Testing competing caldera models using U/Pb geochronology: intrusive history of the Questa caldera, Latir Volcanic Field, New Mexico, USA

A compelling new model for caldera evolution challenges the standard depiction of large-scale volcanism. Here, I test these competing models by establishing the temporal and chemical relationship between ignimbrite and potential cogenetic plutons related to the Questa caldera, Latir volcanic field, New Mexico. Results from zircon U/Pb geochronology indicate that the majority of intrusive rocks formed after ignimbrite eruption. The Rio Hondo pluton was assembled over a minimum of 500 k.y., and crystallization progressed from the structurally highest levels downward, consistent with top-down incremental assembly of the pluton. Trace-element modeling demonstrates that the plutonic rocks are not the residua of crystal fractionation, and the predicted systematic difference in trace-element chemistries of plutonic and volcanic rocks is not observed. Finally, the data presented here mostly support the new model for caldera evolution and is broadly inconsistent with the standard caldera model

Petrochronological Close-Up on the Thermal Structure of a Paleo-Subduction Zone (W. Alps)

International audienceWe present the results from a study combining zoned geochronology, mineral trace element geochemistry and phase equilibria modeling of a mafic lithology from the Zermatt Saas Ophiolite in order to elucidate the tectonic rates and thermal structure of the Western Alps paleo-subduction interface. The Zermatt-Saas Ophiolite represents dismembered slices derived from an eclogitized, 60-km wide coherent fragment of Tethyan oceanic lithosphere. Two cm-sized garnet crystals from a pyrite-rich chlorite-talcschist (representative of sub-seafloor hydrothermally-altered basalts) from the Servette mine locality (St. Marcel Valley, Italy) were microdrilled to separate core and rim growth generations and dated using Sm-Nd geochronology to determine the overall duration of garnet growth. Garnet cores were dated to 46.9 ± 1.6Ma, signifying the timing of the initiation of garnet growth. Garnet rims were dated to 43.5 ± 1.3Ma, signifying the timing of burial to peak depths (constrained to ∼75 km). Major and trace element zoning in garnet suggest two distinct generations of garnet growth. Garnet crystal cores display evidence for rapid nucleation and growth, while garnet crystal rims suggest growth at relatively slow (tectonic) rates. The duration of garnet growth (3.4 ± 2.1Ma), when coupled to constraints from phase equilibria modeling and Zr-in-rutile thermometry, provides estimates on the burial and heating rate of 4.7 (+ 7.

The Northwest Africa 8159 martian meteorite: Expanding the martian sample suite to the early Amazonian

International audienceNorthwest Africa (NWA) 8159 is an augite-rich shergottite, with a mineralogy dominated by Ca-, Fe-rich pyroxene, plagioclase, olivine, and magnetite. NWA 8159 crystallized from an evolved melt of basaltic composition under relatively rapid conditions of cooling, likely in a surface lava flow or shallow sill. Redox conditions experienced by the melt shifted from relatively oxidizing (with respect to known Martian lithologies, similar to QFM) on the liquidus to higher oxygen fugacity (similar to QFM + 2) during crystallization of the groundmass, and under subsolidus conditions. This shift resulted in the production of orthopyroxene and magnetite replacing olivine phenocryst rims. NWA 8159 contains both crystalline and shock-amorphized plagioclase (An(5062)), often observed within a single grain; based on known calibrations we bracket the peak shock pressure experienced by NWA 8159 to between 15 and 23 GPa. The bulk composition of NWA 8159 is depleted in LREE, as observed for Tissint and other depleted shergottites; however, NWA 8159 is distinct from all other martian lithologies in its bulk composition and oxygen fugacity. We obtain a Sm-Nd formation age of 2.37 +/- 0.25 Ga for NWA 8159, which represents an interval in Mars geologic time which, until recently, was not represented in the other martian meteorite types. The bulk rock Sm-147/Nd-144 value of 0.37 +/- 0.02 is consistent with it being derived directly from its source and the high initial epsilon(143)(Nd) value indicates this source was geochemically highly depleted. Cr, Nd, and W isotopic compositions further support a unique mantle source. While the rock shares similarities with the 2.4-Ga NWA 7635 meteorite, there are notable distinctions between the two meteorites that suggest differences in mantle source compositions and conditions of crystallization. Nevertheless, the two samples may be launch-paired. NWA 8159 expands the known basalt types, ages and mantle sources within the Mars sample suite to include a second igneous unit from the early Amazonian.(C) 2017 Elsevier Ltd. All rights reserved