HR: 0800h
AN: V51A-0330    [Abstracts]
TI: Trace Element Analyses of Quartz-Two Wrongs Don't Make a Right
AU: * Lowers, H
EM: hlowers@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center MS973, Denver, CO 80225, United States
AU: Rusk, B
EM: bgrusk@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center MS973, Denver, CO 80225, United States
AU: Koenig, A
EM: akoenig@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center MS973, Denver, CO 80225, United States
AB: Microanalysis of trace elements in hydrothermal quartz can improve understanding of the chemical dynamics of ore-forming systems. Quartz is expected to vary in composition because of the wide range of formation temperatures and parent fluid compositions. Compositional variations are evident in the variations in cathodoluminescence (CL) intensity of precipitated quartz, which is a result of structural defects including those caused by trace element substitutions. Quantification of trace elements in quartz remains difficult because of their low concentrations and interference from mineral and fluid inclusions. We compared EPMA and LA-ICP-MS trace element analyses of quartz from hydrothermal ore deposits that formed at temperatures between 100 and 700 degrees C to evaluate the suitability of each analytical method. Whereas LA-ICP-MS allows the user to simultaneously acquire data on over 40 elements at less than 10 ppm detection limits, these analyses are of low spatial resolution relative to CL textures. LA-ICP-MS also included incidental ablation of fluid and mineral inclusions in the quartz. Although far fewer elements are detectable at concentrations as low as LA-ICP-MS, the spatial resolution of EMPA allows for direct comparison between CL textures and TE abundances. In particular, EPMA analysis of quartz acquired at 20 keV, 100 nA, and 20 minute count times for Ti and Al on multiple spectrometers achieves detection limits that approach that of LA-ICP-MS (7 ppm and 4 ppm, respectively) while retaining less than 5 micrometer resolution. Determination of low-level Al using EPMA is sensitive to the background correction model used. In several samples containing concentrations of Al less than 100 ppm, linear and exponential background models yield Al concentrations below detection. Analyses run using only the high background (to avoid Si tailing on the low background side of the Al peak) produce detection limits of 300 ppm. Using a polynomial fit of the background, Al concentrations measured by EPMA range from 4 to 32 ppm where LA-ICP-MS values ranged from 2 to 33 ppm. Excellent agreement between the two techniques strongly suggests that the polynomial background fit is appropriate for quantification of Al in very small quantities. Ti concentrations determined by EPMA and LA-ICP-MS also show excellent agreement. In quartz samples from two porphyry copper deposits that were mostly featureless in CL, EPMA concentrations ranged from 9-25 ppm and 26-83 ppm whereas LA-ICP-MS analyses ranged from less than 13-30 ppm and 48-69 respectively. As with Al, the complex CL texture correlates well with the Ti content variations apparent with the spatial resolution of EPMA. LA-ICP-MS analyses however show variations in Li, Na, Mg, K, Ga, Rb, Sr, Cs, and Ba, that are not detectable with EPMA, but correlate with broad variations in CL intensity. While EPMA background modeling of Ti is not an issue because of the absence of the Si tail, the analyst must also consider secondary fluorescence effects from included minerals, particularly rutile. Ti in quartz can be detected as far as 150 micrometers away from adjacent rutile (Wark 2006).
DE: 0394 Instruments and techniques
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting