Physics
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusm.v52b..07d&link_type=abstract
American Geophysical Union, Spring Meeting 2002, abstract #V52B-07
Physics
3630 Experimental Mineralogy And Petrology, 3640 Igneous Petrology, 3670 Minor And Trace Element Composition, 3672 Planetary Mineralogy And Petrology (5410)
Scientific paper
Garnet (Gt) and its higher-pressure form, majorite (Mj), are thought to play important roles in the generation of terrestrial Archaean komatiites and of Martian meteorite compositions. Earlier work by Ohtani, Kato, and coworkers on majoritic Gt partitioning at 16-24 GPa showed that DHREE for Mj were somewhat lower, and DLREE somewhat higher, than those for lower-pressure Gt. Thus, REE signatures of Mj fractionation are less pronounced than those for ordinary Gt, and because DHf is less affected, Lu/Hf will be less strongly perturbed by Mj fractionation than by Gt fractionation. In this study, we were motivated to measure Gt-melt trace element partitioning in more iron-rich systems than were used in these earlier studies, which are more relevant to early Martian magmatism. However, our results have general applicability and form a useful comparison with the studies of van Westrenen and coworkers (e.g. 2001, CMP 142:219), who developed predictive relationships for 3+ cation partitioning into the X sites of lower-pressure Gt. We doped a synthetic ultramafic composition, based on the silicate portion of Homestead L5 ordinary chondrite, with various subsets of Nd, Sm, Tb, Yb, Lu, Y, Sc, Zr, and Hf (max total dopant ~2 wt%) for multianvil experiments between 5 and 9 GPa. Gt begins to exhibit the transformation to Mj at 5 GPa, a lower pressure than has been previously reported. This transformation is evidenced by decreasing Al and excess Si per formula unit (e.g. 3.1 to 3.2 Si per 12 oxygens), becoming progressively more majoritic (increasing Si, decreasing Al) with increasing pressure to ~3.3 Si per 12 oxygens at 9 GPa. We have reported elsewhere (Xirouchakis et al., 2002, XXXIII LPSC, 1316) that the Mj transformation is favored at these lower pressures by the presence of silicate melt and the absence of clinopyroxene. We find that DHREE,Y and DTb decrease markedly as Gt begins the transformation to Mj, but that the LREE are less affected. DHf,Zr undergo almost no change, and DSc shows a slight decrease with increasing pressure. Ds for 3+ cations into the X sites follow very closely the functional form of the lattice-strain model of Blundy and Wood (1994, Nature 372:452), in similar fashion to the data of van Westrenen et al. However, the observed D values do not match their predictions, suggesting that the transformation to Mj involves structural changes that have an important effect on trace element partitioning. Our data should improve the predictive power of the lattice-strain model with respect to the Mj transformation. To help model 4+ cation partitioning into the Y sites using the same formulation, we sought to estimate Ds for SiVI between Gt and melt. We estimated octahedral SiO2 in Gt using the expression [(Si per 12 oxygens - 3)/(Si per 12 oxygens)] x (wt% SiO2 in garnet), and then simply divided that quantity by the SiO2 content of the quench phase to obtain DSiVI. Combining those values with Ds for Hf, Zr, and Ti results in fits to the Blundy and Wood model that make some qualitative sense: site elasticity E, ideal partition coefficient D0, and ideal site-size r0 are all smaller than those for the X sites. However, there is an indication that elasticity decreases with increasing pressure, and r0 values are substantially higher than those found by van Westrenen et al. for Ca-poor Gt (75 to 80 pm compared to 67 pm). These results may be artifacts of having data on fewer elements from which to calculate the fits compared to the 3+ situation. Accordingly, we require further work that adds a larger cation such as Th to our element set before robust quantitative assessments of our 4+ data can be made.
Agee Carl B.
Draper David S.
Xirouchakis Dimitrios
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