The non-igneous genesis of angrites: support from trace element distribution between phases in D´Orbigny.

MARIA EUGENIA VARELA; KURAT GERO,; ZINNER ERNST,; HOPPE PETER,; NTAFLOS T,; NAZAROV M,

METEORITICS & PLANETARY SCIENCE

METEORITICAL SOC

Año: 2005 vol. 40 p. 409 - 430

1086-9379

D?Orbigny is an exceptional angrite. Chemically it resembles other angrites like Asuka 881371, Sahara 99555, LEW 87051 and LEW 86010, but its structure and texture are peculiar. It is made of  a dense and a porous lithology and has abundant glasses and augite-bearing druses and chemical and mineralogical properties, which are highly unusual for igneous rocks. Our previous studies did conduct us to a new view angrites: possibly, they can be seen as CAIs, which grew larger than the ones we know from carbonaceous chondrites. Thus, angrites may bear a record of rare and special conditions in some part of the early solar nebula. Here we report about the trace element contents of D?Orbigny phases. In this study trace element data were obtained from both, the porous and the compact part of this meteorite. We have confront our results with the popular igneous genetic model considering that, if all phases of D?Orbigny had crystallized from the same system, as the igneous model implies, this should be visible in the distribution of trace elements among early and late phases. Our results show that the trace element distribution of the two contemporaneous phases (e.g., olivines and plagioclase that form the back bone of the rock) seem to required liquids of different composition. Abundances of highly incompatible elements in all olivines, including the megacrysts, indicate non-equilibrium with the bulk rock and suggest liquids very rich in these elements (> 10,000 x CI), much richer than any fractional crystallization could possibly produce. In addition, trace element contents of late phases evidence that they cannot have formed from the bulk system?s residual melt. These results add extra severe constraints to the many conflicts reported previously between an igneous model for the origin of angrites and the mineralogical and chemical observations. These new database of trace element contents supports our previous findings and give strength to the new genetic model that considers that D? Orbigny (and possibly all angrites) could have been formed in the solar nebula under changing redox conditions more akin to chondritic constituents (e.g., CAIs) than to planetary differentiated rock.