Polymict Eucrites Research Articles

The polymict eucrites

The polymict eucrites are a suite of about 14 polymict achondritic meteorites that contain at least 90% (vol) of eucritic material and are found both in Antarctica and elsewhere. They are part of a continuum of polymict breccias that includes howardites and polymict diogenites. Rare gas data suggest that some are gas‐rich regolith breccias. The clasts contained in polymict eucrites sample a variety of lithologies from their parent body and include eucritic clasts, alkali‐enriched eucrites, pyroxene‐cumulate eucrites, two‐pyroxene mafic rocks, possibly feldspar‐cumulate eucrites, diogenites, fayalitic lithologies, and a variety of other mineral clasts. The eucritic clasts are probably samples of near surface lithologies on the parent body, but the presence of two‐pyroxene mafic rocks is considered to be evidence of magmatic activity at pressures in excess of 1 kbar in the parent body. Partial melt models may explain some of the diversity among the clasts, but cumulus processes are believed to have been important. If pressures greater than 1 kbar are confirmed, then the basaltic achondrite parent body (BAP) must have a diameter in excess of 200 km. The variety of lithologies suggests that not all basaltic achondrites are samples of igneous rocks formed by single‐stage melting of a chondritic asteroid but are instead samples of an evolved and diverse crust produced by serial magmatism.

Comparison of Yamato and Victoria Land polymict eucrites: A view from mineralogical and isotopic studies

New interior samples of four Yamato polymict eucrites (Y74159, Y74450, Y75011, and Y75015) have been studied by electron microprobe, transmission electron microscope, and X ray diffraction techniques, and compared with several samples of the Victoria Land polymict eucrites. These same samples have been analysed using Rb‐Sr and Nd‐Sm isotopic systematics. Several grains of inverted pigeonite, with blebby augite similar to those in Binda and Moama, have been identified in all four Yamato eucrites. Coarse‐grained mesostasis‐rich subophitic basalts, which contain Mg‐rich pigeonite (with Fe‐rich olivine veinlets) zoned outward to a subcalcic ferroaugite rims, have also been found. These unique clasts were not found in ALH76005, 77302, 78040, 78158, and 78165 and EET eucrites. The tight grouping of Rb/Sr and Sm/Nd ratios, and similar modal compositions of the Yamato group indicate that they are most likely to be pieces from a single fall, and distinct from the ALH and EET groups. However, the Yamato, Allan Hills, and Elephant Moraine groups may sample a few distinct magmas or similar but different source regions on the same parent body.

THE NOMENCLATURE OF POLYMICT BASALTIC ACHONDRITES

The subgroups within the basaltic achondrite suite are defined using the structural criterion of Wahl (1952). The ‘monomict’ meteorites are samples of a single lithology while the polymict meteorites are those containing two or more lithologies. The ‘monomict’ subgroups eucrites, cumulate eucrites and diogenites are subdivided into both brecciated and unbrecciated meteorites. The polymict achondrites sample a petrological‐compositional continuum that contains both mafic and ultramafic rock types and may be subdivided into several groups. Two groups of polymict basaltic achondrites, the polymict eucrites and howardites are separated using an arbitrarily defined criterion. The recommended criterion is based on the amount of magnesian ortho‐pyroxenite (diogenite) component in the meteorite. Howardites contain more than 10% and polymict eucrites contain less than 10%.The criteria proposed (perhaps with minor variations), appear to reconcile the ambiguities caused by the polymict eucrites. These meteorites, using earlier structural criteria, are howardites, but using mineralogical‐chemical criteria are eucrites. As a subgroup of the polymict achondrites, their relationship with the howardites is clear, and the preservation of the term ‘eucrite’ in their name highlights their modal affinity to the monomict eucrites.

PETROLOGY OF IGNEOUS LITHIC CLASTS FROM POLYMICT EUCRITES ALHA76005 AND ALHA77302

A total of seven lithic clasts from the polymict eucrites ALHA76005 and ALHA77302 have been studied petrographically and analyzed with the electron microprobe. All clasts are composed predominantly of pyroxene and plagioclase, ± ilmenite, troilite, Fe‐Ni metal, mesostasis, and silica. Pyroxene compositions in unequilibrated clasts and clast bulk compositions, calculated by modal recombination, indicate that the clasts originally crystallized under similar conditions and that they may be genetically related to each other by fractionation of pigeonite and plagioclase.

PETROLOGY OF EETA79006 AND IMPLICATIONS FOR THE FORMATION OF POLYMICT EUCRITES

A newly found polymict eucrite, EETA79006, is described. Lithic clasts are similar to those found in howardites and fall into four groups: fine‐grained (aphanitic), coarse‐grained, basaltic, and cataclastic. All have eucritic compositions and differ mainly in cooling and deformation histories. Some basaltic clasts cooled faster than others and may be impact melts. Analysis of pyroxene and feldspar in the matrix and in 20 lithic clasts indicates that the matrix was not derived from the observed lithic clast population. This meteorite and similar polymict eucrites may have formed by addition of younger more fractionated lithic clasts to the regolith of the parent body.

The Antarctic achondrite ALHA 76005: a polymict eucrite

ALHA 76005 is a basaltic achondrite containing few. if any, orthopyroxenes. Its bulk major and trace element composition is like that of a non-cumulate eucrite, and unlike that of a howardite. It contains a variety of igneous clasts which differ in their textures, pyroxene/plagioclase ratios and pyroxene and plagioclase compositions. One clast, No. 4, was found to have the REE pattern of a cumulate eucrite and an oxygen isotopic composition different from that of the bulk meteorite. Both the chemical and oxygen isotopic composition of clast No. 4 suggest that it was derived from a source different from its host. These observations lead to the conclusion that ALHA 76005 is a polymict eucrite.