The effect of titanium on the partitioning behavior of high-field strength elements between silicates, oxides and lunar basaltic melts with applications to the origin of mare basalts
Felipe P. Leitzke, Raúl O.C. Fonseca, Lina T. Michely, Peter Sprung, Carsten Münker, Alexander Heuser, Henrik Blanchard
Chemical Geology
In Press, Accepted Manuscript, Available online 18 July 2016
“Highlights
• DHFSEcrystal/melt were calculated from experiments with 0 to 20 wt.% melt TiO2.
• At ca. IW − 1.6 generally higher values of DMcrystal/melt for Mo, W and U are found.
• Using the new data, fractional melting models of the lunar mantle were performed.
• Metal and Fesingle bondTi oxides are need at source to match observed high-Ti basalts trends.”
“A specific feature of some basaltic lunar rocks is that their TiO2 contents can reach concentrations as high as 16 wt.%. The high-field strength elements (HFSE) group, which includes Ti, may provide valuable information of the processes that occurred in the lunar mantle to generate high-Ti mare basalts. To assess the effect of such high TiO2 concentrations on the partitioning of Zr, Hf, Nb, Ta, U, Th, Mo and W between major silicate and oxide phases and silicate melts, we present results from experiments at one atmosphere and 1100 °C–1305 °C, under controlled oxygen fugacity. With the exception of Nb, all DHFSEcpx/melt show a strong negative correlation with the TiO2 content of the silicate melt. Olivine/Silicate melt partition coefficients for Zr, Hf, Nb, Ta and Th decrease slightly from 0 to ca. 5 wt.% TiO2, above which they remain constant up to ca. 20 wt.% TiO2 in the silicate glass. In addition, redox sensitive elements, i.e. U, Mo, and W show clearly distinct DMsilicates/melt at different fO2, implying that these elements are relatively more compatible at reduced (ca. IW − 1.8) than at oxidized (FMQ and air) environments. Iron-rich and Mg-rich armalcolite show contrasting patterns of DMcrystal/melt, with the latter exhibiting slightly higher values of partition coefficient for all analyzed elements, except Th, which is equally incompatible in both end-members. Finally, the new dataset of DHFSEcrystal/melt was used to perform simple melting models of the lunar mantle cumulates. Results indicate that to reproduce the fractionation of W from the HFSE, as well as U and Th observed in lunar mare basalts, metal saturation and the presence of Fesingle bondTi oxides in the mantle sources is required.”
