MSA DANA MEDAL LECTURE: NEW FRONTIERS IN METAMORPHIC PETROLOGY AND PALEOECOLOGY

The Dana medal is named for James Dwight Dana and his son Edward Salisbury Dana. The elder Dana is celebrated for his Manual of Mineralogy, which has remained in print (in revised form) for over 70 years. However, Dana’s interests spanned not only mineralogy, volcanology and tectonics, he was also a foremost expert in the zoology of Crustacea and "zoophytes." In the spirit of Dana’s profoundly diverse research interests, this talk will present results of recent work in disparate fields within both hard-rock and soft-rock geology.

Metamorphism: Crystal nucleation theory demands reaction overstepping and disequilibrium growth of metamorphic minerals, especially garnet, but the affected crystal volume remains unknown. Overstepping implies that chemical zoning in garnets may not reflect equilibrium pressure-temperature conditions, potentially compromising efforts to recover metamorphic P-T paths from chemically zoned garnets. New quartz-in-garnet elastic barometry from rocks whose P-T paths have been calculated using garnet chemical zoning suggest that overstepping can be minimal, such that robust P-T paths can be recovered from chemical zoning alone.

Paleoecology: Trace elements of tooth enamel have been proposed as a means to identify positions in food chains (trophic levels) of fossil hominins. Combined with complementary research in stable isotopes, these data potentially improve dietary reconstruction and allow testing of evolutionary and niche partitioning models. Calibration of trophic level partitioning shows that many elements do biopurify or biomagnify between herbivores and carnivores, but primary zoning in tooth enamel and potential post-burial alteration can complicate interpretations. New 20 °C experimental data indicate that trace elements diffuse ~2 orders of magnitude more slowly in enamel than in bone, as predicted theoretically. These rates suggest that in cool climates tooth enamel should retain original biogenic concentrations of trace elements at timescales up to ~100 ka, but will be altered at timescales exceeding ~1 Ma.