Experimental Investigations on Mineral-Melt Partioning Behaviour of Trace Elements at High Temperature Conditions: Implications for High-Titanium Lunar Basalts

EXPERIMENTAL INVESTIGATIONS ON MINERAL-MELT PARTITIONING BEHAVIOUR OF TRACE ELEMENTS AT HIGH TEMPERATURE CONDITIONS: IMPLICATIONS FOR HIGHTITANIUM LUNAR BASALTS. Tyagi, V.1 (vaish[email protected]), Rai, N.1, 2, Narendranath, S.3 1Department of Earth Sciences, Indian Institute of Technology, Roorkee, 2Centre for Space Science and Technology, Indian Institute of Technology, Roorkee, 3Space Astronomy Group, U.R. Rao Satellite Centre, ISRO, Bengaluru. Introduction: Trace elements, including First Row Transition Elements (FRTEsSc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) and High Field Strength Elements (HFSEsZr, Hf, Nb, Ta, W and Mo), can provide significant clues on the source and petrogenesis of crustal rocks since their concentrations get significantly affected by factors like oxygen fugacity (fO2) and melt composition (FeO, TiO2 abundance in the melt), yet they can remain independent of the extent of partial melting and metasomatism [1, 2]. The aim of this study is to make usage of such trace elements as excellent indicators of the conditions relevant to the generation of lunar mare basalts, specifically high-titanium ones. Elevated abundance of titanium in the melt composition can play a key role in controlling the partitioning behavior of many incompatible elements. For example, there seems to be a strong inverse correlation between the DHFSEmineralmelt ratios and TiO2 content of the melt [3]. The observed correlation has been attributed to the formation of Fe-O-Ti complexes in the melt. The structure of this molten Fe-O-Ti complex is not constrained by a crystal lattice and thus elements of varying sizes and charges can easily be accommodated by the melt [3], thus facilitating for the substitution of Ti by HFSE group elements. New experiments were conducted to determine the partition coefficients between olivine and silicate melt of selected FRTEs, HFSEs, Ge and Ga. Their partitioning behavior was investigated based on varying TiO2 content of the silicate melt at different oxygen fugacity conditions and FeO contents, in order to gain a deeper understanding of the melt’s role. Although, recent studies have been carried out to understand how differently FRTEs, Ge and Ga behaved under varying oxygen fugacity and FeO content of the melt for lowtitanium basalts [4], there still are significant gaps in the research concerning hightitanium basalts in light of the above discussed strong dependence of certain trace elements on titanium content of the melt. This study aims to investigate how trace element partitioning differs between low-Ti and high-Ti basalts through mineral-melt partitioning experiments. The HFSEs were selected because of their geochemical similarity with titanium and the competition they create for Ti sites in the Fe-O-Ti oxide structure of the melt [3]. Additionally, selected FRTEs were chosen, specifically, chromium and vanadium due to their inverse correlation with TiO2 content of the melt, suggesting partial substitution into Ti sites in Fe-Ti oxides [3]. Other than these, manganese was also chosen as DMnOl-melt is strongly controlled by temperature and melt composition [4], including TiO2 content. Furthermore, Mn is a compatible element in olivine, and examining the effect of melt TiO2 on its partitioning behavior can help quantify its compatibility. Additionally, cobalt, nickel, germanium and gallium were tested as their partitioning behavior with varying FeO content has been studied before for low Tibasalts but remains to be tested for high Ti-ones, thus driving the need for their investigation. Methodology: The experiments were conducted using the ANTS-i 1-atm gasmixing vertical furnace at temperatures up to 1400°C and carried out under redox conditions ranging from ΔIW-2 to ΔIW+2, in order to characterise lunar mantle conditions. Bulk compositions of starting mixtures varied in TiO2 content from 0.2 wt% to 17 wt%, and the FeO content varied from 15 wt% to 25 wt%. All experiments were heated to 1400°C and maintained at this temperature for 48 hours before being quenched to room temperature. The analytical techniques employed for the analysis of the quenched material involves SEM, FE-EPMA and LA-ICP-MS. Results & Discussion: Initial investigation of the experimental runs reveals a strong negative correlation between TiO2 melt abundance and the partition coefficients of all selected HFSEs, Cr, V, and Mn, while Co and Ni showed a weak negative correlation. Although DCrOl−melt ratios have been studied under varying FeO conditions for low Ti basalts [4], the mineral-melt partitioning behavior of Cr had yet to be investigated for highTi basalts. Our results indicate that the DCrOl−melt ratios depend not only on FeO content [4], but also on the content of titanium in the melt. This influence of TiO2 plays a significant role in determining the extent of chromium partitioning into the later crystallized mare basalts. Consequently, the study provides new insights into the genesis of higher bulk chromium content of hightitanium lunar basalts as compared to terrestrial midocean ridge basalts and ocean-island basalts, given a Cr-enriched shallow mantle source for lunar basalts [4]. The new results from this study are expected to shed light on the underlying causes of Cr enrichment in lunar basalts compared to their terrestrial counterparts and to provide new insights into the source composition of high-Ti basalts through their trace element behavior. These new findings will address key research gaps and will be presented at the conference. References: [1] Humayun, M. et al. (2004). Science 306 (5693), 91–94. [2] Le Roux, V. et al. (2010). GCA 74 (9), 2779–2796. [3] Dygert, N. et al. (2013). GCA 106, 134-151. [4] Jing, J. J., et al. (2024). GCA 373. 211-231. Abstract #1402.