Horoman Peridotite Complex Research Articles

Structural evolution of the Horoman peridotite complex in conjunction with the formation of the Hidaka Metamorphic Belt, Hokkaido

We conducted crystal-fabric analyses of peridotites within the Horoman peridotite complex in the Hidaka metamorphic belt, Hokkaido, Japan. Over fifty oriented peridotite samples were collected and analyzed for olivine fabric strength (J-index) and crystallographic preferred orientations (CPOs). The peridotites contained four olivine CPOs: A, E, D, and AG types. We confirmed that olivine CPOs presented a transitional distribution from E to A to AG type from south to north. Previous experimental studies have demonstrated that E type CPO can merge under hydrous conditions. In addition, magnetotelluric observations suggested that the subducting oceanic plate supplies pore fluid to the basal thrust in the region (the Hidaka Main Thrust). Therefore, we infer that the E type CPO was originated from a local water infiltration event. AG type CPO, on the other hand, was considered as a secondary product enhanced by the heating event in the northern (geological upper) part, combined with the pressure-temperature path inferred by previous petrological studies. Furthermore, we reconstructed the senses of shear strain using the microstructure and olivine crystal-fabrics. Then we built the tectonic history of the Horoman peridotite complex integrating the structural development of the Hidaka Metamorphic Belt.

Deformation and melt–rock interaction in the upper mantle: Insights from the layered structure of the Horoman peridotite, Japan

To obtain a better understanding of melt–rock interactions in the upper mantle, microstructural and petrological analyses were conducted on deformed mantle peridotites from the Horoman peridotite complex, Hokkaido, Japan. The Horoman peridotite complex is lithologically heterogeneous and contains various kinds of ultramafic and mafic rocks. We studied an outcrop of 3 × 70 m in size that contains layered spinel harzburgite, plagioclase lherzolite, and mafic rocks. The results indicate that reactive melts migrated preferentially along the foliation in the already deformed peridotite, and that these melt-rich zones became especially prone to further deformation. This inference is supported by (1) the parallelism of the boundaries of rock layers and foliation in the deformed peridotite, and the shape and crystallographic preferred orientations (SPOs and CPOs) of olivine in the peridotites; (2) the diffusive trends of magnesium and modal compositions of pargasite grains near the boundaries between peridotite and mafic layers; (3) variations in the NiO content of olivine crystals; (4) variations in olivine CPOs with orthorhombic (010)[100] slip system patterns and weak fiber-[010] patterns; and (5) the strong pargasite SPOs, the cuspate shapes of the pargasites, and the absence of intercrystallite deformation. The results, combined with previously reported P–T conditions for the Horoman peridotite complex, indicate that the deformed peridotites and mafic rocks with a layered structure represent temperatures of 1050–1150 °C and pressures of 0.7–1.5 GPa. Our results suggest that a decrease in pressure led to the transition from a melt-free to a melt-bearing system with a consequent change in the deformation mechanism, from dislocation creep in the melt-free system to diffusion creep in the melt-bearing system, with strain localization in the fine-grained melt-rich layers. The change in deformation mechanism is likely to have occurred in the uppermost mantle beneath a mid-ocean ridge, where strong rheological contrasts are controlled by spatial variations in the melt fraction.

Variation in olivine crystal-fabrics and their seismic anisotropies in the Horoman Peridotite Complex, Hokkaido, Japan

We examined the microstructures and crystal-fabrics of peridotites within a large area (6×5km) of the Horoman Peridotite Complex in the Hidaka metamorphic belt of Hokkaido, Japan. Thirteen peridotite samples were analyzed for olivine and orthopyroxene grain sizes, fabric strength (J-index), and crystallographic preferred orientations (CPOs). Mean grain sizes of olivine and orthopyroxene were ranged in 295–497µm and in 257–537µm, respectively. The olivine fabric strength values decreased from the lower to the upper part of the complex, whereas the orthopyroxene fabric strength values showed no systematic trends. The peridotites contained three different olivine CPOs, previously known as A, E, and AG types. Combined with a previous study, we found that olivine CPOs showed a transitional distribution from E to A to AG type from south to north. E type peridotites occur at the basement of the complex in the south, suggesting that local water infiltration might occur at the basement of the complex. The A type peridotites occurred mainly in the middle of the studied area and subsequently the AG type peridotites occurred towards the north. Moreover, we calculated the seismic properties of peridotite as olivine 100% aggregates and mixed (olivine and orthopyroxene) aggregates. It showed that orthopyroxene CPOs reduce P-wave anisotropies of peridotite (0.2–2.2%) without modification of the P-wave propagation patterns.

Relationships between intensity of deformation induced Cr–Al chemical zoning and geometrical properties of spinel: An approach applying machine learning analyses

Chromian spinel [(Mg, Fe2+) (Cr, Al, Fe3+)2O4], a fundamental petrogenetic indicator in Earth's crust and upper mantle systems, carries a unique Cr–Al chemical zoning. It has been postulated that a specific variant of this Cr–Al chemical zoning, particularly when influenced by deformation, can offer considerable insights into the processes of deformation of peridotites, and further advancing our understanding of spinel deformation. In our study, we observed distinct Cr and Al distributions in spinel grains showing the Cr–Al chemical zoning within a dunite sample from the Horoman Peridotite Complex, contingent on their geometric properties such as grain size and aspect ratio. Our observations and previous studies suggest a strong correlation between geometric properties and Cr–Al chemical zoning, but the complexity of these relationships has not been fully explored. To bridge this research gap, we utilized a combination of electron backscatter diffraction (EBSD) and energy-dispersive X-ray spectroscopy (EDS) to provide detailed grain descriptions and analyze origin of the observed Cr–Al zoning. Furthermore, Machine learning algorithms were applied to elucidate the relationships between Cr–Al zoning and geometric properties. This integrated approach illuminated intricate relationships between the geometrical properties of spinel grains and their Cr–Al chemical zoning. The results of this study suggests that lattice diffusion is primarily activated in coarser grains (cluster 1), moderately in intermediate-sized grains (cluster 2), and not active in finer grains (cluster 3). We propose a model where coarser grains undergo deformation predominantly via dislocation-assisted diffusion creep, with lattice diffusion as the primary underlying mechanism. Conversely, finer grains seem to deform mainly through diffusion creep, where grain boundary diffusion plays a significant role. This study offers a novel model, developed via machine learning, for understanding variations in the intensity of Cr–Al chemical zoning and the respective deformation mechanisms in spinel grains.

Remarkably fresh abyssal peridotites from Sibuyan island, Romblon Island Group, Philippines: Markers of young arc-continent collision

The bulk of Sibuyan island in the Romblon Island Group, Philippines is underlain by remarkably fresh spinel lherzolites, harzburgites and dunites collectively called as the Sibuyan Ultramafics. Based on petrographic and geochemical characteristics, the Sibuyan Ultramafics can be classified into: (1) Group I peridotites composed of spinel lherzolites and harzburgites with spinel Cr# < 0.35, (2) Group II harzburgites and dunites with intermediate spinel Cr# = 0.40–0.65, and (3) Group III dunites with very high spinel Cr# (>0.75). Increase in spinel Cr# is accompanied by decreasing Al2O3 contents and increasing Mg# in the pyroxenes from the Group I to Group II and Group III peridotites. Chondrite-normalized clinopyroxene data revealed various trace-element patterns for the three groups suggestive of different processes (e.g. anhydrous and hydrous melting, mantle-melt interaction) involved in their formation. The Sibuyan Ultramafics is closely associated with the similarly fresh Calaton Hill metamorphic/plutonic complex in Tablas island which is thought to be representative of the lower crust of the Philippine island arc. At a more regional context, the Sibuyan Ultramafics and the Calaton Hill metamorphic/plutonic complex can be used as markers for the young arc-continent collision in western Philippines. Rapid uplift and emplacement due to the collision of the Philippine Mobile Belt and the Palawan Microcontinental Block during the Miocene or even later may have led to the remarkably fresh state of these lithologies. This occurrence is similar to the Horoman peridotite complex in the Hidaka metamorphic belt in Northern Japan. The Sibuyan Ultramafics and the Calaton Hill metamorphic/plutonic complex therefore provide a rare and fresh glimpse into the upper mantle and lower crust in the Philippines due to their young ages of emplacement.

Preferential Orthopyroxene Serpentinization and Implications for Seismic Velocity Interpretation in the Fore‐Arc Mantle

AbstractFor better interpretation of seismic tomography data of the hydrated fore‐arc mantle, we examined conditions of nonstoichiometric hydration of peridotite reported by a previous experimental study (i.e., preferential orthopyroxene (Opx) serpentinization and resultant talc formation) at 500–580 °C and under 0.8–1.8 GPa. The corresponding seismic velocity was calculated by considering the talc production. Talc formed under all the experimental conditions even in the olivine‐dominated system, in which the olivine to Opx ratio was 7:3 by weight. The amount of talc produced was comparable to that of serpentine at high temperature and low pressure and increased with increasing temperature and decreasing pressure due to decreasing contribution of olivine in the reaction. We also present a natural case of preferential Opx reaction in the Horoman peridotite complex, Japan, on the basis of its strong similarity in textures and mineral phases to our experimental products obtained at 0.8–1.3 GPa. Calculations on seismic velocities of hydrated harzburgite using the Voigt‐Reuss‐Hill averaging scheme showed that the formation of talc along with antigorite leads to a decrease in P wave (Vp) and S wave velocities (Vs) but no remarkable increase in Vp/Vs due to the lower Vp/Vs value of talc (~1.60) compared with that of antigorite (~1.77). Such a seismic anomaly has been observed in the shallow mantle wedge of the southern Western Alps subduction zone, where suppressed water supply from the subducted continental lithosphere might allow the persistence of talc produced by the preferential Opx reaction.

Dunite channels within a harzburgite layer from the Horoman peridotite complex, Japan: Possible pathway for magmas

AbstractTwo discordant dunite channels in a polished slab were collected from a harzburgite layer in the Horoman orogenic lherzolite massif, Hokkaido, Japan. The dunite channels show extreme grain‐size variations, and include very large olivine megacrysts up to 1.6 m long. Spinel‐rich pyroxenite veins or veinlets occasionally cross‐cut the olivine megacrysts. Mineral compositions differ between channels, and the mineral compositions and textures of the dunite channels and the host harzburgite suggest a replacive origin for the dunites. The Sr and Nd isotopic ratios of clinopyroxene separates from the pyroxenite veins in two dunite channels are different, but these ratios are generally similar to those of mid‐ocean ridge basalts. Assuming the channels formed by the through‐flow of melts, then the trace element compositions of the melts estimated from the clinopyroxenes are broadly similar. The melts have similar features such as enriched in incompatible elements and depleted in heavy rare earth elements, but there are differences in the depletions in Ti and Ba, indicating melts that were different but of similar origins. A Rb–Sr crystallization age (~ 52 Ma) was obtained for clinopyroxene–orthopyroxene pairs in a dunite channel and its host harzburgite. The distinctive dunite channels with megacrysts formed as a result of the through‐flow of mafic magma(s), and those magmas had trace and minor element contents that are indicative of contributions of a melt derived from subducted oceanic lithosphere.

COPPER ISOTOPE CHARACTERISTICS OF COPPER-RICH MINERALS FROM THE HOROMAN PERIDOTITE COMPLEX, HOKKAIDO, NORTHERN JAPAN

The copper isotope ratios of primary and secondary native copper grains in well-characterized plagioclase lherzolites from the Horoman peridotite complex, Hokkaido, northern Japan, were measured by femtosecond-pulsed laser ablation-multiple collector-inductively coupled plasma-mass spectrometry (fs-LA-MC-ICP-MS). The whole-rock copper isotope value of the fresh fertile plagioclase lherzolite was also determined. The primary native copper and whole-rock samples yield homogeneous δ 65 Cu (where δ 65 Cu = [{( 65 Cu/ 63 Cu) sample /( 65 Cu/ 63 Cu) NIST-SRM976 } − 1] × 1,000) values between −0.03 and 0.14‰, implying that there is no significant copper isotope fractionation during high-temperature magmatic processes. These and previously published results suggest that the copper isotope range of terrestrial igneous copper is from −0.27 to 0.27‰. In contrast, secondary native copper grains affected by serpentinization have obviously lower δ 65 Cu values (−0.67 to −0.41‰) than primary native copper and whole-rock samples, though the compositional differences between primary native copper and secondary native copper are significantly small, suggesting that selective preferential oxidation or leaching of the heavier copper isotope occurs during low-temperature serpentinization processes. Because secondary low-temperature processes probably disturb the original copper isotope values of primary copper-rich minerals, in situ determination of copper isotope values for intact mineral grains by microanalytical techniques such as LA-MC-ICP-MS has become increasingly important to understand primary isotope signatures of rock and ore samples.

Subhorizontal tectonic framework of the Horoman peridotite complex and enveloping crustal rocks, south‐central Hokkaido, Japan

AbstractGeological observations in the Horoman area, south‐central Hokkaido, show that the Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about 1200 m thick. The peridotite slab is intercalated into a gently east‐dipping structure. The underlying unit is a Cretaceous–Paleogene accretionary complex. Riedel shear planes in the sedimentary layers of the accretionary complex near the structural bottom of the peridotite slab indicate top‐to‐the‐west (thrust) displacement. The overlying unit is composed of felsic–pelitic gneisses and mafic–felsic intrusive rocks (the Hidaka metamorphic rocks). The boundary surface between the peridotite complex and metamorphic rocks forms a domal structure. Microstructures of sheared metamorphic rocks near the structural top of the peridotite slab indicate top‐to‐the‐east (normal) displacement. The results combined with previous studies suggest that the Horoman peridotite complex was emplaced onto the Asian margin (Northeast Japan) during the collision between the Asian margin and the Hidaka crustal block.

Origin of layering observed in the Horoman peridotite complex, Japan

Origin of layering observed in the Horoman peridotite complex, Japan

Formation and deformation mechanisms of pyroxene-spinel symplectite in an ascending mantle, the Horoman peridotite complex, Japan: An EBSD (electron backscatter diffraction) study

Symplectites, fine vermicular intergrowth (< 10 μm) of orthopyroxene, clinopyroxene, and spinel, and the sur- rounding lenticular coarser-grained (100-200 μm) aggregate (seam) with the same mineral assemblage of the symplectites define remarkable foliation and lineation in spinel lherzolites of the Horoman complex, northern Japan. They are inferred to be products of reaction between garnet and olivine during decompression of the host peridotite accompanying deformation. Microstructure of a symplectite was investigated with automated electron backscattered diffraction (EBSD) analysis using a field -emission gun SEM (FE-SEM) in order to clarify reaction and deformation mechanisms and thereby better constraining mechanical interaction between the Earth's upper mantle and lower crust. The symplectite is composed of two segments with a large misorien- tation angle of ~ 60° only for spinel, and the two spinel crystals are in mirror symmetry with the segment boundary approximately parallel to the mirror plane. The segment boundary is interpreted as spinel law twin formed during phase transition from garnet. Each segment is further subdivided into several sectors with grad- ual lattice distortion smaller than a few degrees/mm and intra-sector misorientation mostly smaller than 25° for all constituent minerals and with misorientation axes nearly perpendicular to the lineation and parallel to the foliation. The sector boundaries are inferred to be subgrain boundaries formed by dislocation creep of py- roxenes and spinel in the spinel stability field. The spinel twin suggests that a garnet was decomposed directly into entangled aggregate of pyroxenes and spinel, which grew from an embryo nucleated on the surface of the reactant garnet. The symplectite minerals in each sector show systematic crystallographic orientations (topo- taxy) with each other. The topotaxial relationship in the fine intergrowth with subgrain structure demonstrates that the systematic crystallographic orientations were acquired when the crystals grew by decomposing the garnet and were later modified by deformation during the consecutive ascent of the complex.

Occurrence and chemical composition of amphiboles and related minerals in corundum-bearing mafic rock from the Horoman Peridotite Complex, Japan

A corundum-bearing mafic rock in the Horoman Peridotite Complex, Japan, was derived from upper mantle conditions to lower crustal conditions with surrounding peridotites. The amphiboles found in the rock are classified into 3 types: (1) as interstitial and/or poikilitic grains (Green amphibole), (2) as a constituent mineral of symplectitic mineral aggregates with aluminous spinel at grain boundary between olivine and plagioclase (Symplectite amphibole) and (3) as film-shaped thin grains, usually less than 10 μm in width, at grain boundary between olivine and clinopyroxene (Film-shaped amphibole). The Film-shaped amphibole is rarely associated with orthopyroxene extremely low in Al 2O 3, Cr 2O 3 and CaO (Low-Al OPX). These minerals were formed by infiltration of SiO 2- and volatile-rich fluids along grain boundaries after the rock was recrystallized at olivine-plagioclase stability conditions, i.e. the late stage of the exhumation of the Horoman Complex. Chondrite-normalized rare earth element patterns and primitive mantle-normalized trace-element patterns of the Green amphibole and clinopyroxene are characterized by LREE-depleted patterns with Eu positive and negative anomalies of Zr and Hf. These geochemical characteristics of the constituent minerals were inherited from original whole-rock compositions through a reaction involving both pre-existing clinopyroxene and plagioclase. We propose that the fluids were originally rich in a SiO 2 component but depleted in trace-elements. Dehydration of the surrounding metamorphic rocks in the Hidaka metamorphic belt, probably related to intrusion of hot peridotite body into the Hidaka crust, is a plausible origin for the fluids.

Trace element and isotopic (Sr and Nd) compositions of clinopyroxenes in dunite channels of the Horoman peridotite complex, Hokkaido, Japan

Trace element and isotopic (Sr and Nd) compositions of clinopyroxenes in dunite channels of the Horoman peridotite complex, Hokkaido, Japan

Deformation history and exhumation process of the Horoman Peridotite Complex, Hokkaido, Japan

The Horoman Peridotite Complex is an Alpine-type orogenic peridotite massif in the Hidaka metamorphic belt, Hokkaido, Northern Japan. Because of wide exposure and extremely limited serpentinization, the complex provides important information on uplift and emplacement processes of an Alpine-type peridotite massif into the crust. Based on microstructures, the massif can be divided into five structural units parallel to the lithological layering as follows; (1) Equigranular Zone, (2) Internal Shear Zone (ISZ), (3) Transition Zone, (4) Porphyroclastic Zone and (5) Basal Shear Zone (BSZ). A top-to-the-north sense of shear deformation in the Porphyroclastic Zone and the Basal Shear Zone implies that the Horoman Peridotite Complex had uplifted from the upper mantle to the lower crust along a northward dipping extensional shear-zone systems. After incorporation of the mantle peridotite with lower crustal rocks, the upper part of the massif (i.e. the Equigranular Zone and the Internal Shear Zone) was overprinted by a top-to-the-south sense of shear deformation that was comparable with the sub-horizontal displacement of the crustal granulite sequences in the Hidaka metamorphic belt under transpressive tectonic environment.

Possible Non-melted Remnants of Subducted Lithosphere: Experimental and Geochemical Evidence from Corundum-Bearing Mafic Rocks in the Horoman Peridotite Complex, Japan

. Morishita, T. & Arai, S. (2001). Petrogenesis of corundum-bearing mafic rock in the Horoman Peridotite Complex, Japan. Journal of Petrology 42, 1279---1299. Morishita, T. & Kodera, T. (1998). Finding of corundum-bearing gabbro boulder possibly derived from the Horoman Peridotite Complex, Hokkaido, northern Japan. Journal of Mineralogy, Petrology and Economic Geology 93, 52---63. Morishita, T., Arai, S. & Gervilla, F. (2001). High-pressure aluminous mafic rocks from the Ronda peridotite massif, southern Spain: significance of sapphirineand corundum-bearing mineral assemblages. Lithos 57, 143---161. Morishita, T., Arai, S. & Green, D. H. (2003a). Evolution of low-Al orthopyroxene in the Horoman peridotite, Japan: an unusual indicator of metasomatising fluids. Journal of Petrology 44,

Thermal History of the Horoman Peridotite Complex: A Record of Thermal Perturbation in the Lithospheric Mantle

The ascent history of the Horoman peridotite complex, Hokkaido, northern Japan, is revised on the basis of a detailed study of large ortho- and clinopyroxene grains 1 cm in size (megacrysts) in the Upper Zone of the complex. The orthopyroxene megacrysts exhibit distinctive M-shaped Al zoning patterns, which were not observed in porphyroclastic grains less than 5 mm in size described in previous studies. Moreover, the Al and Ca contents of the cores of the orthopyroxene megacrysts are lower than those of the porphyroclasts. The Upper Zone is inferred to have resided not only at a higher temperature than previously suggested but also at a higher pressure (1070C, 23 GPa) than the Lower Zone (950C, 19 GPa), in the garnet stability field, before the ascent of the two zones. The Horoman complex probably represents a 12 5k m thick section of lithospheric mantle with an 10 8C/km vertical thermal gradient. The current thickness of the Horoman complex is3 km, which is a result of shortening of the lithospheric mantle by 025 01 during its ascent. The Upper Zone appears to have experienced a heating event during its ascent through the spinel stability field, with a peak temperature as high as 1200C. The effect of heating decreases continuously towards the base of the complex, and the lowermost part of the Lower Zone underwent very minor heating at a pressure higher than 05 GPa. The uplift and associated deformation, as well as heating, was probably driven by the ascent of a hot asthenospheric upper-mantle diapir into the Horoman lithosphere.

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 peridotite from the Horoman Peridotite Complex, Japan. Al2O3 ,C r 2 O 3and CaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter) are 5025 wt%, 5004 wt% and 503 wt%, respectively, and are distinctively lower than those in orthopyroxene porphyroclasts. The Low-Al OPX occurs in two modes, both at the margin of olivine. The first mode of occurrence is as the rim of a large orthopyroxene porphyroclast in contact with olivine. This type of Low-Al OPX occurs only locally (15mm 45mm), and the orthopyroxene rim in contact with olivine more commonly has normal Al2O3 contents (42 wt%). In the second mode of occurrence, the Low-Al OPX occurs as a thin film, 5mm 50mm in dimension, at a grain boundary between olivine and clinopyroxene. Trace element compositions of porphyroclast clinopyroxene in the sample indicate that the sample having the Low-Al OPX underwent metasomatism although there are no hydrous minerals around the Low-Al OPX. Petrographic observations and trace element compositions of clinopyroxene combined with an inferred P‐T history of the Horoman peridotite suggest that the Low-Al OPX was formed through a very local reaction between peridotite and invasive fluids, probably formed by dehydration of a subducted slab, in a late stage of the history of the Horoman peridotite. Crystallization of orthopyroxene, representing addition of silica to mantle lherzolite via a CO2a H2O-bearing fluid phase, is a mechanism for metasomatic alteration of mantle wedge peridotite.

Evolution of spinel–pyroxene symplectite in spinel–lherzolites from the Horoman Complex, Japan

We examined the textural and geochemical characteristics of spinel–pyroxene symplectites in spinel–lherzolites collected from the lowest, middle, and upper parts (LZ1, MZ, and UZ1, respectively) of the Horoman Peridotite Complex, Japan. The modal proportion of the minerals within symplectite is almost the same, i.e., orthopyroxene:clinopyroxene:spinel = 2:1:1. The size of the symplectite minerals increases from the lowest through to the middle to the upper parts in the complex. The reconstructed major element composition of the bulk symplectites is intermediate between pyrope-rich garnet and olivine. The model garnet compositions of the LZ1satisfies garnet stoichiometry and those of the MZ and UZ1 are not consistent with garnet stoichiometry. The primitive mantle-normalized pattern in trace elements for the LZ1 symplectite is similar to that of pyrope-rich garnet from fertile peridotites, particularly in its enrichment of HREE and a positive Zr anomaly. Thus, the LZ1 symplectite has inherited both major and trace element signatures from pre-existing garnet whereas the compositions of the MZ and UZ1 symplectites were modified during and/or after breakdown of pre-existing garnet. Geochemical and textural variations of symplectites might basically correspond to temperature differences within the complex during upwelling of the Horoman Complex. The basal part of the complex (LZ1) experienced the lowest temperature decompression path in the complex, which resulted in less textural and chemical modification. On the other hand, the higher part of the complex (UZ1) experienced a relatively higher temperature decompression path than other parts of the complex, resulting in chemical equilibration among the constituent minerals and coarsening of the symplectite minerals. Selective enrichment of Sr and LREE in the symplectite may indicate that the metasomatism by a Sr- and LREE-rich melt/fluid occurred during and/or after the formation of symplectite.

Observations of three-dimensional microstructures of symplectite minerals in the Horoman Peridotite Complex using a high-resolution X-ray CT system at Spring-8

X-ray computerized tomography (CT) has become a powerful tool for completely non-destructive examination of the interiors of geological objects. An X-ray CT image is a map of the spatial distribution of values related to the linear attenuation coefficient (LAC), which depends on mass density, chemical composition and incident X-ray energy (Nakano et al., 2000). In this report we show the application of this method to a symplectite consisting of irregular fine-grained orthopyroxene, clinopyroxene and spinel from the Horoman peridotite, Japan. The symplectite in the Horoman peridotites is interpreted to be of pyropic garnet origin. Textural characteristics of symplectite from the Horoman complex will therefore provide opportunities to investigate the phase transition from garnet to spinel lherzolite. We imaged symplectites using a highresolution X-ray CT system at SPring-8 (Uesugi et al., 2001) with a monochromatic X-ray beam of 15 keV (Fig.1). Brighter regions on the CT images correspond to higher values of LAC. Three-dimensional structures of the symplectites were obtained from 1018 slices of the CT images with 1000 × 1000 matrix (Figs.2, 3). The voxel size is 0.5 μm × 0.5 μm × 0.5 μm, and the effective spatial resolution is about 1 μm. Clusters of orthopyroxene, clinopyroxene and spinel in the symplectites were clearly identified using the contrast in the CT images, which coincided with the theoretical LAC values of these minerals (Fig.1).Acknowledgements: The authors thank to Greg Yaxley to improve the English of the manuscript. Constructive review by Susumu Ikeda improved the manuscript.

Lead isotope systematics of the Horoman peridotite complex, Hokkaido, northern Japan

Lead isotope systematics of the Horoman peridotite complex, Hokkaido, northern Japan