Cooling history of lunar Mg-suite gabbronorite 76255, troctolite 76535 and Stillwater pyroxenite SC-936 : the record in exsolution and ordering in pyroxenes

We have determined cooling rates of orthopyroxene crystals from two Mg-suite lunar samples (gabbronorite 76255 and troctolite 76535) and one terrestrial sample (orthopyroxenite SC-936 from the Stillwater Complex), on the basis of their Fe-Mg ordering states. In addition, a cooling rate of 76255 was determined by modeling the formation of exsolution lamellae in pyroxenes. The M1-M2 site occupancies of the orthopyroxene crystals were determined by single crystal X-ray diffraction and the rate constant for the ordering reaction was used along with calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature to calculate cooling rates. The closure temperatures (TC) of cation ordering are ∼525 °C for 76255, ∼500 °C for 76535 and ∼350 °C for SC-936 corresponding to cooling rates of ∼4 × 10-2 °C/year at the closure temperature for the lunar samples and ∼10-6 °C/year for the Stillwater sample. A cooling rate for 76255, determined by simulating the exsolution process, is 1.7 × 10-2 °C/year at a closure temperature for exsolution of 700 °C. The Fe-Mg ordering cooling rate determined for 76535 reflects a complex thermal history superimposed on the initial plutonic provenance established for this sample [McCallum, I.S., Schwartz, J.M., 2001. Lunar Mg suite: thermobarometry and petrogenesis of parental magmas. J. Geophys. Res. 106, 27969-27983]. The preservation of a crystallization age of 4.51 Ga and a metamorphic age of 4.25 Ga for 76535 is consistent with a model in which excavation of this sample from the lower lunar crust took place while the sample was at a temperature above the closure temperatures for the Sm-Nd, U-Pb and Ar-Ar isotopic systems. Temperatures in excess of the isotopic closure temperatures (i.e., >600 °C) in the lower lunar crust were maintained by heat diffusing from concentrations of U- and Th-rich KREEP material at the base of the crust. On the other hand, 76255 formed at a much shallower depth in the lunar crust (∼2 km) and was well below its isotopic closure temperatures at the time of excavation, most likely during the Serenitatis basin-forming impact event. Both lunar samples were reheated during transport to the surface and deposition in hot ejecta blankets. The reheating was short lived but apparently sufficient to redistribute Fe and Mg in M sites in orthopyroxenes. For the lunar samples, the cooling rates based on Fe-Mg ordering represent final stage cooling within an ejecta blanket.