Constraining fluid flow events using in situ measurements of Li isotopes in garnet
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.
We have begun focusing our efforts on understanding the behavior of Li in garnet in order to use in situ measurements of Li isotopes in garnet to constrain the duration of metamorphic fluid flow events and to decipher fluid chemistry.
The data published in our 2020 paper demonstrates the feasibility of making these measurements and shows that there are indeed significant variations in Li isotopic composition within garnets in eclogite and amphibolite from the Franciscan Complex. The observations indicate a role for fluids fluxing through the slab to create the observed variability, and the isotopic patterns observed suggest a role for intracrystalline diffusion. The data suggest that some aspect of the chemistry of the subduction-related fluids may have controlled the diffusion; likely candidates are either pH or Eh of the fluid.
SIMS instrument at Universite de Lausanne.
Mg X-ray map of garnet showing measured d7Li values.
