A dynamic melting model for the origin of Apollo 15 olivine-normative and quartz-normative mare basalts

Details A dynamic melting model for the origin of Apollo 15 olivine-normative and quartz-normative mare basalts A dynamic melting model for the origin of Apollo 15 olivine-normative and quartz-normative mare basalts

May 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993bvapcreptr....v&link_type=abstract

In its Basaltic Volcanism and Ancient Planetary Crusts 2 p (SEE N94-24363 06-91)

Other

Basalt, Dynamic Models, Melting, Olivine, Petrography, Petrology, Quartz, Trace Elements, Breccia, Fractionation, Liquidus, Metal Oxides, Pyroxenes

Scientific paper

Early studies of mare basalts from the Apollo 15 site established that two distinct groups are represented: the olivine-normative basalts (ONB) and the quartz-normative basalts (QNB). The ONB and QNB suites are distinguished petrographically by their phenocryst assemblages (the ONB's are olivine-phyric, the QNB's are generally pyroxene-phyric) and chemically by their major element compositions: the QNB's are higher in SiO2 and MgO/FeO, and lower in FeO and TiO2 than ONB's with similar MgO contents. Experimental data show that the QNB suite is derived from a more magnesian, olivine-normative parent magma, a conclusion which is supported by the recent discovery of high-SiO2 olivine-normative basalt clasts in breccia 15498. The high-SiO2 ONB's fall on olivine control lines with primitive QNB's, and least-squares mixing calculations are consistent with the high-SiO2 ONB's being parental to the more evolved QNB suite. These high-SiO2 ONB's are included as part of the 'QNB suite'. Our major element modeling results also are consistent with the conclusions of earlier studies which showed that the ONB and QNB suites cannot be related to one another by low pressure crystal fractionation. The combination of high Mg#, high SiO2, and low TiO2 in the QNB suite precludes a relationship to the ONB suite by simple removal of liquidus minerals (olivine and pigeonite). Despite these significant differences in petrography and major element composition, both groups have nearly identical trace element concentrations and chondrite-normalized abundance patterns. The major question to be addressed by any petrogenetic model for Apollo 15 mare basalts is how to form mare basalt suites with distinctly different major element characteristics but nearly identical trace element compositions. The similarity in trace element concentrations imply compositionally similar source regions and similar percent melting, but these conclusions are not easily reconciled with the observed differences in major element compositions, which require sources with distinct mineralogies or large differences in percent melt.

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