Jacob J. Hanley

Evidence from meimechites and other low-degree mantle melts for redox controls on mantle-crust fractionation of platinum-group elements.

Understanding of the geochemistry of the chalcophile elements [i.e., Os, Ir, Ru, Pt, Pd (platinum-group elements), and Au, Cu, Ni] has been informed for at least 20 years by the common assumption that when crust-forming partial melts are extracted from the upper mantle, sulfide liquid in the restite sequesters chalcophile elements until the extent of partial melting exceeds ≈25% and all of the sulfide has been dissolved in silicate melt [Hamlyn, P. R. & Keays, R. R. (1985) Geochim. Cosmochim. Acta 49, 1797–1811]. Here we document very high, unfractionated, chalcophile element concentrations in small-degree partial melts from the mantle that cannot be reconciled with the canonical residual sulfide assumption. We show that the observed high, unfractionated platinum-group element concentrations in small-degree partial melts can be attained if the melting takes place at moderately high oxygen fugacity, which will reduce the amount of sulfide due to the formation of sulfate and will also destabilize residual monosulfide solid solution by driving sulfide melts into the spinel-liquid divariant field. Magmas formed at high oxygen fugacity by small degrees of mantle melting can be important agents for the transfer of chalcophile elements from the upper mantle to the crust and may be progenitors of significant ore deposits of Pt, Pd, and Au.

Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts

Deformation and melting of the crust during the formation of large impact craters must have been important during the Earths early evolution, but such processes remain poorly understood. The 1.8-billion-year-old Sudbury structure in Ontario, Canada, is greater than 200 km in diameter and preserves a complete impact section, including shocked basement rocks, an impact melt sheet and fallback material. It has generally been thought that the most voluminous impact melts represent the average composition of the continental crust, but here we show that the melt sheet now preserved as the Sudbury Igneous Complex is derived predominantly from the lower crust. We therefore infer that the hypervelocity impact caused a partial inversion of the compositional layering of the continental crust. Using geochemical data, including platinum-group-element abundances, we also show that the matrix of the overlying clast-laden Onaping Formation represents a mixture of the original surficial sedimentary strata, shock-melted lower crust and the impactor itself.

Paleohydrogeological and thermal events recorded by fluid inclusions and stable isotopes of diagenetic minerals in Lower Cretaceous sandstones, offshore Nova Scotia, Canada

Upper Jurassic–Lower Cretaceous sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. Previous studies of fluid inclusions in Jurassic limestones and apatite thermochronology of Cretaceous sandstones have suggested a late Mesozoic thermal event. Fluid inclusions in different authigenic cements were analyzed to determine the temperature and composition of basinal fluid at the time of precipitation and the timing of hydrocarbon migration and entrapment. Fifty-one sandstone samples were analyzed for stable isotope composition (18O and 13C) of carbonate cements. Trapping temperatures for primary aqueous inclusions hosted in quartz overgrowths (89–175C) and in late carbonate cements (138–173C) are higher than predicted by two-dimensional modeling from burial alone based on postrift geothermal gradients. These inclusions contain high-salinity fluids (mostly 19–22 wt. % NaCl equivalent). Second, predominantly aqueous inclusions have much lower salinities (5.2–6.7 wt. % NaCl equivalent), and some contain liquid hydrocarbons. Late Fe-calcite cement in Hauterivian sandstone shows negative values of (13.17 to 9.2), whereas cements in deeper and shallower sandstones have higher 13C values. These data indicate that high-temperature, high-salinity fluids preceded hydrocarbon migration. The oldest and youngest rocks studied show less effect of high temperatures than do rocks that were buried to depths of at least 2 km (1.2 mi) after 135 Ma and before 100 Ma. Both fluid inclusions and 13C of carbonate cement suggest that highest temperatures were achieved during burial at 115 to 105 Ma. This Early Cretaceous thermal event had a geothermal gradient of at least 55C/km. It is earlier than the previously reported event based on apatite fission tracks but is consistent with the fission track data. The event is synchronous with regional evidence of volcanism, and its peak coincides with rapid salt-tectonic deformation in the deep basin. Hydrocarbon charge to the outer-shelf wells occurred after this thermal event and thus not before the Late Cretaceous.

Halogens in Terrestrial and Cosmic Geochemical Systems: Abundances, Geochemical Behaviors, and Analytical Methods

The aims of this review chapter are to (i) summarize the distribution of halogens in different fluid (surficial, formation and crystalline shield waters, metamorphic, magmatic-hydrothermal-geothermal) and solid (oceanic and continental crust, mantle and core) domains of the Earth, and various extra-terrestrial materials and bodies (meteorites, planets and moons, and the Sun); (ii) briefly discuss characteristic fractionation processes; and direct the reader to other chapters in this volume; (iii) provide an estimate of the total halogen abundance for the Earth and in its dominant reservoirs contributing to the Earth’s halogen endowment; and (iv) discuss some missing observations that could further improve our understanding of halogen abundances and geochemical systematics. Determination of the distribution of the non-radioactive halogen elements (fluorine, F; chlorine, Cl; bromine, Br; and iodine, I) in, and the geochemical processes controlling their mass transfer between, solid and fluid repositories on Earth and in extraterrestrial environments has seen increasing attention in recent years. In part, this has been enabled by the development of dedicated analytical methodologies (e.g., in situ beam methods, secondary ion mass-spectrometer [SIMS], laser ablation-inductively coupled mass-spectrometer [LA-ICPMS], combined noble gas-halogen methods) that can provide a low detection level, accurate and precise determinations of halogen concentrations, and their isotope systematics in complex matrices (e.g., fluid inclusions, glasses, and minerals). However, a key motivation for this method development stems from an increased awareness of the value in halogen characterization for studying specific processes in Earth’s hydrosphere, crust, mantle, and core (e.g., crustal and mantle metasomatism; ore metal transfer; magmatic differentiation and volatile exsolution; fluid reservoir contamination and fluid mixing; mineral-melt-fluid partitioning; and basinal fluid evolution) in which the chemical and isotopic properties of the halogens provide significant advantages over other element groups. These properties include their (i) differential (i.e., temperature- and melt composition-dependent) incompatibility during fluid-melt and mineral-melt partitioning; (ii) collectively highly mobile and volatile nature but with only a few processes capable of fractionating the halogens from one another or leading to significant halogen mass transfer from one repository to another (e.g., the formation of evaporites, fluid phase separation [immiscibility, boiling], crystallization and degassing of magmas, subduction devolatilization and metamorphism); and (iii) strong systematic covariance of Cl and Br, but commonly differential behaviors of F and I (in response to organic processes) in most fluids in the hydrosphere, sediments, crustal rocks in general, the mantle, and mantle-derived lavas. Mass balance calculations show that F is dominantly hosted by mantle and crust, while Cl and Br show nearly identical distribution patterns in which a total of the seawater, formation waters, and evaporites comprise more than half of the Earth’s halogen budget. Experimentally determined metal-silicate partition coefficients suggest that a significant quantity of I is potentially hosted by the Earth’s core.