Glass in the submarine section of the HSDP2 drill core, Hilo,
Hawaii
Edward Stolper
Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-25, Pasadena, California
91125, USA (ems@gps.caltech.edu)
Sarah Sherman and Michael Garcia
Department of Geology and Geophysics, University of Hawaii at Manoa, 2525 Correa Road, Honolulu, Hawaii 96822,
USA (bean@soest.hawaii.edu; garcia@soest.hawaii.edu)
Michael Baker and Caroline Seaman
Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-25, Pasadena, California
91125, USA (mikeb@gps.caltech.edu)
[
1
]
The Hawaii Scientific Drilling Project recovered
3 km of basalt by coring into the flank of Mauna
Kea volcano at Hilo, Hawaii. Rocks recovered from deeper than
1 km were deposited below sea level
and contain considerable fresh glass. We report electron microprobe analyses of 531 glasses from the
submarine section of the core, providing a high-resolution record of petrogenesis over ca. 200 Kyr of
shield building of a Hawaiian volcano. Nearly all the submarine glasses are tholeiitic. SiO
2
contents span a
significant range but are bimodally distributed, leading to the identification of low-SiO
2
and high-SiO
2
magma series that encompass most samples. The two groups are also generally distinguishable using other
major and minor elements and certain isotopic and incompatible trace element ratios. On the basis of
distributions of high- and low-SiO
2
glasses, the submarine section of the core is divided into four zones. In
zone 1 (1079–
1950 mbsl), most samples are degassed high-SiO
2
hyaloclastites and massive lavas, but
there are narrow intervals of low-SiO
2
hyaloclastites. Zone 2 (
1950–2233 mbsl), a zone of degassed
pillows and hyaloclastites, displays a continuous decrease in silica content from bottom to top. In zone 3
(2233–2481 mbsl), nearly all samples are undegassed low-SiO
2
pillows. In zone 4 (2481–3098 mbsl),
samples are mostly high-SiO
2
undegassed pillows and degassed hyaloclastites. This zone also contains
most of the intrusive units in the core, all of which are undegassed and most of which are low-SiO
2
. Phase
equilibrium data suggest that parental magmas of the low-SiO
2
suite could be produced by partial melting
of fertile peridotite at 30–40 kbar. Although the high-SiO
2
parents could have equilibrated with
harzburgite at 15–20 kbar, they could have been produced neither simply by higher degrees of melting of
the sources of the low-SiO
2
parents nor by mixing of known dacitic melts of pyroxenite/eclogite with the
low-SiO
2
parents. Our hypothesis for the relationship between these magma types is that as the low-SiO
2
magmas ascended from their sources, they interacted chemically and thermally with overlying peridotites,
resulting in dissolution of orthopyroxene and clinopyroxene and precipitation of olivine, thereby
generating high-SiO
2
magmas. There are glasses with CaO, Al
2
O
3
, and SiO
2
contents slightly elevated
relative to most low-SiO
2
samples; we suggest that these differences reflect involvement of pyroxene-rich
lithologies in the petrogenesis of the CaO-Al
2
O
3
-enriched glasses. There is also a small group of low-SiO
2
glasses distinguished by elevated K
2
O and CaO contents; the sources of these samples may have been
enriched in slab-derived fluid/melts. Low-SiO
2
glasses from the top of zone 3 (2233–2280 mbsl) are more
alkaline, more fractionated, and incompatible-element-enriched relative to other glasses from zone 3. This
excursion at the top of zone 3, which is abruptly overlain by more silica-rich tholeiitic magmas, is
reminiscent of the end of Mauna Kea shield building higher in the core.
G
3
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Geochemistry
Geophysics
Geosystems
Published by AGU and the Geochemical Society
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Geochemistry
Geophysics
Geosystems
Article
Volume 5
,Number7
30 July 2004
Q07G15, doi:10.1029/2003GC000553
ISSN: 1525-2027
Copyright 2004 by the American Geophysical Union
1 of 67
Components:
46,680 words, 24 figures, 3 tables
.
Keywords:
basaltic glass; Hawaii; HSDP; Mauna Kea; petrogenesis.
Index Terms:
3640 Mineralogy and Petrology: Igneous petrology; 3655 Mineralogy and Petrology: Major element
composition; 8439 Volcanology: Physics and chemistry of magma bodies.
Received
19 March 2003;
Revised
5 March 2004;
Accepted
22 April 2004;
Published
30 July 2004.
Stolper, E., S. Sherman, M. Garcia, M. Baker, and C. Seaman (2004), Glass in the submarine section of the HSDP2 drill core,
Hilo, Hawaii,
Geochem. Geophys. Geosyst.
,
5
, Q07G15, doi:10.1029/2003GC000553.
————————————
Theme:
Hawaii Scientific Drilling Project
Guest Editors:
Don DePaolo, Ed Stolper, and Don Thomas
1. Introduction
[
2
] An important feature of the lithologic section
recovered by the Hawaii Scientific Drilling Proj-
ect in 1999 is that its lower
2000 m is submarine
(see summary in
Hawaii Scientific Drilling Project
[2001]). Consequently, this lower part of the sec-
tion contains considerable glass, reflecting rapid
cooling of magmatic liquids when they come in
contact with seawater. Furthermore, due to the
generally low level of alteration of the core, fresh
glass can usually be recovered from any given unit.
From the perspective of petrological and geochem-
ical investigations, the availability of fresh glass
spanning the lower 2000 m of the HSDP2 core is
advantageous in that in contrast to investigations of
whole rocks, with their accumulations of pheno-
crysts and xenocrysts and their susceptibility to
alteration, fresh glasses provide unambiguous in-
formation on the compositions of magmatic
liquids. (We use the acronym HSDP to refer to
the Hawaii Scientific Drilling Project. The pilot
hole, or HSDP1, was drilled in 1993. The 1999
drillingisreferredtoasHSDP2.)Suchliquid
compositions provide the primary information on
which petrogenetic hypotheses are constructed and
tested, so studies of submarine glasses from this
drill core are an important source of data on
petrogenetic processes affecting erupted Hawaiian
magmas over a substantial fraction of the lifetime
of a single volcano. Since glass is available essen-
tially continuously in the submarine section, it
provides a particularly high-resolution record of
petrogenesis. In the context of the integrated HSDP
project, this high resolution record provides the
framework for interpretation of the less frequently
sampled whole rock samples that have been sub-
jected to a wide range of high-precision chemical
and isotopic analyses.
[
3
] This paper focuses on electron microprobe
analyses of 531 glasses from the submarine sec-
tion of the HSDP2 core, presumed to represent
output from the Mauna Kea volcano [
Hawaii
Scientific Drilling Project
, 2001]. There are actu-
ally two independent data sets, one from the
University of Hawaii and one from Caltech, but
in the interests of a coherent presentation, we
have chosen to report them in a single publica-
tion. Consequently, there are considerable data
and petrogenetic information, amounting to an
unprecedented time series of the evolution of a
Hawaiian volcanic system.
2. Samples
[
4
] Glasses were analyzed from each of the four
rock types identified in the submarine section of
the drill core (see Figure 1) [
Hawaii Scientific
Drilling Project
, 2001]: hyaloclastites, massive
basalts, pillow basalts, and intrusives. Interlayered
hyaloclastites and massive basalts dominate the
upper part of the submarine section from the
subaerial-submarine transition at 1079 mbsl to a
depth of 1883 mbsl. The first intrusive unit
appears at 1883 mbsl, and from this depth to
the bottom of the drill core, pillow basalts and
hyaloclastites are interlayered with relatively
thin intrusive units cross-cutting both pillow and
hyaloclastite units.
[
5
] The glasses analyzed in this study represent
two independent sampling and analytical pro-
grams. Most of the first suite was collected at the
drill site by the core loggers. As each box of core
(containing
10 feet of core) was described petro-
graphically, the logger selected one or two fresh,
0.1–1 cm glass fragments from representative,
interesting, and/or most easily sampled parts of the
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core. These samples were sent to the University of
Hawaii for electron microprobe analysis. The
on-site sample suite was collected in a reconnais-
sance fashion before the overall stratigraphy of the
core was known. Consequently, some sections of
the core were resampled at Caltech and the glasses
sent to the University of Hawaii to resolve ques-
tions about the precise locations and unit types of
certain samples, and other sections were assigned
revised unit types (e.g., changed from pillow to
intrusive or from hyaloclastite to intrusive breccia)
on the basis of reexamination of the originally
sampled section. In particular, some units (e.g.,
U0343, sections a–e) that were originally de-
scribed as pillow basalts in the deeper parts of
the submarine section are now believed to contain
thin (1–10 cm) fingers of intrusive units between
some of their pillow margins. These intrusive
fingers have lithologies and major element chemical
compositions distinct from the surrounding pillows
Figure 1.
Simplified lithologic column of the HSDP2 core hole [after
Hawaii Scientific Drilling Project
, 2001].
The depths of the individual glass samples analyzed at the University of Hawaii and Caltech are indicated, as are
the depths of samples in the whole rock reference suite. Key depths are also shown: the Mauna Loa-Mauna Kea
boundary; the occurrence of alkalic lavas at the top of the Mauna Kea section; the subaerial-submarine boundary;
the first occurrences of intrusives and pillows; and the divisions of the submarine section into zones 1–4 (see
section 4.2).
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(e.g., SR0969-5.1 versus SR0969-5.0 and SR0975-
4.95 versus SR0975-5.0), but are sufficiently thin
that they were not identified at the time the
samples were collected on site. In addition, some
lithic fragments and glass in units identified as
hyaloclastites near intrusives show lithologies
and/or major element chemical compositions sim-
ilar to the nearby intrusives, suggesting that they
may be related to the intrusives; indeed, some of
these units may be actually intrusive breccias
(e.g., SR0857-20.2 from U0308c). Complexities
and uncertainties of this sort are pointed out in the
footnotes to Table 1.
[
6
] In all, 475 samples from this first suite were
analyzed at the University of Hawaii. Their distri-
bution within the core is shown in Figure 1. From
these glasses, 91 were also analyzed by infrared
spectroscopy for water at Caltech, and of these, 50
were also analyzed for carbon dioxide [
Seaman et
al.
, 2004].
[
7
] The second suite, comprising 56 samples,
was collected and analyzed at Caltech after
drilling was completed. The principal objective
was to create a reference suite of glasses (52 sam-
ples) comparable to the whole rock reference
suite distributed for chemical and isotopic analy-
ses. Most of the glass reference samples were
chosen to be roughly evenly spaced (
50 m),
and every effort was made to produce clean,
fresh glass separates of 0.5–1 g. We also col-
lected reference samples that on that basis of the
stratigraphy of the core, whole rock analyses or
the analyses of the on-site glass collections
would be desirable to have in larger quantities.
Samples for the reference suite were coarsely
crushed to
1–2 mm in a piston mortar, washed
in deionized water, dried for several hours in a
100
C drying oven, and sorted to eliminate
visibly altered fragments and fragments contain-
ing crystals. In addition, olivine separates from a
number of these samples were prepared. In the
case of hyaloclastite samples, all glass fragments
that make up each reference sample are from the
same glassy clast. In addition to the reference
samples, we also collected chips from four hya-
loclastites in the 1765–1810 mbsl interval for
analysis at Caltech in order to extend the sam-
pling of this ‘‘excursion’’ (see section 7). Elec-
tron microprobe analyses were done at Caltech
along with infrared spectroscopic analyses for
water and carbon dioxide [
Seaman et al.
,
2004]. Samples from this reference suite were
also analyzed by laser-ablation ICP-MS at Boston
University (M. B. Baker et al., Trace elements in
submarine glasses from the HSDP2 core, manu-
script in preparation, 2004) (hereinafter referred
to as Baker et al., manuscript in preparation,
2004a) and by ion microprobe for carbon
at Lawrence Livermore National Laboratory
[
Seaman et al.
, 2004]. Aliquots of these samples
were distributed to the E
́
cole Normale Supe
́rieure
de Lyon and the University of California, Berke-
ley for selected isotopic analyses [
Bryce and
DePaolo
, 2000;
Blichert-Toft et al.
, 2003].
[
8
] Table 1 presents a complete list of the samples
and analyses from this study. In addition to the
531 samples analyzed by electron microprobe,
Table 1 also lists 25 additional samples that
were analyzed for H
2
OandCO
2
by infrared
spectroscopy but not analyzed by electron micro-
probe [
Seaman et al.
, 2004].
3. Analytical Techniques
3.1. University of Hawaii
[
9
] Glass compositions were measured at the Uni-
versity of Hawaii using a five-spectrometer Cameca
SX-50 electron microprobe with an accelerating
voltage of 15 keV, a current of 10 nA, and a 15
m
m
beam. Smithsonian VG2 and A99 glass standards
were used to calibrate the major elements, and
mineral standards were used to calibrate the minor
elements (orthoclase for K
2
O, apatite for P
2
O
5
, and
troilite for S). Glass standards were analyzed before
and after the samples. Na
2
O was analyzed first in
order to minimize Na loss during analyses. Count-
ing times for the major elements were 60–130 s,
except for Na
2
O (40 s). Counting times for minor
elements were 50 s for K
2
O, 110 s for P
2
O
5
and 120 s
for S. A PAP-ZAF matrix correction was applied to
all analyses. Each analysis listed in Table 1 repre-
sents an average of 5–15 points per sample.
3.2. Caltech
[
10
] Glass compositions were determined with a
five-spectrometer JEOL 733 electron microprobe at
Caltech using a 15 keV accelerating voltage, a
10 nA beam current, a 10
m
m spot size, and glass,
mineral, and oxide standards (Si, VG-2; Ti, TiO
2
;
Al, Ca, synthetic anorthite; Cr, Cr
2
O
3
; Fe, synthetic
fayalite; Mn, synthetic Mn-olivine; Mg, synthetic
forsterite; Na, Amelia albite; K, Asbestos micro-
cline; P, Durango apatite; S, pyrite). Peaks were
counted for 30 s, high and low backgrounds were
counted for 15 s, and data were reduced using a
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Table 1 (Representative Sample).
Major Element and Volatile Concentrations
a
(The full Table 1, which also includes P
2
O
5
,S,H
2
O total, H
2
O molecular, CO
2
(IR),
CO
2
(SIMS), Cl, and F, is available in the HTML version of this article at http://www.g-cubed.org)
Sample
Depth, mbsl
Unit
Rock Type
Footnote
SiO
2
TiO
2
Al
2
O
3
FeO*
MnO
MgO
CaO
Na
2
OK
2
O
SR0458-0.80
1102.9
U0180
hyaloclastite
51.13
2.65
13.51
11.71
6.46
11.12
2.27
0.38
SR0463-0.40
1109.6
U0181
massive
50.77
2.65
13.47
11.75
6.48
11.04
2.26
0.38
SR0465-2.50
1110.9
U0183
massive
51.28
2.76
13.44
11.79
6.38
10.82
2.21
0.37
SR0471-1.10
1119.8
U0184
hyaloclastite
b
51.87
2.58
13.37
11.62
0.18
6.36
10.93
2.24
0.38
SR0475-0.40
1126.4
U0185
massive
51.15
2.76
13.53
11.47
6.50
10.97
2.21
0.38
SR0475-0.40
1126.4
U0185
massive
51.55
2.47
14.98
10.62
5.90
11.16
2.34
0.35
SR0485-0.90
1138.7
U0187
hyaloclastite
b
51.74
2.59
13.29
11.55
0.16
6.51
10.89
2.31
0.39
SR0495-0.90
1234.5
U0190
hyaloclastite
b
51.83
2.73
13.37
11.44
0.17
6.21
10.91
2.30
0.46
SR0508-8.00
1283.5
U0191
massive
SR0508-8.60
1283.7
U0192
hyaloclastite
b
51.91
2.60
13.67
10.93
0.17
6.45
11.05
2.31
0.43
SR0517-8.40
1310.8
U0195
massive
SR0518-6.30
1313.2
U0196
hyaloclastite
SR0523
1328.3
U0196
hyaloclastite
b, n
51.75
2.80
13.07
12.04
0.17
6.01
10.45
2.32
0.45
SR0523-6.80
1328.9
U0196
hyaloclastite
52.00
2.34
13.98
10.91
6.55
11.22
2.23
0.36
SR0539-4.30
1376.8
U0198
hyaloclastite
49.48
2.79
14.59
11.18
7.06
11.57
2.53
0.45
SR0540-6.80
1379.2
U0198
hyaloclastite
49.50
2.65
14.48
11.06
7.01
11.50
2.47
0.44
SR0541-8.10
1382.6
U0198
hyaloclastite
49.71
2.81
14.59
11.21
7.04
11.58
2.53
0.44
SR0542-7.50
1385.5
U0198
hyaloclastite
SR0544-5.30
1391.2
U0198
hyaloclastite
b
49.50
2.88
14.20
11.15
0.17
7.05
11.49
2.60
0.46
SR0546-2.85
1396.0
U0198
hyaloclastite
49.27
3.01
14.36
11.32
6.81
11.61
2.56
0.47
SR0550-0.00
1406.1
U0199
massive
SR0556-2.70
1421.2
U0202
hyaloclastite
SR0561-3.30
1436.9
U0202
hyaloclastite
b
49.85
2.77
14.08
11.13
0.17
6.89
11.51
2.51
0.45
SR0572-7.30
1469.9
U0202
hyaloclastite
SR0582-1.90
1494.9
U0205
massive
SR0595-6.40
1523.5
U0214
hyaloclastite
b
51.51
3.29
13.00
12.24
0.16
5.93
10.28
2.33
0.53
SR0619-3.50
1570.5
U0218
hyaloclastite
SR0631-4.30
1607.0
U0224
massive
SR0646-6.30
1652.1
U0229
hyaloclastite
51.05
3.39
13.15
12.03
5.95
10.23
2.39
0.57
SR0646/647
1653.2
U0231
hyaloclastite
b, n
51.55
3.32
13.05
11.99
0.18
5.85
10.35
2.42
0.58
SR0653-0.75
1671.2
U0238
hyaloclastite
50.84
2.41
13.66
10.89
7.29
11.10
2.24
0.33
SR0657-8.90
1685.2
U0238
hyaloclastite
50.17
2.97
13.93
10.84
6.63
11.21
2.37
0.56
SR0658-0.90
1685.8
U0238
hyaloclastite
b
50.59
3.13
13.86
10.84
0.16
6.49
11.45
2.42
0.57
SR0659-5.50
1690.3
U0238
hyaloclastite
50.38
2.90
13.90
10.89
6.66
11.34
2.41
0.58
SR0660-1.00
1692.0
U0238
hyaloclastite
50.40
2.95
13.90
10.64
6.64
11.45
2.38
0.54
SR0661-5.20
1696.3
U0238
hyaloclastite
50.27
3.07
13.96
10.91
6.70
11.36
2.43
0.54
SR0668-2.25
1715.8
U0238
hyaloclastite
50.92
3.07
14.00
10.95
6.70
11.45
2.41
0.55
SR0668-9.95
1718.2
U0241
hyaloclastite
50.42
3.03
14.02
10.90
6.80
11.41
2.38
0.53
SR0669-2.50
1719.0
U0241
hyaloclastite
50.84
3.04
13.97
10.72
6.70
11.41
2.37
0.51
SR0672-0.95
1727.8
U0242
hyaloclastite
50.70
3.02
13.61
11.26
6.45
10.93
2.33
0.53
SR0674-3.70
1734.9
U0242
hyaloclastite
51.35
2.68
13.58
11.25
6.55
10.88
2.31
0.39
SR0675-8.80
1739.4
U0243
hyaloclastite
b
51.87
2.55
13.62
11.09
0.20
6.45
11.05
2.29
0.38
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modified ZAF procedure (CITZAF [
Armstrong
,
1988]). In general, each probe mount consisted of
5–6 pieces of glass (
1 mm in size) and three
analyses were collected on each of 3–4 of the
pieces. Four analyses each of two USGS pre-
pared glasses (BHVO-2g and BCR-2g; lacking
an accepted analysis for each glass, we used the
accepted compositions for the two powders; see
http://minerals.cr.usgs.gov/geo_chem_stand/) and
VG-2 [
Jarosewich et al.
, 1979] were collected
at the beginning, during (generally after three to
five unknowns had been analyzed; VG-2 was
sometimes passed over), and at the end of each
analytical session. The mean BHVO-2g compo-
sition from each session coupled with the accepted
composition of this glass were used to reprocess all
the k-ratios except those of S for all the analyses
from that session; the mean VG-2 composition was
used to reprocess the S data. The reprocessed glass
compositions were averaged, and the mean compo-
sitions are listed in Table 1; individual analyses
with oxide sums <98 and >100.4% (excluding
H
2
O) were not included in the averages. (All
concentrations are in weight percent, unless other-
wise indicated.)
[
11
] With one exception (SR0979-1.3; this sam-
ple is a pillow margin containing an ‘‘intrusive
finger’’), analyses from different chips of the
same sample are identical within analytical un-
certainty. Glass from one of the analyzed chips
of SR0979-1.3 differs from the three other ana-
lyzed chips (5.6 versus 7.6% MgO, 51.7 versus
49.1% SiO
2
). This compositional difference is
consistent with the petrographic observation that
the low-MgO, high-SiO
2
chip contains micro-
phenocrysts of olivine, plagioclase, and clinopy-
roxene, while the other chips contain only olivine
microphenocrysts; it may also reflect the sam-
pling of both the pillow and the intrusive finger.
Only the average of the analyses of the higher-
MgO chips is reported in the body of Table 1;
the low-MgO analysis is listed in a footnote to
Table 1 but is not shown in the figures or
included in the various counts of samples by
lava type and compositional group.
[
12
] The mean BCR-2g and VG-2 glass compo-
sitions based on data from all the analytical
sessions are listed in Table 2 along with the
sample standard deviations for each oxide. For
both glasses, the mean concentrations for most
oxides overlap with the accepted values at the 1
s
level. The absolute deviation between the mean
MgO concentration and the accepted value for
VG-2 is 0.21%, well outside the 1
s
value of
0.05%, but the deviation in MgO for BCR-2g is
only 0.01%;
Clague et al.
[1990] and
Dixon et al.
[1991] report mean MgO values for VG-2 that
overlap with our analyzed MgO concentration at
the 1
s
level. Percent errors for each oxide (i.e.,
100
1
s
/mean concentration) based on 73 BCR-
Notes to Table 1.
a
Unless otherwise indicated, samples are from the University of Hawaii glass suite collected during core logging in 1999. All concentrations are
in weight percent, except CO
2
(ppm). Numbers in parentheses for the H
2
O, molecular H
2
O, and CO
2
analyses are 1
s
based on multiple analyses.
Samples without reported errors were analyzed only once. A dash (‘‘-’’) indicates a measured concentration nominally <0.
b
Samples are part of a Caltech glass reference suite collected during the summer of 2000 after core logging had been completed and revisions
had been made. See text for description of sample preparation.
c
Samples were collected at Caltech and analyzed at the University of Hawaii (2000–2002) to clear up ambiguity about samples collected on site
in 1999.
d
Rock type is different than that listed in core log, but unit number matches original lithologic unit assigned during core logging. Most of these
samples are thin (2–10 cm) intrusive units that were not recognized as such when the core was logged.
e
Samples were collected and analyzed at Caltech but are not part of the glass reference suite.
f
CaO-K
2
O-rich glasses (see section 7 of the text).
g
Glasses from 2233–2280 mbsl excursion (see section 6 of the text).
h
CaO-Al
2
O
3
-rich glasses (MgO
7%; both original glass compositions and those adjusted to be in equilibrium with Fo90.5 olivine form a
coherent group on plots of CaO-SiO
2
and Al
2
O
3
-SiO
2
; see section 4.6 of the text).
i
Suspected of alteration due to elevated Cl or molecular H
2
O content; see
Seaman et al.
[2004].
j
This sample has an anomalous Na
2
O content, attributed to alteration (see Figure 6g and
Seaman et al.
[2004]). It has been left off most figures.
k
One glass chip from this sample has a distinct composition (see section 3.2 of the text): SiO
2
51.67; TiO
2
3.26; Al
2
O
3
13.16; FeO* 12.66; MnO
0.21; MgO 5.61; CaO 10.25; Na
2
O 2.38; K
2
O 0.55; P
2
O
5
0.40; S 0.009.
l
Detectable molecular CO
2
based on absorption at 2350 cm
1
.
m
‘‘Pillow breccia’’; see
Hawaii Scientific Drilling Project
[2000].
n
Box disturbed during handling, depth from midpoint of box.
o
Close to intrusive.
p
Glass from intrusive margin, originally logged as massive.
q
Originally logged as hyaloclastite.
r
Finger of intrusion at SR0859-1.00 not noted on original log; originally logged as pillow.
s
Two lobes of aphyric basalt between SR0864-15.80 and 17.20 not noted on original log; originally logged as pillow.
t
Originally logged as pillow.
u
Originally logged as pillow basalt; no unit number assigned.
v
Originally logged as pillow breccia (footnote m).
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2g analyses are 0.64, SiO
2
; 5.3, TiO
2
; 0.88, Al
2
O
3
;
1.7, FeO* (i.e., total Fe as FeO), 14.3, MnO; 1.4,
MgO; 1.1, CaO; 3.5, Na
2
O; 2.2, K
2
O; 13.9, P
2
O
5
.
3.3. Interlaboratory Comparison
[
13
] Five sets of pillow glasses collected indepen-
dently from the same pillow margins (typically a
1–4 cm-thick glassy rind) from nominally identical
depths were analyzed by both laboratories
(SR0763-14.9, SR0831-2.3, SR0866-5.3,
SR0907-2.8, and SR0949-8.2; see Table 2). In
the three cases where the standard deviations are
available for the University of Hawaii analyses, the
mean concentrations for each oxide in the two sets
of analyses for each sample overlap at the 1
s
level,
except SiO
2
and FeO* in SR0831-2.3 and SiO
2
,
Al
2
O
3
, and CaO in SR0907-2.8, all of which
overlap at the 2
s
level.
4. High- and Low-Silica Magma Types
4.1. Definition of the High- and Low-Silica
Magma Types
[
14
] We begin our examination of chemical varia-
tions in the HSDP2 glass suite with a subdivision
of the glasses into two groups (Figure 2). The first
group, which we refer to as the low-SiO
2
group, is
shown in all figures as blue symbols; it is defined
by SiO
2
<50%. The second, referred to as the high-
SiO
2
group, is shown as pink symbols in all
figures; it is defined by SiO
2
50%. The high-
SiO
2
group comprises 69% (367 of 531) of the
glasses; on the basis of the spacing of the samples,
we estimate that an essentially indistinguishable
fraction of the submarine volume of the core (70%)
is made up of high-SiO
2
material. The MgO con-
tents of the two groups overlap, but the average
MgO content of the high-SiO
2
group (6.75 ± 0.51
(1
s
)%) is lower than that of the low-SiO
2
group
(7.41 ± 0.68 (1
s
)%). Comparison of histograms of
MgO contents (not shown) demonstrates that the
distribution of MgO contents in the low-SiO
2
group is shifted to higher MgO relative to the
high-SiO
2
group.
[
15
] Figure 3 shows SiO
2
contents versus H
2
O and
S contents of all analyzed glasses. Full discussion
and interpretation of these volatile components is
presented in a companion paper [
Seaman et al.
,
2004], but it is well known that H
2
O and S degas
significantly from Hawaiian liquids only at rela-
tively low total pressures such as those pertaining
to subaerial or shallow submarine environments or
to high-level magma chambers connected to the
Table 2.
Interlaboratory Comparison of Microprobe Data
a
Sample (Number of Analyses) Depth, mbsl
SiO
2
TiO
2
Al
2
O
3
FeO*
MgO
CaO
Na
2
OK
2
OP
2
O
5
S
Sum
SR0763-14.90 CIT (12)
2131.4
50.32(26) 2.59(8) 14.10(6) 10.96(13) 6.80(6) 11.88(13) 2.36(5) 0.39(2) 0.24(5)
0.015(8) 99.63
SR0763-14.90 UH (6)
2131.5
50.46(13) 2.49(8) 14.21(10) 10.78(12) 6.89(10) 11.92(4) 2.37(4) 0.36(2) 0.191(12) 0.021(7) 99.69
SR0831-2.30 CIT (12)
2438.4
49.30(23) 2.68(14) 14.18(9) 11.49(14) 6.97(10) 11.54(10) 2.50(5) 0.41(2) 0.23(4)
0.062(14) 99.36
SR0831-2.30 UH (10)
2438.5
49.63(6) 2.69(5) 14.22(5) 11.18(6)
6.98(5) 11.57(9) 2.50(4) 0.39(2) 0.20(2)
0.058(9) 99.42
SR0866-5.30 CIT (12)
2639.7
51.33(18) 2.69(17) 13.50(5) 10.94(15) 6.84(7) 11.13(10) 2.30(5) 0.42(2) 0.24(3)
0.092(18) 99.45
SR0866-5.30 UH
2639.6
51.38
2.62
13.56
11.02
6.82
11.24
2.26
0.41
0.22
0.098
99.63
SR0907-2.80 CIT (9)
2789.4
51.59(28) 2.85(11) 13.18(12) 12.36(8) 6.06(5) 10.46(5) 2.31(5) 0.47(2) 0.27(5)
0.060(15) 99.62
SR0907-2.80 UH (5)
2789.5
51.16(13) 2.83(7) 13.43(9) 12.23(13) 6.11(4)
10.64(4) 2.28(2) 0.42(3) 0.23(2)
0.078(5) 99.41
SR0949-8.20 CIT (11)
2979.7
49.43(14) 2.80(19) 14.18(7) 11.62(9)
6.63(9) 11.76(10) 2.49(4) 0.42(2) 0.26(4)
0.10(1)
99.27
SR0949-8.20 UH
2979.9
49.20
2.77
14.35
11.50
6.58
11.90
2.41
0.41
0.23
0.11
99.46
VG-2 CIT (57)
50.83(16) 1.85(9) 13.94(9) 11.84(14) 6.92(5) 11.14(10) 2.66(5) 0.20(2) 0.21(4)
0.131(13) 99.96
BCR-2g CIT (73)
54.75(35) 2.28(12) 13.63(12) 12.50(21) 3.62(5) 7.19(8)
3.12(11) 1.84(4) 0.36(5)
99.50
a
Abbreviations: CIT, Caltech; UH, University of Hawaii; mbsl, meters below sea level. Values in parentheses after each oxide value are sample standar
d deviations; read 50.32(26) as 50.32 ± 0.26; FeO* is all
Fe as FeO. Number of analyses and oxide standard deviations are not available for SR0866-5.30 UH and SR0949-8.20 UH. Mean MnO values for VG-2 and BCR-2g
are 0.21(3) and 0.21(3), respectively. Given
the CIT operating conditions and counting time, sulfur in BCR-2g was below the microprobe detection limit.
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atmosphere. For example, on the basis of the
results of
Dixon and Stolper
[1995], basaltic mag-
mas with
0.7% H
2
O (i.e., corresponding roughly
to the upper end of the distribution of water
contents shown in Figure 3) would only degas
significant amounts of water at pressures less than
45 bars; samples with 0.5 and 0.2% H
2
O would
only degas significant amounts of water at pres-
sureslessthan
25 and 5 bars, respectively.
Figure 2.
MgO versus SiO
2
(wt.%) for glasses from
the HSDP2 core: (a) pillow-rim glasses; (b) hyaloclastite
glasses; (c) glasses from intrusive and massive units. In
each panel, the small dots are all samples not assigned to
the group being emphasized with the large symbols.
Altered samples, based on elevated molecular water or
Cl contents [
Seaman et al.
, 2004], are indicated by
superimposed gold-colored dots.
Figure 3.
(a) H
2
O and (b) S contents (wt.%) versus
SiO
2
(wt.%) for HSDP2 glasses. The definitions of
undegassed, partially degassed, and degassed samples
are given in section 4.1. Symbol shapes indicate rock
types, and colors indicate the chemical group to which
the glasses are assigned (see the legend). Gray-filled
symbols represent glasses with <7% MgO; black-filled
symbols represent glasses with
7% MgO. Altered
samples, based on elevated molecular water or Cl
contents [
Seaman et al.
, 2004], are indicated by
superimposed gold-colored dots.
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Following
Moore and Clague
[1992],
Garcia and
Davis
[2001], and
Sherman et al.
[2002], we divide
our samples into an ‘‘undegassed’’ group (H
2
O
0.45% and S
0.09%), a ‘‘partially degassed’’
group (0.45 > H
2
O
0.21%; 0.09 > S
0.04%),
and a ‘‘degassed’’ group (H
2
O < 0.21% and S <
0.04%). Although the exact boundaries of this
classification scheme are arbitrary, the point is that
the ‘‘degassed’’ samples lost water and sulfur at
pressures approaching atmospheric, whereas the
‘‘undegassed’’ samples did not experience pres-
sures of less than
40–50 bars for sufficient time
to vesiculate significantly. Figure 3 shows that the
undegassed and partially degassed glass samples
Figure 4.
(a and b) SiO
2
(wt.%) and (c) alkalinity versus depth (mbsl) for glasses from the HSDP2 core and the
HSDP pilot hole [
Garcia
, 1996]. The horizontal green line shows the position of the unconformity between Mauna
Loa and Mauna Kea subaerial lavas. Horizontal red lines show divisions of the core into subaerial samples and
zones 1–4 (see section 4.2). Symbol shapes indicate rock types, and colors indicate the chemical group to which the
glasses are assigned (see the legend). In Figure 4a, gray-filled symbols represent glasses with <7% MgO; black-filled
symbols represent glasses with
7% MgO. Filled symbols in Figures 4b and 4c represent partially degassed or
undegassed glasses; open symbols represent degassed glasses; see section 4.1 for the definitions of degassed versus
partially degassed and undegassed glasses. In Figure 4b, superimposed red dots indicate Ca-Al-rich glasses (see
section 4.6). In Figure 4c, superimposed gold-colored dots indicate altered samples [
Seaman et al.
, 2004].
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define a low-SiO
2
group (106 samples; 48.3–
49.8% SiO
2
) and a high-SiO
2
group (135 samples;
50.5–52.0% SiO
2
) separated by a gap in SiO
2
in
which there are no undegassed or partially
degassed samples. In contrast, degassed samples
span the full range of SiO
2
content and, in partic-
ular, fill the gap in SiO
2
content defined by the
undegassed samples.
[
16
] Figure 4 shows SiO
2
content and alkalinity
versus depth for the HSDP2 glasses (alkalinity is
defined as the vertical distance of a sample to the
Macdonald-Katsura line on a silica-alkalies dia-
gram [
Carmichael et al.
, 1974]). Figures 4a and
4b show SiO
2
contents of the glasses using
distinct symbols for the different sample types
(pillows; hyaloclastites; massives; intrusives;
‘‘excursions’’; see below). In Figure 4a, light
gray versus black filled symbols indicate glasses
with <7% and
7% MgO; in Figures 4b and 4c,
open symbols indicate degassed glasses, and
filled symbols indicate undegassed/partially
degassed samples. Note that all of the glasses
plot on the tholeiitic side of the Macdonald-
Katsura line (i.e., the alkalinity is negative).
(We refer to most of these glasses as tholeiites
since they plot below the Macdonald-Katsura
line. Some authors would define most (or all)
of the low-SiO
2
glasses as ‘‘transitional’’ [
Wolfe
et al.
, 1995;
Sisson et al.
, 2002], but we restrict
use the term ‘‘transitional’’ to compositions es-
sentially on the Macdonald-Katsura line. Note
that all glasses from this study have normative
hypersthene (CIPW norms were calculated as-
suming 5–10% of the Fe is Fe
3+
).)
[
17
] Figure 5 shows the distributions of SiO
2
con-
tents in histogram form by rock type, distinguishing
degassed and partially degassed from undegassed
samples. Figure 5a shows that the pillow-rim
glasses are mostly undegassed and have a bimodal
distribution of SiO
2
contents; this figure also shows
clearly that degassed pillows nearly all have inter-
mediate SiO
2
contents. Figure 5b shows that al-
though the hyaloclastite glasses span the full range
of silica contents in the core, most are in the high-
SiO
2
group (and degassed) and, in contrast to the
pillow-rim glasses, the distribution of SiO
2
contents
in the hyaloclastite glasses is not as clearly bimodal
(i.e., low- and intermediate-SiO
2
glasses are roughly
equally abundant). Figure 5c shows that all of the
glasses we analyzed from the massive flows in the
upper submarine section are degassed and have high
SiO
2
. Figure 5c also illustrates the predominance of
undegassed, low-SiO
2
glasses among the intrusive
glasses in this study.
[
18
] Figure 6 shows on SiO
2
variation diagrams
the concentrations of several major and minor
elements in glasses from this study. In this figure
and several others we restrict our comparison to
glasses with
7% MgO to minimize the effects of
the fractionation of phases other than olivine
[
Seaman et al.
, 2004]. Glasses from different
depth intervals and degassed versus undegassed/
partially degassed glasses are distinguished by
different symbols in Figure 6. This figure shows
clearly that the distinction between the high- and
low-SiO
2
groups extends to several other chemi-
cal components in addition to silica. Relative to
the high-SiO
2
group, the low-SiO
2
group is higher
in Al
2
O
3
and FeO*. Although the differences are
small, the low-SiO
2
group is also systematically
higher in Na
2
O and TiO
2
. The low-SiO
2
group is
also slightly higher in H
2
O and perhaps slightly
lower in S. The CaO, K
2
O, or P
2
O
5
contents of
the two groups are not distinguishable.
[
19
] Figure 7 shows variations with depth of
several isotopic and elemental ratios in the
HSDP2 samples. Most of the analyses shown
are of whole rocks, but several are for glasses,
including some from the reference suite from
this paper. Isotopic and trace element data on
the HSDP2 samples are fully interpreted else-
where by the authors who report them [
Bryce
and DePaolo
, 2000;
Blichert-Toft et al.
, 2003;
Eisele et al.
, 2003;
Feigenson et al.
, 2003;
Huang
and Frey
, 2003;
Kurz et al.
, 2004;
Rhodes and
Vollinger
, 2004], and some details of the compar-
ison to our data set are presented in Appendix A2.
The point we want to emphasize here is that the
high- and low-SiO
2
groups are generally distin-
guishable isotopically and using certain elemental
ratios. This is particularly evident for He isotope
ratios (Figure 7a). The low-SiO
2
samples typically
have higher
3
He/
4
He, and the range of
3
He/
4
He
of the low-SiO
2
group (
14–25 RA) is larger
than that of the high-SiO
2
group (
10–17 RA).
There is, however, overlap in the He isotope
ratios of these groups in that some of the low-
SiO
2
samples do not have elevated
3
He/
4
He.
Although the effects in other isotopic ratios are
not as large,
e
Nd
(Figure 7b),
176
Hf/
177
Hf
(Figure 7c), and
206
Pb/
204
Pb (Figure 7e) are all
typically lower and
87
Sr/
86
Sr (Figure 7d) is
typically higher in low-SiO
2
samples relative to
high-SiO
2
samples. As with He isotopic ratios,
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