Hydrothermal Precipitation of Chromites

<正>Chromite is a typical refractory igneous mineral,precipitated from mafic magmas at relatively high temperatures.Chromites commonly occur in sedimentary,metamorphic and metasomatic rocks,where they are interpreted as relics of an igneous phase and serve as the source of Cr for low-temperature Cr-bearing minerals.We     

Lunar meteorite Queen Alexandra Range 94281: Glass compositions and other evidence for launch pairing with Yamato 793274

Abstract— Lunar meteorite Queen Alexandra Range 94281 is remarkably similar to Yamato 793274. Pairing in the conventional Earth-entry sense is difficult to reconcile with the 2500 km separation between the find locations for these two samples. Nonetheless, both of these regolith breccias are dominated by very-low-Ti (VLT) mare basalt, the pyroxenes of which feature exsolution lamellae on a remarkably coarse scale (typical lamella width = 0.5–1 μm) by mare standards. The pyroxenes also show similar compositional variations (e.g., Fe# vs. Ti# trends, which confirm parentage from VLT mare basalt). Plots using Al₂O₃ or FeO as a tracer of the highland component indicate indistinguishable internal mare-highland geochemical mixing trends. The same two distinctive glass types dominate the mare glass populations of both breccias. Glass type YQ1 features 0.37–0.63 wt% TiO₂, 10–17 wt% MgO, and 9–11 wt% Al₂O₃. Glass type YQ2 features higher TiO₂ (0.99–1.22 wt%), which is inversely correlated with MgO (12.6–13.8 wt%), and nearly constant (8.8 wt%) Al₂O₃. All of these similarities suggest that Y-793274 and QUE 94281 are a launch pair, which we designate YQ. Most of these similarities also extend to another mare-breccia meteorite, Elephant Moraine 87521. However, the EET 87521 mare basalt is unusually V-poor (～88 μg/g), whereas the YQ mare component contains ～166 μg/g. Queen Alexandra Range 94281 features a variety of textural domains. Discrete patches of dark matrix material appear to represent clods of mature regolith that have been mixed with a coarser, relatively immature material. Interior to a frothy fusion crust are areas of massive glass that probably formed as a splash coating on QUE 94281 when it was still on the Moon. The coarse YQ and EET 87521 pyroxene exsolution features imply relatively slow cooling in either a very shallow sill or an unusually thick (ponded) lava and/or later annealing within a cryptomare. Mare pyroclastic glasses, including the two YQ varieties, are systematically MgO-rich compared to crystalline mare basalts. This disparity may be a consequence of limited survival of graphite—the main fuel for explosive volcanism—during formation of the mare source regions as magma ocean cumulates. Graphite (2.2 g/cm³) survived preferentially in regions that avoided extensive early melting and thus remained MgO-rich. An apparent bimodality in the TiO₂ contents of mare volcanics, especially the pyroclastic glasses, also seems a plausible consequence of petrogenesis by remelting of magma ocean cumulates. Cumulates deposited after the magma ocean evolved to ilmenite saturation had vastly higher TiO₂ contents than cumulates deposited shortly before. The YQ regolith's subequal proportions of mare and highland matter are consistent with derivation from a terrain close to a mare-highland boundary. However, a similar mixture might also develop through vertical mixing in a cryptomare or a region of thin mare coverage. Thus, unfortunately, the YQ bulk composition is not a very useful clue to the identity of the source crater.     

Recent macroids on the Kikai‐jima shelf,Central Ryukyu Islands,Japan

Sandy and gravelly carbonate sediments found off Kikai‐jima, southern Japan, a coral reef‐related island shelf, represent the northernmost sub‐tropical, carbonate deposits in the Central Ryukyu Islands (Ryukyus). On the Kikai‐jima shelf, at water depths of 61 to 105 m, these sediments are characterized by macroid pavements. Since the abundance of very small and of exceptionally large macroids may indicate specific hydrodynamic controls regarding constraints on growth and taphonomy, the detailed analysis of recent and fossil macroid pavements is meaningful ecologically and environmentally. Macroids, ranging in size from ca 25 to 130 mm in diameter, are spheroidal and sub‐spheroidal in shape and consist mainly of the encrusting foraminifer Acervulina inhaerens and subordinate thin encrusting and lumpy coralline algae. Accessory components include bryozoans, serpulids and, to a lesser extent, encrusting arborescent foraminifera (Homotrema and Miniacina). Low sedimentation rates and occasional movement due to current action are indicated by sizes, shapes and growth‐forms of the studied macroids, the Entobia–Gastrochaenolites–Trypanites–Maeandropolydora ichnocoenosis and the ‘Bioerosion Index’ for coated grains (introduced herein). The deep‐water tidally induced current energy was sufficient to maintain multi‐directional growth (spheroidal shapes) of the larger macroids and to initiate macroid growth using the diverse biogenic remnants as nuclei. The asymmetrical inner arrangement suggests possible periods of stability for the macroids. The residence time of the coated grain in its original environment determines the size and morphology of the macroid and the selection of coating organisms. The composition of the coating community is mainly a consequence of component growth rates in relation to turnover time and residence time. Long‐term studies are needed to assess the spatial and temporal resolution of present‐day encrusting communities across biogeographic provinces and shelf to slope regions.     

Four lunar mare meteorites: Crystallization trends of pyroxenes and spinels

Abstract— We studied crystallization trends of pyroxene and spinel in four Antarctic meteorites known to be derived from mare regions of the Moon: Y-793169 and A-881757 (YA meteorites) are unbrecciated igneous basalts, EET 87521 is a fragmental breccia, and Y-793274 is a regolith breccia. All have relatively low bulkrock TiO₂ content, and the YA meteorites are uncommonly ancient. Our electron probe microanalysis (EPMA) data indicate that the YA meteorites and the dominant mare components of Y-793274 and EET 87521 conform to a general trend for Ti-poor (low-Ti and very low-Ti) mare basalts. Their pyroxenes show a strong correlation between Fe/(Fe + Mg) (Fe#) and Ti/(Ti + Cr) (Ti#), both ratios typically increasing from core to rim. These trends presumably reflect local crystallization differentiation of interstitial melt. Previous studies (M. J. Drake and coworkers) have suggested that the detailed configurations of such Fe# vs. Ti# trends may reflect the bulk TiO₂ contents of the parent magmas (basalts). As a more systematic approach to this problem, we plot bulk-rock TiO₂ as a function of the Fe# = 0.50 intercept of each rock's pyroxene Fe# vs. Ti# trend. We call this intercept the Fe#-normalized Ti#. Based on our data for EET 87521, the YA meteorites, and Apollo 12 basalts 12031 and 12064, plus literature data for several other Ti-poor mare basalts, we find a strong correlation between Fe#-normalized Ti# and the bulk TiO₂ content of the parent basalt. This correlation confirms that fragmental breccia EET 87521 is nearly pure very low-Ti (VLT) basalt and that the YA meteorites, for which bulk-rock TiO₂ results scatter due to unusually coarse grain size (A-881757) or scarcity of available sample (Y-793169), are pieces of an uncommonly Ti-poor, but not quite VLT, variety of low-Ti mare basalt. Extrapolating from this correlation, the dominant mare component of regolith breccia Y-793274 is probably of VLT affinity. Besides the normal mare pyroxene trend of strong correlation between Fe# and Ti#, Y-793274 includes two additional pyroxene compositional trends, both showing a wide range of Ti# despite relatively constant (and low, by mare standards) Fe#. The most magnesian of these trends consists of a single clast with a mode of orthopyroxene + MgO-rich ilmenite. These two trends are of uncertain origin. Possibly one or both represents the highland component of this regolith breccia, although, unlike most highland pyroxenes, these appear relatively unaltered by impact brecciation and metamorphism. Compositions of spinels in the coarse-grained A-881757 show an extraordinary distribution: chromite and ulvöspinel components vary among grains but are nearly constant within grains. Despite its old age and unusually coarse grain sizes, mineralogical evidence (i.e., heterogeneity within both pyroxene and spinel; typical pyroxene exsolution scale very coarse by mare standards but exceeded by the pyroxenes of EET 87521 and Y-793274) indicates that A-881757 was cooled only slightly more slowly than typical mare basalts and may have formed near the center of an uncommonly thick lava flow. Both of the VLT basaltic lunar meteorite breccias, EET 87521 and Y-793274, are composed dominantly of pyroxenes with exsolution coarser than normal for mare basalts. Possibly VLT basalt flows tend to be systematically thicker, and thus more slowly cooled, than more Ti-rich flows.     

Evolution of Low-Al Orthopyroxene in the Horoman Peridotite, Japan: an Unusual Indicator of Metasomatizing Fluids

Unusually alumina-poor orthopyroxene is found in a spinel peridotitefrom the Horoman Peridotite Complex, Japan. Al₂O₃, Cr₂O₃ andCaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter)are <0·25 wt %, <0·04 wt % and <0·3wt %, respectively, and are distinctively lower than those inorthopyroxene porphyroclasts. The Low-Al OPX occurs in two modes,both at the margin of olivine. The first mode of occurrenceis as the rim of a large orthopyroxene porphyroclast in contactwith olivine. This type of Low-Al OPX occurs only locally (15µm x 45 µm), and the orthopyroxene rim in contactwith olivine more commonly has normal Al₂O₃ contents (>2wt %). In the second mode of occurrence, the Low-Al OPX occursas a thin film, 5 µm x 50 µm in dimension, at agrain boundary between olivine and clinopyroxene. Trace elementcompositions of porphyroclast clinopyroxene in the sample indicatethat the sample having the Low-Al OPX underwent metasomatismalthough there are no hydrous minerals around the Low-Al OPX.Petrographic observations and trace element compositions ofclinopyroxene combined with an inferred P–T history ofthe Horoman peridotite suggest that the Low-Al OPX was formedthrough a very local reaction between peridotite and invasivefluids, probably formed by dehydration of a subducted slab,in a late stage of the history of the Horoman peridotite. Crystallizationof orthopyroxene, representing addition of silica to mantlelherzolite via a CO₂ + H₂O-bearing fluid phase, is a mechanismfor metasomatic alteration of mantle wedge peridotite. KEY WORDS: Horoman Peridotite Complex; low-Al orthopyroxene; metasomatism; mantle wedge     

Platinum–Group Minerals in Podiform Chromitite from the Kamuikotan Zone, Hokkaido, Northern Japan

Abstract: Ru–Os–Ir alloys have been found in two podiform chromitites located at the Chiroro and Bankei mines in the Sarugawa peridotite complex in the Kamuikotan zone, Hokkaido, Japan. This is the first report on the occurrence of PGM (= platinum-group minerals) from chromitites in Japan. The Ru–Os–Ir alloys most typically form polyhedra associated with other minerals (Ni–Fe alloys and heazlewoodite) in chromian spinel. The PGM are possibly pseudomorphs after some primary PGM such as laurite and are chemically highly inhomogeneous, indicating a low-temperature alteration origin. This is consistent with intense alteration (formation of serpentine, uvarovite and kämmererite) imposed on the Kamuikotan chromitites. High-temperature primary PGE (platinum–group elements)–bearing sulfides were possibly recrystallized at low temperatures into a new assemblage of PGM, Ni-Fe alloys and sulfides. Placer PGM around the peridotite complexes are chemically different from the PGM in dunite and chromitite possibly due to the, as yet, incomplete search for the rock-hosted PGM. The PGE content in chromitites is distinctly higher in those in the Kamuikotan zone than in those in the Sangun zone of Southwest Japan, consistent with the more refractory nature (Cr# of spinel, up to 0.8) of the former than the latter (Cr# of spinel, 0.5).     

Dunite--Harzburgite--Chromitite Complexes as Refractory Residue in the Sangun--Yamaguchi Zone, Western Japan

Dunite, harzburgite and chromitite of alpine-type ultramaficcomplexes emplaced in the Paleozoic sediments in the Sangun—Yamaguchizone, western Japan, are massive and almost lacking in layeredstructure. Constituent minerals are more or less deformed andequilibrated at a relatively low temperature, about 700 °Cor lower. Chromian spinels in ultramafic rocks from dunite—harzburgite—chromititecomplexes in the Sangun—Yamaguchi zone are characterizedby the uniformity of the Cr/Cr + Al ratio, around 0.5, regardlessof locality and rock type, which is in contrast to the widevariation of the ratio of chromian spinel from the ordinaryalpine-type dunite—harzburgite complex. Mg/Mg + Fe" ratioof chromian spinel, on the other hand, is variable in parallelto the volume per cent of chromian spinel in ultramafic rocks.Olivine in ultramafic rocks is uniform in chemical composition,from Fo₈₈ to Fo₉₂, except for that in chromitite, of which itis Fo₉₅ to Fo₉₇. Primary chemical compositions (especially the Mg/Mg + Fe" ratio)of the constituent minerals have been modified to some extentby element redistribution at low temperature (700 °C orlower), the degree of modification depending on the volume ratioof the minerals. For example, the Mg/Mg + Fe" ratio of chromianspinel in peridotitic rocks has been lowered substantially,and inversely, that of olivine in chromitite has been raised.Primary Mg/Mg + Fe" ratios of olivine and chromian spinel canbe estimated, assuming a certain value of high temperature,e.g. 1200 °C, and the partition coefficient between olivineand chromian spinel at that temperature. As a result, the Mg/Mg+ Fe" ratio of olivine (0.88 to 0.92), and that of chromianspinel (0.78 to 0.80) were uniform, irrespective of rock typeand locality, at the high-temperature stage. Dunite—harzburgite—chromitite complexes in the Sangun—Yamaguchizone were accumulated as refractory residue after the relativelylarge-scaled partial fusion of some primordial peridotites,which resulted in the chemical uniformity of the residual minerals.They may have initially constituted the lowest part of an ophioliticsuite as ultramafic tectonite and been emplaced as dismemberedportions after the disruption of the parent body.     

Orogenic Lherzolites and Mantle Processes: Editorial

The papers in this thematic issue of Journal of Petrology weredelivered at the Fourth International Workshop on Orogenic Lherzolitesand Mantle Processes, which was held in Samani, Hokkaido, Japan,between August 26 and September 3, 2002. Fifty oral presentationswere given and 45 posters were displayed during the meeting,and the research papers in this issue provide an     

Remnants of an ancient forest on the continental shelf of northwest Japan

Tree stumps in a standing position have been discovered in 20 to 40 meters of water in Toyama Bay, Japan. Radiocarbon dated samples from two of these stumps yielded ages of 8.480±70 and 9.060±80 years B.P., indicating that the stumps are remnants of a forest that existed during thc early Holocene.