HR: 10:35h
AN: V22B-02    [Abstracts]
TI: Processing micronuggets-containing LA-ICP-MS data
AU: * Keller, N S
EM: nicole.keller@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, 0200, Australia
AU: Mavrogenes, J A
EM: mavro@ems.anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, 0200, Australia
AU: Mavrogenes, J A
EM: mavro@ems.anu.edu.au
AF: Deparment of Earth and Marine Sciences, Australian National University, Canberra, 0200, Australia
AB: In many experimental studies of solubility of metals in silicate melts, there are metal nuggets in the glass at the end of the run regardless of the technique used. They are often so small that the only way of discovering them is by examining the spectra obtained by mass spectrometry technique such as LA-ICP-MS, where the spectra (cps of element of choice vs. ablation time) show spikes and bumps. There is no common agreement on how to process data that contains micronuggets, as it is unclear how and when they form. If the nuggets are in equilibrium with the melt during the run, they need to be removed from the signal as including them would lead to an overestimated solubility. Yet if they are exsolved on quench, the metals were dissolved in the melt at run conditions and the nuggets need to be included in the data in order to yield the total solubility data. Most authors have considered them to be quench phase and included them in their analysis. The study presented here was done on a series of Cu, Au and Pd solubility experiments in a minimum An-Di melt as a function of oxygen fugacity (fO₂). All runs were conducted in a 1-atm furnace at 1300°C using a standard loop technique, over a range of logfO₂'s from 0 to -11. 3 to 6 spots per sample were analysed with LA-ICP-MS. The spectra show that the nugget density increases from no nuggets at high fO₂ to high density at low fO₂. The data was evaluated both by integrating all nuggets and by determining a baseline on the spectra. Each set was plotted against fO₂, assuming that whichever dataset appears to be best would produce a strong argument in favour of the correct method. The data agreeing best with the general trend of decreasing solubility with decreasing fO₂ was obtained when choosing the overall lowest point of a series of analysis on a given sample; the results indicate that including the nuggets in the laser signal overestimates the solubility by over an order of magnitude at high nugget density. This result implies that the nuggets are equilibrium phases at run conditions. The fact that nugget formation is so strongly fO₂ dependent may present an additional argument for this hypothesis, as quench processes are unlikely to be very strongly affected by fO₂.
DE: 3630 Experimental mineralogy and petrology
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting