Rayleigh fractionation of heavy rare earths and yttrium during metamorphic garnet growth

OTAMENDI, J.E., DE LA ROSA, J.D., PATIÑO DOUCE, A.E., AND CASTRO, A.

GEOLOGY

GEOLOGICAL SOC AMER, INC

Año: 2002 vol. 30 p. 159 - 162

0091-7613

We present laser-ablation microprobe inductively-coupled mass spectrometry analyses of Y, Yb, Er, Dy, and Gd in garnet crystals from high-grade metamorphic and migmatitic rocks from the Sierras Pampeanas of Argentina. These elements are abundant in large porphyroblasts ( ³4 mm), and  are zoned; rim concentrations are typically one order of magnitude lower that those in the cores. A notable feature of these concentration profiles is the inversion of Yb/Er and Yb/Dy ratios, from >1 in the cores to <1 in the rims. We show quantitatively that the spatial distribution of these trace elements in garnet can be most simply and effectively explained as arising from Rayleigh fraction during garnet growth. We also analyzed a small garnet crystal (<2 mm) representative of a garnet population that is associated to migmatitic leucosomes. This crystal displays uniformly low concentrations of all trace elements that are virtually identical in both absolute and relative magnitudes to those in the rims of the larger garnet porphyroblasts. The small crystals, and perhaps part of the porphyroblast rims, are likely to be peritectic products of incongruent melting reactions. We argue that melts formed from garnet-bearing sources may generally be unable to equilibrate with metamorphic garnet cores and may thus be more strongly depleted in heavy rare earths and Y than modeling based on bulk source-rock abundances of these elements would indicate.