Elemental Diffusion in Eclogite Garnets from the North Qaidam (U)HP Metamorphic Belt, China, and Its Implications for Thermobarometry

April 14th, 2012

The northeastern region of the Tibetan plateau is composed of multiple terranes and tectonically altered zones, including the North Qaidam (ultra)high-pressure metamorphic belt (NQMB).  The NQMB is roughly 15 km wide, extends 350 km NW-SE, and contains well-exposed continental collision-type (ultra)high-pressure metamorphic rocks. Eclogites in the NQMB occur as blocks, boudins or layers within the host orthogneiss. The eclogites hold pressure indicator minerals in fluid-reduced conditions and slow reaction rates making them ideal rocks for pressure-temperature (PT) studies. Thermobarometry on eclogites and (U)HP gneisses has been used previously to decipher PT histories along the NQMB; however, many of the eclogites do not contain an ideal mineral assemblage for conventional thermobarometry. Eclogite sample E9814B1 contains such a mineral assemblage of only grt+cpx+bt+qtz, but the size of the garnets (3-5 mm diameter), the lack of inclusions in garnet, and the overall lack of retrograde metamorphism make it ideal for detailed element diffusion studies. We analyzed major and trace elements in sample E9814B1 garnets by electron microprobe and laser-ablation inductively coupled plasma mass spectrometry. Major and trace element patterns reveal two zones of retrograde equilibration “rims”, implying two stages of garnet growth. Major elements, (Ca, Mg, Mn, Fe), LREEs and MREEs show bell-shaped distribution profiles indicating element distribution through Rayleigh fractionation, whereas HREEs and Y exhibit M-shaped profiles, likely designating diffusion-limited distribution (Skora et al., 2006). We show here how major element zoning and diffusion rates allow for calculation of PT conditions of the sample, while trace element zoning patterns show the source rock and environment.

Chelsea Nissen, ’12
Horseshoe Bay, TX
Major: Geology

Sponsor: Emily Walsh

 

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