White micas in subducted crust: their role in the subduction zone nitrogen cycle and as a tracer of fluid processes

Abstract

Subduction is the primary mechanism for transferring volatiles from the Earth’s surface to the mantle. White micas (muscovite, phengite, paragonite) can be significant mineral hosts for a number of volatiles, including water, nitrogen, boron and halogens, in a range of subducted crustal lithologies. This thesis explores the role of white micas in transporting nitrogen and other fluid-mobile elements in subducted crust, and, more generally, their utility as a recorder of fluid-rock interaction, through the study of formerly subducted and subsequently exhumed metamorphic rocks.

I present the first in situ measurements of N concentrations in minerals from subduction-related rocks, using secondary ion mass spectrometry. Mineral N concentrations in a suite of rocks representing formerly subducted oceanic crust confirm that white micas contain the highest N concentrations of any common high pressure mineral. However, when the concentrations and modal abundances of each mineral are accounted for, omphacite, amphiboles and chlorite also host a significant fraction of the N budget in some samples. The role of paragonite as a host for N, and other volatile elements, may be underappreciated compared to phengite, which is the more common white mica phase. Comparison of bulk rock N concentrations with mineral N concentrations reveals that non-mineral hosts for N such as fluid inclusions can also be significant.

By comparing the behaviour of N to other fluid mobile elements in samples that record high pressure fluid-rock interaction, I am able to place constraints on the fluid-mica partition coefficient of N at high pressure. These are the first natural constraints on this parameter and they suggest that the assumption made by some previous studies, that N behaves similarly to Rb, is incorrect, and N is in fact more fluid mobile. The stability of micas during fluid-rock interaction has a major effect on the retention or loss of N, B and other mica-hosted trace elements during subduction. I also present the first inter-mineral partition coefficients amongst white micas, omphacite, Na-Ca amphiboles and chlorite, showing that N partitions strongly into white micas over a range of subduction-related pressure-temperature conditions.

Through several case studies of rocks that record high pressure fluid-rock interaction, I demonstrate that the analysis of in situ N concentrations, B concentrations and B isotopes, and other fluid-mobile element concentrations in white micas can be an effective tracer of fluid sources in high pressure rocks, particularly for sediment-derived fluids. These tracers are then applied to a suite of formerly subducted oceanic crustal rocks and show that fluid-mediated N transfer from sediments into oceanic crust during subduction appears to be a relatively common process, but some samples can also retain N that was acquired during seafloor alteration. Different styles of seafloor alteration may increase or decrease the ability of oceanic crust to retain N during subduction, depending on their effect on the high pressure mineralogy.