New Insights into Mantle Dynamics from Helium Isotopes and Argon Geochronology

Graduation date: 2017Earth’s mantle extends to nearly 3000 km depth, comprises >80 % of Earth’s total volume, and has the largest influence on the primordial and radiogenic heat budget. Despite its importance, the structure and composition of the mantle is still debated. There are three primary models for Earth’s mantle structure that contain some degree of overlap. First, the Burke model invokes both small scale convection in the upper mantle coupled with larger scale, whole mantle convection. This model implies that long lived intraplate volcanic island chains are derived from upwelling plumes that originate near the edges of thermochemical piles in the lower mantle, while smaller and more ephemeral volcanic provinces originate from melting in the upper mantle. Second, the Courtillot model is similar to the Burke model, except multiple plume types exist, some originating from the core-mantle boundary and some separating from massive lower mantle upwellings that stall near the mantle transition zone at 400-660 km. Third, the Anderson model invokes upper and lower mantle isolation throughout Earth’s history, with volcanism originating through melting of a chemically heterogeneous upper mantle. The origin and evolution of intraplate volcanism are therefore central to understanding the dynamics and evolution of the mantle. In this dissertation, I present new constraints on Earth’s mantle using geochemical, geochronological and geodynamical investigations of intraplate volcanism in both continental and oceanic settings. \ud \ud The first study focuses on the origin of the enigmatic intraplate volcanic fields (harrats) of Saudi Arabia. Helium isotope compositions and trace element concentrations were determined for mantle xenoliths and lava flows from Harrat Hutaymah, plus ³He/⁴He in several xenoliths from Harrat Al Birk, Al Kishb, and Ithnayn. Harrat Hutaymah is the most northeastern Arabian volcanic field, found off of the main axis of Arabian volcanism, referred to as the Makkah-Medinah-Nafud (MMN) line. Hutaymah has uniform ³He/⁴He of 7.5 Rᴀ (where Rᴀ is the atmospheric ratio) in nearly all xenolith types. The uniformity is explained by volatile equilibration between the xenoliths and the host magma through the trapping of fluids/gases along grain defects during magma ascent, followed by re-annealing at lower pressure. Anhydrous spinel lherzolites from Hutaymah, Ithnanyn and Al Birk are notably different, having lower ³He/⁴He of 6.8 Rᴀ and distinctly depleted light rare earth element signatures. The widespread presence of this 6.8 Rᴀ signature in the lherzolites appears to be representative of the Arabian Proterozoic lithosphere prior to any metasomatic overprinting associated with later volcanic/tectonic activity. The origin of volcanism associated with the harrats located off the MMN line appears to involve mixing of delaminated, ancient enriched lithosphere with shallow depleted asthenosphere. Elevated ³He/⁴He signatures associated with a deep mantle plume origin are absent in the peripheral harrats, although they appear to be weakly present in some volcanic formations along the MMN line. \ud \ud The second study investigated intraplate volcanism in the Marquesas Islands, French Polynesia. This archipelago represents the surface expression of a low buoyancy flux mantle plume in a region of oceanic lithosphere marked by deep seated fracture zones. The Marquesas plume appears to contain an intrinsically large isotopic and chemical heterogeneity, both geographically and through time. The study uses ³He/⁴He results, major and trace element compositions of whole rocks, ⁴⁰Ar/³⁹Ar age determinations, and chemical analyses of olivine-hosted melt inclusions (OHMIs). The new results indicate that magmas feeding Marquesas volcanism are chemically diverse and are derived by melting of small-scale heterogeneities in the source regions. Variation in Cl/K among the OHMIs suggests that both the melting of recycled altered crust and interaction of magmas with brine at shallow levels potentially affects the lavas erupted in this region. Lavas at Hiva Oa show an increase in ³He/⁴He from shield (8.3 Rᴀ; 2.6 Ma) to late-shield stage (10.4-14.5 Rᴀ; 2.2 Ma), followed by a slight decrease (9.0-11.4 Rᴀ) during the post-shield phase (<1.8 Ma). Elevated ³He/⁴He ratios (>10 Rᴀ) are restricted to the central islands of Nuku Hiva and Hiva Oa in the Marquesas chain, suggesting a concentrically zoned mantle plume that has primitive and hot material concentrated near its central axis. \ud \ud The third study provides a refined history of the Rurutu hotspot, a long-lived mantle plume situated beneath the Pacific plate. Provided are new ⁴⁰Ar/³⁹Ar age determinations for seamounts from the Tuvalu Islands region of the west Pacific. These islands and seamounts range in age from 64 to 47 Ma and are isotopically similar to some young volcanoes in the Cook-Austral region. Using age constrained seamounts from the Rurutu chain (0-10 Ma; 47-72 Ma) in combination with the Pacific Louisville hotspot (0-79 Ma) and Hawaiian hotspot (0-78 Ma), the relative motions of these hotspots were computed. These motions confirm that Hawaii is unique, having an abrupt, southward plume drift from 60-47 Ma, while both Louisville and Rurutu show similar, eastward trending motions. Using geodynamic models of plume motions based primarily on seismic tomography and mantle viscosity models, the conditions required to account for the observed motions were tested. The relative motions are best reproduced when the plumes are both rooted near the core-mantle boundary and have roots that are able to move by mantle flow. However, none of the model parameters can properly reproduce the rapid motions of the Hawaiian hotspot, indicating additional processes may be responsible. The relative hotspot motions cannot be reproduced when the plumes are sourced at the upper/lower mantle boundary and thus indicate a deeper origin for long-lived plumes. \ud \ud The studies presented herein generally support the Burke model for mantle structure. The results from age-progressive Pacific intraplate volcanism support a plume source that is derived from dense thermochemical piles in the lower mantle. These source regions have somewhat mobile edges and contain a component of primitive material. Geochemical results for the Arabian volcanic fields indicate that secondary convection in the upper mantle may generate melts near the lithosphere-asthenosphere boundary, and therefore also be a cause of intraplate volcanism