Abstract
A trace element fractionation model originally designed to relate
plutonic rocks containing a trapped glass component to a
complementary volcanic rock suite was applied to olivine and
opaque-oxide gabbroic xenoliths from the summit cone of Mauna Kea,
Hawaii.
The mathematical model which is based on the Rayleigh
Fractionation Law was extended to include the various phases
present in these two groups of gabbros and was generalized to
treat multiple trace elements, in this case four. These results
were incorporated in a new FORTRAN program using the
interval-bisection method to greatly improve the convergence time
in the numerical solutions.
Calculated results of the model include the concentrations of four
trace elements - Cr, Zr, Ni and Sr - in the glass phase; the
weight fractions of all phases in the gabbro; the fraction of the
liquid that remained in the magma chamber when some of it was
trapped; and the starting compositions for Ni and Sr in the
initial liquid before fractional crystallization. The choice of
partition coefficients and trace element starting compositions
appears to be crucial for the model results, as indicated by
sensitivity calculations.
To test the model results, the glass phase in the xenoliths was
analyzed for trace elements by electron microprobe. A comparison
between the analyses and the values calculated by the model
suggests two different possibilities for the large amount of Zr
and Sr accumulating in the glass phase: Highly evolved glass might
not be a true trapped liquid phase but rather was injected from
the hawaiitic host during ascent to the surface. However, tiny
patches of cognate melt might possibly also lead to an
amplification in the amount of incompatible trace elements. A
definitive solution could not be obtained by use of the model.
Eberle, U., 1990. Application of a trace element fractionation
model to cumulate gabbroic xenoliths of Mauna Kea, Hawaii.
Unpublished MSc. thesis, State University of New York at Albany.
99 pp., +ix
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 40 Z899 1990 E34
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