TRACKING AUTHIGENIC MINERAL CEMENTS IN FOSSIL BONES FROM THE UPPER CRETACEOUS (CAMPANIAN) TWO MEDICINE AND JUDITH RIVER FORMATIONS, MONTANA

ABSTRACT Previous studies have yielded mixed results as to whether authigenic mineral infill in fossilized bone tracks specific lithologies or depositional environments, with most suggesting weak to no correlation between infill composition and host lithofacies. This study documents infill patterns in a suite of fossil bones from the Upper Cretaceous Two Medicine and Judith River formations of Montana. The composition and distribution of void-filling materials, including authigenic mineral precipitates (e.g., calcite, chlorite, iron oxides/hydroxides, sulfides, and sulfates) and sedimentary detritus, were identified (petrography and SEM-EDS), imaged (photomicrographs, BSE maps), and quantified on false color maps using ImageJ. The authigenic cement content of fossil bone is distinct at the formation scale, with Two Medicine specimens characterized by pervasive calcite infill (non-ferroan followed by ferroan phase) and local chlorite infill. In contrast, Judith River specimens are characterized by abundant unfilled primary void space, with iron oxides and sulfides, along with rare sulfates, present in all bones, albeit in low abundance. Calcite infills are rare, chlorite is absent, and detrital infill is more abundant in Judith River specimens, presumably reflecting the rapid and more complete permineralization of Two Medicine bones. The sequencing of mineral cements in voids is generally consistent within formations, but is more complex in Two Medicine specimens. Authigenic cement content does not serve to effectively distinguish among facies or localities in either formation. This study compliments previous work on rare earth element (REE) content in the same general suite of fossil bones. In the previous study, patterns in REE uptake also served to readily distinguish fossils at the formation scale, and proved more effective than authigenic cements at differentiating fossils recovered from different facies.

Volcanic and Saline Lithium Inputs to the Salar de Atacama

The Li-rich brine contained within the halite body of the Salar de Atacama is uncommon for two reasons: First, it has an exceptionally high Li concentration, even compared to other closed basins in the Li triangle of South America; and second, it is widespread within the halite nucleus and not restricted to a localized area. This study focusses on the southern half of the salar where Li production occurs and draws comparisons with its northern neighboring basin through which the Loa river flows. Concentration and isotope data for water inflowing to this part of the salar were obtained from surface inflow as well as wells located within the alluvial fans on its eastern margin. Lithium varies between 0.2 and 20 mg/L before reaching the salar where small amounts of the brine and or salts that precipitated from it can increase its concentration up to 400 mg/L or higher. The δ7Li of the inflow water varies between +4.9‰ and +11.2‰ and increases to +12.6‰ within the salar margin, consistent with salar brine based on reported measurements. Boron isotopes indicate that it is unlikely that solutes are derived from sedimentary evaporites or mineral cements, unlike the situation in the adjacent Loa basin. Water that flows through an aquifer laterally confined by a basement block and a line of volcanoes has a notably higher δ7Li than other inflow water, around +9‰, and increasing to +10.5‰. δ7Li values are overall higher than were measured in the adjacent Loa basin, indicating that here the water–rock reactions for Li are more evolved due to longer residence times. Lithium concentrations increased with sodium and chloride, but sedimentary evaporites are shown to be unimportant from δ11B. This is accounted for two ways: evaporated saline inflow leaks from higher elevation basins and inflows are partly derived from or modified by active volcanic systems. Active and dormant volcanoes plus the massive Altiplano–Puna magmatic body are important as heat sources, which enhance water–rock reactions. The large topographic difference between the mean elevation of Altiplano on which these volcanoes sit and the salar surface allows hydrothermal fluids, which would otherwise stay deep below the surface under the modern arc, to uplift at the salar.

Geochemical modelling of diagenetic reactions in a sub-arkosic sandstone

A reaction path model was constructed in a bid to simulate diagenesis in the Magnus Sandstone, an Upper Jurassic turbidite reservoir in the Northern North Sea, UKCS. The model, involving a flux of source rock-derived CO2 into an arkosic sandstone, successfully reproduced simultaneous dissolution of detrital K-feldspar and growth of authigenic quartz, ankerite and illite. Generation of CO2 occurred before and during the main phase of oil generation linking source rock maturation with patterns of diagenesis in arkosic sandstones and limiting this type of diagenesis to the earlier stages of oil charging. Independent corroborative evidence for the model is provided by formation water geochemical data, carbon isotope data from ankerite and produced gas phase CO2 and the presence of petroleum inclusions within the mineral cements. The model involves a closed system with respect to relatively insoluble species such as SiO2 and Al2O3 but is an open system with respect to CO2. There are up to seven possible rate-controlling steps including: influx of CO2, dissolution of K-feldspar, precipitation of quartz, ankerite and illite, diffusive transport of SiO2 and Al2O3 from the site of dissolution to the site of precipitation and possibly the rate of influx of Mg2+ and Ca2+. Given the large number of possible controls, and contrary to modern popular belief, the rate of quartz precipitation is thus not always rate limiting.