Sr-Nd-Hf-Pb Isotopic Composition of Diamond-Grade MORB
Abstract
Accurate models describing chemical geodynamics of the subduction cycle depend critically upon the choice of end-members used in calculations. This is especially true in the metamorphic realm of a subduction zone, where the broad spectrum of seafloor alteration processes occurring prior to high-pressure metamorphism can be difficult to discern from the fractionation and recycling processes occurring along the subduction-zone prograde path. Furthermore, metamorphic rocks that record the prograde path are often overprinted during exhumation. These metamorphic variables result in uncertainties that significantly impact our ability to describe arc volcanism, recycling during subduction, and mantle evolution. Ultrahigh-pressure metamorphic suites offer the best opportunity to extract these records of interest. The intensely-studied metabasaltic eclogites from Lago di Cignana (western Italian Alps) have served as a target for numerous petrologic, geochemical, geochronological, and geodynamic investigations of the subduction cycle. Preservation of coesite- and diamond-bearing assemblages at Lago di Cignana is especially important, indicating that eclogites exposed at Lago di Cignana were subducted deeper than the metamorphic processes promoting arc volcanism, that there is little retrograde interference of this record, and that geochemical compositions recorded here are appropriate for use as post-subduction eclogites recycled into the mantle. We have examined the isotopic compositions of Sr (TIMS, UNC-Chapel Hill) and Nd-Hf-Pb (MCICPMS, Washington State University) of six exquisitely preserved metabasaltic eclogites from Lago di Cignana. These samples encompass the (relatively small) range of metamorphic parageneses from the UHP metamorphic stage. The overall range in isotopic values for all samples is small and indicate a depleted MORB protolith that enjoyed only minor pre-subduction seafloor alteration. Our values are: 87Sr/86Sr = 0.7037 to 0.7038, ɛNd = +7.35 to +8.10; ɛHf = +12.1 to +12.8; 206Pb/204Pb = 18.388 to 18.832; 207Pb/204Pb = 15.529 to 15.604; 208Pb/204Pb = 37.950 to 38.345. These data support the use of the Lago di Cignana eclogites as an excellent proxy for investigation of ultrahigh-pressure metamorphic processes occurring beneath volcanic arcs and for oceanic crustal recycling into the mantle.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V43C2862K
- Keywords:
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- 1030 GEOCHEMISTRY / Geochemical cycles;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 3613 MINERALOGY AND PETROLOGY / Subduction zone processes;
- 3654 MINERALOGY AND PETROLOGY / Ultra-high pressure metamorphism