Precise initial abundance of Niobium-92 in the Solar System and implications for p-process nucleosynthesis

The niobium-92–zirconium-92 (92Nb–92Zr) decay system with a half-life of 37 Ma has great potential to date the evolution of planetary materials in the early Solar System. Moreover, the initial abundance of the p-process isotope 92Nb in the Solar System is important for quantifying the contribution of p-process nucleosynthesis in astrophysical models. Current estimates of the initial 92Nb/93Nb ratios have large uncertainties compromising the use of the 92Nb–92Zr cosmochronometer and leaving nucleosynthetic models poorly constrained. Here, the initial 92Nb abundance is determined to high precision by combining the 92Nb–92Zr systematics of cogenetic rutiles and zircons from mesosiderites with U–Pb dating of the same zircons. The mineral pair indicates that the 92Nb/93Nb ratio of the Solar System started with (1.66 ± 0.10) × 10−5, and their 92Zr/90Zr ratios can be explained by a three-stage Nb–Zr evolution on the mesosiderite parent body. Because of the improvement by a factor of 6 of the precision of the initial Solar System 92Nb/93Nb, we can show that the presence of 92Nb in the early Solar System provides further evidence that both type Ia supernovae and core-collapse supernovae contributed to the light p-process nuclei.

Geothermobarometry of spinel peridotites from southern British Columbia: implications for the thermal conditions in the upper mantle

Spinel lherzolite xenoliths within alkali basalts exposed at Rayfield River and Big Timothy Mountain, south-central British Columbia, represent samples of the underlying lithospheric mantle. Electron microprobe analysis shows that most xenoliths comprise compositionally homogeneous grains of olivine, orthopyroxene, clinopyroxene, and spinel. We applied the following mineral-pair geothermometers to these rocks: orthopyroxene–clinopyroxene, spinel–orthopyroxene, and spinel–olivine. Temperatures calculated using the Brey and Köhler calibration of two-pyroxene thermometry were constrained in pressure by being required to lie on a model geotherm we develop for this region of B.C. The model geotherm is constrained to produce a temperature at the Moho (33 km) of 825 ± 25 °C to match the lowest temperature peridotite xenoliths recovered in this study. Although the overall effect of pressure on the temperature calculations is negligible (∼2 °C for 0.1 GPa), the simultaneous solution of the model geotherm and the pressure-dependent Brey–Köhler two-pyroxene thermometry removes the need for adopting an arbitrary pressure. We take these temperatures to represent peak mantle lithosphere temperatures. Fourteen Rayfield River xenoliths return two-pyroxene temperatures between 841 and 962 °C corresponding to depths of 34–42 km. Orthopyroxene–spinel and olivine–spinel results are 889 ± 60 and 825 ± 88 °C, respectively. Five Big Timothy xenoliths have two-pyroxene temperatures spanning 840–1058 °C and corresponding to depths of 34–48 km. Mean orthopyroxene–spinel and olivine–spinel temperatures are 844 ± 63 and 896 ± 232 °C, respectively. We argue that the differences in ranges of temperature do not represent closure temperatures imposed during cooling either in the mantle or during transport by the magma. Rather, these differences reflect differences in the original calibrations of the geothermometers or different degrees of equilibration in exchange reactions in dry rocks. Isochemical phase diagrams (pseudosections) constrain the pressure–temperature (P–T) field in which spinel is stable. These diagrams suggest that the spinel-bearing peridotites equilibrated at pressures ranging from ∼9.6 to 14 kbar (10 kbar = 1 GPa).

Colombian-style emerald mineralization in the northern Canadian Cordillera: integration into a regional Paleozoic fluid flow regime

Emerald in the Mackenzie Mountains is hosted in extensional quartz–carbonate veins cutting organic-poor Neoproterozoic sandstones and siltstones within the hanging wall of a thrust fault that emplaced these strata above Paleozoic rocks. Isotopic compositions of water extracted from emerald are typical of evolved sedimentary sulphate brines. Fluid inclusion studies indicate two saline fluid populations: a CO2–N2-bearing, high-salinity brine (20.4–25.8 wt.% NaCl equivalent), and a gas-free, saline brine (7.6–15.3 wt.% NaCl equivalent). Both populations display evidence of post-entrapment volume changes. δ18OVSMOW (VSMOW, Vienna standard mean ocean water) values for emerald, quartz, and dolomite yield averages of 17.3‰ (±0.9), 19.6‰ (±1.5), and 18.1‰ (±1.0), respectively. Dolomite δ13CVPDB (VPDB, Vienna Pee Dee belemnite) averages –6.8‰ (±1.0). Two pyrite samples returned δ34SCDT (CDT, Cañon Diablo troilite) values of 5.1‰ and 11.2‰. Triply concordant mineral equilibration temperatures determined from mineral pair δ18OVSMOW equilibration (quartz–emerald, quartz–dolomite, emerald–dolomite) range from 380 to 415 °C. Depth calculations based on mineral pair isotope equilibration and typical geothermal gradient indicate vein formation at 6–11 km depth. A Re–Os isochron age of 345 ± 20 Ma from pyrite indicates that mineralization was contemporaneous with estimated ages of some northern Cordilleran Zn–Pb occurrences. Emerald mineralization resulted from inorganic thermochemical sulphate reduction via the circulation of warm basinal brines through siliciclastic, carbonate, and evaporitic rocks. These brines were driven along deep basement structures and reactivated normal faults during the development of a trans-tensional back-arc basin during the late Devonian to middle Mississippian. The Mountain River emerald occurrence thus represents a variant of the Colombian-type emerald deposit model requiring thermochemical sulphate reduction.

Jadeite–K-feldspar rocks and jadeitites from northwest Turkey

Blueschist-facies rocks with jadeite-K-feldspar-lawsonite paragenesis occur as exotic blocks in Miocene debris flows in the blueschist belt of northwest Turkey. The jadeite-K-feldspar rocks have a very fine grain size and although recrystallized locally retain a relict porphyritic volcanic texture. The former nepheline microphenocrysts, recognized from their characteristic shapes, are pseudomorphed by jadeite and K-feldspar, while the relict magmatic aegirine has rims of jadeite. The matrix of the rock consists of very fine-grained aggregates of jadeite, K-feldspar and lawsonite. In some blocks, jadeite makes up >60% of the mode. Jadeite, K-feldspar and lawsonite in the blocks are essentially pure end-member in composition. P-T estimates for these rocks are 8 ± 2 kbar and 300 ± 50°C. The preserved volcanic texture, relict aegirine and the bulk rock composition indicate that these rocks represent metamorphosed phonolites. The paragenesis in these rocks shows that jadeite-K-feldspar is a stable mineral pair in blueschist-facies P-T conditions.

Sulfur Isotope Partitioning in Metallic Sulfide Systems

Sulfur isotopic fractionation factors involving pairs of pyrite, pyrrhotite, sphalerite, chalcopyrite, and galena have been determined experimentally over the temperature range 250 °C to 600 °C.Since chalcopyrite and pyrrhotite are not stable at higher PS2 conditions, buffer assemblages were necessary to control PS2 in experiments with these minerals. Since low PS2 values and low temperatures are unfavorable to rapid isotope exchange, techniques were devised whereby equilibrium constants could be estimated indirectly in systems where direct measurements are not possible because of the time factor.Current data place the sulfide minerals in the following order of 34S enrichment under equilibrium exchange conditions: pyrite > (pyrrhotite [Formula: see text]sphalerite) > chalcopyrite > galena in agreement with theoretical predictions. In agreement with theory the equilibrium exchange constant K for a given mineral pair depends upon temperature as follows: 1000 ln [Formula: see text], where A denotes a constant. The A values for various mineral pairs have been determined with ± 10% uncertainties as follows: 11.0 × 105 (py–gn), 8.0 × 105 (sp–gn), 6.5 × 105 (cp–gn), 4.5 × 105 (py–cp), 3.0 × 105 (py–sp, py–po), 1.5 × 105 (sp–cp, po–cp), and [Formula: see text] 0 (sp–po).