HR: 0800h
AN: V41C-1399    [Abstracts]
TI: An Empirical Calibration of an Al-in-olivine Geothermometer for Mantle-derived Materials
AU: * de Hoog, J C
EM: cees-jan@gvc.gu.se
AF: Dept. of Earth Sciences G$\ddot{o}$teborg University, Guldhedsgatan 5A, Gothenburg, 413 20 Sweden
AB: The concentration of aluminum in olivine from mantle peridotites is variable and strongly temperature dependent, and can therefore be used as a geothermometer. A suite of fresh mantle peridotite xenoliths from the Kaalvallei kimberlite (South-Africa) was used for calibration of the thermometer. The samples contain olivine, opx, cpx, garnet $\pm$ spinel. Aluminum contents of the olivines, determined by LA-ICP-MS, range from 8-140 ppm, with a few anomalous values up to 310 ppm. Mg\# lie between 91.5 and 93.3. {\it P-T} conditions for the peridotites were estimated with the Al-in-opx geobarometer and cpx-opx geothermometer [1], and plot on a conductive geotherm of ca. 40 mW/m$^{2}$. The temperature range is 900-1380$\deg$C. Only samples plotting on the geotherm were considered, as the others showed disequilibrium features. The expression for the thermometer is: {\it T}$_{Al-in-ol }$($\deg$C) = 11390 / [ 11.88 - ln(ppm Al) ] - 273 with an average residual of 12$\deg$C. As the compositional range of mantle olivine is very small, no correction for major chemical components is necessary. In addition, no correction for Al activity of the system is necessary, as long as an Al-saturated phase such as garnet is present. Combined with the Ca-in-olivine barometer [2], the new thermometer has the potential to determine {\it P-T} conditions of single olivines. As olivine is an abundant component of heavy mineral separates from kimberlites, it could serve as a new tool for diamond exploration. Vanadium and Cr show similar temperature-dependent variations to Al, but to a lesser degree, and would therefore yield less accurate geothermometers. In addition, partitioning of these elements is sensitive to variations in oxidation state. The pressure dependence of the thermometer is the subject of future research. Considering the significant pressure effect on Ca and Ti partitioning into olivine, it is recommended that the Al-in-olivine thermometer in its current form is applied to rocks derived from comparable (cratonic) geotherms only. [1] Brey and Kohler, 1990, J Petrol, 31, 1353-1378 [2] Kohler and Brey, 1990, GCA, 54:2375-2388
DE: 3620 Crystal chemistry
DE: 3670 Minor and trace element composition
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting