Geochemistry and petrogenesis of volcanic rocks in the Kuril island arc

Newly obtained precise analytical data on trace elements and radiogenic Sr, Nd, and Pb isotopes testify to anomalous geochemical characteristics of mafic and intermediate Quaternary lavas in Paramushir (in the north of the Kuril arc), Kunashir and Iturup (in the south) islands, which are the largest three islands of the Kuril island arc. The high K and LREE concentrations in the volcanic products in Paramushir Island resulted from the southward expansion of the mantle thermal anomaly of the Kamchatka Peninsula and the involvement of melts related to the melting of oceanic sediments in magma generation. The depleted characteristics of the mafic volcanics are explained by the relatively young tectono-magmatic events during the opening of the Kuril backarc basin. The Kuril island-arc system developed on a heterogeneous basement. The northern islands are a continuation of the volcanic structures of southern Kamchatka, which were formed above an isotopically depleted and hot lithospheric mantle domain of composition close to that of the Pacific MORB type. The southern islands were produced above an isotopically enriched and cold lithospheric domain of the Indian-Ocean MORB type, which was modified in relation to relatively young backarc tectono-magmatic processes. Although issues related to the genesis of the transverse geochemical zoning were beyond the originally formulated scope of our research, the homogeneous enough isotopic composition of the rear-arc lavas in the absence of any mineralogical and geochemical lines of evidence of crustal contamination suggests an independent magmatic source.     

SHRIMP U–Pb zircon geochronology and Sr–Nd isotopic systematic of the Neoproterozoic Ghimbi-Nedjo mafic to intermediate intrusions of Western Ethiopia: a record of passive margin magmatism at 855 Ma?

The reworked Pre-Neoproterozoic and juvenile Neoproterozoic terrane of the Western Ethiopian Shield (WES) consists of three N–S trending terranes. These are the western migmatitic gneissic terrane, the central metavolcano sedimentary terrane (CVST) and the eastern migmatitic gneissic terrane. The eastern part of the CVST mostly consists of suture-related ultramafic-metasedimentary complexes, whereas metavolcanics predominate in the western part. Gabbroic to granitic intrusions frequently occur in the CVST and in adjacent areas. New zircon SHRIMP U–Pb ages for two gabbros and three diorites in the Ghimbi-Nedjo region of the WES indicate magmatic crystallization ages. Two pulses of magmatism, at 860–850 and 795–785 Ma, are documented with the former for the first time. The tholeiitic Kemashi diorite and Bikilal-Ghimbi gabbros have oceanic affinities and yield U/Pb zircon ages of 856.3 ± 9.8 and 846.0 ± 7.6 Ma, respectively. The calc-alkaline Gebeya Kemisa pyroxene diorite, and the Senbet Dura hornblende diorite plus the tholeiitic Wayu Meni gabbro, which collectively have arc-back arc characteristics are indistinguishable at ages of 794.3 ± 9.4, 787.7 ± 8.8 and 778.1 ± 6.3 Ma, respectively. Positive εNd (4.5–7.0) and low initial ⁸⁷Sr/⁸⁶Sr (0.7029 ± 0.0002) and a mean T _DM model age of 0.95 Ga for the Ghimbi-Nedjo region (mean T _DM model age of 0.95 Ga for the WES overall) indicate that the magmas were generated from juvenile Neoproterozoic depleted mantle sources, with no discernable involvement of pre-Neoproterozoic continental crust. The occurrence of gabbros and diorites with oceanic tholeiite affinities combined with the new ages suggests that the intrusions were emplaced in the earliest stages of the rifting of Rodinia. This event in the WES led to the development of a passive margin and associated plume-type magmatism at ~855 Ma. The two intrusive groups with differing magma chemistry and ages suggest that the earliest magmatism was tholeiitic and associated with the passive margin system followed by continental breakup to form the Mozambique Ocean. The combination of tholeiitic and calc-alkaline magmatism was related to arc and back-arc basin formation and later terrane accretion (~830–690 Ma).     

Simulations of dense planetary rings: IV. Spinning self-gravitating particles with size distribution

Previous self-gravitating simulations of dense planetary rings are extended to include particle spins. Both identical particles as well as systems with a modest range of particle sizes are examined. For a ring of identical particles, we find that mutual impact velocity is always close to the escape velocity of the particles, even if the total rms velocity dispersion of the system is much larger, due to collective motions associated to wakes induced by near-gravitational instability or by viscous overstability. As a result, the spin velocity (i.e., the product of the particle radius and the spin frequency) maintained by mutual impacts is also of the order of the escape velocity, provided that friction is significant. For the size distribution case, smaller particles have larger impact velocities and thus larger spin velocities, particularly in optically thick rings, since small particles move rather freely between wakes. Nevertheless, the maximum ratio of spin velocities between the smallest and largest particles, as well as the ratio for translational velocities, stays below about 5 regardless of the width of the size distribution. Particle spin state is one of the important factors affecting the temperature difference between the lit and unlit face of Saturn's rings. Our results suggest that, to good accuracy, the spin frequency is inversely proportional to the particle size. Therefore, the mixing ratio of fast rotators to slow rotators on the scale of the thermal relaxation time increases with the width of the particle size distribution. This will offer means to constrain the particle size distribution with the systematic thermal infrared observations carried by the Cassini probe.     

Sylvite and halite on particles recovered from 25143 Itokawa: A preliminary report

We observed cross sectional ultra‐thin sections near the surface of 12 particles recovered from the S‐type asteroid Itokawa by the Hayabusa spacecraft in 2010, using spherical aberration–corrected STEM and conventional TEM. Although their mineralogy is almost identical to the equilibrated LL chondrites and therefore basically anhydrous, micrometer‐to‐submicrometer‐sized sylvite was identified on the surface of Itokawa particle RA‐QD02‐0034. Separately, micrometer‐sized halite was also identified on the surface of Itokawa particle RA‐QD02‐0129. Detailed inspection of the sample processing procedures at the JAXA's Planetary Materials Sample Curation Facility and textural observation of the sylvite and halite indicate that they were clearly present on two Itokawa particles before they were removed from Clean Chamber #2 at JAXA. However, there is no direct evidence for their extraterrestrial origin at present. If the sylvite and halite are extraterrestrial, their presence suggests that they may be more abundant on the surface of S‐type asteroids than previously thought.     

Dust in the solar system and in extra-solar planetary systems

Among the observed circumstellar dust envelopes a certain population, planetary debris disks, is ascribed to systems with optically thin dust disks and low gas content. These systems contain planetesimals and possibly planets and are believed to be systems that are most similar to our solar system in an early evolutionary stage. Planetary debris disks have been identified in large numbers by a brightness excess in the near-infrared, mid-infrared and/or submillimetre range of their stellar spectral energy distributions. In some cases, spatially resolved observations are possible and reveal complex spatial structures. Acting forces and physical processes are similar to those in the solar system dust cloud, but the observational approach is obviously quite different: overall spatial distributions for systems of different ages for the planetary debris disks, as opposed to detailed local information in the case of the solar system. Comparison with the processes of dust formation and evolution observed in the solar system therefore helps understand the planetary debris disks. In this paper, we review our present knowledge of observations, acting forces, and major physical interactions of the dust in the solar system and in similar extra-solar planetary systems.     

Laboratory production of monophase pyrrhotite grains using solid-solid reaction and their characteristic infrared spectra

A new method of producing pyrrhotite grains, which are most commonly found in cometary material and interplanetary dust particles, was developed. Pyrrhotite grains in the monophase having a 7C structure were predominately produced using a solid-solid reaction between iron and sulfur grains at room temperature. The characteristic infrared peaks were observed at 602, 563, and 397 cm⁻¹ (16.6, 17.8, and 25.2 μm).     

Yamato 792947, 793408 and 82038: The most primitive H chondrites,with abundant refractory inclusions

Abstract— In this paper we report petrological and chemical data of the unusual chondritic meteorites Yamato (Y)‐792947, Y‐93408 and Y‐82038. The three meteorites are very similar in texture and chemical composition, suggesting that they are pieces of a single fall. The whole‐rock oxygen isotopes and the chemical compositions are indicative of H chondrites. In addition, the mineralogy, and the abundances of chondrule types, opaque minerals and matrices suggest that these meteorites are H3 chondrites. They were hardly affected by thermal and shock metamorphism. The degree of weathering is very low. We conclude that these are the most primitive H chondrites, H3.2–3.4 (S1), known to date. On the other hand, these chondrites contain extraordinarily high amounts of refractory inclusions, intermediate between those of ordinary and carbonaceous chondrites. The distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula. The mineralogy, chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in CO and E chondrites.     

Acid‐susceptive material as a host phase of argon‐rich noble gas in the carbonaceous chondrite Ningqiang

Abstract— A fine‐grained dark inclusion in the Ningqiang carbonaceous chondrite consists of relatively pristine solar nebular materials and has high concentrations of heavy primordial rare gases. Trapped 36Ar concentration amounts to 6 times 10^?6 cc STP/g, which is higher than that of Ningqiang host by a factor of three. Light HF‐HCl etching of the dark inclusion removed 86, 73, and 64% of the primordial ³⁶Ar, ⁸⁴Kr, and ¹³²Xe, respectively. Thus, the majority of the noble gases in this inclusion are located in very acid‐susceptive material. Based on the elemental composition, the noble gases lost from the dark inclusion during the acid‐treatments are Ar‐rich, and the noble gases remaining in the inclusion are Q and HL gases. Transmission electron microscopy showed that the acid treatments removed thin Si, Mg, and Fe‐rich amorphous rims present around small olivine and pyroxene grains in the dark inclusion, suggesting that the Ar‐rich gases reside in the amorphous layers. A possible origin of the Ar‐rich gases is the acquisition of noble‐gas ions with a composition fractionated relative to solar abundance favoring the heavy elements by the effect of incomplete ionization under plasma conditions at 8000 K electron temperature.