The Albedo of Ryugu: Evidence for a High Organic Abundance, as Inferred from the Hayabusa2 Touchdown Maneuver

The Hayabusa2 mission successfully collected samples from the asteroid Ryugu last year and will return these to Earth in December 2020. It is anticipated that the samples will enable the analysis of terrestrially uncontaminated organic matter and minerals. Such analyses are in turn expected to elucidate the evolution of organic matter through Solar System history, including the origination and processing of biogenically important molecules, which could have been utilized by the first organisms on Earth. In anticipation, studies have made predictions concerning the properties of Ryugu, including its composition. The spectral characteristics of Ryugu, such as albedo, have been employed to relate the asteroid to members of the carbonaceous chondrite group that have been identified on Earth. However, the recent Hayabusa2 touchdown highlights a disparity between the color of surfaces of displaced platy fragments, indicating a brightening trend for the surface exposed to space compared to that facing into the body. Here we present a mass balance calculation with reference to data from the literature, which indicates that Ryugu may contain a significantly higher abundance of organic matter (likely >50%) than the currently most accepted meteorite analogues. A high organic content may result in high levels of extractable organic matter for the second touchdown site, where the spacecraft sampled freshly exposed material. However, high abundances of insoluble aromatic/graphitic rich organic matter may be present in the first touchdown site, which sampled the surface of Ryugu that had been exposed to space. Moreover, we suggest that the potentially high organic abundance and the rubble-pile nature of Ryugu may originate from the capture of rocky debris by a comet nucleus and subsequent water-organic-mineral interactions and sublimation of water ice

Mantle-derived trace element variability in olivines and their melt inclusions

Trace element variability in oceanic basalts is commonly used to constrain the physics of mantle melting and the chemistry of Earth's deep interior. However, the geochemical properties of mantle melts are often overprinted by mixing and crystallisation processes during ascent and storage. Studying primitive melt inclusions offers one solution to this problem, but the fidelity of the melt-inclusion archive to bulk magma chemistry has been repeatedly questioned. To provide a novel check of the melt inclusion record, we present new major and trace element analyses from olivine macrocrysts in the products of two geographically proximal, yet compositionally distinct, primitive eruptions from the Reykjanes Peninsula of Iceland. By combining these macrocryst analyses with new and published melt inclusion analyses we demonstrate that olivines have similar patterns of incompatible trace element (ITE) variability to the inclusions they host, capturing chemical systematics on intra- and inter-eruption scales. ITE variability (element concentrations, ratios, variances and variance ratios) in olivines from the ITE-enriched Stapafell eruption is best accounted for by olivine-dominated fractional crystallisation. In contrast, ITE variability in olivines and inclusions from the ITE-depleted Háleyjabunga eruption cannot be explained by crystallisation alone, and must have originated in the mantle. Compatible trace element (CTE) variability is best described by crystallisation processes in both eruptions. Modest correlations between host and inclusion ITE contents in samples from Háleyjabunga suggest that melt inclusions can be faithful archives of melting and magmatic processes. It also indicates that degrees of ITE enrichment can be estimated from olivines directly when melt inclusion and matrix glass records of geochemical variability are poor or absent. Inter-eruption differences in olivine ITE systematics between Stapafell and Háleyjabunga mirror differences in melt inclusion suites, and confirm that the Stapafell eruption was fed by lower degree melts from greater depths within the melting region than the Háleyjabunga eruption. Although olivine macrocrysts from Stapafell are slightly richer in Ni than those from Háleyjabunga, their overall CTE systematics (e.g., Ni/(Mg/Fe), Fe/Mn and Zn/Fe) are inconsistent with being derived from olivine-free pyroxenites. However, the major element systematics of Icelandic basalts require lithological heterogeneity in their mantle source in the form of Fe-rich and hence fusible domains. We thus conclude that enriched heterogeneities in the Icelandic mantle are composed of modally enriched, yet nonetheless olivine-bearing, lithologies and that olivine CTE contents provide an incomplete record of lithological heterogeneity in the mantle. Modally enriched peridotites may therefore play a more important role in oceanic magma genesis than previously inferred.</p

Bilateral Heterogeneity in an Upwelling Mantle via Double Subduction of Oceanic Lithosphere

Vietnam is a major field of Cenozoic volcanism in Southeast (SE) Asia. Two contrasting models have been proposed to explain the mantle upwelling and volcanism in this region; collision of the Indian and Eurasian continents or subduction of the Pacific or Indo-Australian oceanic lithosphere. To place constraints on the origin of the intraplate volcanism in SE Asia, new geochronological and geochemical data for Cenozoic basalts in Vietnam are presented. Based largely on Sr-Nd-Pb isotope systematics, it was found that the sources of basalts from Central and Southern Vietnam are chemically distinct forming a sharp boundary at 13.5°N. The basalts north of the boundary show isotopic features similar to Enriched Mantle type 2 (EM2) ocean island basalts. Whereas the basalts south of the boundary show isotopic features similar to Enriched Mantle type 1 (EM1) ocean island basalts. The EM1 and EM2 basalts display positive Sr anomalies and elevated Pb/Ce and Th/La ratios, respectively. Such features suggest the origins of the sources through the recycling of deeply-subducted crustal lithologies. Furthermore, subduction of dense oceanic lithosphere can induce a convecting cell in the upper mantle. Therefore, we suggest that the chemically different basalts from Central and Southern Vietnam represent the surface expression of melting in two different convecting cells, one is driven by subduction of the Pacific plate and the other by subduction of the Indo-Australian plate

Desensitization of delayed-type hypersensitivity in mice: suppressive environment

The systemic injection of high doses of antigen into a preimmunized animal results in transient unresponsiveness of cell-mediated immune responses. This phenomenon is known as desensitization. Serum interleukin 2 (IL-2) activity was found transiently in desensitized mice at 3 h after the antigen challenge. These mice could not reveal antigen nonspecific delayed-type hypersensitivity (DTH) 1 d after the challenge. Specific suppression of DTH was observed at later stages. Sera from 3 h desensitized mice showed suppressive effects on DTH in preo immunized mice. Administration of recombinant IL-2 into preimmunized mice led to the failure of development of DTH to antigens. These observations suggest that IL-2 plays an important role in the suppressive environment

Mineralogical alterations in calcite powder flooded with MgCl2 to study Enhanced Oil Recovery (EOR) mechanisms at pore scale

Seawater injection into chalk-reservoirs on the Norwegian Continental Shelf has increased the oil recovery and reduced seabed subsidence, but not eliminated it. Therefore, understanding rock–fluid interactions is paramount to optimize water injection, predict and control water-induced compaction. Laboratory experiments on onshore and reservoir chalks have shown the need to simplify the aqueous chemistry of the brine, and also the importance of studying the effect of primary mineralogy of chalk to understand which ions interact with the minerals present. In this study, the mineralogy of the samples tested, are simplified. These experiments are carried out on pure calcite powder (99.95%), compressed to cylinders, flooded with MgCl2, at 130 °C and 0.5 MPa effective stress, for 27 and 289 days. The tested material was analysed by scanning and transmission electron microscopy, along with whole-rock geochemistry. The results show dissolution of calcite followed by precipitation of magnesite. The occurrence and shape of new-grown crystals depend on flooding time and distance from the flooding inlet of the cylinder. Crystals vary in shape and size, from a few nanometres up to 2 μm after 27 days, and to over 10 μm after 289 days of flooding and may crystallize as a single grain or in clusters. The population and distribution of new-grown minerals are found to be controlled by nucleation- and growth-rates along with advection of the injected fluid through the cores. Our findings are compared with in-house experiments on chalks, and allow for insight of where, when, and how crystals preferentially grow

Origin of ocean island basalts in the West African passive margin without mantle plume involvement

The geochemical variabilities in intraplate basalts (IB) from the West African passive margin (WAPM) region, have generally been employed to indicate the presence of recycled materials in an associated upwelling mantle plume. However, the absence of time-progressive linear hotspot tracks in WAPM-IB make it difficult to explain their genesis solely by the mantle plume hypothesis. Here, we show that the Sr–Nd–Hf–Pb isotopic variations in basalts from most of the WAPM-IB could have mainly attributed to the derivation from two types of fusible regions of the refertilized subcontinental lithospheric mantle (SCLM) and the sub-lithospheric mantle. The locations and magma genesis of WAPM-IB are strongly related to the distance from the Mesozoic rift axis and the structure of the rifted SCLM. The melting of the source region can possibly be attributed to small-scale mantle convection at the base of the SCLM without the involvement of a mantle plume

Temperature-Dependent Anomalies in the Structure of the (001) Surface of LiCu2O2

Surface corrugation functions, derived from elastic helium atom scattering (HAS) diffraction patterns at different temperatures, reveal that the Cu2+ rows in the (001) surface of LiCu2O2 undergo an outward displacement of about 0.15 {\AA} as the surface was cooled down to 140 K. This is probably the first time that isolated one-dimensional magnetic ion arrays were realized, which qualifies the Li1+Cu2+O2-2 surface as a candidate to study one-dimensional magnetism. The rising Cu2+ rows induce a surface incommensurate structural transition along the a-direction. Surface equilibrium analysis showed that the surface Cu2+ ions at bulk-like positions experience a net outward force along the surface normal which is relieved by the displacement. Temperature-dependent changes of the surface phonon dispersions obtained with the aid of inelastic HAS measurements combined with surface lattice dynamical calculations are also reported.Comment: 4 pages, 7 figure