Fluid-Rock Interactions from the Lithosphere to Earth’s Surface

Citation

Swindle, Carl Raymond (2024) Fluid-Rock Interactions from the Lithosphere to Earth’s Surface. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5mbh-jw64. https://resolver.caltech.edu/CaltechTHESIS:09282023-195100516

Abstract

Fluids can cycle and migrate through planetary bodies, transporting soluble ions and influencing physical properties of the surrounding rock or magma, such as fracture toughness, seismic wave velocity, melting point, viscosity, and more. Precipitated minerals, fluids trapped in inclusions, and free pore fluids can be used to constrain fluid provenance, mixing relationships, and paleoenvironmental information such as temperature, pressure, redox conditions, salinity, and pH. In my thesis, I discuss my research on topics pertaining to the geochemistry associated with fluid-rock interactions that occur from the depths of the lithospheric mantle to Earth’s surface. Broadly, these chapters address open questions pertaining to 1) the retention timescales and metasomatic overprinting of fluids sourced from the mantle in obducted peridotites, 2) the capacity for pedogenic Mg-carbonates to preserve palaeohydrological information with implications for Martian carbonates, and 3) the influences hydrous fluids have on lithospheric magmas and minerals.

Helium isotopes are arguably the best tracer for fluid sources in Earth materials at the planetary scale. ³He/⁴He ratios of the Earth’s 1) continental crust, 2) atmosphere, 3) upper mantle, and 4) core or deep isolated mantle (mantle plume source) vary by over two orders of magnitude, offering considerable dynamic range compared to measurement precision. While helium isotope signatures in Earth’s mantle have been determined almost exclusively by the analysis of helium retained in mantle xenoliths, phenocrysts, erupted glasses, and vent gases, this selection introduces a sampling bias towards fluids that have been transported to Earth’s surface by eruptive processes. In contrast, residual mantle peridotites take much longer to arrive at Earth’s surface and are therefore more susceptible to metasomatic processes that can overprint primary helium isotopic signatures. In Chapter 1, I use concentrations and isotopes of helium and argon along with concentrations of U and Th to place constraints on the sources and siting of helium retained in exhumed mantle peridotites collected from Twin Sisters Mountain of the Northern Cascades in Washington State, USA. Helium isotope ratios of peridotites from the Twin Sisters Mountain span from 0.8 to 6 times the atmospheric ratio (1RA=1.4*10⁻⁶ ³He/⁴He). Fluid inclusions in these peridotites capture a two-component mixture that included a mantle-like endmember (~6 RA) and a serpentinizing endmember (1.0 ± 0.5 RA) that is consistent with a mixture of surface-derived groundwater, leached crustal radiogenic helium and reworked mantle helium. While these components are not effectively isolated by extraction using vacuum crushing and powder fusion, step-heating analysis reveals that the serpentinizing endmember is released at lower temperatures (<1000°C) and the mantle-like endmember is released at higher temperatures. Results demonstrate that helium signatures can be retained in lithospheric peridotites against both diffusive loss and radiogenic ingrowth over at least 10⁸-year timescales but can be greatly modified by cryptic metasomatic processes during emplacement.

Mg-carbonates have become increasingly relevant in the scientific community due to their orbital and in situ detection on the Martian surface. Like Ca-carbonate on Earth, Martian Mg-carbonates may preserve paleoenvironmental information associated with their formation on Mars billions of years ago, shedding light on habitability. Yet, unlike Ca-carbonates, the capacity for Mg-carbonates to preserve paleoenvironmental information through trace element signatures associated with their source fluids has not been well established for surficial magnesite samples on Earth. In Chapter 2, I 1) develop a digestion protocol to selectively digest Mg-carbonates (magnesite ± dolomite) while obviating influences of contaminant phases and ions adsorbed to mineral surfaces, 2) validate a method to analyze trace elements with Mg-matrix by solution ICP-MS, and 3) apply these procedures to determine trace element concentrations of pedogenic Mg-carbonates sampled along a depth profile in the Kunwarara open pit magnesite mine in Queensland, Australia. Results from this study confirm that the method we implemented selectively digests magnesite ± dolomite. A relationship between negative Ce anomaly in the carbonates and Fe/Mn-oxides/hydroxides in corresponding host sediment collected along the depth profile demonstrates that pedogenic magnesites can capture redox gradients in the soil column. This finding implies that Ce anomaly in carbonates can potentially be used to place constraints on the paleo-redox conditions associated with Mg-carbonate formation on ancient Mars.

Numerous questions in Earth science depend on quantitative understanding of how elements fractionate during melting and crystallization. To name a few: assessment of how lithospheric fluids influence geodynamical processes, constraining mechanisms that led to the formation of the Earth’s continental crust, evaluation of elemental fluxes from the mantle to Earth's surface, calibration of a reliable crustal barometer, and gauging how magmatism and plate tectonics differed with the higher geothermal gradients of a younger Earth. MELTS thermodynamic software is a widely available free tool utilized by geoscientists to both test hypotheses and model the geochemistry of magmatic processes. However, minerals of the amphibole supergroup, although common in magmatic systems, rarely crystallize in MELTS simulations, even when well controlled experiments demonstrate that they should. The decrease in the Gibbs energy needed to stabilize amphibole in MELTS is often on the order of the configurational entropy contribution to the Gibbs energy associated with minor elements that are not present in any of the current amphibole solution models used in MELTS but are frequently incorporated in the amphibole crystal lattice. In Chapter 3, I outline a framework for a volume model for monoclinic amphiboles that can be used in an expanded amphibole solution model to be incorporated in MELTS software. A volume model is prerequisite to calibrating the other model terms because it accounts for differences in pressure among experimental constraints. The framework I develop extends the model to include minor components that are not present in existing versions of the MELTS amphibole models. I calibrate a preliminary model using a dataset composed of x-ray refinements that supply amphibole volume and site occupancy data. Results reveal regions in parameter space where data is limited and the sensitivity that model coefficients have to uncertainties in the data, suggesting that filtering the dataset to remove outliers may be necessary.

Thesis Files