Mineral growth and reactions in some eclogites: an in-situ trace element study using Laser Ablation ICP-MS

N.J. Pearson1, S.Y. O'Reilly1 and W.L. Griffin1,2

1. GEMOC, Macquarie, 2. CSIRO EM

Studies of mineral zoning in high-grade metamorphic rocks provide important constraints on the nature and time scales of thermal events and enable the determination of P-T histories and diffusion rates. Numerous studies have used major element zonation in garnet to construct P-T histories for individual grains. Unfortunately, major element diffusion is relatively rapid in terms of the duration of tectonic processes. Thus, the interpretation of major element zonation patterns is often ambiguous because of the dependence of diffusion rates and major element distribution on the numerous intensive parameters that control metamorphism.

Trace element zoning has the potential to become an extremely powerful method for investigating mineral growth and reaction mechanisms and in interpreting the early history of tectonic processes commonly overprinted by later thermal events. Of particular interest is the distribution of Nd-Sm, Lu-Hf and U-Pb, as isotopes of these elements are used to date individual garnets, thereby constraining the rates and timing of events, such as heating, cooling, burial and uplift.

Laser ablation ICP-MS is rapidly developing as a powerful microbeam technique that can produce high precision determinations of trace elements at sub-ppm detection limits. The laser microprobe at Macquarie University incorporates a Q-switched, frequency quadrupled Nd:YAG laser. Under typical operating conditions of 4Hz and 1-3 mJ per pulse, the laser would produce a spot diameter of 50 µm or less and drill rates of 0.5-1 µm per sec. An analysis of 30 trace elements routinely takes approximately 5 minutes, making this both a rapid and cost effective method.

A trace element study has been initiated as part of on-going investigations aimed at understanding the stability of eclogites beneath cratons and craton margins. The sub-cratonic lower crust is dominated by eclogite and mafic granulite rock types, as evidenced by the xenolith population in kimberlites. Reaction microstructures and major element compositional zoning preserved in many of the granulite xenoliths record the transition from granulite to eclogite facies P-T conditions. The trace element data provide new information to interpret reaction mechanisms and progress.

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