I) Trace element and strontium isotopic geochemistry of accessory mineral phases and the origin of epizonal silicic intrusive igneous rocks. II) Electrochemistry of lead-doped molten silicate glasses and the development of new ion sources for thermal ionization mass spectrometry.

摘要

Exactly how silicic magmas are generated in the shallow continental crust remains a significant issue. This study addresses the role of accessory minerals and crystal-liquid separation in producing silicic magmas associated with the Organ Needle pluton (ONP), NM. In-situ trace element determinations were used to assess the origin of titanite from the inequigranular syenite (IEQ) of the ONP. The IEQ was previously interpreted as a cumulate related to production of silicic melt at the ONP. Titanites from the IEQ are light rare earth element (LREE) enriched ((La/Yb)N= 10-60). The removal of titanite could account for LREE depletion observed in associated silicic igneous rocks. Eu/Eu* and Sr concentrations suggest that titanites are not in chemical equilibrium and represent cumulates formed at different times and from parental magma batches that were juxtaposed after crystallization.;In-situ Sr isotopic analysis via thermal ionization mass spectrometry (TIMS) was performed to determine the source for volcanic units at the ONP, the Squaw Mt. Tuff (SMT), and the West Side Lavas (WSL), and to confirm the hypothesis that the alkali feldspar granite (AFG) was produced by progressive assimilation followed by transport. 87Sr/ 86Sr(i) vs. 1/Sr shows evidence that the WSL cores isotopically match plutonic units: IEQ, AFG, and a mafic enclave. In-situ Sr data also supports the hypothesis that the AFG is isotopically equivalent to the SMT. In both cases, data points to differentiation and magma removal at depth.;We also initiated efforts to generate a high-efficiency TIMS ion source using electrochemical techniques. Typical ionization efficiencies range from 0.1% to 10%. We produced voltammograms in a volatilizing electrolyte at high temperatures under vacuum conditions and argon atmosphere. Under argon, the voltammograms display peaks that may correspond to the Pb to Pb + reaction. An initial condition is that the glass is a molten conductor. Resistivities of these glasses range from 428.15 O-m (unleaded borosilicate, vacuum, 1390 deg C) to 9.73 O-m (leaded borosilicate, argon, 1200 deg C), an order of magnitude higher than reported values. High resistivities could be due to bubble formation in those glasses. Suggestions for future users include miniaturizing the cell to minimize bubble formation and to mimic conditions of the TIMS.