ABSTRACT
The lunar picritic volcanic glasses have been identified as being
quenched
samples of primary magmas extruded onto the lunar surface via
fire-fountains.
The study of the composition of these glasses is of extreme
importance
for the understanding of the Moon's mantle composition and
petrogenetic
processes.
Based on their chemical signature (low-Ti and high Mg#) and
physical
characteristics, the lunar picritic glasses are believed to
represent primary
magmas. Experimental data suggest that these melts formed at
360-500 km
depth (18-25 kbar) in the lunar mantle, and were transported onto
the lunar
surface through a "channel" network (McKenzie, 1985b) that
isolated these
magmas and minimized fractionation. The aim of this work is to
obtain high
precision electron microprobe analysis in order to more
confidently model
melting processes that may have occurred deep within the lunar
mantle.
For this study, Apollo 14 volcanic green glasses A and B were
analyzed.
These glasses (A and B) show large enrichments of incompatible
elements
(e.g. K, Na, Ti) and an intragroup trend that does not follow
olivine (olivine
is known to be the liquidus phase for all pressures less than
about 20
kbar), low-Ca pyroxene, augite or plagioclase fractionation
trends. Although
batch melting, mantle source hybridization and/or assimilation of
KREEP
in the magma source region have been considered, these processes
fail to
explain the behavior of major elements.
More recently, it has been suggested (Delano, 1996; Delano and
Fernandes,
1998) that the trends observed reflect deep-seated magmatic
processes resulting
from dynamic melting of an ascending mantle diapir (Delano and
Fernandes,
1998). This model, the dynamic partial melting process, involves
the differential
flow of melt and residual matrix (Eggins, 1992). In the melt
region, the
degree of melting will increase as a function of decompression,
therefore
of height above the adiabat and peridotite (source) solidus
intersection.
The amount of melt present (i.e. porosity) at a particular height,
however
will be less than the degree of melting due to the more rapid
buoyancy-driven
ascent of melt compared to the matrix. The magnitude of this
melt-filled
porosity (phi) will depend upon the velocity of the melt relative
to the
matrix and upon the rate of melting. The amount of melt created
will dictate
the abundance of elements in a specific melt fraction.
Based on dynamic melting modeling, the degree of partial melting
involved
in the origin of these melts (the glasses) is within the range of
values
(5 to 25%) expected for conventional petrogenetic processes (e.g.
batch
melting). The model results for Al, Ca, K, Na, and Ti show a wide
range
of porosity and distribution coefficients that may have been
involved in
the origin of these melts. The little sensitivity to the model
shown by
K and Na suggest that these two elements are important in the
identification
of the degree of partial melting undergone by the source mantle
diapir.
These two elements may provide important information that will
allow all
of the other elements, including Ti, to be better constrained in
the model.
Vera Assis Fernandes, V.A., 1999. Major- and minor-element
analysis
of Apollo 14 volcanic green glasses B, and petrogenic modeling of
Apollo
14 green glasses A and B. Unpublished MSc. thesis, State
University of
New York at Albany. 147 pp., +xi
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1999 F47
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