Monazite CHIME/EPMA dating of Erinpura granitoid deformation: Implications for Neoproterozoic tectono-thermal evolution of NW India

The in-situ “chemical” Th–U–Pb dating of monazite with the electron microprobe is used to unravel the Neoproterozoic tectono-thermal history of the “Erinpura Granite” terrane in the foreland of the Delhi Fold Belt (DFB) in the NW Indian craton. These granitoids are variably deformed and show effects of shearing activity. Monazites from the Erinpura granitoids recorded two main events; (1) protolith crystallization at 863 ± 23 Ma and (2) recrystallization and formation of new Th-poor monazite at 775 ± 26 Ma during shear overprint. Some components of the Erinpura granitoids, such as the Siyawa Granite and granites exposed near Sirohi town, show evidence of migmatization. This migmatization event is documented by anatexis and associated monazite crystallization at 779 ± 16 Ma. The age data indicate an overlap in timing between anatectic event and ductile shear deformation. The end of the tectono-thermal event in the Sirohi area is constrained by a 736 ± 6 Ma Ar–Ar muscovite age data from the ductile shear zone.     

The cloud phase discrimination from a satellite

A new technique to identify mixed-phase clouds but also clouds with supercooled water droplets using satellite measurements is proposed. The technique is based on measurements of the backscattered solar light at wavelengths 1.55 and 1.67 /spl mu/m in combination with cloud brightness temperature measurements at 12 /spl mu/m. For the first time, the concept of the phase index-temperature correlation plot (the P-T diagram) is introduced in the cloud remote sensing. Retrievals of cloud temperature and cloud phase index are performed using data from the Advanced Along Track Scanning Radiometer (AATSR) and Scaning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) both onboard the Envisat platform.     

Calibration of SCIAMACHY Using AATSR Top-of-Atmosphere Reflectance Over a Hurricane

This letter investigates the synergy between the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) and the Advanced Along Track Scanning Radiometer (AATSR) onboard the ENVISAT platform for reflectance calibration purposes. This calibration study was mainly performed over a portion of a hurricane corresponding to fully cloudy SCIAMACHY and AATSR pixels. Results show that SCIAMACHY underestimates the top-of-atmosphere (TOA) reflectance by up to 23% (at 870 nm) as compared to AATSR for a nadir viewing geometry. Specifically, considering AATSR calibration as accurate, which is confirmed by comparison with the Medium Resolution Imaging Spectrometer, the SCIAMACHY TOA reflectances should be multiplied by 1.21, 1.19, 1.23, and 1.10 for wavelengths at 550, 670, 870, and 1600 nm, respectively, ahead of satellite retrieval schemes based on the measurements of TOA reflectance     

Curation protocol of Phobos sample returned by Martian Moons eXploration

Japan Aerospace Exploration Agency's Martian Moons eXploration (MMX) mission will launch a spacecraft in 2024 to return samples from Phobos in 2029. Curatorial work for the returned Phobos samples is critical for the sample allocation without degrading the sample integrity and subsequent sample analysis that will provide new constraints on the origin of Phobos and the evolution of the circum-Mars environment. The Sample Analysis Working Team of the MMX is designing the sample curation protocol. The curation protocol consists of three phases: (1) quick analysis (extraction and mass spectrometry for gases), (2) pre-basic characterization (bulk-scale observation), and (3) basic characterization (grain-by-grain observation and allocation of the sample aliquots). Nondestructive analyses within the clean chamber (e.g., visible and near-infrared spectral imaging) and outside the chamber (e.g., gas mass spectrometry) are incorporated into the curation flow in coordination with the MMX mission instrument teams for ground-truthing the remote-sensing data sets. The MMX curation/sample analysis flow enables the seamless integration between the sample and remote-sensing data sets to maximize the scientific value of the collected Phobos samples.     

Age calibration of the Fish Canyon sanidine Ar/Ar dating standard using primary K-Ar standards

The ⁴⁰Ar/³⁹Ar dating technique requires the use of neutron fluence monitors (standards). Precise calibrations of these standards are crucial to decrease the uncertainties associated with ⁴⁰Ar/³⁹Ar dates. Optimal calibration of ⁴⁰Ar/³⁹Ar standards should be based on K/Ar standards having independent isotope dilution measurements of ⁴⁰K and ⁴⁰Ar*, based on independent isotope tracers (spikes) because this offers the possibility to eliminate random interlaboratory errors. In this study, we calibrate the widely used Fish Canyon sanidine (FCs) standard based on four primary K/Ar standards (GA-1550, Hb3gr, NL-25, and GHC-305) on which K and Ar* concentrations have been determined in different labs with independently calibrated tracers. We obtained a mean age of 28.03 ± 0.08 Ma (1σ; neglecting uncertainties of the ⁴⁰K decay constants) for FCs, based on the decay constant recommended by Steiger and Jäger [Steiger R.H., Jäger. E. 1977. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett.36, 359-362.]. This age corresponds to a mean ⁴⁰Ar*/⁴⁰K value of (1.6407 ± 0.0047) × 10⁻³. We also discuss several criteria that prevent the use of previous calibrations of FCs based on other primary standards (LP-6, SB-3 and MMhb-1). The age of FCs obtained in this study is based on the ⁴⁰K decay constants of Steiger and Jäger (1977) but we anticipate the imminent need for revision of the value and precision of the ⁴⁰K decay constants (representing the main source of uncertainties in ⁴⁰Ar/³⁹Ar dating). The ⁴⁰Ar*/⁴⁰K result of FCs obtained in this study allows therefore a rapid calibration of the age of FCs with uncertainties at the 0.29% level but perhaps more importantly this value is independent of any particular value of the ⁴⁰K decay constants and may be used in the future in conjunction with revised decay constants.     

Closure of the Clymene Ocean and formation of West Gondwana in the Cambrian: Evidence from the Sierras Australes of the southernmost Rio de la Plata craton,Argentina

The formation of Gondwana took place across a series of Brasiliano–Pan African suture zones that record late Neoproterozoic to earliest Paleozoic collisions between Precambrian cratons. In South America, an internal suture zone marks the disappearance of the Clymene Ocean that separated the Amazon craton from the São Francisco and Rio de la Plata cratons. New geochronological data from the southern end of this massive collision zone in the Sierras Australes of central-eastern Argentina document Paleoproterozoic crust and suggest an Ediacaran age for the oldest sedimentary rocks. These two observations extend the known limit of the Rio de la Plata craton at least 200 km SW of previous estimates. New data also confirm the occurrence of late Ediacaran to late Cambrian magmatism in the Sierras Australes. The age of these hypabyssal to volcanic rocks corresponds to igneous events in the Pampean belt along the western margin of the Rio de la Plata craton, although the shallow level magma emplacement in the Sierra da Ventana study area contrasts with the deeply exhumed rocks of the Pampean orogeny type locality. These new age data are compared with a broad compilation of geochronological age Clymene collision belts to the north, the Paraguai and Araguaia belts. The close overlap of the timing of orogenesis indicates the age of Clymene ocean closure in its northern reaches. In the south, the Pampean belt was unconfined, allowing continued tectonic activity and crustal accretion throughout the Paleozoic.     

Mesoproterozoic fluid events affecting Archean crust in the northern Olympic Cu–Au Province,Gawler Craton: insights from 40Ar/39Ar thermochronology

The Olympic Cu–Au Province, Gawler Craton, is host to the Olympic Dam and Prominent Hill iron oxide–copper–gold (IOCG) deposits. Both of these deposits and the region between the two are covered by Neoproterozoic to Cenozoic sediment, making inferences about prospectivity in this portion of the Olympic Domain reliant on geophysical interpretation and sparse drill hole information. We present new U–Pb zircon sensitive high resolution ion microprobe (SHRIMP) dates from two basement intersecting drill holes in the region between Olympic Dam and Prominent Hill that show bimodal volcanism occurred at 2555 ± 5 Ma, and was followed by intrusion of tonalite at 2529 ± 6 Ma. Laser ⁴⁰Ar/³⁹Ar dating of biotite and muscovite from the tonalite yields ages around ca 2000 Ma, consistent with slow cooling trends observed in Archean rocks elsewhere in the northern Gawler Craton. Step heating experiments on K-feldspar from the same tonalite yields an age spectrum with older ages around 1740 Ma from the highest temperature steps becoming progressively younger to a minimum of 1565 Ma in the lowest temperature heating steps; this is consistent with either Paleoproterozic cooling to final closure of K-feldspar by 1565 Ma or a reheating event at ca 1565 Ma, with the latter more likely, given the evidence for sub-solidus alteration of the K-feldspar. Sericite within hematite–sericite–chlorite altered portions of the tonalite yield a poorly defined age of ca 1.6 Ga. Taken together the ⁴⁰Ar/³⁹Ar data providing evidence for a fluid event affecting this region between Olympic Dam and Prominent Hill during the early Mesoproterozoic. Low temperature quartz–carbonate–adularia veins occur in <10 cm wide fractures within basalt in one drill hole in this region. Adularia from these veins yields ⁴⁰Ar/³⁹Ar ages that span from ca 1.3–1.1 Ga. This age range is interpreted to approximate either the timing of adularia formation during a hydrothermal event or the timing of resetting of the ⁴⁰Ar/³⁹Ar systematics within the adularia as a result of fluid flow in this sample. This is evidence for a mid-Mesoproterozoic fluid event in the Gawler Craton and necessitates a reconsideration of the long-term stability of the craton, as it appears to have been affected, at least locally, by fluid flow related to a much larger event within the Australian continent, the Musgrave Orogeny.     

The formation of IIE iron meteorites investigated by the chondrule‐bearing Mont Dieu meteorite

A 435 kg piece of the Mont Dieu iron meteorite (MD) contains cm‐sized silicate inclusions. Based on the concentration of Ni, Ga, Ge, and Ir (8.59 ± 0.32 wt%, 25.4 ± 0.9 ppm, 61 ± 2 ppm, 7.1 ± 0.4 ppm, respectively) in the metal host, this piece can be classified as a IIE nonmagmatic iron. The silicate inclusions possess a chondritic mineralogy and relict chondrules occur throughout the inclusions. Major element analysis, oxygen isotopic analysis (Δ¹⁷O = 0.71 ± 0.02‰), and mean Fa and Fs molar contents (Fa_{15.7 ± 0.4} and Fs_{14.4 ± 0.5}) indicate that MD originated as an H chondrite. Because of strong similarities with Netschaëvo IIE, MD can be classified in the most primitive subgroup of the IIE sequence. ⁴⁰Ar/³⁹Ar ages of 4536 ± 59 Ma and 4494 ± 95 Ma obtained on pyroxene and plagioclase inclusions show that MD belongs to the old (~4.5 Ga) group of IIE iron meteorites and that it has not been perturbed by any subsequent heating event following its formation. The primitive character of MD sheds light on the nature of its formation process, its thermal history, and the evolution of its parent body.     

Ediacaran terrane accretion within the Arabian–Nubian Shield

The Ad Dawadimi Terrane is an Ediacaran basin of the Arabian Nubian Shield (ANS), Saudi Arabia. This basin terrane is situated in the far eastern part of the ANS and represents the youngest accretion event of the exposed ANS. Therefore, the timing of events within the basin is key to understanding both the closure of the Mozambique Ocean and the amalgamation of Gondwana along the northern East African Orogen. Here we present U/Pb detrital zircon data for the Abt Formation, the principle basin sediments of the Ad Dawadimi Terrane, along with ⁴⁰Ar/³⁹Ar ages on muscovite and whole rock Sm/Nd data. These data indicate that deep-water deposition in the Abt Basin did not end until after ca. 620 Ma and that deformation and greenschist-facies metamorphism of the Abt Formation occurred at 620 ± 3 (2σ) Ma along an active margin. This is the youngest terrane amalgamation event reported so far in the Arabian–Nubian Shield, but we suggest even younger sutures lie further east beneath the Phanerozoic cover of eastern Saudi Arabia. Our results suggest that the Ediacaran basins of the eastern ANS were not part of the Huqf basin in Oman, which was instead part of a passive margin of Neoproterozoic India, separated from the active margin of Africa by the Mozambique Ocean that probably did not close until the late Ediacaran or early Cambrian.