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1. Introduction
Lunar magma ocean (LMO) crystallization experiments and simulations performed by earlier workers have revealed that at the last stage, Fe-Ti-rich cumulates are formed [1,2,3], which have been linked to the varied TiO2 contents of the lunar basalts [4]. These last stage cumulates are postulated to be dense enough to sink down through the underlying Mg-rich mantle [5], possibly till depths of the lunar core-mantle boundary [6]. To understand this interaction between the Fe-Ti cumulates and the underlying mantle, new high-pressure, high-temperature, layered experiments have been performed between an iron-titanium-rich silicate composition signifying the Fe-Ti cumulates (6.22 wt.% TiO2 and 40.54 wt.% FeO) and forsteritic olivine (Mg# ~ 92) signifying the lunar mantle composition, taken in nearly 1:4 weight ratio (Fig. 1). These experiments were performed between 1-3 GPa and 1100-1525 °C pressure and temperature ranges, respectively.
Fig. 1: Run product showing the two layers of starting material and the reaction zone
2. Results
The results show that it is not necessary that the cumulates would sink through the underlying mantle. Even if they do, the Fe-Ti cumulates may undergo melting at the pressures and temperatures that were prevalent in the early lunar mantle. These would produce partial melts of various TiO2 contents, depending on the dissolution of different phases, degree of melting and extent of interaction with the lunar mantle. These different melts have different densities, which may or may not be negatively buoyant with respect to the ambient lunar mantle. Thus, these melts may be stranded inhomogeneously at various depths in the lunar interior. The assimilation of these melts with lunar basaltic melts may perhaps explain the TiO2 variation in lunar basalts.
3. References
[1] J. F. Rapp and D. S. Draper. Meteorit. Planet. Sci. 53 (2018) 1432.
[2] G. A. Snyder, L. A. Taylor and C. R. Neal. Geochim. Cosmochim. Acta 56 (1992) 3809.
[3] B. Charlier, T. L. Grove, O. Namur and F. Holtz. Geochim. Cosmochim. Acta 234 (2018) 50.
[4] S. M. Brown and T. L. Grove. Geochim. Cosmochim. Acta 171 (2015) 201.
[5] P. C. Hess and E. M. Parmentier. Earth Planet. Sci. Lett. 134 (1995) 501.
[6] S. Zhong, E. M. Parmentier and M. T. Zuber. Earth Planet. Sci. Lett. 177 (2000) 131.
