Experimental study of incongruent evaporation kinetics of enstatite in vacuum and in hydrogen gas

Variations in bulk Mg/Si ratios in the various groups of chondritic meteorites indicate that Mg/Si fractionation occurred in the primitive solar nebula. Enstatite (MgSiO₃) evaporates incongruently forming forsterite (Mg₂SiO₄) as an evaporation residue; therefore, evaporation of enstatite produces Mg/Si variations in solid (Mg-rich) and gas (Si-rich) and must be considered as a probable process responsible for Mg/Si fractionation recorded in chondrites. To understand the evaporation kinetics of enstatite, incongruent evaporation experiments on enstatite single crystals have been carried out in vacuum and in hydrogen gas at temperatures of 1300 to 1500°C. A polycrystalline forsterite layer is formed on the surface of enstatite by preferential evaporation of the SiO₂ component, both in vacuum and in hydrogen gas. The thickness of the forsterite layer in vacuum increases with time in the early stage of evaporation and later the thickness of the forsterite layer remains constant (several microns). This is due to the change in the rate limiting process from surface reaction plus nucleation and growth to diffusion in the surface forsterite layer. The activation energy of the diffusion-controlled evaporation rate constant of enstatite is 457 (±58) kJ/mol. A thinner forsterite layer is formed on the surface of enstatite in hydrogen gas than in vacuum. Evaporation of enstatite in hydrogen gas is also considered to be controlled by diffusion of ions through the forsterite layer. The thin forsterite layer formed in hydrogen gas is ascribed to the enhanced evaporation rate of forsterite in the presence of hydrogen gas.The results are applied to incongruent evaporation under the solar nebular conditions. The steady thickness of the forsterite of nebular pressure-temperature conditions is estimated to be submicron because of the enhanced evaporation rate of forsterite under hydrogen-rich nebular conditions if evaporated gases are taken away immediately and no back reaction occurs (an open system). Because enstatite grains in the solar nebula would be comparable to the estimated steady thickness of forsterite, evaporation of such enstatite grains under kinetic conditions could play an important role in producing variations in Mg/Si ratios between solid and gas in the solar nebula.     

Overprinting porphyry‐type veinlets on the intrusive rocks and phreatomagmatic breccias in the Southwest prospect,southwestern Sto. Tomas II (Philex), Baguio District,Philippines

The Southwest prospect is located at the southwestern periphery of the Sto. Tomas II porphyry copper–gold deposit in the Baguio District, northwestern Luzon, Philippines. The Southwest prospect hosts a copper‐gold mineralization related to a complex of porphyry intrusions, breccia facies, and overlapping porphyry‐type veinlets emplaced within the basement Pugo metavolcanics rocks and conglomerates of the Zigzag Formation. The occurrences of porphyry‐type veinlets and potassic alteration hosted in the complex are thought to be indications of the presence of blind porphyry deposits within the Sto. Tomas II vicinity. The complex is composed of at least four broadly mineralogically similar dioritic intrusive rocks that vary in texture and alteration type and intensity. These intrusions were accompanied with at least five breccia facies that were formed by the explosive brecciation, induced by the magmatic–hydrothermal processes and phreatomagmatic activities during the emplacement of the various intrusions. Hydrothermal alteration assemblages consisting of potassic, chlorite–magnetite, propylitic and sericite–chlorite alteration, and contemporaneous veinlet types were developed on the host rocks. Elevated copper and gold grades correspond to (a) chalcopyrite–bornite assemblage in the potassic alteration in the syn‐mineralization early‐mineralization diorite (EMD) and contemporaneous veinlets and (b) chalcopyrite‐rich mineralization associated with the chalcopyrite–magnetite–chlorite–actinolite±sericite veinlets contemporaneous with the chlorite–magnetite alteration. Erratic remarkable concentrations of gold were also present in the late‐mineralization Late Diorite (LD). High X_Mg of calcic amphiboles (>0.60) in the intrusive rocks indicate that the magmas have been oxidizing since the early stages of crystallization, while a gap in the composition of Al between the rim and the cores of the calcic amphiboles in the EMD and LD indicate decompression at some point during the crystallization of these intrusive rocks. Fluid inclusion microthermometry suggests the trapping of immiscible fluids that formed the potassic alteration, associated ore mineralization, and sheeted quartz veinlets. The corresponding formation conditions of the shallower and deeper quartz veinlets were estimated at pressures of 50 and 30 MPa and temperatures of 554 and 436°C at depths of 1.9 and 1.1 km. Temperature data from the chlorite indicate that the chalcopyrite‐rich mineralization associated with the chlorite–magnetite alteration was formed at a much lower temperature (ca. 290°C) than the potassic alteration. Evidence from the vein offsetting matrix suggests multiple intrusions within the EMD, despite the K‐Ar ages of the potassic alteration in EMD and hornblende in the LD of about the same age at 3.5 ± 0.3 Ma. The K‐Ar age of the potassic alteration was likely to be thermally reset as a result of the overprinting hydrothermal alteration. The constrained K‐Ar ages also indicate earlier formed intrusive rocks in the Southwest prospect, possibly coeval to the earliest “dark diorite” intrusion in the Sto. Tomas II deposit. In addition, the range of δ³⁴S of sulfide minerals from +1.8‰ to +5.1‰ in the Southwest prospect closely overlaps with the rest of the porphyry copper and epithermal deposits in the Sto. Tomas II deposit and its vicinity. This indicates that the sulfides may have formed from a homogeneous source of the porphyry copper deposits and epithermal deposits in the Sto. Tomas II orebody and its vicinity. The evidence presented in this work proves that the porphyry copper‐type veinlets and the adjacent potassic alteration in the Southwest prospect are formed earlier and at a shallower level in contrast with the other porphyry deposits in the Baguio District.     

Temporal variations in the abundance and sinking flux of diatoms under fast ice in summer near Syowa Station, East Antarctica

To investigate the fate of ice algae released from sea ice, we investigated the abundance, species composition, and sinking flux of diatoms in the water column under fast ice near Syowa Station, Antarctica during the summer of 2005/2006. The diatom assemblage in the water column consisted of chain-forming planktonic species, in contrast to the under-ice assemblage dominated by pennate species reported from this site in the past; this dissimilarity suggests the presence of an unconsolidated platelet ice layer under the congelation ice, within which planktonic species can bloom. Among the dominant diatoms, Porosira pseudodenticulata and Pseudo-nitzschia cf. turgiduloides were dominant in the water column, and their water column stocks were higher than their mass sedimentation. These species apparently maintain their populations in the surface layer, as their production remains in the water column. In contrast, Fragilariopsis kerguelensis and Thalassiosira australis were scarce in the water column but rich in the flux, indicating active sinking and export of their production to the benthic ecosystem. This distinction in buoyancy control and sinking characteristics of the dominant diatoms on release from the fast ice influences the diatom species composition and carbon flow under the ice.     

The nature of the CM parent asteroid regolith based on cosmic ray exposure ages

Cosmic ray exposure (CRE) ages of CM chondrites have been found to have multiple peaks (as many as four), in stark contrast to other groups of chondrites (Nishiizumi and Caffee 2012; Herzog and Caffee 2014). In this study, we sought correlations between the CRE ages and petrography of CM chondrites, and we conclude that the degree of aqueous alteration does appear to vary with the CRE ages—the CMs displaying the most aqueous alteration all have relatively short exposure ages. However, some CMs with low degrees of alteration also have short exposure ages—thus, this apparent correlation is not exclusive. We also found a definite inverse relation between the number of distinctive lithologies in a CM and its exposure age, which could indicate different responses of homogeneous and heterogeneous meteoroids to the space environment between their onset of exposure (exhumation and ejection from the parent body) and arrival at Earth. Breccias have more internal surfaces of lithologic discontinuity, possibly resulting in weaker meteoroids that disintegrate more readily than their more homogeneous counterparts. Our results suggest that CM chondrite regoliths consist of numerous genomict lithologies in a breccia with millimeter‐ to decimeter‐scale clasts, with varying degree of heating/metamorphism.     

Crustal structure in the Mizuho Plateau, East Antarctica, by a two-dimensional ray approximation

Studies on the crustal and upper-mantle structure in Antarctica have been one of the major contributions to Antarctic research since the International Geophysical Year of 1957–1958. Many refraction surveys with small charge size have been conducted in Antarctica, but long-range experiments were also made in 5 regions on the margin of the Antarctic continent.In 1979–1981, the scientific program of the Japanese Antarctic Research Expedition was focused on the earth sciences, and in particular, an explosion seismic experiment along a long survey line was the major item during these years. An experiment along a 300 km-long line with three shots and 27 observation stations was successfully made in the northern Mizuho Plateau, East Antarctica.From the analysis of travel times and the amplitude study of synthetic seismograms, the crustal structure of the northern Mizuho Plateau was determined. The depths of the Conrad and the Moho discontinuties were determined as 31 km and 42 km, respectively. The P-wave velocity and depth relation was determined as 6.0 km/s on the surface of the bedrock, 6.3 km/s at a depth of 2 km, 6.8 km/s at the Conrad and 7.9 km/s at the Moho. The velocity in the crust increases gradually. The crustal structure obtained is representative of East Antarctica.     

Deep-sea submersible survey in the Suruga, Sagami and Japan Trenches: preliminary results of the 1985 Kaiko cruise, Leg 2

Nine submersible dives were made in three trenches off central Japan, between 2990 and 5900 m of water depth. Our observations confirm the interpretation that Daiichi-Kashima Seamount is a Cretaceous guyot formed on the Pacific plate that has traveled into the Japan Trench. We also confirmed the previous interpretation of a large normal fault that splits the seamount in two halves, the lower one being now subducting beneath the Japan margin. Compressional deformation was identified within the lower part of the inner slope in front of the seamount. The pattern of deformation that affects Quaternary sediments is in agreement with the present kinematics of the convergence between the Pacific plate and Japan. Deep-water (5700 m) clam colonies are associated with advection of fluids, driven by the subduction-related overpressures. In the northern slope of the Boso Canyon, along the Sagami Trough system (Philippine Sea plate-Japan boundary), the deformation affecting a thick upper Miocene to lower Pliocene sequence indicates two directions of shortening: a N175°E direction which is consistent with the present relative motion along the Sagami Trough (N285–N300°E) and a N30°E direction which could be related to a more northerly direction of convergence that occured during the early Quaternary and earlier.     

Petrology and shock history of the first depleted-like poikilitic shergottite Asuka 12325

Asuka (A) 12325 is the first poikilitic shergottite having a depleted pattern in light rare earth elements (REE). Compared with known poikilitic shergottites, A 12325 has smaller but more abundant pyroxene oikocrysts with remarkable Fe-rich pigeonite rims, indicating that A 12325 cooled relatively faster at a shallower part of the crust. The redox condition (logfO₂ = IW + 0.6-IW + 1.7) and Fe-rich chemical compositions of each mineral in A 12325 are close to enriched shergottites. The intermediate shergottites could not form by a simple mixing between parent magmas of A 12325 and enriched shergottites. Although A 12325 contains various high-pressure minerals such as majorite and ringwoodite, plagioclase is only partly maskelynitized. Therefore, the maximum shock pressure may be within 17–22 GPa. Thermal conduction and ringwoodite growth calculation around a shock vein revealed that the shock dwell time of A 12325 is at least 40 ms. The weaker shock pressure and longer shock dwell time in A 12325 may be attained by an impact event similar to those of nakhlites and Northwest Africa (NWA) 8159. Such a weak shock ejection event may be as common on Mars as a severe shock event recorded in shergottites. Alteration of sulfide observed in A 12325 may imply the presence of magmatic fluid in its reservoir on Mars. A 12325 expands a chemical variety of Martian rocks and has a unique shock history among poikilitic shergottites while A 12325 also implies that poikilitic shergottites are common rocks on Mars regardless of their sources.     

Mineral chemistry of MUSES‐C Regio inferred from analysis of dust particles collected from the first‐ and second‐touchdown sites on asteroid Itokawa

The mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES‐C Regio were characterized by synchrotron‐radiation X‐ray diffraction and field‐emission electron microprobe analysis. Olivine and low‐ and high‐Ca pyroxene, plagioclase, and merrillite compositions of the first‐touchdown particles are similar to those of the second‐touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES‐C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first‐ and second‐touchdown samples, indicate that dust particles of MUSES‐C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES‐C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K‐feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact‐induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL6 ordinary chondrite where Na‐K distributions of plagioclase have been disturbed.     

Anharmonic effect on the equation of state (EoS) for NaCl

We find clear intrinsic anharmonicity in the NaCl-B1 phase by examining the equation of state (EoS) based on previous ultrasonic velocity data for pressures up to 0.8 GPa and temperatures up to 800 K. The experimental EoS for this phase shows that its specific heat at constant volume (C _V ) is significantly smaller than that based on a harmonic model. Also, the sign of $\left( {{{\partial C_{V} } \mathord{\left/ {\vphantom {{\partial C_{V} } {\partial P}}} \right. \kern-0pt} {\partial P}}} \right)_{T} ,$ which is normally negative in the quasi-harmonic approximation, is unexpectedly positive. The thermodynamic Grüneisen parameter (γ), which has frequently been assumed to be a single-variable function of molar volume, shows not only volume dependence but also negative temperature dependence. To understand these features of C _V and γ, we introduce a thermodynamic model including positive quartic anharmonicity. To make an anharmonic model advancing the ordinarily quasi-harmonic approximation model, we introduce two parameters: anharmonic characteristic temperature (θ _a ) and its volume derivative. In the anharmonic model, the value of C _V is calculated along an isochore using classical statistical mechanics and a harmonic quantum correction. At high temperatures, the decrease in C _V from the Dulong-Petit limit is related to the value of T/θ _a . For infinitely large θ _a , the system is approximately quasi-harmonic. The temperature dependence of γ is related to C _V by the thermodynamic identity $\left( {{{\partial C_{V} } \mathord{\left/ {\vphantom {{\partial C_{V} } {\partial \ln V}}} \right. \kern-0pt} {\partial \ln V}}} \right)_{T} = C_{V} \left( {{{\partial \gamma } \mathord{\left/ {\vphantom {{\partial \gamma } {\partial \ln T}}} \right. \kern-0pt} {\partial \ln T}}} \right)_{V} + \gamma \left( {{{\partial C_{V} } \mathord{\left/ {\vphantom {{\partial C_{V} } {\partial \ln T}}} \right. \kern-0pt} {\partial \ln T}}} \right)_{V}.$ Even though our modification of the quasi-harmonic approximation is simple, our anharmonic model succeeds in reproducing the experimental γ and C _V simultaneously for the NaCl-B1 phase.     

Ten Years of GIAHS Development in Japan

Approximately ten years have passed since Globally Important Agricultural Heritage Systems (GIAHS) was introduced to Japan in 2011, with 11 GIAHS sites designated so far. The Japan Nationally Important Agricultural Heritage Systems (J-NIAHS), which considers resilience, multi-stakeholder participation and sixth industrialization, was subsequently established in 2016, and has designated 15 J-NIAHS sites. GIAHS sites can be classified into three major types: Landscape, farming method, and genetic resource conservation types, and most Japanese GIAHS sites are of the landscape type. Since there is almost no national subsidy for GIAHS or J-NIAHS, designated sites are expected to secure funding for conservation from their own efforts. For this reason, a voluntary network of the Japanese GIAHS sites has been active in promoting cooperation on GIAHS conservation. The priorities of the Japanese GIAHS have focused on raising public awareness about GIAHS and J-NIAHS, improving livelihoods, as well as fostering the international exchange of experience and knowledge regarding Agricultural Heritage Systems, especially among Japan, China and Korea.