EXPERIMENTAL INSIGHTS ON TABULAR OLIVINE GROWTH DURING CONTACT METAMORPHISM OF DOLOMITE

In the Ubehebe Peak dolomitic contact aureole, a mapped olivine shape ‘isograd’ corresponds to a change from an equant habit near the intrusion to a tabular habit farther away (Roselle et al., 1997); similar observations have been made in ultramafic contact aureoles (LaFay et al., 2019). This change almost certainly corresponds to spatiotemporal variation in reaction kinetics, but the details remain to be clarified.

Many tabular Ubehebe forsterites are twinned; {011} is the most common twin law, but {021} and {012} twin laws are also present. All forsterite is mantled by a corona of calcite. Both tabular and equant morphologies have parallel growth textures.

Electron backscatter diffraction shows that for rocks with either tabular or equant forsterite there is no preferred orientation in forsterite, coronal calcite, or dolomite. However, matrix calcite have a moderate preferred c-axis orientation that we interpret as having pre-dated contact metamorphism. Grain size analysis reveals that coronal calcite crystals are bigger than matrix calcite crystals. Similarly, dolomite crystals in contact with coronal calcite are systematically larger than dolomite crystals not touching coronal calcite. The general sequence of mean crystal size is: coronal calcite > forsterite > dolomite touching coronal calcite > dolomite not touching coronal calcite > matrix calcite.

Preliminary isobaric cold seal pressure vessel experiments at 1.7 kbar and peak temperature of 650 ℃ suggests that tabular forsterite nucleates during heating. The crystals have well-developed (100) faces and lack skeletal growth features. In addition to the tabular morphology, experimental forsterite display all of the twin laws observed in the Ubehebe, as well as {031} trillings, which consist of three individuals 59.3° from one another. The relative crystal size distributions seen in Ubehebe samples is also reproduced in the experiments.

1. Lafay, R., Baumgartner, L. P., Putlitz, B., & Siron, G. (2019). Oxygen isotope disequilibrium during serpentinite dehydration. Terra Nova, 31(2), 94-101.
2. Roselle, G. T., Baumgartner, L. P., & Chapman, J. A. (1997). Nucleation-dominated crystallization of forsterite in the Ubehebe Peak contact aureole, California. Geology, 25(9), 823-826.