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28 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 8.—Homogeneity of basaltic glasses (mean and 95% confidence interval (2 sd) on examples
from text; numbers in parentheses indicate total number of analyses; each analysis
represents five 10-second point counts, except for USNM 111230, where the 91 analyses are for
single 10-second point counts)
Constituent
SiO2
A12O3
FeO .
MgO . .
CaO . ,
Na20 .
K20 ..
TiO2 ,
P 2°5 '
Total
Standard glass
VG-2
(77)
50.81 + 0.70
Ik.06 + 0.30
11.8k + 0.3 1 *
6.71 + 0.22
11.12 + 0.30
2.62 + 0.08
0.19 + 0.0^
1.85 + 0.12
0.17 i o.oU
99-37
usra #111230
(91)
50.30 ± 1.0U
15.68 + 0.51
10.57 + 0.23
7.50 + 0.51
11.07 + 0.27
3.0U + 0.10
0.26 + 0.05
1.97 + 0.11
0.26 + 0.06
100.65
Analyses of 76 glassy pillow fragments from the
Galapagos Ridge demonstrate the homogeneity of
glasses within a single dredge haul. Table 8 gives
the mean and two standard deviations for the glass
analyses. Comparing the standard deviation for
this dredge against that of VG-2 we find very close
agreement. The increase in 2o- for Al, Mg, and Ca
is partially due to the higher contents of these
oxides in the dredged glasses. Similarly the lower 2a
for K and P are due to lower concentrations. The
extreme homogeneity of the glasses from this dredge
haul, which yielded several hundred pounds of
glassy pillow basalt, is not unusual in the dredges
that have been extensively analyzed by this laboratory
(Melson, et al., 1976).
A second example of the homogeneity of glass
in pillow basalts is demonstrated by variations in
chemistry in a 70-meter interval of core from Leg
24, Site 238, DSDP (Melson, et al., 1976). Glass
from each glassy horizon was analyzed and, along
with XRF analyses (Dmitriev, 1974) from the same
core, plotted in Figure 1 as a function of depth.
The glasses are clearly much more homogeneous
than the XRF analyses of the crystalline material.
Fe, Mg, and, to a lesser extent, Ca are depleted
in the crystalline material while K and Na are
enriched. These chemical variations in the crystalline
material and the concurrent homogeneity of
Galapagos
DeSt. 5
(76)
50.13 ± 0.88
16.2U + 0.5k
9.02 + 0.38
8.U3 + 0.37
12.50 + 0.37
2.20 + 0.08
0.06 + 0.03
1.03 + 0.12
0.07 + 0.02
99.68
DSDP Site 238
(21)
U9.1+7 i 0.73
15.Ik + O.Ul
11.15 + 0.17
7.58 + 0.3U
12.61 + 0.17
2.U5 + 0.07
0.07 + 0.03
1.22 + 0.09
0.08 + 0.02
99-77
the glasses again document the superiority of glass
analyses.
In Table 8 the mean and standard deviations of
the Galapagos dredge and DSDP Site 238 are compared
to VG-2. The standard deviations are very
close to being only a function of analytical error
and differences in concentration. This indicates that
these analyses reflect single eruptive events and that
on a larger scale glass analyses could be very important
in chemical stratigraphy. Melson, et al.
(1976) present data from this laboratory indicating
the extent to which the chemically homogeneous
groups from one DSDP site or dredge location can
be distinguished and compared to groups within or
between locations. This technique seems quite
promising because of the precision of the analyses
used. A data file of chemically distinct groups of
glass analyses is being built, which will be used in
studying regional differences in basaltic chemistry.
Preliminary data on subaerial glasses indicate a
homogeneity similar to that of abyssal basaltic
glasses. The Hawaiian tholeiite glass standard
(VG-A99) and the rhyolite glass standard (VG-568)
used here have been checked extensively by probe
for homogeneity. Two suites of fractionally crystallized
Hawaiian tholeiite glasses are currently being
studied at this lab. Using a least squares liquid
line of descent to compare the glass analyses they