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
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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.
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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
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