ALBERT

Description: The strong dependence of vanadium partitioning between olivine and silicate melt (DVOl-M) on redox conditions (fO2) can be used as sensitive oxybarometer in magmatic systems. Here we extend the experimental database on DVOl-M, obtained so far at high temperatures (mainly above 1250 °C), to lower temperatures that are typical for island-arc basalts. Crystallization experiments were performed using a composition from Mutnovsky volcano (Kamchatka), and the investigated temperature, pressure, and oxygen fugacity ranges were 1025–1150 °C, 0.1 and 0.3 GPa, and ΔQFM of –0.5 to +3.2, respectively. The water content in melts ranged from 0.6 to ∼6.5 wt% H2O. The data demonstrate a strong negative correlation between DVOl-M and oxygen fugacity, similar to the behavior observed previously at higher temperatures and in MgO-rich compositions. The correlation between DVOl-M and ΔQFM in the range from –0.5 to +3.2 is described for melts with MgO 〈 12 wt% and Na2O 〈 4 wt% at temperatures ≤1250 °C by the empirical equation: ΔQFM = −3.07−0.29+0.26 logDVOl-M – 3.34−0.49+0.40 with the standard error (SE) as a function of logDVOl-M: 2SE(ΔQFM) = –0.275logDVOl-M + 0.4. We suggest that this equation can be used as an oxybarometer, which is particularly well applicable to the hydrous island-arc magmas at relatively low temperature. Application of the equation to the composition of melt inclusions and their host olivine phenocrysts from basalts of Mutnovsky volcano, containing vanadium concentrations in the range of 250–370 and 4–6 ppm, respectively, reveals an oxygen fugacity in the range ΔQFM +1.9 to +2.3. The estimates are in a good agreement with olivine-spinel oxybarometry for Mutnovsky basalts and may be typical for moderately evolved island-arc magmas.

Description: Olivine is a major rock-forming mineral in various magmatic and metamorphic rocks and upper mantle. In this paper, we present first high-precision analyses of olivine from 15 samples of magnesian skarns and silicate marbles (MSSM) from the collection of the Fersman Mineralogical Museum (Moscow, Russia). Mg# (Mg/(Mg+Fe2+)*100, mol.%) of olivine from the samples studied varies from 86 to nearly 100. The main distinctive features of the olivine are anomalously low contents of Co (〈51 μg/g), Cr (〈 5 μg/g) and Ni (〈 44 μg/g) and high content of B (23-856 μg/g), which correlate with host-rock compositions. Phosphorus (5-377 μg/g) incorporation in olivine is charge-balanced by the incorporation of Li (0.15-61 μg/g) and Na (〈14.3 μg/g). Y and REE contents exhibit positive correlations with Na, which suggest that REE incorporation into MSSM olivine could occur via charge-balanced coupled subs titution with Na at low temperature and low aSiO2 conditions during MSSM formation. The documented compositional features of olivine from magnesian skarns and silicate marbles can help reconstruct the genesis of the host-rocks and identify xenocrysts of MSSM olivine in magmatic rocks.