HR: 0830h
AN: V41C-0314    [PDF]
TI: Application of Olivine-Spinel Fe-Mg Thermometers to Olivine - Cr-Spinel Pairs in a Shallow Tahitian Basanite Dike, and a Comparison with 1-atm Experimental Data
AU: Fedele, L
EM: lfedele@vt.edu
AF: Dipart. Geof. Vulcanol., Univ. Federico II, Via Mezzocannone 8, Naples, 80138 Italy
AU: * Tracy, R J
EM: rtracy@vt.edu
AF: Dept. Geological Sciences, VPI and SU, 4044 Derring Hall, Blacksburg, VA 24061 United States
AB: Application of spinel-olivine empirical and theoretical thermometers has mainly been confined to plutonic rocks, although spinel-olivine pairs are also common in basaltic (broadly defined) volcanic rocks and in shallow mafic intrusions (dikes and sills). In this work we have applied olivine spinel Fe-Mg thermometers to samples of a mantle xenolith-bearing basanite dike collected near Papeete, Tahiti. This leucite-normative basanite is notable for presence of a suite of mantle-derived ultramafic nodules (dunites, wehrlites and lherzolites), a relatively rare occurrence on Tahiti that probably reflects very rapid ascent. The basanite contains abundant olivine, titanaugite, plagioclase and titanomagnetite phenocrysts in a very fine-grained, and occasionally glassy, matrix. Zoned olivine phenocrysts (Fo88 cores to Fo60 rims) commonly include small euhedral to subhedral spinel inclusions that range from Mg-chromite to aluminous Ti-magnetite, depending on core-to-rim location within olivine. Systematic variations in composition of spinel and adjacent olivine record entrapment during the magmatic crystallization interval from near-liquidus ($>$ 1300 C) to near-solidus (ca. 1125 C) temperature. Three published olivine-spinel Fe-Mg thermometers (Roeder et al, 1979; Sack and Ghiorso, 1991; Jianping et al., 1995) were used to evaluate the T dependence of olivine-spinel Fe-Mg partitioning in analyzed pairs. Calculations show that the olivine-spinel pairs undoubtedly record thermal evolution of the basanite magma during crystallization, however all thermometers appear to underestimate absolute crystallization temperatures. This is particularly evident for the final stages (at lower T) of the co-crystallization of the olivine-spinel pairs, probably due to significant concentrations of the usp end member in the most fractionated magmatic spinels (approx. 70 mol% usp). Ti is not taken into account in calibrating two of the thermometers (Roeder et al. and Jianping et al.) and apparently is not fully accounted for in the spinel solid solution model (Sack and Ghiorso). A direct test of calculated T is possible due to a set of 1-atm crystallization experiments done on the basanite some years ago (in collaboration with E. Stolper). Experimental runs contain olivine microcrystals containing spinel inclusions, and both minerals were analyzed. T calculated using the three thermometers was compared with the known T of each crystallization experiment. Data show approximately linear relationships between Ti content and T- underestimation for all 3 thermometers.
DE: 3620 Crystal chemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting