HR: 14:35h
AN: V12F-04    [PDF]
TI: Application of Garnet-Accessory Phase Thermometry: a Combined EMP and SIMS Study
AU: * Pyle, J M
EM: pylej@rpi.edu
AF: Rensselaer Polytechnic Institute, EES-JRSC 1W09 110 8th St., Troy, NY 12180 United States
AU: Daniel, C G
EM: cdaniel@bucknell.edu
AF: Bucknell University, Department of Geology Bucknell University, Lewisburg, PA 17837 United States
AU: Spear, F S
EM: spearf@rpi.edu
AF: Rensselaer Polytechnic Institute, EES-JRSC 1W09 110 8th St., Troy, NY 12180 United States
AU: Layne, G D
EM: glayne@whoi.edu
AF: Woods Hole Oceanographic Institute, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AB: Fe-Mg exchange thermometry involving garnet and other phases requires appropriate bulk compositions, and is often compromised by bulk diffusion within garnet and retrograde net-transfer reactions that dissolve garnet or shift biotite compositions. Application of garnet-accessory phase thermometry provides a robust, multi-faceted alternative. Garnet-accessory phase pairs are resistant to diffusive reequilibration of Y and HREE, display evidence for an approach to compositional equilibrium, record metamorphic events in detail with yttrium distribution, and provide temporal constraints for such events via correlated monazite or xenotime age estimates. Precise garnet-accessory phase thermometry is extendible to low-Y garnets using secondary-ion mass spectrometry (SIMS), which furnishes additional information on trace element residence in major phases. Empirical garnet-xenotime thermometry requires only measurement of the yttrium concentration in the garnet of interest; garnet-monazite thermometry requires calculation of molar components in garnet, plagioclase, apatite, and monazite, plus a fluid fugacity estimate. Establishing criteria for selection of appropriate mineral pairs is crucial. Electron Microprobe (EMP) garnet-accessory phase thermometry is applied to three samples from the Mesoproterozoic of New Mexico (Grt-Bt-St schist, Grt-Bt-Sil migmatite, Grt-Ms-St (biotite-absent) schist). In the former samples, Grt-Bt thermometry estimates are compromised by garnet resorption and diffusive reequilibration, and the latter sample is unsuitable for Grt-Bt thermometry. Correlation of appropriate garnet-monazite pairs yields peak T estimates of 550 to $590\deg$C for the Grt-Bt-St schist, compared to 525-$535\deg$C for Grt-Bt thermometry, and T estimates of 610-$650\deg$C for pre-melting garnet-monazite pairs in the migmatite, compared to Grt-Bt peak T estimates of 580-$620\deg$C. Additionally, T estimates for earlier-formed garnet-monazite pairs (490-$525\deg$C, Grt-St-Bt schist; 465-$530\deg$C, migmatite) are recovered. Apatite and plagioclase are absent from the Grt-Ms-St rock, precluding application of the garnet-monazite thermometer, but garnet-xenotime thermometry yields T estimates of ca. $530\deg$C for garnet cores, and 550-$620\deg$C for post staurolite-in garnet. All temperature estimates are linked to monazite ages, as determined by EMP chemical methods, ranging from 1440 to 1300 Ma. Garnet-monazite thermometry is extended to selected migmatites (British Columbia (BC), New England (NE)) via SIMS analysis of low-Y garnet. Garnet rim (152$\pm$0.5 ppm Y) - high Y monazite pairs from the BC migmatite yield T estimates of ca. $660\deg$C, considerably lower than the peak T estimate of $820\deg$C; this mineral pair is interpreted as pre-melting garnet and monazite. In the NE migmatite, very low Y (3.4$\pm$0.12 ppm) anatectic garnet correlated with 350 Ma high grade monazite domains yields a T range of 720-$755\deg$C, consistent with previous peak T estimates of $740\deg$C, and formation of monazite on melt crystallization.
DE: 1020 Composition of the crust
DE: 3655 Major element composition
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting