HR: 1330h
AN: V12C-0603    [PDF]
TI: Trace Element Partitioning Between low-Ca Pyroxene and Ultracalcic Liquids.
AU: * Pertermann, M
EM: maik.pertermann@erdw.ethz.ch
AF: Institut fuer Mineralogie und Petrographie, ETH-Zentrum, Zuerich, 8092 Switzerland
AU: Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Institut fuer Mineralogie und Petrographie, ETH-Zentrum, Zuerich, 8092 Switzerland
AU: Pettke, T
EM: thomas.pettke@erdw.ethz.ch
AF: Institut fuer Isotopengeologie und Mineralische Rohstoffe, ETH-Zentrum, Zuerich, 8092 Switzerland
AB: Low-Ca pyroxene or pigeonite ($\sim$0.25-0.35 Ca per formula unit, pfu) is an important residual phase during high temperature melting of refractory mantle (e.g., ankaramite formation). High-Ca cpx ($>$0.6-0.7 Ca pfu) may be residual to relatively low temperature melting of fertile mantle (MOR and OI), but the opx-cpx solvus narrows considerably at higher temperatures ($>$1330-1350$\deg$C), leading to coexisting opx and low-Ca cpx. Little is known about the trace element partitioning of such low-Ca cpx at upper mantle conditions. Our new partitioning experiments investigate the role of low-Ca cpx during melting of depleted peridotite. Nominally anhydrous experiments with graphite-lined Pt-capsules were conducted at 1.4 GPa and 1360-1370$\deg$C. The synthetic starting material is close in composition to an ultracalcic liquid saturated in opx+pigeonite+olivine+spinel. The experiments yielded assemblages of glass, low-Ca cpx, ol, and minor Cr-spinel; opx is absent. The low-Ca clinopyroxenes have 0.20 and 0.32 Ca pfu at 1370 and 1360$\deg$C, respectively, and tetrahedral Al of 0.046 and 0.067 pfu. The liquids have $\sim$50 wt% SiO$_{2}$, $\sim$12.5 wt% CaO and CaO/Al$_{2}$O$_{3}$ of 1.44-1.54. Pyroxenes and glasses were analyzed for trace elements (La, Ce, Nd, Sm, Eu, Gd, Dy, Er Yb, Lu, Sc, Y, Sr, Zr, Hf, V, Cr, Mn, Co, Zn) by LA-ICP-MS using a 193 nm ArF excimer laser coupled to an Elan 6100 mass spectrometer. Ablation occurred in He, and ablation spot sizes were 15-30 $\mu$m for minerals and 50 $\mu$m for glasses. Trace element concentrations in pyroxenes were low for most 3+ and 4+ cations. This resulted in small mineral/melt partition coefficients ({\it D}-values), approximately an order of magnitude lower than those for high-Ca cpx associated with peridotite melting, thus making the low-Ca cpx partitioning behavior rather similar to the behavior of peridotitic opx. Cpx with 0.32 Ca pfu has slightly elevated {\it D}-values for 3+ cations when compared to the 0.20 Ca pfu cpx: {\it D}$_{Sc}$ = 0.45, {\it D}$_{Y}$ = 0.11, {\it D}$_{Sm}$ = 0.054 and {\it D}$_{Yb}$ = 0.141, compared to values of 0.32, 0.065, 0.016 and 0.075, respectively. Regardless of Ca content, V, Mn, Co and Zn are mildly incompatible ({\it D}s $\sim$0.5-0.9). Therefore, only Sc, V, Cr, Mn, Co, Ni and Zn are retained during partial mantle melting with a pigeonite+opx+ol residue. Most other elements partition strongly into the melt, and barring exotic residual minerals, the composition of ultracalcic liquids reflects the incompatible trace element budget of the depleted peridotite source.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting