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Abyssal Basaltic Glasses as Indicators of Magma Compositions
Gary R. Byerly, William G. Melson,
Joseph A. Nelen, and Eugene Jarosewich
ABSTRACT
A review of the literature on alteration of abyssal
basalts shows that often portions of the glassy rind
are the most representative, or least altered, material
present. Separating this glass from the palagonite
and other alteration products provides the
material closest in chemistry to the original magma
chemistry. Use of the electron microprobe allows
for analysis of only the freshest portions of the
glass. Analyzing glasses by electron microprobe,
with glass standards internally mounted in the
sample discs, is shown to be a very precise technique.
Examples of the homogeneity of dredged and cored
abyssal basalts, based on glass analyses by electron
microprobe, are presented.
Introduction
Pillow basalts are the most common igneous rock
recovered from dredges or cores taken from the
ocean depths. Chemical analyses of these basalts
have played key roles in many petrogenetic and
plate tectonic models and yet considerable controversy
still exists over the degree to which these
analyses reflect primary igneous processes rather
than abyssal alteration processes. Many workers
have investigated the variation in chemistry within
single pillows (Paster, 1968; Corliss, 1970; Hart
1969; Melson, Thomson, and Van Andel, 1968;
Muehlenbachs and Clayton, 1972; Matthews, 1971;
Moore, 1965; Melson and Thompson, 1973; Shido,
Miyashiro, and Ewing, 1974; and Hart, Erlank, and
Gary R. Byerly, William G. Melson, Joseph A. Nelen, and
Eugene Jarosewich, Department of Mineral Sciences, National
Museum of Natural History, Smithsonian Institution,
Washington, D.C. 20560.
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Kable, 1974; Melson, 1973; Thompson, 1973) or in
abyssal basalts as a function of age (Hart, 1973;
Hekinian, 1971). All workers agree that basalt is
rapidly altered in the ocean-bottom environment
but there is no general agreement on the mechanisms
of alteration, the products formed, or the
basaltic material that might be most representative
of the primary chemistry. Many of these studies
have concluded that alteration varies from a highly
altered glassy rim to slight alteration in pillow
cores, making the core the most popular candidate
for chemical analysis to represent the primary
chemistry. It is the theme of this paper that portions
of the glassy rim often remain unaltered and
when these portions can be analyzed separately they
are the material most representative of the primary
igneous chemistry. Since most abyssal basalts are
sparsely phyric (usually less than 5 percent) the glass
will be very close to the chemistry of the total rock.
Late stage gravity or flow differentiation of phenocrysts
are reflected in the whole rock chemistry and
may confuse the primary partial melting or differentiation
trends. Glass chemistry appears more homogeneous
in single igneous events and apparently is
much less effected by late stage igneous differentiation.
We present a model for the alteration of
pillow basalts and a technique for electron-microprobe
analysis of basaltic glasses. Data is presented
on the homogeneity of basaltic glass within a single
thin section and within single eruptive flows.
Model for Alteration of Abyssal Pillow Basalts
Most basaltic pillows range in size from 10 cm to
several meters in diameter and are characterized by
a concentric zonal structure consisting of an outer
glassy selvage (up to 2 cm in thickness), an intcr-