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Rosenbaum, J.M., 1990

Trace element and isotopic effects of fluids in mantle monoliths

Bibliographic Reference

Rosenbaum, J.M., 1990, Trace element and isotopic effects of fluids in mantle monoliths: New York City, New York, Columbia University, Ph.D. dissertation, 157 p.

Abstract

The trace-element composition of the fluid trapped in fluid inclusions in two lherzolite nodules from San Carlos, AZ, and Nunivak Island, AK, and a cumulate wehrlite from Salt Lake Crater, HI was constrained by direct measurements on inclusion-bearing and inclusion-free mineral separates. Fluids trapped in the two lherzolites contain significant amounts of K, Cs, Rb, Ba, Sr, Pb, U, and Th; differ principally in their K and Ba concentrations; and resemble carbonatites and kimberlites in trace element composition (K~12,100 and ~43,000, Cs~1, Rb~15, Ba~3,000 and ~1,200, Sr~10,000, Pb~35, U~8, Th~25 in ppm). Olivine from the fluid-bearing lherzolites analyzed is enriched in 18O relative to coexisting pyroxene. A series of oxygen isotope exchange experiments demonstrates that this is indicative of isotopic disequilibrium at mantle conditions. The observed stable isotope compositions of pyroxene and olivine can be reproduced using an open system exchange model wherein a fluid phase isotopically exchanges simultaneously with both phases. Reaction kinetics of oxygen exchange between matrix and fluid constrain the duration of fluid/rock interaction to < ~1.5 My. Rb-Sr, Sm-Nd, and U-Th-Pb isotopic data also suggest that fluid influx is recent. Stable isotopic data are consistent with both fluid and matrix originating from an initially homogenous diapir. Fluid may be generated by destabilization of volatile-bearing phases such as mica, amphibole, and carbonate during diapiric ascent. The trace element composition of fluid trapped in the cumulate is very dilute relative to fluids trapped in the lherzolites (K~2,300 ppm, Cs~0.26 ppm, Rb~7 ppm, Ba~600 ppm, Sr~200 ppm, Pb~3 ppm, U~0.5 ppm, Th~3 ppm). The fluid and the melt in equilibrium with the cumulate may be related to a common parent fertile enough to produce an alkali basalt. The fluids characterized in this study contain all components of the U-Th-Pb and Rb-Sr isotopic systems and the incompatible trace elements typically depleted in lherzolites relative to estimated primitive upper mantle compositions. For most incompatible elements, these fluids dominate the trace element budget of fluid/silicate mixtures at the sub-percent (> 0.01 wt%) level and must be considered when modeling trace-element distribution in the mantle or basalt petrogenesis.

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