti, Zr,
P, K, and Cl, and a negative correlation with
Ti/Zr. Partial melting will initially produce
magmas characterized by high (La/Sm)n and
low Ti/Zr ratios due to the relatively low
partition coefficients for La and Zr. As melting proceeds, subsequent lavas will have
lower (La/Sm)n and higher Ti/Zr values. As
Geochemistry
Geophysics
Geosystems
3
500
450
400
350
300
250
200
150
100
50
0
a
0
0.1
0.2
0.3
K2Ofc
0.4
0.5
0.6
0.25
0.20
0.15
Clfc
P2O5fc
1999GC000017
NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
Clfc
Clfc
G
0.10
0.05
0.00
0.0 0.2 0.4 0.6 0.8 1.0 1.2
1.4 1.6 1.8
TiO2 fc
500
450
400
350
300
250
200
150
100
50
0
500
450
400
350
300
250
200
150
100
50
0
c
Averages of melt inclusions from
individual samples
0
0.1
0.2
0.3
K2Ofc
0.4
0.5
JDF O-2
Galap
PL0213-12
EPR
R3-2
SEIR
D69 Gorda
D9-2
0.6
d
Trendlines for single sample
inclusion populations
ASP
D3
SEIR
D70 EPR
Axial R32-5
Smt
SEIR
EPR D112 SEIR
R28-7
D123
Blanco
3R
ASP
D67
SEIR
D145
Blanco 11R
AMAR 829-12
Blanco 12R
JDF E-32
Blanco 5R
Blanco 9R
0
0.1
0.2
0.3
K2Ofc
0.4
0.5
SEIR D69
SEIR D70
SEIR D123
SEIR D112
SEIR D131
SEIR D113
SEIR D145
EPR R32-5
Gorda D9-2
EPR P55
JDF E32
EPR R34-3
JDF O-2
EPR R3-4
PL02-13-12
EPR R28-3
EPR P113-4
EPR R3-2
EPR R28-7
EPR 93-3
ASP D3
ASP D67
ASP D48
Axial TT170
Blanco 1R
Blanco 3R
Blanco 5R
Blanco 9R
Blanco 10R
Blanco 11R
Blanco 13R
AMAR 829-12
0.6
Figure 5. Global systematics of minor element distribution for melt inclusions in ocean floor lavas. The
scale for Cl is expanded compared to Figure 4. All values are fractionation compensated. (a) Correlation of
Cl and K2O. Note the lack of a coherent global pattern and that EMORB lavas have the lowest slope while
NMORB have the steepest slope. (b) Correlation of P and Ti. Note the general positive correlation of all
inclusion data. (c) Single sample averages of melt inclusion analyses. SEIR: Southeast Indian Ridge (R. E.
Sours-Page, unpublished data, 1999); Gorda, Gorda Ridge [Nielsen et al., 1995a]; JDF, Endeavour Segment
of the Juan de Fuca Ridge [Sours-Page et al., 1999]; Galap, Galapagos Platform [Sinton et al., 1993]; EPR,
East Pacific Rise (R. E. Sours-Page and R. L. Nielsen, manuscript in preparation, 2000); ASP, Amsterdam-St
Paul Platform [Douglas, 1998]; Axial, Axial Seamount, Juan de Fuca Ridge (R. L. Nielsen, unpublished
data, 1999); Blanco, East Blanco Depression (R. L. Nielsen, unpublished data, 1999); AMAR, Atlantic MidOcean Ridge [Frey et al., 1993; R. L. Nielsen, unpublished data, 1999].
clinopyroxene is exhausted at high extents of
melting (>20%), the rise in Ti/Zr accelerates
owing to the increasing difference in the
partition coefficients of Ti and Zr as the Ca
content of the pyroxene drops [Nielsen et al.,
1992; Forsythe et al., 1994; Longhi and
Bertka, 1996; Wood and Blundy, 1997; Salters and Longhi, 1998].
[30] The HFSE data are consistent with variable
degrees of partial melting. Specifically, the Ti
and Zr contents suggest that the most depleted
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
10000
Ti in cpx (calc)
Ion probe data on Abyssal
Peridotite CPX
Johnson et al., 1990
1000
Endeavour O-2 inclusions
Galapagos Plat PL02-13-12 inclusions
Galapagos Plat PL02-13 lavas
100
0.01
0.1
1
10
100
Zr in cpx (calc)
Figure 6. Calculated equilibrium clinopyroxene for melt inclusion compositions from samples O-2, an
NMORB from the Endeavour Segment of the Juan de Fuca Ridge [Sours-Page et al., 1999] and PL02-13-12,
a depleted ocean floor lava dredged from the NE Galapagos Platform [Sinton et al., 1993]. Note that the
measured clinopyroxene compositions from abyssal peridotites overlie the calculated pyroxene compositions.
Assumed partition coefficients (DTi 0.4; DZr 0.15).
inclusions are the product of high extents of
melting, probably past the point at which
clinopyroxene was exhausted from the source.
However, the relatively high (La/Sm)n, but low
LREE content, variable K, and extremely high
Cl contents in the inclusions from the Endeavour and Galapagos Platform lavas are inconsistent with such a process. This is similar to
the patterns exhibited by some clinopyroxenes
from abyssal peridotites [Johnson et al., 1990]
and has been attributed to later stage reintroduction of enriched material to the peridotites
[Elthon, 1992], after an initial episode of partial
melting and melt extraction.
[31] We can examine this issue in detail by first
comparing the composition of abyssal perido-
tite clinopyroxene with clinopyroxene compositions calculated from melt inclusion
compositions as done by Nielsen et al.
[1995a] and Sours-Page et al. [1999]. To compare the melt inclusions with abyssal peridotites, we must first calculate the composition of
clinopyroxene in equilibrium with those melts.
This procedure is complicated by the fact that
we know that the clinopyroxene-melt partition
coefficients for the REE and HFSE are dependent on the major element composition of the
pyroxene as a function of degree of melting, a
parameter we do not know with any precision.
However, if we assume that we begin with a
moderately depleted lherzolite, and use partition coefficients from existing experimental
data [Hack et al., 1994; Forsythe et al., 1994],
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
we can constrain the values of the initial partition coefficients. However, REE and HFSE
partition coefficients should drop as melting
proceeds and the source becomes depleted in
Na, Ca, and Al. The relative values of those
partition coefficients (i.e., DTi/DZr) should stay
largely constant up to the point where clinopyroxene is near exhaustion. Unfortunately, we do
not know how the clinopyroxene partition coefficients change as a function of that transition in
melting mode [Longhi and Bertka, 1996; Shimizu et al., 1997]. We do know that for an
orthopyroxene-dominated mode, the relative
partition coefficients for TiO2 and Zr (DTi/
DZr) will be dramatically higher than for lherzolite, resulting in a rapid rise in Ti/Zr in
subsequent melts of the now harzburgite source
[Nielsen et al., 1992; Forsythe et al., 1994;
Shimizu et al., 1997].
[32] We have chosen a conservative approach
in our calculations in that we have assumed
that the partition coefficients do not change as
a function of degree of melting. Therefore the
calculated equilibrium clinopyroxene compositions from the melt inclusions are almost
certainly more enriched than they would be
if we could predict the partition coefficients
with more precision. The results of these
calculations (Figure 6) demonstrate that the
melt inclusions from the Galapagos Platform
and Endeavour Segment are similar to melts
one would predict to be in equilibrium with a
wide range of abyssal peridotites. Most importantly, calculated values extend below those
observed for peridotites with <3% clinopyroxene [Johnson et al., 1990]. Given that the
actual equilibrium concentrations would be
lower than these calculated values, we speculate that the most depleted inclusions are the
products of harzburgite melting. This group
represents ~5 - 10% of the inclusion population in the NMORBs we have studied but is
absent from the inclusion populations from
EMORB lavas.
1999GC000017
Both abyssal peridotites and the Galapagos
and Endeavour melt inclusions are relatively
enriched in (La/Sm) and Na2O compared to
what should be found in a material that had
undergone up to 25% near fractional melting
[Johnson et al., 1990; Elthon, 1992]. As liquids, the melt inclusions also carry information about enrichment in other incompatible
elements, specifically K2O and Cl, that are
not characteristic of high degree melts of dry
peridotite. In addition, the major element compositions of these melt inclusions are relatively
homogeneous and appear to be near multiple
saturation at low pressure with olivine, plagioclase, and aluminous spinel [Nielsen et al.,
1995a; Sours-Page et al., 1999].
[33]
To summarize, melt inclusions from depleted ocean floor magmas exhibit a wide
range of Ti and Zr contents and Ti/Zr ratios,
even occasionally for inclusions from single
phenocrysts. The values of the most depleted
inclusions indicate that melting continued past
the point at which clinopyroxene was exhausted in the source [Sours-Page et al.,
1999]. In contrast, the LREE, K, and Cl
compositions from those same inclusions are
inconsistent with generation by simple progressive partial melting of a dry lherzolite
protolith. Finally, variations in the major element composition of the melt inclusions are
independent of the trace element variability,
and reflect low-pressure phase equilibria.
[34]
This paradox is further complicated by the
question of how such depleted melts can be
produced and extracted. As isobaric melting of
a lherzolite source proceeds, melt production
should drop dramatically as clinopyroxene is
exhausted and melting changes from clinopyroxene- to orthopyroxene-dominated melting
reactions. This can be demonstrated either from
constraints derived from simple systems [Presnall, 1969] or from theoretical calculations
[Asimow et al., 1997; Hirschmann et al.,
[35]
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
1999]. The loss of clinopyroxene will also
result in reduced productivity as the mantle
rises (adiabatic productivity). The result should
be greatly reduced melt production in the
depleted tip of the rising melting regime. However, the extremely high Ti/Zr ratios of the most
depleted inclusions are consistent with melt
production significantly past the point at which
clinopyroxene is controlling the trace element
contents. This raises the question of if and how
such refractory material can produce significant
volumes of melt.
There are several possible sources for the
relatively high incompatible element contents
found in the depleted inclusions. Enrichment of
an incompatible element in an otherwise depleted source material is a commonly cited
process for explaining the pattern of enrichment in Na and the LREE observed in abyssal
peridotites [Elthon, 1992; Shiano and Bourdon,
1999]. Incompatible element enriched mantle
fluids have been observed as inclusions in
mantle nodule olivines. These contain up to
2000 ppm Cl [Shiano and Bourdon, 1999] but
are also enriched in other components (e.g.,
LILE) not seen in the melt inclusions.
[36]
[37] Assimilation of altered crust has been cited
as one means by which Cl can be added to an
evolving magma system [Michael and Schilling, 1989]. However, late-stage addition to the
magma is difficult to resolve with the fact that
both the melt inclusions and the host phenocryst are extremely primitive in composition.
Even more difficult to explain by simple assimilation processes is the observation that the
inclusions most depleted in Ti, Zr, and REE
are those most enriched in Cl and (La/Sm).
This would require that the assimilation process
actually deplete the magma in incompatible
elements or that only ultradepleted magmas
are involved in assimilation. Neither model
satisfactorily explains why all ultradepleted
inclusions contain high Cl contents or the
1999GC000017
demonstrated differences in trajectory of the
Cl-K trends for NMORB and EMORB (Figure
6). We do not contend that simple assimilation
of altered crust is not an important process in
the evolution of MORB, only that the signal
described above for ultradepleted melts is not
readily attributable to such a process.
[38] A fluid could be involved in the origin of
the observed inclusion chemistry if it transported the LREE and Cl into the peridotite,
resulting in high-temperature alteration of the
uppermost mantle. In addition to serving as the
mechanism for input of low levels of an enriched signature, this fluid and the alteration
mineralogy would provide a flux, increasing
the melt productivity of the harzburgite
[Hirschmann et al., 1999; Stalder et al.,
1999]. The effect of this fluid in more enriched
mantle would be diluted by the normally higher
productivity of lherzolite. The presence of a
small population of high-Cl NMORB-like inclusions in the transitional basalts (Figures 3
and 4) suggests that the fluid signature may be
present in most NMORB and TMORB. The
low trajectory for Cl-K2O for EMORB suggests that the Cl in enriched lavas may have a
different source, one reflecting the composition
of enriched materials. It is also important to
reiterate that high Cl inclusions are absent in
the most enriched lavas (EMORB) but are more
common in the inclusion populations from the
lavas characterized by low K2O (NMORB).
Intermediate composition lavas exhibit intermediate numbers of high Cl inclusions. In
addition, EMORB has generally been inferred
to be the product of melting at greater depth
than NMORB, which is generally assumed to
be the product of high extent of melting in the
shallower parts of the melting regime [Plank
and Langmuir, 1992]. Taken as a whole, these
observations support the view that whatever the
process, the source of the high Cl signal in the
ultradepleted inclusions is derived from above,
rather than from below.
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We contend that we can describe at least
three components present in NMORB lavas on
the basis of the minor and trace element signatures of their melt inclusions: (1) a component derived from variable degrees of melting
of a lherzolite source, dominating the NMORB
parent magma array; (2) a component derived
from melting of a uniformly enriched source
resulting in EMORB magmas; and (3) a component derived from melting of a harzburgite,
fluxed by the presence of alteration minerals
and/or a fluid derived from a hydrothermal
source, resulting in ultradepleted magmas.
The distinctly different trends exhibited by melt
inclusions in highly depleted NMORB versus
EMORB, as well as the presence of both trends
in inclusions from transitional MORB lavas,
and the close spatial association of ultradepleted and NMORB melt inclusions supports
the contention that there are multiple distinct,
contemporaneous sources contributing to the
array of parental magmas. As of yet it is
unclear how the melt inclusion source compositions may differ isotopically. That study
awaits the development of a new generation
of microanalytical instrumentation that will
allow us to obtain accurate isotopic information
on melt inclusions.
[39]
[40] What we propose is similar to the process
described by Benoit et al. [1999] to explain
ultradepleted mineralogy associated with the
Oman Ophiolite. Field relations, isotopic, and
trace element data collected on gabbronorites
and websterites demonstrated that there were
two distinct rock types. One exhibits ``normal''
trace element compositions (N-gabbronorites)
and the other group exhibits depleted trace
element compositions, anorthitic plagioclase,
and radiogenic Sr isotopes (D-gabbronorites).
These bodies are found as massive units in the
harzburgite surrounding a fossil mantle diapir
associated with the ophiolite. They concluded
that these cumulates were formed from melts
produced by reheating of hydrothermally al-
1999GC000017
tered harzburgite at a depth of ~3 - 5 km.
They devised a scenario wherein periodic diapiric fluctuations resulted in alternating episodes of hydrothermal alteration and melting.
Melting of this serpentinized harzburgite was
inferred to produce ultradepleted melts that are
not sampled at the surface due to their low
volumes and the likelihood that they become
mixed with less depleted melts en route to the
surface. Owing to the low concentration of
incompatible elements characteristic of these
``D-gabbronorites,'' addition of even a small
amount of normal basalt will overwhelm their
distinct chemical signature [Nielsen et al.,
1995a; Sours-Page et al., 1999].
[41] The melt inclusion data are generally consistent with such a scenario. However, the
relative homogeneity of the major element
composition of the melt inclusions strongly
suggests that the entire suite of parent magmas
represented by the melt inclusion population
have been buffered by transit thorough a significant section of the deep crust. This in turn
suggests a depth or origin for the ultradepleted
magmas that is greater than that suggested by
Benoit et al. [1999]. Such a depth of origin and
proximity to the ridge magma supply dictated
by the proximity of ultradepleted and normal
melt inclusions in single phenocrysts would
require deep circulation of fluids very close to
the ridge axis.
[42] Recent modeling of seismic tomography
by Cherkaoui et al. [1999] has suggested that
at fast spreading rate ridges such as the East
Pacific Rise (EPR), the isotherms are nearly
vertical near the ridge (within 2±4 km), with
the 6008C isotherm reaching the base of the
crust close to the ridge axis. They infer that
deep hydrothermal circulation is sufficiently
active to cool the entire crust and perhaps some
unknown part of the upper mantle below the
brittle/ductile transition. For ridge segments
such as Endeavour the lower and more episod-
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
ic magma supply could result in periodic reheating of sections of the lower crust and upper
mantle that had undergone hydrothermal alteration. Part of the difficulty in understanding the
possible extent of this process is that we do not
really know what the physical nature of the
upper mantle will be under such circumstances,
such as the maximum depth and temperature
for brittle failure in harzburgite and therefore of
the maximum possible depth of penetration of
fluids.
[43] In spite of this support for the existence of
deep hydrothermal circulation near the ridge,
there are three issues that remain unknown
regarding the role of such a fluid. First, we
do not know the water content of the hydrothermal fluid. It is probable that any seawater
derived fluid would have been highly modified
by reaction with the crust to produce zeolites,
chlorite, amphibole, serpentine, and other hydrous minerals. The nature of the fluid generated
during remelting of this alteration mineralogy
could potentially be determined by study of
melt inclusions. However, quantitative information of the nature of the flux may not be
recoverable owing to the permeability of feldspar to hydrogen (Hauri, personal communication, 1999). Specifically, even if there were
water present in the inclusions, it almost certainly would have been lost to the host magma
during transport after mixing. In addition, the
totals for microprobe analysis are near 100,
suggesting that the amount of water remaining
after rehomogenization will be <1%.
[44] Second, we do not know the effectiveness
of the Cl-bearing alteration mineralogy as a
flux of harzburgite. The maximum Cl content
observed in inclusions is ~0.25 wt%. This
provides a value for the minimum Cl concentration of the flux. Even if it is only as effective
as water as a flux, addition of only a small
amount of fluid derived by dehydration could
result in a significant reduction of the solidus
1999GC000017
and the production of considerable additional
melt, with the amount dependent on temperature.
[45] Owing to our uncertainty regarding the
amount or general chemical character of this
fluxing agent, its effects on the physical state/
nature/composition of the upper mantle remains a matter of speculation. The presence
of small amounts of fluid, either the hydrothermal fluid, the alteration mineralogy, or the
magma generated by them, would certainly
have an effect on the rheology of the upper
mantle.
[46] Third, the contrasting compositional diversity of these melt inclusions compared to their
relatively homogeneous major element compositions, and the fact that their phase equilibria
reflect low-pressure differentiation suggests a
petrogenesis involving multiple processes. If
the ultradepleted magmas are the product of
the fluxing action of alteration minerals or a
hydrothermal fluid, one might expect those
melts to have a distinct major element character. Benoit et al. [1999] inferred that the composition of the magma produced by the melting
of altered harzburgite was Si-rich in addition to
being incompatible element poor. In the case of
the Oman gabbronorites the depth of melting
was sufficiently shallow that plagioclase was
stable at the site of melting. Since we do not
know the depth of melting of the ultradepleted
melts that are the focus of this work, we can
only infer that whatever the major element
differences between NMORB and the ultradepleted melts, they were eliminated by subsequent mixing and buffering reactions.
[47] In earlier work, Nielsen et al. [1995a] and
Sours-Page et al. [1999] ascribed the homogeneity of the major element contents to buffering
by equilibration with olivine and plagioclase
cumulates in the lower and middle crust during
transport. The diversity of incompatible trace
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NIELSEN ET AL.: ROLE OF CL-BEARING FLUX
element contents is attributed to the small size
of the magma packets traveling along independent pathways, yet never reequilibrating with
mantle clinopyroxene. Our recent observations
confirm this proposal in that the inclusions
have preserved both the trace element diversity
and evidence for independent incompatible
element sources represented in the parent magma array. However, the plagioclase-phyric
character of many magmas that contain ultradepleted inclusions may suggest a possible link
to a hydrothermal fluid if that fluid was waterrich. The presence of water, either in the form
of alteration minerals in the harzburgite or as a
free, supercritical fluid, could result in the
suppression of plagioclase stability and the
production of aluminous melts [Muentener et
al., 1999]. This is further supported by the fact
that ultradepleted inclusions found in olivine
are relatively rare and do not contain anomalous Cl or water contents [Sobolev et al., 1994;
A. V. Sobolev, personal communication, 1999].
Regardless, existence of plagioclase-phyric
EMORB, although relatively uncommon, suggests that if such a link exists, it is not the only
mechanism for the production of plagioclaserich magmas.
6. Conclusions
High Cl and (La/Sm)n together with low
HFSE and K2O in some inclusions from depleted lavas coupled with uniformly high incompatible element concentrations in enriched
lavas present an apparent paradox in our understanding of mantle melt generation processes.
These geochemical variations can, however, be
attributed to the fact that MORB magmas are
composed of a combination of three distinct
magma sources produced by discrete processes
and source materials. Correlation of Ti/Zr with
Cl in the ultradepleted inclusions supports the
contention that, whatever the source, the paradox is a mantle phenomenon and not a result of
alteration at or near the seafloor. In addition,
[48]
1999GC000017
the absence of ultradepleted inclusions from
EMORB lavas and the distinct Cl-K trends for
inclusion populations from NMORB and
EMORB lavas demonstrate that the source of
Cl in the ultradepleted inclusions is not mixing
with EMORB.
[49] The most intriguing possibility suggested
by this observation is that there is a significant
component in depleted ocean floor magmas
that is generated by the fluxing of a harzburgite
source with either alteration minerals or with a
Cl and LREE-bearing fluid derived from deep
hydrothermal circulation. That depleted component comprises ~5 - 10% of the array of
magmas that make up NMORB magmas in the
areas we have studied. If this is true, it has
important implications for the thermal and mass
budget as well as the rheology of the upper
mantle and lower crust.
Acknowledgments
We wish to thank Jill Karsten, Fred Frey, Dave
Graham, Dave Clague, Emily Klein, and Rodey Batiza for
guidance in sample selection. We also wish to thank Paul
Robinson and Claire McKee for help with the electron
microprobe analyses and data reduction. Reviews by
Marc Hirschmann and an anonymous reviewer significantly improved this manuscript. The ion probe data were
measured at the University of New Mexico/Sandia
National Laboratory SIMS facility, a national multi-user
facility supported in part by NSF grant EAR 95-06611.
This work was supported by NSF grants OCE-9503782
and OCE-9730079, as well as NSF grant CHE-9996136
(K.S.H.).
[50]
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