Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation?

Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ⁵⁶Fe values up to +0.35‰ relative to the solar system mean. On average, the δ⁵⁶Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ⁵⁶Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ⁵⁶Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ¹⁸O (0.9–1.5‰) than their host chondrites, and the same is true for Δ¹⁷O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an ¹⁶O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts.     

New data on the Late Cenozoic basaltic volcanism in Syria,applied to its origin

New data on geology and 21 K–Ar dates of the Late Oligocene–Quaternary basalts in Syria, combined with analysis of the new and previous data are used to reconstruct the volcanic history and relations between it and tectonic events. Volcanism began at the end of Oligocene (26–24 Ma) and was concentrated in the Late Oligocene–Early Miocene along a N-trending band, which stretches from the Jebel Arab (Harrat Ash Shaam) up to Kurd Dagh and southern Turkey. Activity waned in the Middle Miocene (17–12 Ma), but was resumed in the same band in the Tortonian and increased in the Messinian and Early Pliocene (6.3–4 Ma), when volcanism spread to the Shin Plateau and its coastal extension. After a brief hiatus ～ 4–3.5 Ma, volcanism became still more intensive and spread from the N-trending band to the east into the northern margin of the Mesopotamian Foredeep and to the west into the Dead Sea Transform zone. Additional eruptions continued into the Holocene.Volcanism lasted > 25 million years in the Jebel Arab Highland and > 15 million years in the Aleppo Plateau. The long duration of volcanism in the same parts of the moving Arabian plate and absence of records of one-way migration of the activity mean that the magmatic sources moved together with the plate, i.e., they were situated within the lithosphere mantle. Coincidence of the tectonic and volcanic stages of the Arabian plate development proves that volcanic activity depended on the geodynamic situation, caused by the plate motion. Situated within the lithosphere, magmatic sources within this transverse band were possibly caused by thermal and deforming influences of the asthenospheric lateral flow, moved laterally from the Ethiopia–Afar deep superplume.     

Fe3+ in shocked olivine crystals of the ALHA 77005 meteorite

ALHA 77005, a shocked achondrite of the shergottite group, is unique in containing cumulus olivine crystals which display a brownish color in thin section. Mössbauer, EPR, and optical spectroscopic studies show the presence of Fe³⁺ in the olivine. Approximately 4.5 wt.% of the total iron in the olivine (24 wt.% as FeO) is in the trivalent state. The Fe³⁺ ions preferentially occupy M2 lattice positions. Charge transfer between Fe³⁺ and Fe²⁺ as well as between Fe³⁺ and oxygen ligands causes a broad absorption band in the optical spectrum at high energies which is the reason for the olivine color. It is concluded that the color of the ALHA 77005 olivine is preterrestrial and may be the result of shock-induced oxidation of the olivine crystals on the meteorite parent body.     

P–T evolution of kyanite eclogite from the Pirin Mountains (SW Bulgaria): implications for the Rhodope UHP Metamorphic Complex

A new occurrence of kyanite eclogite in the Pirin Mountains of southwestern Bulgaria within the rocks belonging to the Obidim Unit of the Rhodope Metamorphic Complex is presented. This eclogite provides important information about the peak–pressure conditions despite strong thermal overprint at low pressure. Textural relationships, phase equilibrium modelling and conventional geothermobarometry were used to constrain the metamorphic evolution. Garnet porphyroblasts with inclusions of omphacite (up to 43 mol.% Jd), phengite (up to 3.5 Si p.f.u.), kyanite, polycrystalline quartz, pargasitic amphibole, zoisite and rutile in the Mg‐rich cores (X_Mg = 0.44–0.46) record a prograde increase in P–T conditions from ～2.5 GPa and 650 °C to ～3 GPa and 700–750 °C. Maximum pressure values fall within the stability field of coesite. During exhumation, the peak–pressure assemblage garnet + omphacite + phengite + kyanite was variably overprinted by a lower pressure one forming symplectitic textures, such as diopside + plagioclase after omphacite and biotite + plagioclase after phengite. The development of spinel (X_Mg = 0.4–0.45) + corundum + anorthite assemblage in the kyanite‐bearing domains at ～1.1 GPa and 800–850 °C suggests a thermal overprint in the high‐pressure granulite facies stability field. This thermal event was followed by cooling at ～0.8 GPa under amphibolite facies conditions; retrograde kelyphite texture involving plagioclase and amphibole was developed around garnet. Our results add to the already existing evidence for ultra high pressure (UHP) metamorphism in the Upper Allochthon of the Rhodope Metamorphic Complex as in the Kimi Unit and show that it is more widespread than previously known. Published age data and field structural relations suggest that the Obidim Unit represents Variscan continental crust involved into the Alpine nappe edifice of the Rhodopes and that eclogite facies metamorphism was Palaeozoic, in contrast to the Kimi Unit where age determinations suggest a Jurassic or Cretaceous age for UHP metamorphism. This implies that UHP metamorphism in the Upper Allochthon of the Rhodopes may have occurred twice, during Alpine and pre‐Alpine orogenic events, and that two independent HP/UHP provinces of different age overlap in this area.     

Tectonic control on third‐order sequences in a siliciclastic ramp‐style basin: An example from the Roper Superbasin (Mesoproterozoic), northern Australia

TThe Roper Group is a cyclic, predominantly marine, siliciclastic succession of Calymmian (Early Mesoproterozoic) age. It has a distribution of at least 145 000 km² and a maximum known thickness of ～5000 m. In the Roper River district the middle part of the Roper Group (～1300 m thick) is characterised by the cyclical alternation of mudstone and sandstone units, and can be divided into six third‐order depositional sequences. A typical sequence is broadly progradational in aspect, and comprises a lower, mudstone‐rich, storm‐dominated shelf succession (up to 330 m thick), and a sequence‐capping unit dominated by tidal‐platform cross‐bedded sandstone (up to 80 m thick); both are interpreted as highstand systems tracts. Transgressive strata are poorly represented but where present are characterised by paralic to fluvial redbed assemblages that include ooidal ironstone. Roper Group sequences lack a distinct condensed section and sequence boundaries are mostly conformable. Erosional contacts separate mud‐rich shelf facies from sequence‐capping sandstones. We infer that these erosion surfaces were generated by episodic flexural tectonism, which also generated the accommodation and sediment supply for Roper sequences.     

Evidence of gas hydrate accumulation and its resource estimation in Andaman deep water basin from seismic and well log data

2D and 3D seismic reflection and well log data from Andaman deep water basin are analyzed to investigate geophysical evidence related to gas hydrate accumulation and saturation. Analysis of seismic data reveals the presence of a bottom simulating reflector (BSR) in the area showing all the characteristics of a classical BSR associated with gas hydrate accumulation. Double BSRs are also observed on some seismic sections of area (Area B) that suggest substantial changes in pressure–temperature (P–T) conditions in the past. The manifestation of changes in P–T conditions can also be marked by the varying gas hydrate stability zone thickness (200–650 m) in the area. The 3D seismic data of Area B located in the ponded fill, west of Alcock Rise has been pre-stack depth migrated. A significant velocity inversion across the BSR (1,950–1,650 m/s) has been observed on the velocity model obtained from pre-stack depth migration. The areas with low velocity of the order of 1,450 m/s below the BSR and high amplitudes indicate presence of dissociated or free gas beneath the hydrate layer. The amplitude variation with offset analysis of BSR depicts increase in amplitude with offset, a similar trend as observed for the BSR associated with the gas hydrate accumulations. The presence of gas hydrate shown by logging results from a drilled well for hydrocarbon exploration in Area B, where gas hydrate deposit was predicted from seismic evidence, validate our findings. The base of the hydrate layer derived from the resistivity and acoustic transit-time logs is in agreement with the depth of hydrate layer interpreted from the pre-stack depth migrated seismic section. The resistivity and acoustic transit-time logs indicate 30-m-thick hydrate layer at the depth interval of 1,865–1,895 m with 30 % hydrate saturation. The total hydrate bound gas in Area B is estimated to be 1.8 × 10¹⁰ m³, which is comparable (by volume) to the reserves in major conventional gas fields.     

Diagnostic Analysis of the Solar Proton Flares of September 2017 by Their Radio Bursts

The powerful solar flares that occurred on September 4–10, 2017 are analyzed based on a quantitative diagnostics method for proton flares developed at the Institute of Terrestrial Magnetism, the Ionosphere and Radio-Wave Propagation (IZMIRAN) in the 1970–1980s. We show that the fluxes and energy spectra of the protons reached the Earth with the energies of tens of MeV qualitatively and quantitatively correspond to the intensity and frequency spectra of the microwave radio bursts in the range of 2.7–15.4 GHz. Specifically, the flare of September 4 with a peak radio flux S ~ 2000 sfu at the frequency f ~ 3 GHz (i.e., with the soft radio spectrum) was accompanied by a significant proton flux J (>10 MeV) ~100 pfu and a soft energy spectrum with the index γ ~3.0, while the strong flare on September 10 with S ~ 21000 sfu at f ~ 15 GHz (i.e., with the hard radio spectrum) led to a very intense proton event with J (>10 MeV) ~1000 pfu with a hard spectrum (γ ~ 1.4), including the ground level enhancement (GLE72). This is further evidence that microwave radio data can be successfully used in diagnostics of proton flares independently of a specific source of particle acceleration at the Sun, in particular, with the IZMIRAN method.