Date of Award

6-30-2016

Document Type

Open Access Thesis

Department

Art

Sub-Department

Geological Sciences

First Advisor

Michael Bizimis

Abstract

The platinum group elements (Os, Ir, Ru, Rh, Pt, Pd) are important petrogenetic tracers of mantle processes and the Re-Os isotope system an important tool for tracing ancient depletion and refertilization processes in the mantle. A key characteristic of these elements is that they are siderophile and chalcophile, and their abundance in the Earth’s mantle is thought to be controlled by sulfides. Existing thermodynamic data suggests that at reducing conditions similar to those found at the early stages of serpentinization, PGE may exist as alloys in the mantle. While numerous studies report on the bulk peridotite PGE and sulfide PGE systematics, the effects of serpentinization on PGE systematics have not yet been investigated. This study presents bulk rock and in situ (LA-ICPMS on sulfides, down to a 10 micron beam size) PGE concentrations on five partially serpentinized peridotites from the St. Elena ophiolite, Costa Rica. The presence of Fe-Ni alloys and native Cu in these peridotites indicate low fO2 and fS2 conditions and low water-rock ratios during serpentinization. Low LREE/HREE ratios, low Ti, and low Al contents in these peridotites suggest variable degrees of depletion (3-14%) with little evidence for melt metasomatism in all but one peridotite. Bulk rock 187Os/188Os range from 0.1233 to 0.126, consistent with an origin from the depleted upper mantle. Sulfides are dominantly pentlandites. PGE-Re concentrations in sulfides are highly variable, ranging from 1 – 100,000 times that of primitive mantle (PM). PM-normalized PGE-Re patterns in the sulfides are dominated by strong Pt depletions relative to Os, Ir, Ru and

Pd. Bulk rock PGE concentrations are roughly similar to PM, and lack the Pt-depletions seen in sulfides. Mass balance reconstructions using in situ sulfide data and bulk rock sulfide S contents reproduce the measured bulk rock Os, Ir, Ru, concentrations within a factor of 3, but highly underestimate that of Pt. This data suggests that while Os, Ir, and Ru are dominantly hosted in analyzed sulfides, Pt is hosted in other phases but has still remained in the rock within the hand specimen scale. Detailed SEM analyses reveal the presence of various micron-sized Cu-Pt-Pd, Pt-Te, and Pt-Te-Au alloys. Laser ablation data revealed transient Pt spikes within sulfides, further confirming the presence of “nugget” phases that are also enclosed in sulfides. These data demonstrate unambiguously that in serpentinized oceanic peridotites, Pt is not controlled by the sulfide mineralogy. The formation of Pt “nuggets” is likely a subsolidus exsolution feature developed during cooling and pentlandite crystallization. Low sulfur and oxygen fugacities may have helped preserve the Pt alloys, and serpentinization fluids possibly redistributed them, but at scales in the order of millimeters. Finally, the relatively low Pb concentrations in the sulfides are inconsistent with the sulfide solution to the Pb paradox.

Rights

© 2016, Jessica A. Holm

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