Petrographic and electron microprobe study of the Monturaqui impactite

Cindery, glassy impactites from the Monturaqui meteorite crater contain unshocked and shocked mineral and rock fragments together with Ni-Fe-Co-P spherules tightly bound in a partly devitrified matrix of clear, green or brown glass. Shock effects range from lightly shocked (100 kb) to intensely shocked (650 kb).Electron microprobe analyses of the metal spherules show that composition is related to grain size, the smallest (2–10 ) spherules being highest in Ni, the largest spherules (0.4 to 1.5 mm) being lowest in Ni. The lowest Ni content of analyzed spherules is 7.8 wt %, which suggests that the original meteorite may have been an octahedrite. Analysis of the mixed matrix glasses indicates extreme compositional differences compared to granite country rock.A series of high-temperature heating experiments, using various heating and cooling rates and atmospheric conditions were conducted to simulate the effects observed in the impactites. Results suggest that (a) higher Ni content in smaller spherules resulted from residual concentration after preferential melting at temperatures greater than 1400° C and oxidation of Fe; and (b) mixed matrix glasses that are low in SiO₂ and high in FeO resulted from preferential melting of lower melting components of the granite at temperatures greater than 1300° C under rapid heating and cooling conditions, together with contamination by Fe, Ni and Co from the meteorite. In addition, Fe enrichment in glasses that surround the larger spherules depends essentially on the amount of oxidation prior to and during incorporation of the spherules into the glass matrix.