Li isotope zoning in garnet from Franciscan eclogite and amphibolite: The role of subduction-related fluids. (1st October 2020)

• Record Type: Journal Article
• Authors: Penniston-Dorland, Sarah C.; Baumgartner, Lukas P.; Dragovic, Besim; Bouvier, Anne-Sophie
• Is Part Of: Geochimica et cosmochimica acta. Volume 286(2020)
• Page Start: 198
• Page End: 213
• Publication Date: 2020-10-01
• Subjects: Subduction -- Lithium isotopes -- Garnet -- Metamorphism -- Fluid-rock interaction -- Diffusion; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.07.025
• Languages: English
• ISSNs: 0016-7037

Abstract

Highlights: In situ SIMS analysis of metamorphic garnets reveals variations in δ 7 Li up to 19‰. Variations in δ 7 Li occur over a distance of a few 100 microns within single garnets. Spatial patterns are consistent with fractionation of Li isotopes due to diffusion within garnets. Variations are associated with infiltration of a serpentinite-derived fluid during subduction. Abstract: Fluids released within subduction zones affect many fundamental Earth processes, but very little is understood about processes of fluid transport within subduction zones and the chemistry of the fluids. Centimeter-scale variations in bulk-rock Li isotope compositions in subduction-related metamorphic rocks are observed associated with fluid-related features. The small spatial scales of these variations suggest very short duration fluid infiltration events – on the order of months to centuries. However, bulk-rock measurements record a time-integrated record, while in situ measurements in metamorphic porphyroblasts such as garnet have the potential to record individual events experienced by the rock. In this study, traverses across garnet in Franciscan eclogite and amphibolite were analyzed for Li isotopes in situ using SIMS to determine the potential for inferring the duration of metamorphic fluid flow and for deciphering fluid chemistry. The Li concentrations of these garnet crystals are very low, ∼5 µg/g. The measured range of δ 7 Li within some Franciscan garnets is 19‰, which falls well outside the uncertainty of the measurement (<∼4‰ 2σ). Observed variations in δ 7 Li occur within crystals over a scale of a few hundred microns, with crystal core-to-rim traverses exhibiting troughs of very low δ 7 Li measurements in the mantle region of the garnet crystals, surrounded by higher values in the garnet crystal rims and cores. Garnet crystal rims have lower δ 18 O values, which have previously been interpreted as a product of the infiltration of serpentinite-derived fluids. These results suggest a role for fluids fluxing through the slab to create the observed variability, and the unusual trough pattern suggests a role for intracrystalline diffusion. The low δ 7 Li troughs are observed only in garnet from one of two Franciscan localities. Nevertheless, garnets are similar in composition from the two localities. This contrast suggests that some aspect of the chemistry of the fluids may be responsible for the differences between the two localities; likely candidates are either pH or Eh of the fluid.