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Interpretation of the trace element evolution requires that we understand how each trace element of interest behaves, which is complicated by the fact that the partitioning behavior is dependent on a number of parameters, including pressure, temperature, oxygen fugacity, and composition. Our understanding of trace element partitioning is largely based upon experimental determinations conducted with a variety of goals and analytical constraints. This results in a database that is variable in compositional space, number of experiments, and experimental conditions. A number of recent investigations have uncovered evidence that the mantle contains ultradepleted regions consistent with a degree of melting past the point where clinopyroxene is exhausted. After that point, the trace element budget will be controlled by orthopyroxene since most mineral-melt partition coefficients for the elements typically used to interpret active processes for orthopyroxene are higher than that of olivine, plagioclase, or spinel. In this contribution, we identified a number of dataset characteristics from existing orthopyroxene/melt partitioning experiments that must be accommodated before we are able to continue to develop quantitative predictive models. Our results showed that the number of experimental determinations is different from element to element and the compositional dependence of the orthopyroxene/melt partition coefficients for the high field strength elements and rare earth elements is strongly correlated to calcium (Ca) content of the orthopyroxene. Further, the reported analytical uncertainty is significantly higher for experiments where the trace element determinations were done by laser ICP-MS compared to SIMS. Finally, the majority of previous experiments did not identify the specific low-Ca pyroxene present in the run products but rather used the Ca content alone to identify orthopyroxene versus pigeonite. Any regression analysis must see through these differences to be applicable to a wide range of composition. Failure to accommodate the uneven distribution of data and the uncertainty introduced by multiphase analyses will result in systematic bias in any predictive model to constrain the orthopyroxene/melt trace element partitioning.
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This content will become publicly available on July 13, 2027
Trace Element Partitioning Experiments in Orthopyroxene: Insights into Ultradepleted Mantle Signatures
Our ability to model igneous processes depends on knowledge of the phases present. In the case of the mantle, the presence or absence of clinopyroxene in the protolith will control the productivity of the mantle as well as the trace element signature. Increasing evidence suggests that the mantle contains regions characterized by extremely depleted trace element signatures (Ti/Zr<140), consistent with a degree of melting (>20%) past the point at which clinopyroxene is exhausted. Until clinopyroxene is consumed, its relatively high partition coefficients control the trace element budget for the rare earth elements (REE) and high field strength elements (HFSE) at pressures below garnet stability. After clinopyroxene exhaustion, the geochemical evolution of subsequent melts is dependent on the trace element partitioning between orthopyroxene and melt. Therefore, understanding the degree of depletion of incompatible elements during melting of the mantle past clinopyroxene exhaustion requires accurate partition coefficients for the REE and HFSE between orthopyroxene and silicate melt for (opx-meltDREE, HFSE). However, the current experimental database for orthopyroxene/melt partitioning is compromised by inconsistencies in the number and distribution of data in temperature, pressure, and composition. We report on a series of phase equilibria and trace element partitioning experiments that aim to further our understanding of the liquidus phase relations at upper mantle conditions and to constrain the opx-meltDREE, HFSE. The starting material is a powder composed of 65% of basaltic glass from Gorda Ridge, 25% En89 Kenya enstatite, and 10% Fo90 Kilbourne Hole olivine. Experiments were conducted in a piston-cylinder press at 5, 10, 15, and 20 kbar and various temperatures for 24 hours to obtain high degrees of partial melting. Major and trace element analyses were performed on the run products to characterize their texture, mineralogical assemblage, and calculate the opx-meltDREE, HFSE. Experiments at 10 and 15 kbars produced olivine and orthopyroxene ± clinopyroxene crystals larger than 50 m which document the behavior of trace elements in low-Ca pyroxenes as a function of pressure, temperature, and degree of partial melting. Our investigation identifies the specific low-Ca pyroxene species in these experiments and discusses the improvement of analytical procedures to minimize analytical error.
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- Award ID(s):
- 2148990
- PAR ID:
- 10700983
- Publisher / Repository:
- Goldschmidt Conference
- Date Published:
- Format(s):
- Medium: X
- Location:
- Montreal, Quebec, Canada
- Sponsoring Org:
- National Science Foundation
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