Distribution and redistribution of Mo and Tl in high pressure-low temperature metamorphic rocks
Abstract
Non-traditional stable isotope systems such as molybdenum (Mo) and thallium (Tl) are useful for tracking mass exchange between the surface and deep interior of the Earth. Both systems can experience significant measurable isotope fractionation in the oceans that produces distinct Earth surface signatures. These signatures are captured in seafloor sediments and during alteration of oceanic crust, both lithologies that ultimately are subducted. The extent to which these signatures are transferred to arc magmas or entrained into the deeper mantle, and the extent to which isotope compositions are modified by subduction-zone metamorphism, are topics of debate. Clarifying these issues requires understanding of the mineral phases governing the budgets of these elements during metamorphism.
Mineral hosts of Mo and Tl were explored in a study of the Schistes Lustres and related UHP rocks at Lago di Cignana investigated by Cook-Kollars et al. (2014; Chem. Geol.). This suite of blueschist to eclogite facies metapelites and metacarbonates exhibits increase in metamorphic grade from SW to NE providing a natural laboratory for study of the distribution and mobility of Mo and Tl during subduction-zone metamorphism. The behavior of Mo and Tl in this suite can be evaluated in the context of detailed petrologic work and an extensive major and trace element and C-O-N isotope dataset for the same rocks. Preliminary results from LA-ICP-MS mineral analyses indicate that the Mo budget is controlled by several phases at different metamorphic grades. In higher-grade rocks, elevated levels of Mo were detected in minerals with low modal abundances including rutile, titanite, and sulfides, with lower but non-trivial amounts of Mo detected in the more modally abundant garnet and carbonate. At lower grades, Mo was detected primarily in sulfides, Fe-oxides, and carbonates. Many of the samples in this suite, across grade, contain graphite and the potential role of this phase in the Mo budget will be explored with future analyses. For Tl, the principle carrier at all grades is unsurprisingly phengite. Oxides and chlorite were also found to contain Tl but at levels one to three orders of magnitude lower than that for phengite. Phengite in higher-grade samples contains slightly more Tl perhaps reflecting the capture of Tl released by chlorite breakdown.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43E0142G
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY;
- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY