Olivine Ca content measured by ICP-OES to estimate P-T

Bradley D Benavides, University of Texas at El Paso

Abstract

Ca is a trace element in olivine and its concentration is pressure and temperature dependent, providing a rare opportunity to estimate pressure conditions at which mantle-derived rocks (xenoliths) last equilibrated. This P-T dependence has previously been calibrated for mantle xenoliths, and we applied this technique to samples from Jasper Seamount (Jasper Seamount), an oceanic intraplate volcano (OIV) off the coast of the Baja California Peninsula of Mexico. Preliminary compositions for xenoliths from this location appear to define a magmatic plumbing system that features magma storage near the bottom of the crust. A more detailed analysis of the samples is required to better estimate P-T conditions, and this is normally achieved with electron microprobe measurements of Ca content in olivine, and Mg and Fe in orthopyroxene and clinopyroxene (Köhler and Brey, 1990). In order to assess and improve precision, O'Reilly et al. (1997) used proton microprobe and electron microprobe data to obtain Ca concentrations for two samples and compared them to compositional estimates from isotope dilution. The results show that isotope dilution yields the highest precision, while the electron microprobe achieves 1% precision, and the proton microprobe yields the worst precision (2.7%). It was found that Ca is strongly correlated with temperature and poorly correlated with pressure, requiring high-precision Ca concentrations to minimize the error on P-T estimates. In this study, the capabilities of ICP-OES to determine the concentration of Ca of a well-studied Kilbourne Hole xenolith (KH7) have been evaluated. Precise ICP-OES concentrations of Fe, Mg, and Ni were also determined, and these were tested as potential internal standards using electron microprobe concentrations of these elements and the ratio of Ca to one of these elements. The best precision was found to result from measuring Ca/Mg ratios, with a given Mg concentration from a different technique. A gravimetrically prepared UTEP Fake Olivine Standard (UFOS) was subsequently measured, having a composition that is close to Jasper Seamount xenoliths. The new Ca/Mg technique was then applied to the xenoliths of Jasper Seamount, bracketed with UFOS measurements for a secondary correction that also includes a drift correction. Jasper Seamount samples define a similar temperature range as was previously found, and overlapping with other OIVs. Pressure estimates only yield values outside of the calibrated range of the applied geothermobarometer, which may result from partial re-equilibration of mineral compositions, alteration of the samples on the seafloor, or by unintentionally including small inclusions of clinopyroxene in the bulk dissolution of olivine.^

Subject Area

Geology

Recommended Citation

Benavides, Bradley D, "Olivine Ca content measured by ICP-OES to estimate P-T" (2013). ETD Collection for University of Texas, El Paso. AAI1551217.
http://digitalcommons.utep.edu/dissertations/AAI1551217

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