This thesis explores the volatile content of the mantle, subducted oceanic crust, and arc magmas as well as the structure of slow spreading ocean crust and the heterogeneity of Earth’s upper mantle. In Chapter 2, I directly explore the halogen (F and Cl) content of mantle minerals in situ, then use these measurements to assess the halogen content of the upper mantle. In Chapter
3, I investigate the volatile content of Raspas eclogites (SW Ecuador), a proxy for deeply subducted oceanic crust, to evaluate volatile transfer from crustal generation at divergent plate boundaries (e.g., mid-ocean ridges) to recycling of ocean crust at subduction zones. In Chapter 4, I use the H2O content of nominally anhydrous minerals in plutonic arc cumulates to elucidate
the H2O content of the melts from which the rocks crystallized. In this way, I assert that primitive arc magmas may contain 4–10 wt.% H2O and through fractional crystallization up to ~20 wt.% H2O, making them far more hydrous than traditional methods (i.e., olivine-hosted melt inclusions) surmise. In Chapter 5, I show that mantle peridotite exposed along the 16oN region of the Mid-Atlantic Ridge originated in an arc setting and has been remixed into subridge mantle, indicating that the sub-ridge mantle is more heterogeneous and depleted than inferences made from mid-ocean ridge basalts suggest. Chapter 6 surveys the life cycle of oceanic core complexes through zircon geochronology and posits an updated framework for understanding the termination of oceanic core complexes, and more broadly oceanic detachment faults. Together, this contribution highlights the chemical heterogeneity of the mantle, and quantifies the full extent of volatiles hosted by mantle and crustal reservoirs.
Abstract A multifaceted study of the slow spreading Mid‐Atlantic Ridge (MAR) at 16.5°N provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high‐resolution mapping using AUV Sentry , seafloor imaging using the TowCam system, and an extensive rock‐dredging program. At different times, detachment faulting was active along ∼50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, noncorrugated massifs, and back‐tilted ridges marking detachment breakaways. High‐resolution Sentry data reveal a new detachment morphology; a low‐angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multiscale corrugations are observed 2–3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low‐angle (6°) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust >2 Ma, indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two‐sided detachment faulting.
Abstract Plate tectonics and mantle dynamics necessitate mantle recycling throughout Earth’s history, yet direct geochemical evidence for mantle reprocessing remains elusive. Here we present evidence of recycled supra-subduction zone mantle wedge peridotite dredged from the Mid-Atlantic Ridge near 16°30′N. Peridotite trace-element characteristics are inconsistent with fractional anhydrous melting typically associated with a mid-ocean ridge setting. Instead, the samples are best explained by hydrous flux melting which changed the melting reactions such that clinopyroxene was not exhausted at high degrees of melting and was retained in the residuum. Based on along-axis ridge depth variations, this buoyant refractory arc mantle is likely compensated at depth by denser, likely garnet-rich, lithologies within the mantle column. Our results suggest that highly refractory arc mantle relicts are entrained in the upper mantle and may constitute >60% of the upper mantle by volume. These highly refractory mantle domains, which contribute little to mantle melting, are under-represented in compilations of mantle composition that rely on inverted basalt compositions alone.
Abstract We present in situ secondary ion mass spectrometry (SIMS) and electron microprobe analyses of coexisting garnet, omphacite, phengite, amphibole, and apatite, combined with pyrohydrolysis bulk-rock analyses to constrain the distribution, abundance, and behavior of halogens (F and Cl) in six MORB-like eclogites from the Raspas Complex (Southern Ecuador). In all cases concerning lattice-hosted halogens, F compatibility decreases from apatite (1.47–3.25 wt%), to amphibole (563–4727 μg/g), phengite (610–1822 μg/g), omphacite (6.5–54.1 μg/g), and garnet (1.7–8.9 μg/g). The relative compatibility of Cl in the assemblage is greatest for apatite (192–515 μg/g), followed by amphibole (0.64–82.7 μg/g), phengite (1.2–2.1 μg/g), omphacite (<0.05–1.0 μg/g), and garnet (<0.05 μg/g). Congruence between SIMS-reconstructed F bulk abundances and yield-corrected bulk pyrohydrolysis analyses indicates that F is primarily hosted within the crystal lattice of eclogitic minerals. However, SIMS-reconstructed Cl abundances are a factor of five lower, on average, than pyrohydrolysis-derived bulk concentrations. This discrepancy results from the contribution of fluid inclusions, which may host at least 80% of the bulk rock Cl. The combination of SIMS and pyrohydrolysis is highly complementary. Whereas SIMS is well suited to determine bulk F abundances, pyrohydrolysis better quantifies bulk Cl concentrations, which include the contribution of fluid inclusion-hosted Cl. Raspas eclogites contain 145–258 μg/g F and at least 7–11 μg/g Cl. We estimate that ~95% of F is retained in the slab through eclogitization and returned to the upper mantle during subduction, whereas at least 95% of subducted Cl is removed from the rock by the time the slab equilibrates at eclogite facies conditions. Our calculations provide further evidence for the fractionation of F from Cl during high-pressure metamorphism in subduction zones. Although the HIMU (high U/Pb) mantle source (dehydrated oceanic crust) is often associated with enrichments in Cl/K and F/Nd, Raspas eclogites show relatively low halogen ratios identical within uncertainty to depleted MORB mantle (DMM). Thus, the observed halogen enrichments in HIMU ocean island basalts require either further fractionation during mantle processing or recycling of a halogen-enriched carrier lithology such as serpentinite into the mantle.
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