Award Date

5-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Geoscience

Department

Geoscience

First Committee Member

Adam Simon, Chair

Second Committee Member

Eugene Smith

Third Committee Member

Oliver Tschauner

Fourth Committee Member

Jean Cline

Graduate Faculty Representative

Clemens Heske

Number of Pages

127

Abstract

This dissertation is comprised of three broadly related experimental petrology projects on phase equilibria and noble metal solubility in hydrous silicate melts. Chapters two and three combine experimental petrology with high precision laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of experimental run products in order to quantitatively constrain the behavior of the investigated metals. Chapter four presents experimental evidence detailing a novel oxidation mechanism for degassing silicate liquids as well as exploring the geochemical consequences of the proposed mechanism.

Chapter two presents the results of an experimental study on Au, Pt, and Pd behavior in coexisting silicate melt-sulfide-oxide phase assemblages. Data from this study suggest the combined effect of oxygen and sulfur fugacity dictates the identity of stable magmatic sulfide phase assemblages, as well as dictating the concentration of Pt and Pd in monosulfide solid solution; both of these factors are critical components that determine metal tenor and the ore-deposit forming potential of a given magma.

Chapter three presents an experimental study of Au solubility in hydrous, chloride rich basaltic liquids as a function of oxygen fugacity (fO2). LA-ICP-MS determined Au concentrations in the quenched melt do not strictly adhere to the relationship between fO2 and Au solubility predicted for a monovalent Au oxide species. The observed relationship between Au and fO2 suggests the existence of alternative, non-oxide species in the melt. The solubility data presented in this chapter constrain the maximum Au concentration of natural hydrous basaltic liquids to values less than 2 μg g-1.

Chapter four presents experimental evidence suggesting a new mechanism for chloride degassing induced auto-oxidation of silicate liquids. The chemical exchange between silicate melts and chloride bearing fluids preferentially removes ferrous iron from the melt relative to ferric iron. The net effect of this preferential scavenging effect is to enrich the residual melt in ferric iron, increasing the melt’s intrinsic fO2. Dynamically changing magmatic oxygen fugacities profoundly affect the stability liquidus silicate phases in addition to potential sulfide phases involved in ore forming processes. (Refer to PDF file for exact formulas.)

Keywords

Gold; Hydrous silicate melts; Magma-oxidation; Oxygen fugacity; Palladium; Phase equilibria; Platinum; Siderophile elements; Sulfur fugacity

Disciplines

Geochemistry | Geology | Volcanology

Language

English


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