Tracing Fluids from Seafloor to Deep Subduction Environments: An In-situ Geochemical Investigation of Fluid-Mobile Elements in Oceanic Crust

Williams, Morgan John

Tracing Fluids from Seafloor to Deep Subduction Environments: An In-situ Geochemical Investigation of Fluid-Mobile Elements in Oceanic Crust

Date

2019

Authors

Williams, Morgan John

Abstract

Hydration reactions within the oceanic crust incorporate significant amounts of water and incompatible and volatile elements from the Earth's surface into the oceanic lithosphere. During subduction, the altered oceanic crust releases a sequence of metamorphic dehydration fluids, which carry with them a significant portion of the fluid-mobile element inventory. Together, fluid cycling and plate tectonics enable geochemical interaction between the outer regions of our planet and its inner depths. However, aqueous fluids are transient and cannot be observed directly in deeper geological environments. Instead, the nature of fluids must be reconstructed from mineral records. This thesis focuses on the use of in-situ and small-scale geochemical analysis of different mineral generations and intra-mineral zonation to resolve the nature and evolution of fluids in oceanic and subduction environments. Noble gas, halogen and SHRIMP oxygen isotope analyses are supplemented with routine analytical methods including SEM, EPMA, and LA-ICP-MS. Boron isotope analysis with SHRIMP II is also tested and developed, allowing precise investigation of natural suites of metamorphic tourmaline. The first part of the thesis documents a geochemical investigation of hydrated lithologies from the upper detachment surface of the Atlantis Massif using samples collected during IODP Expedition 357. In-situ analyses of lizardite-chrysotile serpentinites reveal that multi-stage serpentinization is linked to shifts in δ18O, and that significant isotopic and trace element heterogeneity between polymorphs and serpentinization stages is present at the microscale. Individual SHRIMP analyses have δ18O ranging from 0 to 6‰, and serpentine generations within individual samples have mean δ18O of -0.2 to +4.4‰. Ranges in oxygen isotope composition are similar between sites, and give consistently high estimates for serpentinization temperatures during all stages of hydration (up to 350°C). Halogen and noble gas abundances of serpentinites, hydrated gabbro and talc-amphibole-chlorite schists from the Massif are lower than previously investigated lizardite-chrysotile serpentinites. Serpentinites contain 28-430 μg/g of Cl, and exhibit 40Ar/36Ar up to 538. Amphibole exerts a strong control on halogen abundances in talc schists. Noble gases are fractionated from seawater abundance patterns, and record a variety of processes including radiogenic ingrowth (He), addition of excess Ar and fractionation related to mineral trapping at different alteration temperature. Oxygen isotope ratios and trace element, halogen and noble gas abundances attest to generally high fluid fluxes and dominant high temperature of serpentinization at the Atlantis Massif. The second part of the thesis investigates metamorphic fluid-rock interaction within a section of subducted upper oceanic crust in the UHP Lago di Cignana Unit and underlying Zermatt-Saas serpentinites near Valtournenche (NW Italian Alps). This study reveals previously undocumented trends and shifts in garnet δ18O, serpentine δ18O and tourmaline 11B. Evidence for multiple stages of metasomatism near peak metamorphic conditions is preserved within a select few samples where outer garnet growth zones exhibit major element, trace element and isotopic shifts (up to 15‰ zonation in δ18O) which require infiltration of externally derived fluids. Multiple stages of fluid infiltration are identified. In two samples fluid derived from serpentinized ultramafic lithologies can be confidently identified as a metasomatic agent, likely preceded by fluids derived from nearby mafic units. Unit-scale tourmaline δ11B heterogeneity is observed, with differences of up to 10‰ observed over the cm to dm-scale; variations of similar magnitude are observed within individual samples. Individual tourmalines exhibit zonation in δ11B to slightly higher values (minor) and lower values (most pronounced in calcschists). Variations in sample-mean tourmaline δ11B may largely reflect protolithic δ11B diversity, slightly modified through dehydration reactions towards lighter δ11B. Significant boron isotopic heterogeneity is preserved near the slab interface even at sub-arc conditions. Together these investigations provide novel insights into seafloor hydration and subduction dehydration of the altered oceanic crust, and demonstrate the utility of microscale oxygen isotope analyses coupled with complementary geochemical tracers to provide detailed constraints on hydrothermal alteration conditions and fluid sources in seafloor and subduction settings.

Keywords

geochemistry, isotopes, oxygen, boron, δ18O, δ11B, halogens, noble gases, SHRIMP, in-situ, fluid-rock interaction, metamorphism, metasomatism, hydration, dehydration, subduction, seafloor, serpentinization, serpentine, tourmaline, garnet, Lago di Cignana, Zermatt-Saas, Atlantis Massif, IODP, geochemical cycling