Diffusion Chronometry on Lunar Troctolite 76535 to Resolve Magmatic Cooling History

Any description of lunar evolution requires explanation of the chemically paradoxical magnesian suite (Mg-suite) of lunar highlands rocks, which is characterized by chemical characteristics consistent with extreme differentiation alongside primitive mineral compositions. A better understanding of the magmatic cooling history of these samples is an essential step toward evaluating existing petrogenetic models [1]. An ideal candidate for evaluating magmatic cooling history is the well-studied, highly pristine Apollo 17 troctolite 76535. Subtle compositional zoning in olivine and plagioclase crystals can be leveraged for geospeedometry, in which quantitative line scans alongside x-ray intensity maps are utilized to obtain diffusion timescales. These diffusion timescales are combined with constraints provided by radiometric data [2], in order to obtain possible thermal histories. We tested temperature-time profiles corresponding to continuous, differentiable, and monotonous functions. These functions are not actual cooling paths, but rather end-member cooling styles that provide constraints on overall cooling times. We find the early cooling of this sample was rapid, taking at most 9.7 million years to cool from 1293 to 850 ̊C, which is well within error of the current Sm-Nd age of this sample. Not only was the cooling rapid, but evidence of dissolution and regrowth of olivine suggests at least one excursion out of the olivine stability field. An initially rapid, possibly multi-stage magmatic cooling history for the Mg-suite would be consistent with impact melting or shallow emplacement of a small intrusion.

[1] Shearer C, Elardo S, Petro N, Borg L, McCubbin F, (2015), American Mineralogist, V100, PP204-32

[2] Borg L, Connelly J, Cassata W, Gaffney A, Bizzarro M, (2016), Geochimica et Cosmochimica Acta, V 201, pp 377-391