Monazite trumps zircon: applying SHRIMP U–Pb geochronology to systematically evaluate emplacement ages of leucocratic, low-temperature granites in a complex Precambrian orogen

Although zircon is the most widely used geochronometer to determine the crystallisation ages of granites, it can be unreliable for low-temperature melts because they may not crystallise new zircon. For leucocratic granites U–Pb zircon dates, therefore, may reflect the ages of the source rocks rather than the igneous crystallisation age. In the Proterozoic Capricorn Orogen of Western Australia, leucocratic granites are associated with several pulses of intracontinental magmatism spanning ~800 million years. In several instances, SHRIMP U–Pb zircon dating of these leucocratic granites either yielded ages that were inconclusive (e.g., multiple concordant ages) or incompatible with other geochronological data. To overcome this we used SHRIMP U–Th–Pb monazite geochronology to obtain igneous crystallisation ages that are consistent with the geological and geochronological framework of the orogen. The U–Th–Pb monazite geochronology has resolved the time interval over which two granitic supersuites were emplaced; a Paleoproterozoic supersuite thought to span ~80 million years was emplaced in less than half that time (1688–1659 Ma) and a small Meso- to Neoproterozoic supersuite considered to have been intruded over ~70 million years was instead assembled over ~130 million years and outlasted associated regional metamorphism by ~100 million years. Both findings have consequences for the duration of associated orogenic events and any estimates for magma generation rates. The monazite geochronology has contributed to a more reliable tectonic history for a complex, long-lived orogen. Our results emphasise the benefit of monazite as a geochronometer for leucocratic granites derived by low-temperature crustal melting and are relevant to other orogens worldwide

Deducing the source and composition of rare earth mineralising fluids in carbonatites: insights from isotopic (C, O, 87Sr/86Sr) data from Kangankunde, Malawi

This is the final version of the article. Available from Springer Verlag via the DOI in this record.Carbonatites host some of the largest and highest grade rare earth element (REE) deposits but the composition and source of their REE-mineralising fluids remains enigmatic. Using C, O and 87Sr/86Sr isotope data together with major and trace element compositions for the REE-rich Kangankunde carbonatite (Malawi), we show that the commonly observed, dark brown, Fe-rich carbonatite that hosts REE minerals in many carbonatites is decoupled from the REE mineral assemblage. REE-rich ferroan dolomite carbonatites, containing 8–15 wt% REE2O3, comprise assemblages of monazite-(Ce), strontianite and baryte forming hexagonal pseudomorphs after probable burbankite. The 87Sr/86Sr values (0.70302–0.70307) affirm a carbonatitic origin for these pseudomorph-forming fluids. Carbon and oxygen isotope ratios of strontianite, representing the REE mineral assemblage, indicate equilibrium between these assemblages and a carbonatite-derived, deuteric fluid between 250 and 400 °C (δ18O + 3 to + 5‰VSMOW and δ13C − 3.5 to − 3.2‰VPDB). In contrast, dolomite in the same samples has similar δ13C values but much higher δ18O, corresponding to increasing degrees of exchange with low-temperature fluids (< 125 °C), causing exsolution of Fe oxides resulting in the dark colour of these rocks. REE-rich quartz rocks, which occur outside of the intrusion, have similar δ18O and 87Sr/86Sr to those of the main complex, indicating both are carbonatite-derived and, locally, REE mineralisation can extend up to 1.5 km away from the intrusion. Early, REE-poor apatite-bearing dolomite carbonatite (beforsite: δ18O + 7.7 to + 10.3‰ and δ13C −5.2 to −6.0‰; 87Sr/86Sr 0.70296–0.70298) is not directly linked with the REE mineralisation.This project was funded by the UK Natural Environment Research Council (NERC) SoS RARE project (NE/M011429/1) and by NIGL (NERC Isotope Geoscience Laboratory) Project number 20135

Magnesium isotope evidence that accretional vapour loss shapes planetary compositions

It has long been recognized that Earth and other differentiated planetary bodies are chemically fractionated compared to primitive, chondritic meteorites and, by inference, the primordial disk from which they formed. However, it is not known whether the notable volatile depletions of planetary bodies are a consequence of accretion1 or inherited from prior nebular fractionation2. The isotopic compositions of the main constituents of planetary bodies can contribute to this debate3, 4, 5, 6. Here we develop an analytical approach that corrects a major cause of measurement inaccuracy inherent in conventional methods, and show that all differentiated bodies have isotopically heavier magnesium compositions than chondritic meteorites. We argue that possible magnesium isotope fractionation during condensation of the solar nebula, core formation and silicate differentiation cannot explain these observations. However, isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals, generates appropriate residual compositions. Our modelling implies that the isotopic compositions of magnesium, silicon and iron, and the relative abundances of the major elements of Earth and other planetary bodies, are a natural consequence of substantial (about 40 per cent by mass) vapour loss from growing planetesimals by this mechanism

Volatile and Organic Compositions of Sedimentary Rocks in Yellowknife Bay, Gale crater, Mars

H₂O, CO₂, SO₂, O₂, H₂, H₂S, HCl, chlorinated hydrocarbons, NO and other trace gases were evolved during pyrolysis of two mudstone samples acquired by the Curiosity rover at Yellowknife Bay within Gale crater, Mars. H₂O/OH-bearing phases included 2:1 phyllosilicate(s), bassanite, akaganeite, and amorphous materials. Thermal decomposition of carbonates and combustion of organic materials are candidate sources for the CO₂. Concurrent evolution of O₂ and chlorinated hydrocarbons suggest the presence of oxychlorine phase(s). Sulfides are likely sources for S-bearing species. Higher abundances of chlorinated hydrocarbons in the mudstone compared with Rocknest windblown materials previously analyzed by Curiosity suggest that indigenous martian or meteoritic organic C sources may be preserved in the mudstone; however, the C source for the chlorinated hydrocarbons is not definitively of martian origin

The Petrochemistry of Jake_M: A Martian Mugearite

“Jake_M,” the first rock analyzed by the Alpha Particle X-ray Spectrometer instrument on the Curiosity rover, differs substantially in chemical composition from other known martian igneous rocks: It is alkaline (>15% normative nepheline) and relatively fractionated. Jake_M is compositionally similar to terrestrial mugearites, a rock type typically found at ocean islands and continental rifts. By analogy with these comparable terrestrial rocks, Jake_M could have been produced by extensive fractional crystallization of a primary alkaline or transitional magma at elevated pressure, with or without elevated water contents. The discovery of Jake_M suggests that alkaline magmas may be more abundant on Mars than on Earth and that Curiosity could encounter even more fractionated alkaline rocks (for example, phonolites and trachytes)

Elemental Geochemistry of Sedimentary Rocks at Yellowknife Bay, Gale Crater, Mars

Sedimentary rocks examined by the Curiosity rover at Yellowknife Bay, Mars, were derived from sources that evolved from approximately average Martian crustal composition to one influenced by alkaline basalts. No evidence of chemical weathering is preserved indicating arid, possibly cold, paleoclimates and rapid erosion/deposition. Absence of predicted geochemical variations indicates that magnetite and phyllosilicates formed by diagenesis under low temperature, circum-neutral pH, rock-dominated aqueous conditions. High spatial resolution analyses of diagenetic features, including concretions, raised ridges and fractures, indicate they are composed of iron- and halogen-rich components, magnesium-iron-chlorine-rich components and hydrated calcium-sulfates, respectively. Composition of a cross-cutting dike-like feature is consistent with sedimentary intrusion. Geochemistry of these sedimentary rocks provides further evidence for diverse depositional and diagenetic sedimentary environments during the early history of Mars