Dynamical Geochemistry: Mantle dynamics and its role in the formation of geochemical heterogeneity

Chemical geodynamics is a term coined nearly forty years ago to highlight the important link between Earth's geochemical evolution and plate tectonics & mantle convection. Significant progress in our understanding of this connection has taken place since then through advances in the analytical precision of geochemical measurements, dramatically improved geophysical imaging techniques, application of novel isotope systems, and great advances in computational power. Thee latter especially has improved geodynamical models and data interpretation techniques. We provide a review of these advances and their impact on chemical geodynamics, or perhaps, dynamical geochemistry. To focus this review we will address primarily the role of whole mantle convection and oceanic crust formation and recycling together with an update on our understanding of noble gas systematics

Structural geometry and evolution of the Rukwa Rift Basin, Tanzania: Implications for helium potential

The Rukwa Rift Basin, Tanzania is regarded as a modern example of a cratonic rift zone despite complex polyphase extensional and episodic inversion structures. We interpret 2D seismic reflection data tied to wells to identify and describe structures controlling stratigraphic sequences (Late Carboniferous to Pleistocene) in two main segmented Rukwa Rift domains, A and B, which are controlled by the Chisi and Saza shear zones. Fault geometry and stratal patterns are illustrated in relation to their kinematic interaction with folds. Fold structures reflect both extensional and compressional deformation and were mapped with a particular interest for their helium potential. We illustrate fault bend folds, fault propagation folds and fault propagation monoclines that are related to extension events. Folds related to compression exhibit various structural styles reflecting at least two phases of episodic and widespread inversion. First, Early Jurassic inversion phase which involved multi-faulted anticlines in the Karoo strata. Second, a mild and widespread inversion structures during the Pleistocene which are characterised by both symmetrical and asymmetrical anticlines styles. Taken together, the extensional and compressional fold structures, stratal juxtapositions and unconformities define stratigraphic packages that are widely distributed in the Rukwa Rift Basin, and form potential subsurface traps for helium-nitrogen–rich gases, from which some seep to the surface, evidently documented in thermal springs across the region

Estimating the recharge properties of the deep ocean using noble gases and helium isotopes

The distribution of noble gases and helium isotopes in the dense shelf waters of Antarctica reflect the boundary conditions near the ocean surface: air-sea exchange, sea ice formation and subsurface ice melt. We use a non-linear least-squares solution to determine the value of the recharge temperature and salinity, as well as the excess air injection and glacial meltwater content throughout the water column and in the precursor to Antarctic Bottom Water. The noble gas-derived recharge temperature and salinity in the Weddell Gyre are -1.95 °C and 34.95 psu near 5500 m; these cold, salty recharge values are a result of surface cooling as well as brine rejection during sea ice formation in Antarctic polynyas. In comparison, the global value for deep water recharge temperature is -0.44 °C at 5500 m, which is 1.5 °C warmer than the southern hemisphere deep water recharge temperature, reflecting the contribution from the north Atlantic. The contrast between northern and southern hemisphere recharge properties highlight the impact of sea ice formation on setting the gas properties in southern sourced deep water. Below 1000 m, glacial meltwater averages 3.5 ‰ by volume and represents greater than 50% of the excess neon and argon found in the water column. These results indicate glacial melt has a non-negligible impact on the atmospheric gas content of Antarctic Bottom Water

Gas emissions and subsurface architecture of fault-controlled geothermal systems : a case study of the North Abaya geothermal area

Funding: W. Hutchison is funded by a UKRI Future Leaders Fellowship (MR/S033505/1). E.R.D. Ogilvie was supported by a St Andrews Research Internship Scheme (StARIS) grant from the University of St Andrews.East Africa hosts significant reserves of untapped geothermal energy. Exploration has focused on geologically young (<1 Ma) silicic calderas, yet there are many sites of geothermal potential where there is no clear link to an active volcano. The origin and architecture of these systems are poorly understood. Here, we combine remote sensing and field observations to investigate a fault-controlled geothermal play located north of Lake Abaya in the Main Ethiopian Rift. Soil gas CO2 and temperature surveys were used to examine permeable pathways and showed elevated values along a ∼110 m high fault, which marks the western edge of the Abaya graben. Ground temperatures are particularly elevated where multiple intersecting faults form a wedged horst structure. This illustrates that both deep penetrating graben bounding faults and near-surface fault intersections control the ascent of hydrothermal fluids and gases. Total CO2 emissions along the graben fault are ∼300 t d−1; a value comparable to the total CO2 emission from silicic caldera volcanoes. Fumarole gases show δ13C of −6.4‰ to −3.8‰ and air-corrected 3He/4He values of 3.84–4.11 RA, indicating a magmatic source originating from an admixture of upper mantle and crustal helium. Although our model of the North Abaya geothermal system requires a deep intrusive heat source, we find no ground deformation evidence for volcanic unrest or recent volcanism along the graben fault. This represents a key advantage over the active silicic calderas that typically host these resources and suggests that fault-controlled geothermal systems offer viable prospects for geothermal exploration.Publisher PDFPeer reviewe

Recycled Components in Mantle Plumes Deduced From Variations in Halogens (Cl, Br, and I), Trace Elements, and He-3/He-4 Along the Hawaiian-Emperor Seamount Chain

Halogens are primarily located within surface reservoirs of the Earth; as such they have proven to be effective tracers for the identification of subducted volatiles within the mantle. Subducting lithologies exhibit a wide variety of halogen compositions, yet the mantle maintains a fairly uniform signature, suggesting halogens may be homogenized during subduction to the mantle or during eruption. Here we present halogen (Cl, Br, and I), K, noble gas, and major and trace element data on olivines from three seamounts along the Hawaiian-Emperor seamount chain to determine if the deep mantle source has retained evidence of halogen heterogeneities introduced through subduction. High Ni contents indicate that the Hawaiian-Emperor mantle source contains a recycled oceanic crust component in the form of pyroxenite, which increases from the 46% in the oldest (Detroit) to 70% in the younger seamount (Koko). Detroit seamount retains mid-ocean ridge basalts (MORB)-like Br/Cl and I/Cl, while the Br/Cl and I/Cl of Suiko and Koko seamounts are higher than MORB and similar to altered oceanic crust and dehydrated serpentinite. Helium isotopes show a similar evolution, from MORB-like values at Detroit seamount toward higher values at Suiko and Koko seamounts. The correlation between pyroxenite contributions, Br/Cl, I/Cl, and He-3/He-4 indicates that subducted material has been incorporated into the primordial undegassed Hawaiian mantle plume source. The identification of recycled oceanic crustal signatures in both the trace elements and halogens indicates that subduction and dehydration of altered oceanic crust may exert control on the cycling of volatile elements to the deep mantle

A late Pleistocene–Holocene noble gas paleotemperature record in southern Michigan

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95678/1/grl19236.pd