JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 12
PAGES 2175–2195
2001
Differentiation Processes of Deccan Trap
Basalts: Contribution from Geochemistry
and Experimental Petrology
T. SANO1,2∗, T. FUJII1, S. S. DESHMUKH3, T. FUKUOKA4
AND S. ARAMAKI5
1
EARTHQUAKE RESEARCH INSTITUTE, THE UNIVERSITY OF TOKYO, TOKYO 113-0032, JAPAN
2
INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, NOGUCHIBARU, BEPPU 874-0903, JAPAN
3
PLOT NO. 89, FRIENDS HOUSING SOCIETY LAYOUT-4, DEENDAYALNAGAR, NAGPUR 400 022, INDIA
4
FACULTY OF GEO-ENVIRONMENTAL SCIENCES, RISSHO UNIVERSITY, KUMAGAYA 360-0194, JAPAN
5
DEPARTMENT OF EARTH SCIENCES, NIHON UNIVERSITY, TOKYO 156-0045, JAPAN
RECEIVED JUNE 5, 2000; REVISED TYPESCRIPT ACCEPTED MAY 18, 2001
The Deccan Traps basalts can be divided into sub-groups based
on the inferred type and/or amount of contamination. The elemental
characteristics of Ba, Sr, TiO2 and Zr/Nb are used to classify the
sub-groups; the least-contaminated group has Ba contents <100
ppm, Sr 190–240 ppm and TiO2 2·0–4·0 wt %, and the mostcontaminated group has TiO2 contents <1·5 wt % and Zr/Nb
[ 15. Analyses of 325 basalts, which were collected from 27
well-distributed sections through the Deccan Traps, demonstrate
that the least- and most-contaminated groups are distributed widely.
To understand the shallow-level fractionation of the Deccan Trap
magmas, melting experiments were conducted at atmospheric pressure
(100 kPa) at both fayalite–magnetite–quartz (FMQ ) and nickel–
nickel oxide (NNO) oxygen fugacities for three Mg-rich basalts,
one of which belongs to the least-contaminated group. The results
indicate that the phenocryst assemblage and the chemical trend of
the least-contaminated basalts can reasonably be explained by
fractional crystallization in shallow chambers under FMQ-buffered
conditions. The inferred fractional crystallization process also reproduces the chemical trend of the most-contaminated basalts,
implying that crustal contamination was not accompanied by the
shallow-level fractional crystallization.
INTRODUCTION
KEY WORDS: Deccan Traps; fractional crystallization; crustal contamination; melting experiment; tholeiitic magma
The Deccan Traps province, one of the most voluminous
continental flood basalt provinces on Earth, consists of
basalts with a wide range of compositions not only in
isotopic ratios but also in major and trace element
contents (e.g. Mahoney, 1988). During ascent of a plumerelated magma through thick continental crust, the variety of compositions would be produced by various differentiation processes such as fractional crystallization,
crustal contamination and accumulation of phenocryst
phases (e.g. Sen, 1986; Cox & Mitchell, 1988; Lightfoot
et al., 1990). The range of the compositions could also
be generated, in part, by addition of magmas derived
from continental lithospheric mantle (e.g. Hooper, 1994;
Turner & Hawkesworth, 1995). When we evaluate the
effects of each differentiation process on the chemical
compositions, the evolutionary history of Deccan Trap
magmas can be understood.
The following studies were conducted to explore the
differentiation processes of Deccan Trap magmas. The
Ambenali Formation basalts, which form the thickest
formation in the western Deccan, are regarded to be the
least contaminated by continental crust and the least
affected by the continental mantle lithosphere, on the
basis of Nd, Sr, Pb and O isotopic signatures (e.g. Cox
& Mitchell, 1988; Mahoney, 1988). The major and
Extended dataset can be found at
http://www.petrology.oupjournals.org
∗Corresponding author. Present address: College of Environment and
Disaster Research, Fuji Tokoha University, Ohbuchi 325, Fuji 4170801, Japan. Telephone: 81-545-37-2007. Fax: 81-545-36-2651.
e-mail: [email protected]
 Oxford University Press 2001
JOURNAL OF PETROLOGY
VOLUME 42
trace element compositions of the Ambenali basalts are
controlled by the amount of gabbroic fractionation (Cox
& Devey, 1987; Devey & Cox, 1987; Cox & Mitchell,
1988; Lightfoot et al., 1990; Aoki et al., 1992; Sen, 1995).
The geochemical features of most other Deccan Trap
basalts can be explained by mixing between the leastcontaminated Ambenali-type magma and three types of
material: broadly granitic, possibly upper crust; granulitic
and amphibolitic, probably lower crust; and continental
lithospheric mantle material (Mahoney et al., 1982; Cox
& Hawkesworth, 1985; Matsuhisa et al., 1986; Lightfoot
& Hawkesworth, 1988; Lightfoot et al., 1990; Hooper,
1994; Peng et al., 1994; Peng & Mahoney, 1995). The
Bushe Formation basalts have the highest 87Sr/86Sr and
lowest Nd among Deccan Trap basalts, and are considered to be the most contaminated by broadly granitictype crust.
These conclusions are based on studies of basalts in
the southwestern Deccan (i.e. the Western Ghats area),
but should be extended to cover the basalt piles in the
central and eastern Deccan (e.g. Subbarao et al., 1994;
Peng et al., 1998; Mahoney et al., 2000). Peng et al.
(1998) reported that lavas isotopically and chemically
indistinguishable from the Ambenali basalts form parts
of two sections in the eastern Deccan. Peng et al. (1988)
and Mahoney et al. (2000) also found lavas chemically
similar to the Bushe formation, although these lavas were
isotopically slightly different from the Bushe basalts.
However, the overall distribution of Ambenali-type and
Bushe-type basalts in the central and eastern Deccan has
not been worked out.
Comparison of experimental melting data with naturalrock data is a powerful method to understand the differentiation process. For natural-rock data, we should use
datasets for basalts from the whole Deccan Traps to help
clarify the differentiation processes of the entire Deccan.
In this paper, we select the least-contaminated (Ambenalitype) and most-contaminated (Bushe-type) basalts among
widely distributed sections in the western, central and
eastern Deccan Traps, and examine the distributions of
lava flows of these two sub-groups. Next, the gabbroic
fractionation of the least-contaminated basalts is evaluated on the basis of melting experiments at atmospheric
pressure (100 kPa). Lastly, the chemical effects of crustal
contamination are assessed by comparing variations in
the most-contaminated basalts with those predicted by
the melting experiments.
GEOLOGICAL BACKGROUND
The Deccan Traps province of India covers an area of
5 × 105 km2 (Fig. 1). The basalt flows are generally
10–50 m thick and tabular in form, dipping at <0·5° in
various directions (e.g. Deshmukh, 1977). The thickness
NUMBER 12
DECEMBER 2001
of the lava pile is >2000 m in the Western Ghats area
between Nasik and the southern edge of the Deccan
Traps, and exceeds 1000 m in parts of the eastern Deccan
along the Tapti and Narmada grabens (Fig. 1). In some
southeastern and far-northern regions distant from both
the Western Ghats and the eastern Deccan areas, the
thickness of the lava pile is <100 m. In the Western
Ghats and the eastern Deccan areas, a large number of
dykes occur, in some cases as dyke swarms (Deshmukh
& Sehgal, 1988; Hooper, 1990; Bhattacharji et al., 1994;
Chatterjee & Nair, 1996; Chawade, 1996; Godbole &
Ray, 1996; Sethna et al., 1996). These swarms probably
formed under tensional stress fields associated with two
main fault systems, the Panvel flexure and Tapti–
Narmada grabens (Fig. 1).
The lava pile in the Western Ghats is divided into 11
stratigraphic formations (Fig. 1) based on both field
observations and geochemical characteristics. The uppermost Panhala Formation appears on the southern edge of
the Western Ghats and the lowermost Jawhar Formation
exists in the Igatpuri section, indicating a southward dip
of formational boundaries (e.g. Beane et al., 1986; Devey
& Lightfoot, 1986; Mahoney, 1988).
The main eruption of the Deccan basalts took place
in a geologically short period of time (probably <1 m.y.)
at 66 Ma (e.g. Courtillot et al., 1988; Duncan & Pyle,
1988; Venkatesan et al., 1993; Baksi, 1994; Allègre et al.,
1999), indicating high eruption rates of >1 km3 per
annum. Geological information, such as the absence of
thick intra-flow sedimentary layers in any section and
the lack of erosional profiles developed between successive
eruptions (e.g. Deshmukh, 1977), also indicates a geologically short period of time for the main eruption. Plate
tectonic reconstructions show that the Deccan eruptions
took place when the Indian continent passed over a
plume situated beneath the present site of Réunion (e.g.
Müller et al., 1993).
SELECTION OF THE LEAST- AND
MOST-CONTAMINATED GROUPS
Samples
The samples chosen for this study came from throughout
the present geographical distribution of the Deccan Traps
(Fig. 1). The region between Igatpuri (IG) and Amboli
(AB) is called the Western Ghats area in this study. The
samples were collected from 10 formations (Panhala,
Mahabaleshwar, Ambenali, Poladpur, Bushe, Khandala,
Bhimashankar, Thakurvadi, Igatpuri and Jawhar). For
convenience, we call the area between 74°30′ and 77°E
(BI, ST, BU, AJ, CK and LC) the Central area, and we
call that east of 77°E (P, CH, AN, TL, MG, KV, NY,
NP, NC, NJ and JB) the Eastern area (Fig. 1). To confirm
intra-flow heterogeneity, one group of five samples was
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SANO et al.
DIFFERENTIATION OF DECCAN TRAP BASALTS
Fig. 1. Geological map of the Deccan Traps and locations of the main tectonic features, after Hooper (1990) and Bhattacharji et al. (1994).
The locations of sections used in this study are also shown. AB, Amboli; AG, Amba Ghat; AJ, Ajanta; AK, Alibag–Khopoli; AN, Amravati–Nagpur;
BH, Bor Ghat; BI, Barwaha–Indore; BM, Bhor–Mahad; BU, Buldana; CH, Chikaldara; CK, Chikhali–Khamgaon; IG, Igatpuri; JB,
Jabalpur–Mandla.; KK, Khambatki; KP, Kolhapur; KT, Katraj; KV, Kalalgaon–Vaghapur; LC, Lonar Crater; MA, Mahabaleshwar; MG,
Mahur Ghats; NC, Nagpur–Chhindwara; NJ, Nagpur–Jabalpur; NP, Nagpur; NY, Bori; P, Akot–Harisal; ST, Shahada–Toranmal; TL, Talegaon.
Stratigraphy of the Western Ghats area is shown in the adjacent column (after Cox & Hawkesworth, 1984, 1985; Beane et al., 1986; Devey &
Lightfoot, 1986; Bodas et al., 1988; Khadri et al., 1988; Mitchell & Cox, 1988; Subbarao, 1988; Subbarao & Hooper, 1988; Lightfoot et al.,
1990; Mitchell & Widdowson, 1991; Widdowson & Cox, 1996).
collected from the BI-13 flow in the Barwaha–Indore
(BI) section and another group of 10 samples was collected
from the MA-W flow in the Ambenali Formation of the
Mahabaleshwar (MA) section. The intra-flow heterogeneity for each element is reported below.
The Deccan basalts are largely microporphyritic with
phenocrysts of plagioclase, subordinate augite and rare
olivine. Phenocrysts are set in a groundmass consisting
of plagioclase, augite, rare Fe–Ti oxide minerals and glass.
Some basalts contain glomero-porphyritic aggregates of
augite and plagioclase crystals, occasionally in association
with olivine.
Analytical methods
Analyses of major and trace elements of 325 samples
were carried out by X-ray fluorescence spectrometry
(XRF) with a Rigaku System 3080E3 instrument at the
Earthquake Research Institute of the University of Tokyo,
following the analytical procedures described by Kaneko
(1995). The 1 value of a calibration line for each element
is shown in the Appendix (Table A1).
Samples were ground to a fine powder in an agate
mill. The powders were then dried for 24 h at 105°C.
For major element analysis, 0·4000 ± 0·0004 g of
powdered sample was mixed with 4·000 ± 0·004 g of
anhydrous lithium tetraborate (Li2B4O7). The mixture
was fused at >1100°C in a Pt95Au5 crucible and shaped
into a glass bead, which was then used directly for the
measurements. For trace elements (Rb, Sr, Ba, Y, Zr,
Nb, V, Cr, Ni, Cu, Zn and Ga), 4·0 g of powdered
sample was pressed into a pellet by a 10 ton force from
a hydraulic press.
A non-destructive instrumental neutron activation analysis (INAA) technique was used for the analyses of Th,
Sc and rare-earth elements (REE; La, Ce, Nd, Sm, Eu,
Tb, Yb and Lu) on seven samples (Table A1). The
powders were activated with thermal neutrons for 1 h
with 5·5 × 103 n/cm2 s at the ‘S pipe’ of the JRR4 reactor of Japan Atomic Energy Research Institute.
Simultaneously, JB-1, JR-2 (standard rocks from the
Geological Survey of Japan) and BCR-1 (a standard
rock from the US Geological Survey) were activated as
standards. After a suitable time, the gamma-ray spectra
of activated samples were counted in two different ways.
The first involved use of a Ge detector coupled to a
2048 multi-channel analyser at the Faculty of Sciences,
Gakushuin University. In the second method, use was
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made of another type of Ge detector, this time coupled
to a 4096 multi-channel analyser. The details of the
analytical procedure have been described by Fukuoka et
al. (1987). The analytical error involved in the results is
<±5%.
We collected one fresh basalt (NY-02-51) with fresh
glass inclusions in the olivine phenocrysts, and the H2O
contents in the glass inclusions were analysed by Fourier
transform IR spectroscopy (FTIR-300E, JASCO Corporation) at the Earthquake Research Institute of the
University of Tokyo, following the method of Yamashita
et al. (1997). For the analysis, doubly polished olivine
crystal plates (with inclusions intersected on both sides)
were prepared by hand grinding on 1000-grit paper
rinsed with H2O and then polished with 10 000-grit
paper. The H2O contents of the glass were determined
from the IR spectra, which were calibrated from manometric and spectroscopic measurements of natural and
synthetic basaltic glass samples (Yasuda, in preparation).
The intensities for the 3550 cm−1 band of the basaltic
glass samples were calibrated. The accuracy of the measurements is <±10%, which is small compared with the
intersample variation, as will be shown in the following
section.
NUMBER 12
DECEMBER 2001
Table 1: (a) H2O and (b) major element
compositions of glass inclusion hosted in olivine
phenocrysts in the least-contaminated group of
Deccan Trap basalts (NY-02-51); (c) major
element compositions of the host olivine
phenocrysts; (d) major element compositions of
plagioclase phenocrysts in the NY-02-51 sample
(a)
Peak
Sample
Inclusion height thickness
no.
(abs)a
H2O
(wt %)
(10−6 m)
IC15
0·039 80
0·60
IC17
0·043 80
0·65
IC21
0·035 82
0·54
(b)
SiO2
IC15
51·20
IC17
50·46
IC21
50·84
(c)
TiO2
Al2O3
FeO∗
MgO
CaO
Na2O
3·10
12·75
16·36
4·62
10·64
1·33
3·04
12·39
16·00
5·07
11·27
1·77
3·25
12·88
16·45
5·22
9·96
1·40
SiO2
FeO∗
MgO
CaO
IC15
37·10
28·64
33·95
0·31
67·9
IC17
36·97
28·71
34·05
0·27
67·9
IC21
36·73
29·16
33·81
0·30
67·4
(d)
SiO2
Al2O3
CaO
Na2O
K2O
pl1
51·58
30·65
14·47
3·19
0·11
pl2
52·27
30·40
13·83
3·38
0·12
pl3
53·58
29·27
13·05
3·98
0·12
Fob
Analytical results
Major and trace element compositions of representative
samples are shown in the Appendix (Table A1). The
complete dataset may be downloaded from the Journal
of Petrology Web site at http://www.petrology.
oupjournals.org. The 1 standard deviations were calculated for five samples from the BI-13 flow and 10
samples from the MA-W flow. These values are the same
as or smaller than the 1 values of the calibration lines
for the contents of incompatible trace elements (Rb, Sr,
Ba, Zr, Y and Nb) as shown in Table A1. For some of
the major and compatible trace elements, on the other
hand, the intra-flow variations are greater than those of
the calibration lines.
The H2O contents of glass inclusions in olivine phenocrysts from a fresh basalt (NY-02-51) range between 0·5
and 0·7 wt % (Table 1) and are within the range of
those in mid-ocean ridge basalts (0·1–0·8 wt % H2O;
e.g. Dixon et al., 1988; Jambon & Zimmermann, 1990;
Johnson et al., 1994), and much smaller than those in
arc basalts (up to 6 wt % H2O; e.g. Sisson & Layne,
1993; Stolper & Newman, 1994).
Identification of the least- and mostcontaminated groups
In the Western Ghats area, each of the upper five
formations has been defined based on Ba, Sr and TiO2
a
b
Absorption peak height after subtraction of background.
100 × Mg/(Mg + Fe2+).
contents and Zr/Nb, 87Sr/86Sr and Nd (Cox & Hawkesworth, 1985; Devey & Lightfoot, 1986; Lightfoot &
Hawkesworth, 1988; Lightfoot et al., 1990; Peng et al.,
1994). The Ambenali formation is formed by basalts with
restricted compositions of Ba <100 ppm, Sr 190–240
ppm, TiO2 2·0–4·0 wt %, Zr/Nb = 10–18, 87Sr/86Sr
<0·7050 and Nd >3·0, indicating that these basalts are
less contaminated by continental crust and/or continental
lithospheric mantle than basalts from any other formation. Figure 2 shows that the majority (>90%) of
basalts with Ba <100 ppm, Sr 190–240 ppm and TiO2
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SANO et al.
DIFFERENTIATION OF DECCAN TRAP BASALTS
Fig. 2. Plots of Ba, Sr and TiO2 vs Sr and Nd isotopic ratios for Deccan Trap basalts. Most (>90%) Ambenali basalts have limited ranges
(shaded areas) of Ba, Sr and TiO2 contents, and Nd and Sr isotopic ratios. Data are from Mahoney et al. (1982), Cox & Hawkesworth (1985),
Lightfoot & Hawkesworth (1988), Lightfoot et al. (1990) and Peng et al. (1994).
2·0–4·0 wt % have isotopic compositions of 87Sr/86Sr
<0·7050 and Nd >3. It is thus possible in most cases to
identify probable least-contaminated basalts on the basis
of Ba, Sr and TiO2 when isotope data are not available.
Bushe Formation basalts are regarded as the most
contaminated by broadly granitic-type crust because of
the high 87Sr/86Sr (>0·7120), greater than values for
basalts from any other formation in the Western Ghats
(Figs 2 and 3). Figure 3 shows that the majority (>95%)
of the Bushe basalts have TiO2 contents <1·5 wt % and
Zr/Nb [ 15. We use TiO2 content and Zr/Nb to identify
probable most-contaminated basalts when isotope data
are lacking.
When we select the least-contaminated (Ambenalitype) and most-contaminated (Bushe-type) groups from
the Western Ghats area on the basis of Ba, Sr, TiO2
and Zr/Nb, most Ambenali lavas lie within the leastcontaminated group, and most Bushe lavas fall into the
most-contaminated group (Fig. 4a). This fact demonstrates that the chosen chemical criteria are useful for
identifying the least- and most-contaminated basalts of
the Deccan Traps.
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NUMBER 12
DECEMBER 2001
areas do not constitute a thick (>500 m) lava pile such
as the Ambenali Formation in the Western Ghats area
(Fig. 4), which suggests that distribution of the leastcontaminated basalts in the Central and Eastern areas
is different from that in the Ambenali Formation.
The most-contaminated basalts are also widely distributed in the Central and Eastern areas (Mahoney et
al., 2000); they are present in the ST, P and CH sections
(Fig. 4b). The lava pile of the most-contaminated basalts
overlies the least-contaminated lavas directly at the P
section. At the CH section, the most-contaminated basalts
are interbedded with the least-contaminated lavas. On
the other hand, in the Western Ghats area, the mostcontaminated lavas (Bushe Formation) are located under
the least-contaminated lavas (Ambenali Formation). The
stratigraphic discrepancy indicates that some or all of
the formations in the Western Ghats area do not continue
to the Central and Eastern areas (Peng et al., 1998;
Mahoney et al., 2000). One possible explanation of this
difference is that lava flows of the Central and Eastern
areas did not erupt from a vent located in the Western
Ghats area (Peng et al., 1998). The vents of the Eastern
and Central lava flows may have been dyke swarms
along Tapti–Narmada grabens.
EXPERIMENTAL PETROLOGY
Fig. 3. Plots of TiO2 and Zr/Nb vs Sr isotopic ratio for Deccan Trap
basalts. Bushe basalts have restricted ranges (shaded areas) of TiO2,
Zr/Nb and Sr isotopic ratio. Data sources are the same as for Fig. 2.
Distributions of the least- and mostcontaminated groups in the Central and
Eastern areas
In the Central and Eastern areas, the least-contaminated
basalts are distributed over large regions (Fig. 4b). Most
Ambenali-like lavas reported by Peng et al. (1998) from
the Chikaldara (CH) and Nagpur–Jabalpur (NJ) sections
are classified as least-contaminated basalts. Although we
expected that the least-contaminated basalts in the other
sections would also have geochemical affinities with the
Ambenali lavas, some of the least-contaminated basalts
do not have similar isotopic compositions to the Ambenali
lavas. Mahoney et al. (2000) have reported that basalts
with isotopic compositions similar to the Ambenali lavas
were not present in the Shahada–Toranmal (ST) section,
although we could not distinguish the least-contaminated
basalts at the ST section from the Ambenali basalts based
on major and trace element compositions (Fig. 4b). The
least-contaminated basalts in the Central and Eastern
A goal of the present study was to evaluate the effects of
fractional crystallization and crustal contamination on
Deccan magmas by comparison of natural-rock and
experimental data. Before making the comparison, the
effects of phenocryst accumulation recorded in the major
element compositions of the least-contaminated basalts
should be eliminated so as to evaluate fractional crystallization processes. The effects of phenocryst accumulation can be identified by the following data.
The least-contaminated basalts include aphyric to highly
porphyritic rocks whose phenocryst contents vary from
0 to 30 vol. %. The most voluminous phenocryst is
plagioclase. Some porphyritic basalts have higher Al2O3
and lower FeO∗ contents than the aphyric basalts (Fig.
5), indicating accumulation of plagioclase phenocrysts in
these porphyritic basalts (e.g. Cox & Mitchell, 1988). We
therefore selected only aphyric and sparsely porphyritic
basalts among the least-contaminated basalts.
Melting experiments
Two previous studies conducted melting experiments on
the Deccan Trap basalts (Krishnamurthy & Cox, 1977;
Cohen & Sen, 1994). Krishnamurthy & Cox (1977)
reported melting experiments at atmospheric pressure
(100 kPa), but they used alkalic basalts from the northwestern Deccan Traps (e.g. Krishnamurthy & Cox, 1980;
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SANO et al.
DIFFERENTIATION OF DECCAN TRAP BASALTS
Fig. 4. Sections showing the spatial distributions of the least- and most-contaminated lavas in the Western Ghats (a) and the Central and
Eastern areas (b). The columnar sections are after Deshmukh et al. (1996a) and Sano (1996). The CK, AN, TL and NP sections are not shown
in this figure because fewer than two flows were collected from each of these four sections. The approximate boundaries between formations
are indicated by continuous lines after Sano (1996). Abbreviations for sections are the same as in Fig. 1.
Mahoney et al., 1985; Melluso et al., 1995; Krishnamurthy
et al., 2000) as the starting materials. Their results are
not applicable to tholeiitic magmas, which form the
majority of Deccan Trap basalts. Cohen & Sen (1994),
however, conducted their experiments on tholeiites at
600 MPa, and concluded that the high-pressure liquid
line of descent (LLD) is not adequate to explain the trend
in chemical characteristics of the Ambenali basalts. We
therefore conducted melting experiments at atmospheric
pressure (100 kPa).
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Fig. 5. Plots of Al2O3 and FeO∗ vs wt % MgO for the least-contaminated basalts on the Deccan Traps.
As the H2O content in one of our least-contaminated
samples is negligibly small (Table 1), its fractional crystallization properties were studied under dry conditions.
Starting materials
For melting experiments at atmospheric pressure, we
selected one of the most Mg-rich (MgO 6·7 wt %)
tholeiitic basalts (MA-W-25 from the Ambenali Formation) among the least-contaminated group (Table A1).
Two other Mg-rich (MgO >9 wt %) basalts were also
selected as starting materials to investigate crystallization
processes at high MgO content; one (BH-14 from the
Khandala Formation) belongs to the most-contaminated
group and the other (IG-02 from the Jawhar Formation)
is classified as intermediate in contamination.
BH-14 and IG-02 are sparsely olivine-phyric basalts,
and MA-W-25 is a sparsely olivine–plagioclase–augitephyric basalt. The phenocryst contents of MA-W-25,
BH-14 and IG-02 are 3, 2 and 3 vol. %, respectively
(Table A1).
Experimental and analytical methods
Samples were crushed into millimetre-sized pieces with
an iron mortar and pestle, and visibly altered pieces were
NUMBER 12
DECEMBER 2001
removed by handpicking. The rock chips were placed in
an agate ball mill with acetone and ground into powder.
The powdered sample was dried and pellets of about 4
g in weight were formed with a hydraulic press. The
pellets were then crushed into many small chips
(200–400 mg in weight) which were bound up with Pt
wire of diameter 0·050 mm. The sample was fused into
a glass at a higher temperature (1300°C) than the liquidus
under the oxidation conditions for the target experiments,
and then quenched. The glass beads were then used as
starting samples.
The glass beads on Pt loops were suspended in the hot
spot of a quenching furnace. Temperature was measured
during each run with a Pt/Pt87Rh13 thermocouple with
accuracy of better than ±1·5°C. A mixture of CO2 and
H2, flowing vertically upward through the furnace, was
used to maintain oxygen fugacity ( fO2) at fayalite–
magnetite–quartz (FMQ ) and nickel–nickel oxide (NNO)
buffers. These conditions were selected because most
tholeiitic basalts are considered to have crystallized at
fO2 between these two buffers (e.g. Walker et al., 1979;
Helz & Thornber, 1987). After a sufficient run time
as described below, experiments were terminated by
quenching in water. Experimental conditions are reported in Table 2.
The compositions of phases in run products were
analysed with a JEOL JXA-8800R electron-probe microanalyser (EPMA) at the Earthquake Research Institute
of the University of Tokyo, using a 15 kV accelerating
voltage and a 12 nA beam current. The counting time
was 10 s. The beam size was 10 m for glass analysis
and 1 m for mineral phases. The data were reduced
using the Bence & Albee (1968) matrix correction and
included the modification described by Nakamura &
Kushiro (1970).
Experimental results
The proportions and compositions of phases are summarized in Tables 2 and 3. The phase proportions in
Table 2 were estimated based on a least-squares massbalance calculation of major element compositions for
the phases (Table 3). For MA-W-25, olivine, plagioclase
and augite crystallize simultaneously at 1155°C at the
FMQ buffer. These three mineral phases crystallize even
at temperatures as low as 1114°C (Table 2, Fig. 6). On
the other hand, at the NNO buffer, plagioclase, augite
and Fe–Ti oxide mineral crystallize at 1154°C. The
plagioclase, augite and Fe–Ti oxide mineral are stable at
<1144°C, and low-Ca pyroxene also appears at <1134°C
(Table 2, Fig. 6).
Olivine is the liquidus phase for both BH-14 and IG02 at >1220°C (Table 2), which is consistent with the
petrographical observations. The crystallization sequence
2182
SANO et al.
DIFFERENTIATION OF DECCAN TRAP BASALTS
Table 2: Run conditions and run products for experiments on Deccan Trap basalts
Starting
Run
T (°C)
fO
2
Duration
Run productsa
SSRc
Phase
b
lost
lost
2
2
no.
MA-W-25
26-1
1163
FMQ
44
gl
32-1
1155
FMQ
46
gl, ol, pl, cpx
93:1:5:1
0·04
36-1
1134
FMQ
48
gl, ol, pl, (cpx)e
82:6:12
0·36
25-1
1124
FMQ
44
gl, ol, pl, cpx
64:8:21:7
0·36
37-1
1119
FMQ
61
gl, ol, pl, cpx
53:5:26:16
0·26
35-1
1114
FMQ
52
gl, ol, pl, cpx
50:5:27:18
0·19
29-1
1163
NNO
22
gl
33-1
1144
NNO
47
gl, pl, cpx, sp
68:14:14:4
0·27
18-1
1134
NNO
54
gl, pl, cpx, lpx, sp
28-1
1123
NNO
45
gl, pl, cpx, lpx, sp
39:30:17:8:6
0·04
6-1
1225
FMQ
26
gl
38-1
1214
FMQ
26
gl, ol
98:2
0·34
4
31-1
1190
FMQ
22
gl, ol
96:4
0·10
1
34-1
1175
FMQ
32
gl, ol, pl, cpx
78:7:12:3
0·21
2
26-2
1163
FMQ
44
gl, ol, pl, cpx
68:11:18:3
0·99
4
24-2
1144
FMQ
46
gl, ol, pl, cpx
41:12:29:18
0·01
25-2
1124
FMQ
44
gl, ol, pl, cpx
30:14:35:21
0·21
35-2
1114
FMQ
52
gl, ol, pl, cpx, sp
23:15:38:24:tr
0·56
3
10-1
1215
NNO
23
gl, ol
99:1
0·64
5
4-1
1204
NNO
19
gl, ol
98:2
0·58
5
23-2
1191
NNO
26
gl, ol
96:4
0·54
4
19-1
1174
NNO
51
gl, ol, pl
76:8:16
0·33
1
15-2
1170
NNO
48
gl, ol, pl
2-2
1154
NNO
27
gl, ol, pl, cpx, sp
61:7:21:11
0·01
27-2
1143
NNO
44
gl, ol, pl, cpx, sp
43:12:29:16:tr
0·50
3
28-2
1123
NNO
45
gl, ol, pl, cpx, sp
26:9:35:27:3
0·10
1
21-1
1114
NNO
30
gl, ol, pl, cpx, sp
BH-14
BH-14
IG-02
IG-02
proportions
% Nad
material
MA-W-25
(h)
% Fed
2
2
3
3
4
2
1
39-1
1232
FMQ
27
gl
38-2
1214
FMQ
26
gl, ol
98:2
0·04
31-2
1190
FMQ
22
gl, ol, cpx
95:5:tr
0·11
1
34-2
1175
FMQ
32
gl, ol, cpx
87:4:9
1·15
4
32-2
1155
FMQ
46
gl, ol, pl, cpx
79:4:tr:17
0·89
5
36-2
1134
FMQ
48
gl, ol, pl, cpx, lpx
43:12:29:16:tr
0·32
3
37-2
1119
FMQ
61
gl, ol, pl, cpx, lpx
26:9:35:27:3
0·53
9-1
1225
NNO
24
gl, ol
99:1
10-2
1215
NNO
23
gl, ol
99:1
0·13
4-2
1204
NNO
19
gl, ol
98:2
0·11
13-2
1184
NNO
50
gl, ol, cpx, sp
94:3:3:tr
0·23
29-2
1163
NNO
52
gl, ol, pl, cpx, sp
83:6:1:10:tr
0·31
2
33-2
1144
NNO
47
gl, ol, pl, cpx, sp
56:7:14:23:tr
0·03
1
a
6
4
1
2
Abbreviations: gl, glass; ol, olivine; pl, plagioclase; cpx, augite; lpx, low-Ca pyroxene; sp, Fe–Ti oxide.
Weight proportions of phases estimated by materials balance using phase compositions from Table 3.
c
Residual sum of squares in the materials balance calculation.
d
Weight per cent Fe and Na lost from the whole-rock sample, estimated using the results of materials balance calculations.
Blanks mean the Fe and Na lost are p1.
e
Cpx should be present in 36-1 but was not found in the subsample selected for electron microprobe analysis.
b
2183
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 12
DECEMBER 2001
Table 3: Electron microprobe analyses of run products of the melting experiments reported in Table 2 (oxides
reported as weight per cent)
Phase
Run
n
SiO2
TiO2
Al2O3
FeO∗
MgO
CaO
Na2O
K2O
MA-W-25 (FMQ )
gl
ol
pl
cpx
26-1
6
49·74(29)
2·74(9)
13·61(22)
14·31(20)
6·66(11)
10·51(22)
2·17(10)
0·26(2)
32-1
6
49·71(30)
2·87(6)
13·13(12)
14·93(46)
6·40(9)
10·59(15)
2·11(13)
0·26(1)
36-1
6
50·76(30)
3·38(10)
12·18(20)
14·92(36)
5·60(14)
10·73(14)
2·09(10)
0·34(2)
25-1
5
50·77(45)
4·24(40)
11·30(8)
16·40(71)
4·83(25)
9·60(11)
2·39(4)
0·47(2)
37-1
6
48·86(37)
4·36(35)
10·84(20)
19·29(45)
4·57(17)
9·43(19)
2·20(13)
0·45(4)
35-1
6
48·90(22)
4·82(7)
10·69(24)
19·21(46)
4·35(13)
9·40(7)
2·14(10)
0·49(4)
32-1
4
37·48(26)
24·85(35)
37·36(29)
0·31(2)
36-1
4
37·40(48)
26·80(90)
35·44(57)
0·36(6)
25-1
3
37·00(41)
29·83(1·05) 32·72(1·28)
0·45(2)
37-1
4
35·95(35)
34·52(52)
29·20(10)
0·33(2)
35-1
2
35·75(11)
35·26(90)
28·62(45)
0·37(1)
32-1
4
51·99(35)
0·09(3)
29·52(12)
0·82(7)
0·22(1)
13·95(21)
3·34(4)
0·07(1)
36-1
4
52·74(62)
0·10(1)
28·98(27)
0·91(9)
0·26(2)
13·40(35)
3·52(15)
0·09(1)
25-1
4
52·56(44)
0·12(2)
28·59(33)
1·34(9)
0·32(4)
13·34(22)
3·64(11)
0·09(1)
37-1
4
53·14(31)
0·14(3)
28·33(54)
1·34(22)
0·18(4)
12·96(20)
3·78(7)
0·13(1)
35-1
4
53·37(20)
0·15(4)
28·19(23)
1·44(4)
0·21(3)
12·72(10)
3·78(17)
0·14(2)
32-1
3
50·27(1·23)
1·09(27)
3·50(1·14) 11·48(73)
25-1
3
50·08(60)
1·52(23)
2·93(33)
37-1
4
50·02(1·64)
1·24(35)
35-1
3
49·34(1·99)
1·59(57)
4·76(2·60) 13·69(1·19) 15·44(44)
14·98(74)
17·93(94)
15·59(1·29) 0·14(2)
13·54(1·15) 15·66(24)
16·03(1·53) 0·24(1)
3·73(1·79) 13·30(1·39) 15·98(73)
15·52(1·22) 0·21(4)
0·20(3)
MA-W-25 (NNO)
gl
pl
cpx
lpx
sp
33-1
6
51·69(27)
2·76(14)
12·93(18)
14·70(31)
5·54(15)
9·33(9)
2·47(7)
0·58(23)
18-1
4
52·46(32)
3·64(9)
11·81(16)
14·27(54)
5·26(12)
9·85(11)
2·23(5)
0·48(2)
28-1
6
54·22(53)
3·82(16)
11·88(24)
13·70(50)
4·52(12)
8·81(11)
2·47(21)
0·58(2)
33-1
4
51·98(59)
0·11(2)
29·06(95)
1·30(46)
0·23(3)
13·77(44)
3·31(7)
0·24(9)
18-1
3
52·91(45)
0·19(4)
27·08(66)
2·51(3)
0·39(3)
13·21(32)
3·59(18)
0·12(1)
28-1
3
53·66(39)
0·19(4)
27·71(52)
1·81(21)
0·26(7)
12·47(38)
3·79(13)
0·11(3)
33-1
3
51·26(72)
0·79(8)
3·43(61)
10·28(72)
17·11(75)
16·91(37)
0·22(2)
28-1
3
50·30(86)
1·33(12)
3·31(29)
12·37(64)
14·99(81)
17·46(1·41) 0·24(3)
18-1
1
53·92
0·35
0·79
17·24
24·06
28-1
2
53·13(1·16)
0·59(35)
1·28(69)
16·50(1·04) 22·93(1·60)
5·51(2·71) 0·06(2)
3·59
33-1
2
0·16(7)
9·67(8)
5·60(4)
76·25(33)
5·74(19)
0·18(1)
18-1
3
0·18(2)
9·26(13)
3·74(16)
81·35(61)
5·02(14)
0·19(2)
28-1
3
0·13(2)
12·43(14)
3·42(1)
78·99(50)
4·72(17)
0·12(2)
for BH-14 is olivine → plagioclase → augite → Fe–Ti
oxide mineral at both FMQ and NNO buffers (Table
2, Fig. 6). On the other hand, for IG-02, the second
crystallizing phase is augite, which appears at >1190°C,
followed by plagioclase at <1170°C. The higher crystallization temperature of augite for IG-02 may be due
to its higher CaO/Al2O3 compared with BH-14 (Table
A1). In addition to the above three mineral phases (olivine,
plagioclase, augite), an Fe–Ti oxide also crystallizes at
0·05
Ζ1184°C at the NNO buffer, and low-Ca pyroxene
begins to crystallize at 1134°C at the FMQ buffer in the
IG-02 composition.
Volatilization of Fe and Na from the experimental
charge is a serious problem in the atmospheric pressure
experiments (e.g. Tormey et al., 1987). In the present
experiments, however, serious volatilization was not confirmed because the duration of the experiments was
limited (Ζ61 h). The calculated per cent FeO loss and
2184
SANO et al.
Phase
Run
n
DIFFERENTIATION OF DECCAN TRAP BASALTS
SiO2
TiO2
Al2O3
FeO∗
MgO
CaO
Na2O
K2O
BH-14 (FMQ )
gl
ol
pl
cpx
6-1
6
49·90(16)
1·14(1)
14·55(14)
11·81(34)
8·74(10)
11·57(13)
1·76(2)
0·53(1)
38-1
5
50·15(57)
1·19(5)
15·13(10)
11·25(24)
8·20(11)
11·64(25)
1·91(3)
0·53(1)
31-1
4
50·56(33)
1·17(3)
15·13(15)
11·42(28)
7·50(18)
11·87(18)
1·82(5)
0·53(3)
34-1
6
51·16(52)
1·57(7)
13·82(15)
12·56(24)
6·70(12)
11·52(26)
1·99(6)
0·68(3)
26-2
6
52·35(30)
1·67(8)
13·03(22)
12·56(40)
6·01(13)
11·60(26)
2·00(12)
0·78(5)
24-2
6
51·99(60)
2·52(8)
12·68(24)
15·08(48)
4·91(9)
9·68(18)
2·05(6)
1·09(6)
25-2
6
54·13(37)
2·99(10)
12·85(14)
13·75(49)
4·06(8)
8·25(19)
2·33(2)
1·64(2)
35-2
2
53·88(1·61)
3·32(12)
12·80(22)
14·05(25)
3·77(30)
8·09(41)
2·16(2)
1·93(13)
38-1
4
38·86(44)
16·90(30)
43·86(37)
0·38(3)
31-1
1
38·91
17·52
43·26
0·31
34-1
4
38·11(47)
19·98(85)
41·56(62)
0·35(6)
26-2
4
38·49(15)
21·67(16)
39·48(46)
0·36(3)
24-2
3
37·18(23)
28·68(36)
33·75(26)
0·39(3)
25-2
3
36·87(27)
30·31(33)
32·42(44)
0·40(7)
35-2
3
36·77(29)
32·48(23)
30·34(67)
0·41(3)
34-1
4
50·55(42)
30·26(34)
0·94(24)
0·31(2)
15·08(14)
2·72(16)
0·14(2)
26-2
4
51·25(20)
29·88(29)
1·13(16)
0·37(4)
14·42(27)
2·80(8)
0·15(1)
24-2
3
51·15(29)
30·19(32)
1·30(10)
0·22(1)
14·20(65)
2·74(10)
0·20(1)
25-2
3
51·77(32)
28·86(57)
1·58(18)
0·38(5)
14·07(39)
3·07(15)
0·27(7)
35-2
3
52·96(48)
28·68(33)
1·31(18)
0·19(4)
12·94(18)
3·50(3)
0·42(3)
34-1
2
51·57(42)
0·43(2)
3·60(17)
7·67(62)
18·48(86)
18·09(86)
0·16(2)
26-2
2
51·91(42)
0·83(9)
3·44(25)
8·02(1)
15·38(86)
20·22(35)
0·20(1)
24-2
3
51·58(34)
0·76(13)
2·72(45)
10·51(71)
15·22(25)
19·02(57)
0·19(2)
25-2
3
50·47(1·13)
1·14(28)
3·07(59)
12·14(41)
14·24(62)
18·72(1·08) 0·22(4)
35-2
1
51·10
1·02
2·59
12·18
14·89
17·98
0·24
IG-02 (FMQ )
gl
ol
pl
cpx
lpx
39-1
6
51·45(14)
1·73(2)
12·70(12)
11·17(13)
9·03(5)
11·57(9)
1·78(3)
38-2
6
51·75(29)
1·75(5)
12·79(23)
11·07(46)
8·31(8)
11·88(19)
1·87(10)
0·58(2)
31-2
4
52·30(18)
1·82(9)
13·12(34)
10·75(50)
7·29(15)
12·15(19)
1·95(6)
0·62(4)
34-2
6
52·65(40)
1·78(9)
14·38(33)
10·77(30)
6·73(13)
10·74(21)
2·22(7)
0·73(3)
32-2
6
52·85(56)
1·91(9)
15·07(18)
10·83(27)
6·01(17)
10·03(25)
2·58(13)
0·72(6)
36-2
6
54·55(23)
3·04(6)
13·13(6)
12·43(51)
4·60(6)
8·54(22)
2·43(8)
1·28(4)
37-2
3
54·09(48)
3·34(1)
13·15(9)
13·37(76)
3·88(16)
7·52(25)
2·79(6)
1·86(15)
38-2
4
39·20(74)
15·92(64)
44·52(70)
0·36(2)
31-2
2
39·33(28)
17·52(2)
42·78(95)
0·37(4)
34-2
4
38·67(51)
18·69(20)
42·31(80)
0·33(3)
32-2
2
38·07(21)
20·22(48)
41·43(4)
0·28(2)
36-2
4
37·81(29)
26·57(1·16) 35·26(41)
37-2
4
36·99(73)
28·25(1·12) 34·52(49)
32-2
4
52·76(50)
29·32(37)
0·83(4)
0·29(3)
12·99(52)
3·65(18)
0·16(1)
36-2
4
52·80(21)
28·84(23)
1·26(18)
0·34(7)
13·10(35)
3·42(23)
0·24(4)
37-2
4
53·52(13)
28·48(36)
1·18(6)
0·22(3)
12·39(16)
3·90(6)
0·31(4)
31-2
2
53·02(8)
0·61(6)
3·32(2)
6·69(74)
19·09(12)
17·13(13)
0·14(2)
34-2
2
52·92(23)
0·56(0)
2·86(6)
7·15(62)
19·07(46)
17·25(1·38) 0·19(7)
32-2
3
51·06(46)
0·90(20)
4·65(67)
6·95(56)
16·85(1·46) 19·40(1·77) 0·19(3)
36-2
3
50·92(72)
1·38(37)
3·79(76)
9·42(50)
15·07(34)
19·21(1)
37-2
4
49·35(66)
1·36(25)
6·19(67)
10·08(40)
16·71(3)
16·08(69)
36-2
1
54·85
0·31
0·69
15·88
24·65
3·59
0·03
37-2
1
52·66
0·59
1·17
17·98
22·12
5·36
0·12
2185
0·57(1)
0·36(2)
0·24(1)
0·21(3)
0·23(1)
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 12
DECEMBER 2001
Table 3: continued
Phase
Run
n
SiO2
TiO2
Al2O3
FeO∗
MgO
CaO
Na2O
K2O
BH-14 (NNO)
gl
ol
pl
cpx
10-1
6
50·33(22)
1·13(2)
14·87(21)
11·19(41)
8·67(9)
11·41(24)
1·88(4)
0·52(2)
4-1
6
50·23(22)
1·16(3)
15·11(23)
11·21(74)
8·34(11)
11·45(17)
1·96(7)
0·54(2)
23-2
5
50·74(33)
1·19(3)
15·37(18)
11·18(28)
7·43(8)
11·67(17)
1·87(10)
0·55(3)
19-1
5
51·25(46)
1·48(7)
13·40(17)
13·01(28)
6·83(9)
11·41(20)
1·96(10)
0·66(4)
15-2
4
52·41(62)
1·47(5)
13·24(15)
12·70(87)
6·81(15)
10·99(12)
1·75(6)
0·63(2)
2-2
6
50·88(42)
27-2
6
52·88(17)
28-2
3
56·98(12)
21-1
4
63·13(28)
1·77(12)
13·00(12)
14·79(27)
6·21(18)
10·59(21)
12·79(10)
14·53(32)
5·02(8)
9·44(23)
2·56(2)
13·13(16)
11·69(31)
3·89(15)
7·57(23)
2·16(32)
2·02(6)
2·24(10)
13·00(11)
7·57(27)
3·12(7)
6·57(8)
2·05(7)
2·32(22)
14(19)
2(6)
0·76(4)
2·05(5)
1·15(3)
10-1
3
39·56(30)
13·92(8)
46·24(52)
0·28(2)
4-1
3
39·13(35)
14·67(46)
45·83(52)
0·37(1)
23-2
2
39·61(33)
16·03(17)
44·07(47)
0·29(2)
19-1
2
39·20(1)
16·73(27)
43·79(28)
0·28(2)
15-2
3
39·36(7)
17·82(19)
42·42(52)
0·40(2)
2-2
2
38·54(14)
21·10(2)
40·01(29)
0·35(0)
27-2
3
37·90(50)
25·80(15)
35·98(55)
0·32(4)
28-2
3
37·99(13)
25·36(19)
36·32(30)
19-1
1
50·54
27·32
3·39
1·59
14·78
2·13
0·25
2-2
1
49·84
30·11
1·97
0·88
14·70
2·36
0·14
27-2
2
51·77(21)
29·14(32)
1·51(16)
0·31(3)
14·10(66)
2·97(23)
0·20(4)
28-2
3
51·59(83)
29·13(47)
1·69(20)
0·42(8)
13·99(57)
2·95(29)
0·23(3)
2-2
3
52·15(30)
0·48(6)
2·76(10)
8·39(15)
18·08(50)
17·93(41)
0·21(3)
27-2
3
50·56(38)
0·98(3)
3·52(28)
10·(36)1·130 14·94(14)
19·44(21)
0·20(3)
28-2
3
50·23(86)
1·33(13)
3·31(29)
12·44(64)
17·46(1·42) 0·24(3)
1·00(21)
21-1
3
47·67(94)
lpx
21-1
1
54·40
sp
28-2
3
21-1
1
12·40(13)
7·31
14·99(81)
0·33(5)
9·00(32)
11·90(74)
16·54(1·25) 13·62(1·59) 0·27(8)
4·25
12·57
22·43
3·45(1)
79·12(48)
4·73(16)
0·13(2)
3·80
82·29
6·15
0·20
5·90
0·45
IG-02 (NNO)
gl
ol
pl
cpx
9-1
6
51·74(14)
1·67(5)
12·63(16)
11·01(43)
8·90(11)
11·67(22)
1·82(7)
0·56(3)
10-2
6
51·79(18)
1·68(7)
12·71(9)
10·95(34)
8·81(8)
11·72(8)
1·82(8)
0·52(2)
4-2
6
51·53(54)
1·79(11)
12·70(21)
11·26(36)
8·34(13)
11·92(13)
1·88(4)
0·58(2)
13-2
6
52·17(17)
1·79(8)
13·14(30)
11·39(29)
7·55(21)
11·61(21)
1·76(8)
0·59(2)
29-2
6
52·68(21)
1·93(5)
14·36(44)
11·02(41)
6·05(20)
11·13(20)
2·17(9)
0·66(3)
33-2
6
53·93(39)
2·59(4)
13·11(14)
12·76(40)
5·10(13)
8·99(13)
2·40(6)
1·12(6)
9-1
4
39·61(22)
14·01(36)
46·03(7)
0·35(0)
10-2
4
40·23(43)
12·72(96)
46·66(86)
0·39(3)
4-2
4
39·33(23)
13·11(21)
47·28(36)
0·28(1)
13-2
3
39·78(25)
14·79(42)
45·08(26)
0·35(2)
29-2
3
39·84(9)
17·19(85)
42·61(1·10)
0·36(4)
33-2
4
38·46(8)
22·94(40)
38·29(52)
0·31(2)
29-2
3
51·94(46)
29·34(5)
1·13(9)
0·27(2)
14·17(7)
2·98(14)
0·17(1)
33-2
3
52·53(95)
28·94(63)
1·37(13)
0·25(5)
13·28(72)
3·44(21)
0·19(2)
13-2
2
51·22(74)
0·68(4)
3·42(35)
8·01(35)
17·42(78)
19·03(83)
0·22(0)
29-2
2
51·67(17)
0·91(2)
4·06(6)
7·65(57)
17·02(1·82) 18·50(3·78) 0·19(4)
33-2
4
49·15(50)
1·34(23)
5·74(40)
8·48(76)
14·53(11)
20·52(75)
0·24(5)
Abbreviations used for phases are the same as in Table 2. Numbers in parentheses adjacent to the analyses are 1 SD; e.g.
49·74(29) represents 49·74±0·29 wt %.
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DIFFERENTIATION OF DECCAN TRAP BASALTS
Fig. 6. Results of melting experiments at atmospheric pressure (100 kPa) performed on three Deccan Trap basalts. Χ, wt % MgO and estimated
phase proportions for each experiment. Abbreviations for phases are the same as in Table 2.
per cent Na2O loss in the melting experiments were
within the precision of the EPMA technique (Table 2).
Attainment of equilibrium
The experiments were run as long as practically possible
(Table 2) to achieve homogeneity in the products, but
were limited by Na loss from the samples. Longer duration
experiments than those carried out here (>61 h) are
required to achieve total homogeneity of the phases (e.g.
Grove & Bryan, 1983; Tormey et al., 1987). This is
because the homogenization scale of SiO2 in melt is
>0·15 mm in 61 h, using an interdiffusion coefficient D
of >10−9 cm/s at 1100–1200°C (e.g. Koyaguchi, 1989);
this scale is distinctly smaller than the size of the sample
(2–3 mm in diameter). However, the following facts show
that the experiments at temperatures >1120°C closely
approach equilibrium.
First, quenched liquid, olivine, plagioclase and augite in
high-temperature experiments (>1120°C) are chemically
homogeneous within the precision of the EPMA technique. Because some augite crystals in low-temperature
experiments (<1120°C) were found to be chemically
heterogeneous, these compositions were not used for
evaluating fractional crystallization processes.
Second, the Fe–Mg partitioning between magnesian
phases (olivine and augite) and melts observed in the
present experiments at high temperature (>1120°C) is
constant. Under the conditions of the FMQ buffer, the
Fe–Mg partitioning coefficients (KD) are 0·28–0·34 for
olivine–melt and 0·26–0·32 for augite–melt. The KD
values were calculated by using an Fe3+ content appropriate for FMQ and NNO buffers (Sack et al., 1980),
and are in good agreement with those reported from
previous equilibrium experiments (e.g. Tormey et al.,
1987; Yang et al., 1996). The KD values under the NNO
buffer are also constant (0·24–0·30 for olivine and 0·23–
0·36 for augite), although with a slightly greater range
than those under the FMQ buffer.
Third, almost all of the experimental temperatures
coincide with predicted temperatures calculated using
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NUMBER 12
DECEMBER 2001
Fig. 7. Observed temperatures vs predicted temperatures calculated
using the geothermometer of Ford et al. (1983).
the geothermometer of Ford et al. (1983). Those workers
reported that temperatures could be obtained within
errors of ±10°C by using major element compositions
of magmas equilibrated with olivine crystals. When we
consider the accuracy of temperature measurements in
our experiments (±1·5°C), the majority of the calculated
temperatures are all within 10°C of the experimental
temperature (Fig. 7). However, calculated temperatures
for three runs (24-2, 26-2 and 29-2) are distinctly lower (by
>15°C) than the experimental temperatures, indicating a
thermocouple problem or modification during quenching
in the glasses from these runs. We do not use results of
these three runs in the following discussion.
Comparison of natural-rock and
experimental data
Evaluation of fractional crystallization
The least-contaminated basalts contain phenocrysts of
plagioclase, augite and olivine, and do not possess any
Fe–Ti oxide mineral as a phenocryst phase. The phenocryst assemblage in the least-contaminated basalts is
identical to that crystallizing during the melting of MAW-25, BH-14 and IG-02 at the FMQ buffer. We suggest
that the FMQ-buffered conditions are more representative of Deccan Trap magmas than are those
equivalent to the fO2 of the NNO buffers. This is consistent
with the estimated fO2 for Deccan Trap basalts based on
the compositions of Fe–Ti oxides (Sen, 1986; Sethna &
Sethna, 1988).
To check the validity of the fractional crystallization
processes, compositions of phenocryst phases in the leastcontaminated basalts are compared in Fig. 8 with those
from experiments on a least-contaminated basalt (MAW-25). The minerals obtained from the experiments and
Fig. 8. Plots of major elements vs wt % MgO for olivine, plagioclase
and augite phenocrysts in the least-contaminated basalts. Data are
from Sen (1986) and this study (Table 1). Only rim compositions are
plotted for plagioclase phenocrysts in porphyritic rocks, because some
plagioclase phenocrysts in porphyritic rocks might be cumulate. To
compare the natural-rock data with the experimental results, mineral
and melt phase compositions obtained in the experiments on the leastcontaminated basalt MA-W-25 are also shown.
the natural phenocrysts have nearly identical compositions for olivine, plagioclase and augite. Because
natural olivine compositions are known for one leastcontaminated basalt (NY-02-51 in Table 1), the natural
phenocrysts have restricted compositions compared with
the minerals obtained by the experiments. Figure 8 shows
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SANO et al.
DIFFERENTIATION OF DECCAN TRAP BASALTS
that augite crystals obtained from the experiments have
lower CaO contents than augite phenocrysts in the leastcontaminated basalts. This result is simply due to CaO
depletion of the starting composition (Fig. 9).
The major element compositions of the least-contaminated basalts define certain fundamental chemical
trends: SiO2 is constant, and FeO∗ and K2O show a
gradual increase with decreasing MgO content. In addition, Al2O3 and CaO show a progressive decrease with
decreasing MgO content (Fig. 9). The chemical trends
observed in the least-contaminated basalts can be approximately reproduced by the LLD obtained from the
experiments at both FMQ- and NNO-buffered conditions, indicating that the least-contaminated basalts
experienced fractional crystallization at low pressure
(>100 kPa) under dry conditions. Figure 9 shows that
the FeO∗ content in the experimental melts at NNObuffered conditions progressively decreases, whereas that
in the least-contaminated basalts increases with decreasing MgO content. Therefore, the LLD under FMQbuffered conditions is more favourable than that under
NNO-buffered conditions to explain the chemical trend
of the least-contaminated basalts.
The results of the experiments (Fig. 6) show that one
of the most evolved magmas, with 5·0 wt % MgO, can
be produced by subtraction of 8 wt % of olivine, 20 wt %
of plagioclase and 7 wt % of augite from one of the most
Mg-rich basalts (MA-W-25). This result is consistent with
the earlier conclusions (e.g. Cox, 1980; Cox & Devey,
1987; Devey & Cox, 1987; Cox & Mitchell, 1988;
Lightfoot et al., 1990; Sen, 1995) that the least-contaminated magma evolved its chemical composition by
gabbroic fractionation (olivine, plagioclase and augite) in
shallow magma chambers and/or dykes.
The behaviour of incompatible trace elements during
the fractional crystallization process is further examined
assuming Rayleigh fractionation (e.g. Shaw, 1970). The
element contents expected from fractional crystallization
are shown in Fig. 10 for Ba, Nb, Zr and Y (see Table 4
for the fractional crystallization model used). The trace
element variations observed in the least-contaminated
basalts are reproduced by the modelled fractional crystallization pathways.
Because the MgO content of the majority of the
least-contaminated basalts is 5·0–7·0 wt % (Fig. 9), the
temperature range during the fractional crystallization is
assumed to be 1150–1170°C (Fig. 6). Such a temperature
range is hotter than previous estimates of <1145°C and
1025°C based on pigeonite and Fe–Ti oxide geothermometers, respectively (Sen, 1986; Sethna & Sethna,
1988). The appearance of pigeonite and Fe–Ti oxides
solely as groundmass phases in the least-contaminated
basalts may account for this.
In summary, the comparison of natural-rock and experimental data suggests that the petrography and the
Fig. 9. Plots of major elements vs wt % MgO for the aphyric leastcontaminated basalts. To compare the natural data with the experimental results, melt compositions obtained in the experiments on
the least-contaminated basalt MA-W-25 are also shown.
chemistry of the least-contaminated basalts can be reasonably explained by low-pressure (>100 kPa) fractional
crystallization under dry conditions.
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Table 4: Fractional crystallization calculation for incompatible trace elements
Fraction of each phasea
Chemical composition
ol
pl
cpx
0·08
0·20
0·07
Differentiated magma
0·05
0·20
0·10
Differentiated magma
0·10
0·25
0·15
melt
MgO
Ba
1·00
7·33
56
0·65
5·00
79
1·00
7·19
137
0·65
5·00
194
0·50
5·00
244
Nb
Zr
Y
137
33
208
50
5·3
93
26
8·1
141
39
181
49
Least contaminated
Parental magmab
Differentiated magma
9·0
14
Most contaminated
Parental magmab
11
The distribution coefficients compiled by McKenzie & O’Nions (1995) are used in the calculation. Abbreviations used for
mineral phases are the same as in Table 2.
a
Fraction of each phase for the least-contaminated group is predicted by results of the experiments on MA-W-25 at FMQbuffered conditions; assuming that MgO content in the most differentiated magma is 5·00 wt %, fractions of olivine,
plagioclase, augite and melt are estimated to be 0·08, 0·20, 0·07 and 0·65, respectively (Fig. 6). For the calculation of the
most-contaminated group, two combinations of the fraction of each phase are assumed based on the results of the
experiments on BH-14 and IG-02 samples at FMQ-buffered conditions (Fig. 6).
b
Averaged compositions of Mg-rich (MgO >7·0 wt %) basalts are assumed to be a parental magma whose composition is
used in the calculation.
Evaluation of crustal contamination
Crustal contamination might be accompanied by fractional crystallization for Deccan Trap magmas because
contamination with wall-rock material could be aided by
heat released by partial crystallization of magma. If
so, positive correlations between the amount of crustal
contamination and the degree of fractional crystallization
in magmas erupted through continental crust are expected. Previous workers (e.g. DePaolo, 1981; Kinzler et
al., 2000) reported that differentiated magmas with low
mg-number [100 × Mg/(Mg + Fe2+)] and low MgO
content often can be more contaminated than primitive
magmas (e.g. 87Sr/86Sr of a differentiated magma may
be higher than that of a primitive magma). The contamination (assimilation) accompanied by fractional crystallization is termed assimilation–fractional crystallization
(AFC; e.g. DePaolo, 1981). For the Deccan Traps, however, AFC appears not to apply, because there is no
correlation between the mg-number and 87Sr/86Sr in
Deccan Trap basalts (Devey & Cox, 1987); rather, there
is a rough negative correlation of Nd and mg-number
(Mahoney, 1988; Mahoney et al., 2000).
To examine AFC from the aspect of experimental
petrology, trace element variations of the most-contaminated Deccan basalts are compared with trends
expected from closed-system fractional crystallization.
The trace element contents (Ba, Nb, Zr and Y) in
the most-contaminated basalts gradually increase with
decreasing MgO content, whereas the variation of Ba
contents is slightly scattered (Fig. 10). Figure 10 also
shows the trace element paths expected from lowpressure fractional crystallization under dry conditions
(see Table 4 for fractional crystallization model). The
trace element variations in the most-contaminated
basalts are generally explained by the fractional crystallization model.
The most-contaminated basalts have higher Ba and
lower Nb and Zr contents than the least-contaminated
basalts, and many likely continental crustal contaminants
have low MgO (<4 wt %), higher Ba, and lower Nb and
Zr contents compared with Deccan Trap basalts. If the
most-contaminated basalts contain higher Ba and lower
Nb and Zr contents than expected for fractional crystallization, then an AFC model might apply. However,
AFC is not required for the variations of the mostcontaminated basalts, which suggests the following scenario (Devey & Cox, 1987; Mahoney, 1988; Lightfoot et
al., 1990; Mahoney et al., 2000): the chemical variations
observed in the most-contaminated basalts were generated by fractional crystallization of a gabbroic assemblage in a chamber, without accompanying
contamination. Most of the contamination may have
taken place before most shallow-level fractional crystallization (e.g. Lightfoot et al., 1990).
2190
SANO et al.
DIFFERENTIATION OF DECCAN TRAP BASALTS
the western Deccan have chemical signatures of TiO2
<1·5 and Zr/Nb [ 15. The most-contaminated group
is located under the least-contaminated group in the
western Deccan. On the other hand, the most-contaminated group caps or is interbedded with the leastcontaminated group in parts of the central and eastern
Deccan, as was also concluded by Peng et al. (1998) and
Mahoney et al. (2000), indicating that formations in the
western Deccan do not necessarily continue to the central
and eastern Deccan.
Phenocryst phases of the least-contaminated basalts
(olivine, plagioclase and augite) are reproduced by melting
experiments at atmospheric pressure (100 kPa) under
FMQ-buffered conditions, and the liquid line of descent
under these conditions agrees with the major and trace
element trends of the least-contaminated basalts. The
experimental results also show that the temperature range
during fractional crystallization was 1150–1170°C. The
chemical trends of the most-contaminated basalts are also
reproduced by fractional crystallization at atmospheric
pressure, which suggests that crustal contamination was
not coupled to fractional crystallization in shallow magma
chambers.
ACKNOWLEDGEMENTS
Fig. 10. Plots of incompatible trace elements (Ba, Nb, Zr and Y)
vs wt % MgO for the least- and most-contaminated basalts. Calculated
Rayleigh fractionation paths for two Mg-rich basalts are also shown.
One Mg-rich composition used in the Rayleigh fractionation calculation
is the average of compositions with MgO >7·0 wt % in the leastcontaminated basalts, and another composition used in the calculation
is the averaged composition with MgO >7·0 wt % in the mostcontaminated basalts.
CONCLUSIONS
On the basis of Ba (<100 ppm), Sr (190–240 ppm) and
TiO2 (2·0–4·0 wt %) contents, an inferred leastcontaminated basalt group was selected from samples
covering the whole area of the Deccan Traps. The leastcontaminated basalts are widely distributed in the central
and eastern Deccan and in the western Deccan, where
they appear as the Ambenali Formation. The mostcontaminated basalts that form the Bushe Formation in
We thank A. Yasuda for his help in melting experiments
and electron probe analyses, and K. K. K. Nair and D.
B. Yedeker for providing the geological information. We
are grateful to R. J. Arculus and T. Falloon for their
careful and critical reviews. We are also grateful to J. J.
Mahoney, G. Sen and Y. Tatsumi for their careful and
constructive comments on an earlier version of this
manuscript. K. Kaneko, I. Kaneoka, T. Koyaguchi, S.
Nakada and H. Nagahara are thanked for constructive
discussions. This work was supported by Research Fellowships from the Japan Society for the Promotion of
Science for Young Scientists.
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2193
wt %
SiO2
TiO2
Al2O3
FeO∗f
MnO
MgO
CaO
Na2O
K 2O
P 2O 5
CaO/Al2O3
ppm
Rb
Sr
Ba
Y
Zr
Nb
V
Cr
Ni
Cu
Zn
Ga
Lag
Ceg
Ndg
Smg
Eug
Tbg
Ybg
Lug
Thg
Scg
Phenocryst
Area:c
Group:d
Formation:e
Sample:
17
243
149
41
215
19
502
150
79
257
144
26
1
5
49·46
3·32
13·17
14·92
0·23
5·58
9·91
2·40
0·67
0·34
0·75
AG-1451
W
Cont
Mah
3
150
48
32
115
8
407
92
75
191
110
20
50·28
1·85
13·88
13·46
0·19
6·54
11·25
2·09
0·29
0·17
0·81
KP-0351
W
Cont
Pan
10
217
42
40
176
13
465
206
100
253
124
24
11·73
27·85
20·55
5·74
2·06
1·07
3·33
0·49
1·13
35·79
0
49·86
2·62
13·84
13·25
0·20
6·15
11·07
2·33
0·41
0·27
0·80
AG-1051
W
Least
Amb
2194
4
195
48
34
148
10
373
120
82
171
106
21
10·48
24·57
18·71
5·35
2·04
1·05
3·29
0·48
1·08
39·18
3
48·90
2·77
13·38
14·62
0·21
6·69
10·67
2·19
0·33
0·24
0·80
MA-W25a
W
Least
Amb
8
6
213
62
38
160
10
479
90
81
253
124
26
49·64
2·71
13·95
14·21
0·22
6·10
10·26
2·26
0·40
0·25
0·74
BM-1551
W
Least
Amb
4
214
37
31
132
10
415
128
87
207
120
23
8·83
20·79
15·60
4·54
1·73
0·90
2·85
0·40
0·83
38·08
0
48·69
2·58
13·70
14·15
0·21
6·74
11·37
2·15
0·21
0·20
0·83
KK-0651
W
Least
Amb
8
2
211
29
30
127
8
413
121
92
185
102
21
50·01
2·05
14·14
12·67
0·19
6·58
11·88
2·11
0·19
0·18
0·84
KT-0651
W
Least
Amb
0
16
206
114
38
194
13
480
99
73
250
122
24
50·07
2·97
12·88
14·77
0·20
5·90
9·93
2·44
0·58
0·26
0·77
MA-B51
W
Cont
Pol
8
10
208
133
31
140
9
371
290
118
174
96
21
51·58
1·91
14·47
11·30
0·17
6·91
10·73
2·18
0·56
0·19
0·74
BM-0651
W
Cont
Pol
0
20
189
141
24
101
5
269
305
133
116
80
20
52·30
1·09
14·78
10·55
0·14
7·12
10·95
2·21
0·75
0·11
0·74
MA-00251
W
Most
Bus
4
220
74
35
159
10
435
76
79
216
105
22
13·85
30·78
21·07
5·53
1·99
1·03
3·41
0·51
1·86
41·11
3
49·83
2·33
13·51
14·09
0·18
6·01
11·20
2·40
0·24
0·21
0·83
BU-0551
C
Least
BU I
4
215
67
37
164
11
503
77
78
187
121
22
13·36
29·34
19·98
5·26
1·89
0·99
3·04
0·47
1·77
38·56
0
49·80
2·47
13·50
14·04
0·19
6·08
11·27
2·18
0·26
0·21
0·83
P15-Aa89.2b
E
Least
P III
—
209
36
31
139
9
427
202
116
220
107
16
9·38
22·06
16·27
4·73
1·79
0·93
2·94
0·42
0·88
35·86
0
48·40
2·56
13·61
14·52
0·20
6·94
11·29
2·12
0·14
0·22
0·83
E
Least
CH III
CH-27b
8
217
55
32
145
11
408
143
96
241
108
16
11·40
26·68
18·67
5·02
1·88
0·93
3·06
0·43
1·16
37·16
0
49·19
2·53
13·61
14·25
0·19
6·47
11·25
2·06
0·23
0·22
0·83
CH-32
52b
E
Least
CH V
NUMBER 12
2
6
194
43
33
144
10
426
130
93
220
113
22
49·08
2·52
13·58
14·15
0·22
6·97
10·67
2·21
0·38
0·22
0·79
MA-V55
W
Least
Amb
VOLUME 42
15
14
290
169
28
157
16
314
260
139
153
106
21
50·74
2·31
14·03
12·18
0·18
6·46
10·99
2·46
0·42
0·23
0·78
MA-Z655
W
Cont
Mah
Table A1: Representative whole-rock major and trace element analyses of Deccan Trap basalts
JOURNAL OF PETROLOGY
DECEMBER 2001
2195
20
168
126
25
86
5
284
297
97
136
81
20
17·2
2
51·64
1·15
14·89
10·81
0·17
7·36
11·51
1·77
0·59
0·11
0·77
BM-0251
W
Most
Bus
5
200
81
33
140
8
313
343
155
160
92
21
17·5
5
51·63
1·85
13·44
12·36
0·18
7·34
10·48
2·15
0·39
0·18
0·78
AK-0251
W
Cont
Tha
13
232
91
25
113
8
348
445
142
129
87
16
14·1
1
24
263
70
29
149
11
384
195
99
190
92
20
13·5
12
50·22
2·25
15·55
11·90
0·16
5·85
10·99
2·31
0·55
0·22
0·71
W
Cont
Bhi
W
Cont
Tha
49·61
1·92
13·24
12·56
0·16
8·25
11·58
1·88
0·62
0·18
0·87
BH-10
BH-03
19
180
176
22
76
4
269
500
221
92
88
23
19·0
2
48·74
1·09
14·60
12·62
0·17
9·33
10·86
1·80
0·67
0·12
0·74
W
Most
Kha
BH-14a
14
191
174
27
125
8
268
426
92
56
82
20
15·6
3
51·55
1·70
12·50
11·11
0·15
9·00
11·50
1·80
0·55
0·14
0·92
W
Cont
Jaw
IG-02a
18
256
269
35
189
12
354
217
98
162
89
24
15·8
15
49·56
2·52
14·65
13·18
0·18
5·82
10·34
2·69
0·81
0·25
0·71
W
Cont
Iga
IG-08
—
212
—
37
155
11
410
79
86
232
117
22
14·1
49·73
2·46
13·95
13·78
0·20
5·89
11·25
2·17
0·28
0·29
0·81
C
Least
ST IV
ST-26b
18
137
146
34
109
7
324
133
60
168
84
16
15·6
12
52·01
1·35
14·74
11·91
0·18
5·87
11·19
2·26
0·35
0·14
0·76
E
Most
CH IV
CH-28b
7
224
60
42
189
14
544
81
81
318
133
24
13·5
0
49·04
2·71
13·14
15·37
0·21
5·82
10·62
2·49
0·32
0·28
0·81
KV-0151
E
Least
KV I
—
221
44
29
136
10
393
200
103
204
103
23
13·6
0
49·57
2·34
13·90
12·94
0·18
6·93
11·65
2·16
0·14
0·19
0·84
E
Least
AN-02
12
206
83
40
172
14
582
84
82
295
146
23
12·3
5
48·67
2·83
13·20
16·17
0·21
5·62
10·30
2·26
0·49
0·25
0·78
NY-0251
E
Least
NY I
—
211
61
38
157
11
466
140
77
201
112
25
14·3
2
48·56
2·26
14·32
13·91
0·25
6·77
10·96
2·62
0·12
0·23
0·77
NJ-0951b
E
Least
YD
3
5
14
1
6
1
30
4
4
35
8
1
0·30
0·05
0·20
0·18
0·01
0·28
0·19
0·08
0·08
0·01
0·95
MA-Wh
(1)
W
Least
3
5
7
1
8
1
13
5
4
13
4
1
0·13
0·09
0·23
0·24
0·01
0·15
0·10
0·06
0·02
0·01
0·43
BI-13h
(1)
C
Cont
3
11
18
2
6
1·1
10
13
3·3
2
4
1
0·15
0·01
0·18
0·03
0·01
0·05
0·03
0·04
0·01
0·01
0·17
XRFi
(1)
The full dataset is available from the Journal of Petrology Web site at http://www.petrology.oupjournals.org. The data are calculated to 100 wt % total.
a
Starting materials of melting experiments.
b
The whole-rock analyses of CH, NJ, P and ST sections were previously published (Deshmukh et al., 1996a, 1996b; Godbole et al., 1996; Nair et al., 1996; Yedekar
et al., 1996).
c
Abbreviations of areas: W, Western Ghats area; C, Central area; E, Eastern area.
d
Abbreviations of groups: Least, least-contaminated group; Cont, contaminated group; Most, most contaminated group.
e
Abbreviations of formations in the Western Ghats are the same as in Fig. 4, whereas those in the Central and Eastern areas are after Deshmikh et al. (1996a)
and Yedeker et al. (1996).
f
Total iron as FeO.
g
Elements determined by INAA.
h
The 1 value of compositions for 10 samples collected from MA-W flow and those for five samples from BI-13 flow.
i
Analytical precision of the XRF analysis [the 1 value of compositions for calibration lines from Kaneko (1995)].
wt %
SiO2
TiO2
Al2O3
FeO∗f
MnO
MgO
CaO
Na2O
K 2O
P 2O5
CaO/Al2O3
ppm
Rb
Sr
Ba
Y
Zr
Nb
V
Cr
Ni
Cu
Zn
Ga
Zr/Nb
Phenocryst
Sample:
No.:
Area:c
Group:d
Formation:e
SANO et al.
DIFFERENTIATION OF DECCAN TRAP BASALTS