Garnet geotherms

pressure-temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks

Chris G. Ryan*, William L. Griffin, Norman J. Pearson

*Corresponding author for this work

    Research output: Contribution to journalArticle

    183 Citations (Scopus)

    Abstract

    The temperatures and pressures of equilibration of single peridotitic garnet xenocrysts are estimated using a combination of major- and trace-element data, determined using electron microprobe (EMP) and proton induced X ray emission (PIXE). This new method enables the use of xenocrysts found in kimberlites and other volcanic rocks to determine the local paleogeotherm at the time of eruption of the magma which sampled and transported the xenocrysts. The "Ni thermometer" of Griffin et al. [1989], based on the strong temperature dependence of the partitioning of Ni between garnet and olivine, is refined using an expanded database. Pressure is calculated from garnet composition using an algorithm that combines a modification of the geobarometer of Nickel [1989], based on Cr solubility in coexisting garnet and orthopyroxene, with the composition of a hypothetical coexisting orthopyroxene. The orthopyroxene composition is estimated by inverting the geothermometry equations of Gasparik [1987], Brey and Köhler [1990], and Harley [1984], and combining these with empirical relationships describing Cr in orthopyroxene in Crsaturated peridotite (chromite present). The derived pressure (PCr) gives the equilibration pressure of peridotic garnets provided they were in equilibrium with chromite; garnets from Cr-under-saturated rocks will produce underestimates of pressure. Therefore, the locus of maximum Pcr at a given TNi, defines the "garnet geotherm", and provides a method for the determination of paleogeotherms based solely on PIXE and EMP analyses of garnet grains in concentrates. The assumption of coexisting chromite is tested by comparing the temperature distributions of garnets and chromites from the same concentrate. Chromite equilibration temperature is estimated using the "Zn thermometer", based on the strong temperature dependence of the partitioning of Zn between chromite and olivine. This thermometer is calibrated against the new Ni thermometer using a suite of garnet-chromite intergrowths. The garnet geotherm technique provides an estimate of the geotherm with an accuracy comparable to xenolith-derived geotherms and provides a means of mapping the thermal state of the lithosphere where xenoliths are rare or absent.

    Original languageEnglish
    Pages (from-to)5611-5625
    Number of pages15
    JournalJournal of Geophysical Research
    Volume101
    Issue number3
    DOIs
    Publication statusPublished - 10 Mar 1996

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