HR: 1330h
AN: V13B-08 [Abstracts]
TI: Mg-Fe2+ Exchange Between Olivine and Melt
AU: * Miller, E D
EM: miller.3170@osu.edu
AF: The Ohio State University, Department of Geological Sciences, Columbus, OH 43210 United States
AU: Kelley, D F
EM: kelley.196@osu.edu
AF: The Ohio State University, Department of Geological Sciences, Columbus, OH 43210 United States
AU: Barton, M
EM: barton.2@osu.edu
AF: The Ohio State University, Department of Geological Sciences, Columbus, OH 43210 United States
AB:
Knowledge of exchange of Mg and Fe2+ (Kd) between olivine and melt is crucial for understanding the origin and
evolution of magmas. It is essential to use accurate and precise values for Kd for quantitative studies of melting and
crystallization. However, values of Kd derived from experimental studies are associated with relatively large
uncertainties. Moreover, there is disagreement about the dependence of Kd on fO2, T, and melt composition. To
obtain accurate and precise values of Kd as a function of fO2, T, and melt composition, electron microprobe data
for equilibrium olivine-glass pairs were compiled from published experimental studies of natural samples at 0.1 MPa with
controlled oxygen fugacity. The melt compositions in these experiments include tholeiitic and alkali-olivine basalts,
picrites, andesites, and highly alkaline lavas (nephelinite, leucitites, ugandites). These data confirm that Kd is
independent of fO2 and T as concluded by many previous workers. However, Kd is strongly dependent on melt
composition, in particular on the molar alkalis and silica contents of the melt. These results are consistent with those
obtained by Longhi et al (1978), and suggest that variations in Kd correlate with the degree of polymerization of the
melts. Taking into account the non-ideal behavior of Mg-Fe2+ olivines does not affect this conclusion. An expression
that describes lnKd in terms of molar alkalis and silica recovers experimental values with an accuracy of ±0.0001
and a precision of ±0.007. Comparable accuracy and precision is obtained using the expression given by Gee and Sack
(1988) describing variations in Kd in terms of melt alkalinity. It is not presently clear that these expressions are
applicable to Fe-rich melts (Mg&35; less than about 0.25) or MgO-rich melts such as komatiites. However, they provide a
rigorous test for equilibrium between olivine and melt in the most abundant terrestrial magmas. In addition, accurate values
of Kd allow melt redox state and hence fO2 to be calculated for melts with total Fe reported as FeO. Values of
fO2 calculated for 189 experimental olivine-melt pairs agree with reported values to ñ0.22 log bar units.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly