Implications of near-rim compositional zoning in rutile for geothermometry, geospeedometry, and trace element equilibration
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In principle, compositional profiling of the near-rim region of minerals can provide insight into cooling rates, but presumes that loss or gain of material from the crystal rim is not kinetically restricted. Trace element depth profiles collected for Zr, Hf, Ta, Nb, and U in amphibolite-facies rutile grains of the Catalina Schist, southern California, show significant variability within a single rock: Profiles of the same element among different grains can have significantly different slopes, grains with indistinguishable Zr profiles show vastly different Nb profiles, and grains with indistinguishable Nb profiles show different Zr profiles. Textural and kinetic idiosyncrasies within the matrix apparently affect the ability of specific crystals to accept or release trace elements, and impugn the common assumption that mineral surfaces maintain equilibrium at amphibolite-facies conditions. A new model that limits the flux of Zr from rutile grains helps explain commonly observed compositional profiles, and implies that inversion of compositional profiles assuming equilibrium among grain surfaces will invariably overestimate cooling rates. Few grains may record the low closure temperatures that experimentally determined diffusivities imply. Rather, higher temperatures will be retained, depending on the proximity of reactants and products in the matrix. Silicon diffusion does not control Zr reequilibration in rutile, and relative diffusion coefficients (D’s) of trace elements in rutile are DZr ~ DHf ~ 10DNb ~ 20DTa ~ 40DU.
KeywordsDiffusion Geospeedometry LA-ICP-MS Kinetics HFSE Rutile
This research was funded by NSF Grants EAR-1419865 to MJK and EAR-1419871 to SPD, and by a Government of Brazil undergraduate fellowship to JCSF. We thank Frank Spear for discussions about diffusion modeling approaches, Thomas Zack for providing the R10 rutile standard, Phil Piccoli for EPMA analysis of the Rietfontein rutile, and Marion Lytle for help with LA-ICP-MS analysis, Othmar Müntener for editorial handling, and Mike Jollands and Horst Marschall for extensive and educational reviews that substantially improved the quality of this study. The Catalina Island Conservancy is acknowledged for logistics and support of sample collection of the Catalina Schist.
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