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Hydration, carbonation, and related metasomatism of mantle peridotite play a significant role in the global geochemical cycle. In this study, we combined an analysis of carbonated serpentinite with hydrothermal experiments on carbonation and Ca-metasomatism for samples from the Manlay ophiolite, southern Mongolia to investigate that carbonation mechanism of the serpentinite body after serpentinization. Samples show that the serpentinite was either transected by calcite and dolomite veins or was completely replaced by carbonates (calcite with minor dolomite) and quartz, in which the original mesh texture of serpentinite was preserved. Carbonation occurred after low-temperature serpentinization (lizardite/chrysotile), suggesting that carbonation occurred at temperatures lower than 300 °C. Calcite in the serpentinite showed [delta].sup.13 C.sub.VPDB values ranging from -8.83 to -5.11 â° and [delta].sup.18 O.sub.VSMOW from + 20.1 to + 24.4 â°, suggesting that CO.sub.2 in the fluids could be derived from the degradation of organic material or methanotrophic processes rather than the origin of seafloor limestone. Three batch-type experiments, i.e., single step experiments (1) Olivine + NaHCO.sub.3,aq + CaCl.sub.2,aq and (2) Chrysotile + NaHCO.sub.3,aq + wollastonite (Ca source), and two steps experiment (3) Olivine carbonation and Ca-metasomatism, were conducted at 275 °C and 5.7 MPa to constrain the mechanism of calcite replacement of serpentinite. We found that calcite precipitated from the solution directly in the first two experiments, but replacement of serpentinite by calcite was not observed. In contrast, the third experiment caused the initial carbonation to form magnesite and then changed to calcite by later alteration. The natural occurrences and experiments revealed the possibility that the carbonation of olivine followed by Ca-rich fluid infiltration produced calcite in the carbonated serpentinite. Such Ca-metasomatism of Mg carbonates could easily occur in the ultramafic bodies and significantly affect the global carbon cycle., Author(s): Nomuulin Amarbayar [sup.1] [sup.2], Otgonbayar Dandar [sup.1] [sup.2], Jiajie Wang [sup.1], Atsushi Okamoto [sup.1], Masaoki Uno [sup.1], Undarmaa Batsaikhan [sup.2] [sup.3], Hideko Takayanagi [sup.4], Yasufumi Iryu [sup.4], Noriyoshi Tsuchiya [...]

Infiltration-driven metamorphism has produced the widespread development of forsterite in the siliceous dolomites of the Alta, Utah contact aureole. SIMS (secondary ion mass spectrometry) [delta].sup.18O analyses show that in most of the middle to outer forsterite zone samples, forsterite, calcite and dolomite are homogenous in [delta].sup.18O at the grain-scale, but forsterite and calcite exhibit minor intergranular heterogeneities of < 1â° on the thin-section to hand sample-scales. In contrast, within ~ 300 m of the igneous contact (inner forsterite and periclase zones), grain-scale [delta].sup.18O heterogeneities in forsterite are as large as 3.0â°, but forsterite crystals do not exhibit systematic center-to-edge decreases in [delta].sup.18O due to skeletal growth and infilling. With one exception, forsterite, calcite and dolomite in all samples analyzed yield disequilibrium fractionations. The oxygen isotope disequilibrium commonly recorded among forsterite, calcite and dolomite may reflect the rapid growth of forsterite from increased reaction overstepping as temperature increased, or results from .sup.18O/.sup.16O depletion in the matrix carbonate minerals during continued fluid infiltration after forsterite crystallization was complete, during the initial stages of cooling in the forsterite zone. In a traverse across the boundary between forsterite- and periclase-bearing marble layers in the periclase zone, forsterite and calcite SIMS [delta].sup.18O profiles combined with the trend in forsterite textures indicate that these layers were not infiltrated simultaneously, nor continuously during both prograde reactions responsible for forsterite and periclase. Rather, the texture and SIMS data require infiltration and formation of forsterite first in the forsterite marble layer, followed by infiltration and formation of periclase in the periclase marble layer. As a consequence, the [delta].sup.18O profile recorded by forsterite formed earlier than the [delta].sup.18O profile recorded by calcite, but both profiles are prograde features-that is, both profiles formed in response to prograde reactions forming first forsterite and then periclase. These grain-scale [delta].sup.18O profiles further suggest that these layers experienced significant changes in permeability that are linked to their sequential infiltration-reaction history. The asynchronous growth of these index minerals at the outcrop-scale during prograde heating may be more common during contact metamorphism of carbonate lithologies than currently recognized., Author(s): Carl J. Beno [sup.1], John R. Bowman [sup.1], Kouki Kitajima [sup.2], John W. Valley [sup.2], Patrick C. Loury [sup.3], Lorraine M. Tapanila [sup.4] Author Affiliations: (1) grid.223827.e, 0000 0001 [...]

A record of fluid flow has been documented within a Paleozoic carbonate platform sequence by U--Pb dating of calcite in veins and vugs from rock core sampled througha shallowly dipping sequenceof sedimentary rocks beneath the Bruce nuclear site, Ontario, Canada. Secondary calcite from >650 m deep Ordovician carbonate rocks yields a Silurian age of 434 [+ or -] 5 Ma possibly related to infiltration of seawater from overlying evaporitic basins as well as hydrothermal solutions that infiltrated from below. In contrast, near-surface Devonian rocks mostly give vein infill ages over the range of 80-100 Ma with evidence for younger infill down to 50 Ma. Vein calcite samples previously dated from surface outcrops of Ordovician carbonate exposed up to 500 km to the east yielded similar U--Pb ages. Coincidence of near-surface vein calcite ages indicates widespread vein emplacement synchronous with a change in direction of motion of the North American plate as well as possible erosional unroofing following passage of the region over the Great Meteor hotspot approximately 125 Myr ago. Deeper carbonate formations have remained apparently impermeable to post-Paleozoic disturbance despite these perturbations. Key words: U--Pb calcite dating, hydrochronology, deep geological repository, southern Ontario carbonate platform, Great Meteor hotspot. Un registre de l'ecoulement des fluides a ete obtenu d'une sequence paleozoique de plateforme carbonatee par la datation U--Pb de calcite dans des veines et cavites dans des carottes prelevees d'une sequence de roches sedimentaires de faible pendage sous le site nucleaire de Bruce (Ontario, Canada). De la calcite secondaire provenant de roches carbonatees ordoviciennes de plus de 650 m de profondeur produit un age silurien de 434 [+ or -] 5 Ma possiblement relie a l'infiltration d'eau de mer issue de bassins evaporitiques sus-jacents, ainsi qu'a des solutions hydrothermales infiltrees depuis le bas. En comparaison, des roches devoniennes situees presde la surface produisent des ages de remplissagede veines dans une fourchettede 80 Ma a 100Maet des indices de remplissage plus recent allant jusqu'a 50 Ma. Des echantillons de calcite de veine deja dates tires d'affleurements de carbonates ordoviciens exposes jusqu'a 500 km a l'est donnent des ages U--Pb semblables. La coincidence des ages de calcite de veines situees pres de la surface indique une mise en place synchrone de veines a grande echelle accompagnant un changement de la direction de deplacement de la plaque nord-americaine, ainsi qu'une possible exposition par erosion suivant le passage de la region au-dessusdu point chauddu Grand Meteore vers 125 Ma.Endepit deces perturbations, des formations carbonatees plus profondes sont demeurees apparemment impermeables aux modifications post-paleozoiques. [Traduit par la Redaction] Mots-cles : datation U--Pb sur calcite, hydrochronologie, depot de dechets nucleaires, plateforme carbonatee du sud de l'Ontario, point chaud du Grand Meteor., Introduction Permanent secure storage of nuclear waste from power generation represents a major societal challenge. Because radioactive material must be confined over a time scale of several million years this [...]

We report the first findings of several occurrences of lawsonite and metamorphic aragonite in the meta-sediments from the Combin Zone (Piemonte-Liguria ocean, Western Alps), where the early blueschist-facies episode is poorly documented. New field and metamorphic data (thermodynamic modelling and Raman spectroscopy on carbonaceous material) are used to elucidate the P-T evolution and fluid composition of the Combin Zone and investigate the lawsonite growth and breakdown reactions. Two tectonometamorphic units have been identified within the Combin Zone with distinct geometry, lithological content and P-T conditions. In the higher grade unit, metamorphic aragonite occurs as inclusions in titanite. Lawsonite and garnet were stable at peak P-T conditions (~ 16-17 kbar and 460-480 °C) at very low X(CO.sub.2) values. Lawsonite is systematically pseudomorphed, but preserves hourglass zoning or internal fabric associated with the prograde ductile deformation. The lower grade unit (~ 8 ± 1 kbar ~ 370-400 °C) is discontinuously exposed along the western base of the continental Dent Blanche nappe and records P-T conditions similar to those recorded by the Dent Blanche nappe. A metamorphic discontinuity is, therefore, documented between the largest part of the Combin Zone on the one hand, and the Dent Blanche nappe on the other hand. The discovery of lawsonite and metamorphic aragonite allows a better understanding of the large-scale metamorphic structure of the Western Alps., Author(s): Paola Manzotti [sup.1], Michel Ballèvre [sup.2], Pavel Pitra [sup.2] [sup.3], Federica Schiavi [sup.4] Author Affiliations: (1) grid.10548.38, 0000 0004 1936 9377, Department of Geological Sciences, Stockholm University, , 106 [...]

Abstract. Heliopora coerulea is the only species in the subclass Octocorallia that has a crystalline aragonite skeleton. The skeleton has been reported to contain the blue pigment, biliverdin IX[alpha], which [...]

We have reconstructed the compositional evolution of the silicate and carbonate melt, and various crystalline phases in the subvolcanic reservoir of Kerimasi Volcano in the East African Rift. Trace element concentrations of silicate and carbonate melt inclusions trapped in nepheline, apatite and magnetite from plutonic afrikandite (clinopyroxene-nepheline-perovskite-magnetite-melilite rock) and calciocarbonatite (calcite-apatite-magnetite-perovskite-monticellite-phlogopite rock) show that liquid immiscibility occurred during the generation of carbonatite magmas from a CO.sub.2-rich melilite-nephelinite magma formed at relatively high temperatures (1,100 °C). This carbonatite magma is notably more calcic and less alkaline than that occurring at Oldoinyo Lengai. The CaO-rich (32-41 wt%) nature and alkali-'poor' (at least 7-10 wt% Na.sub.2O + K.sub.2O) nature of these high-temperature (>1,000 °C) carbonate melts result from strong partitioning of Ca (relative to Mg, Fe and Mn) in the immiscible carbonate and the CaO-rich nature (12-17 wt%) of its silicate parent (e.g., melilite-nephelinite). Evolution of the Kerimasi carbonate magma can result in the formation of natrocarbonatite melts with similar composition to those of Oldoinyo Lengai, but with pronounced depletion in REE and HFSE elements. We suggest that this compositional difference results from the different initial parental magmas, e.g., melilite-nephelinite at Kerimasi and a nephelinite at Oldoinyo Lengai. The difference in parental magma composition led to a significant difference in the fractionating mineral phase assemblage and the element partitioning systematics upon silicate-carbonate melt immiscibility. LA-ICP-MS analysis of coeval silicate and carbonate melt inclusions provides an opportunity to infer carbonate melt/silicate melt partition coefficients for a wide range of elements. These data show that Li, Na, Pb, Ca, Sr, Ba, B, all REE (except Sc), U, V, Nb, Ta, P, Mo, W and S are partitioned into the carbonate melt, whereas Mg, Mn, Fe, Co, Cu, Zn, Al, Sc, Ti, Hf and Zr are partitioned into the silicate melt. Potassium and Rb show no preferential partitioning. Kerimasi melt inclusions show that the immiscible calcic carbonate melt is strongly enriched in Sr, Ba, Pb, LREE, P, W, Mo and S relative to other trace elements. Comparison of our data with experimental results indicates that preferential partitioning of oxidized sulfur (as SO.sub.4.sup.2-), Ca and P (as PO.sub.4.sup.3-) into the carbonate melt may promote the partitioning of Nb, Ta, Pb and all REE, excluding Sc, into this phase. Therefore, it is suggested that P and S enrichment in calcic carbonate magmas promotes the genesis of REE-rich carbonatites by liquid immiscibility. Our study shows that changes in the partition coefficients of elements between minerals and the coexisting melts along the liquid line of descent are rather significant at Kerimasi. This is why, in addition to the REE, Nb, Ta and Zr are also enriched in Kerimasi calciocarbonatites. We consider significant amounts of apatite and perovskite precipitated from melilite-nephelinite-derived carbonate melt as igneous minerals can have high LREE, Nb and Zr contents relative to other carbonatite minerals. Electronic supplementary material The online version of this article (doi:10.1007/s00410-014-1093-4) contains supplementary material, which is available to authorized users., Author(s): Tibor Guzmics[sup.1] , Zoltan Zajacz[sup.2] , Roger H. Mitchell[sup.3] , Csaba Szabo[sup.1] , Markus Wälle[sup.2] Author Affiliations: (1) Lithosphere Fluid Research Laboratory, Department of Petrology and Geochemistry, Eotvos University [...]

Although carbonate-carbon (C), an integral part of biochar-C, contributes to the liming properties of that material, it also interferes with the estimation of the stable organic C fraction in biochars. In this study, four methods were compared in order to quantify the carbonate-C in biochars: two direct (a titrimetric procedure and thermogravimetric analysis, TGA), and two indirect (acid treatment with separation by filtration and acid fumigation). The titrimetric method showed a high recovery of added carbonate-C (average 98.8%, range 1.5-38 mg), and the standard deviations of carbonate- C for all biochars tested were 0.3% wt) and when the isotopic signature of organic C in biochars is to be determined. The TGA method (either in [N.sub.2] or a dry air atmosphere) was reliable when calcite was the main carbonate form in biochars, but was inadequate for samples containing a considerable amount of whewellite and certain carbonate-bearing minerals (e.g. magnesite) that decompose at Additional keywords: inorganic C, method development, organic C, total C., Introduction Biochar is a charcoal-like material produced from the pyrolysis of waste biomass. It has been proposed for use as a soil amendment to enhance carbon (C) storage in soils [...]

Byline: Sabine Helene Brodhag (1), Marco Herwegh (1) Keywords: Static grain growth; Second phases; Zener pinning; Analogue experiment; Norcamphor; Growth law; Calcite; Pinning regimes Abstract: The aim of the study was to investigate the effect of rigid second phases on grain growth of a matrix phase. For this purpose, variable mixtures of norcamphor as the matrix phase, with glass beads (0.08--0.51 volume fraction) as second phase, were used to perform see-through rock-analogue experiments under static conditions at constant temperatures (50degC). Irrespective of the second-phase content, grain-size evolution of all mixtures can be subdivided into a stage of continuous grain growth, a transient stage and a stage of a finally stabilized grain size. On the grain-scale, the second phases affect the migrating grain boundaries either by pinning by single particles, by multiple particles or even by particle clusters. Summed up over the entire aggregate, these pinning regimes affect the average bulk grain size of the matrix grains, such that the changes in matrix grain size directly correlate with the amount of second phases, their dispersion and their degree of clustering. In this way, the matrix grain size decreases with increasing second-phase content, which can be expressed as a Zener relationship. Originating from the modification of an ordinary grain growth law, a new mathematical expression is defined, which allows the calculation of changes in the matrix grain size as a function of different second-phase volume fractions and particle sizes. Such models will be helpful in the future to predict microstructural changes in polymineralic rocks at depth. Author Affiliation: (1) Institute of Geological Sciences, University of Bern, Bern, Switzerland Article History: Registration Date: 26/11/2009 Received Date: 14/01/2009 Accepted Date: 26/11/2009 Online Date: 24/12/2009 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0474-6) contains supplementary material, which is available to authorized users.

The contribution of carbonate-producing benthic organisms to the global marine carbon budget has been overlooked, the prevailing view being that calcium carbonate (CaC[O.sub.3]) is predominantly produced and exported by marine plankton in the 'biological pump.' Here, we provide the first estimation of the global contribution of echinoderms to the marine inorganic and organic carbon cycle, based on organism-level measurements from species of the five echinoderm classes. Echinoderms' global CaC[O.sub.3] contribution amounts to ~0.861 Pg CaC[O.sub.3]/yr (0.102 Pg C/yr of inorganic carbon) as a production rate, and ~2.11 Pg CaC[O.sub.3] (0.25 Pg C of inorganic carbon) as a standing stock from the shelves, slopes, and abyssal depths. Echinoderm inorganic carbon production (0.102 Pg C/yr) is less than the global pelagic production (0.4-1.8 Pg C/yr) and similar to the estimates for carbonate shelves globally (0.024-0.120 Pg C/yr). Echinoderm CaC[O.sub.3] production per unit area is ~27.01 g CaC[O.sub.3] x [m.sup.-2] x [yr.sup.-1] (3.24 g [C x [m.sup.-2] x [yr.sup.-1] as inorganic carbon) on a global scale for all areas, with a standing stock of ~63.34 g CaC[O.sub.3]/[m.sup.2] (7.60 g C/[m.sup.2] as inorganic carbon), and ~7.97 g C/[m.sup.2] as organic carbon The shelf production alone is 77.91 g CaC[O.sub.3] x [m.sup.-2] x [yr.sup.-1] (9.35 g C x [m.sup.-2] x [yr.sup.-1] as inorganic carbon) in contrast to 2.05 g CaC[O.sub.3] x [m.sup.-2] x [yr.sup.-1] (-0.24 g C x [m.sup.-2] x [yr.sup.-l] as inorganic carbon) for the slope on a global scale. The biogeography of the CaC[O.sub.3] standing stocks of echinoderms showed strong latitudinal variability. More than 80% of the global CaC[O.sub.3] production from echinoderms occurs between 0 and 800 m, with the highest contribution attributed to the shelf and upper slope. We provide a global distribution of echinoderm populations in the context of global calcite saturation horizons, since undersaturated waters with respect to mineral phases are surfacing. This shallowing is a direct consequence of ocean acidification, and in some places it may reach the shelf and upper slope permanently, where the highest CaC[O.sub.3] standing stocks from echinoderms originate. These organism-level data contribute substantially to the assessment of global carbonate inventories, which at present are poorly estimated. Additionally, it is desirable to include these benthic compartments in coupled global biogeochemical models representing the 'biological pump' and its feedbacks, since at present all efforts have focused on pelagic processes, dominated by coccolithophores. The omission of the benthic processes from modeling will only diminish the understanding of elemental fluxes at large scales and any future prediction of climate change scenarios. Key words: benthic compartments; CaC[O.sub.3] budget; carbon cycle; carbonate production; echinoderms; ocean acidification; standing stock.

Byline: M. Santosh (1,2), Toshiaki Tsunogae (3), Hisako Shimizu (3), Jean Dubessy (4) Keywords: Eclogite; High pressure granulite; Mineral phase equilibria; Fluid inclusions; CO.sub.2; Southern India; Gondwana Abstract: Eclogite-facies rocks and high-pressure granulites provide windows to the deeper parts of subduction zones and the root of mountain chains, carrying potential records of fluids associated with subduction-accretion-collision tectonics. Here, we report petrological and fluid inclusion data on retrogressed eclogite and high-pressure granulite samples from Sittampundi, Kanji Malai and Perundarai in southern India. These rocks occur within the trace of the Cambrian collisional suture which marks the final phase of amalgamation of the Gondwana supercontinent. The garnet--clinopyroxene assemblage in the eclogites preserves relict omphacite, whereas the high-pressure granulites are characterized by an assemblage of garnet and clinopyroxene in the absence of omphacite and with minor plagioclase, orthopyroxene, and quartz. Phase relations computed for the eclogite assemblage yield peak P--T conditions of 19 kbar and 1,010degC. The mafic granulites also preserve the memory of high to ultrahigh-temperature metamorphism followed by an isothermal decompression. Systematic fluid inclusion optical, microthermometric and laser Raman spectroscopic studies were conducted in garnet and plagioclase from the eclogite--high pressure granulite suite. The results suggest that the early fluids were a mixture of CO.sub.2, CH.sub.4 and N.sub.2 probably derived from decarbonation and devolatilization reactions in a subduction setting during the prograde stage. The later generation inclusions, which constitute the dominant category in all the samples studied, are characterized by a near-pure CO.sub.2 composition with moderate to high densities (up to 1.154 g/cm.sup.3). The highest density fluid inclusions recorded in this study occur within the mafic granulites from Sittampundi (0.968--1.154 g/cm.sup.3) and Kanji Malai (1.092--1.116 g/cm.sup.3). In some cases, carbonate minerals such as dolomite and calcite are associated with the CO.sub.2-rich fluid inclusions. The composition and densities of the later generation fluids closely match with those of the CO.sub.2-bearing fluid inclusions reported from ultrahigh-temperature granulites occurring proximal to the eclogite--high pressure granulite suite within this suture zone, and suggest a common tectonic link for the fluid regime. We evaluate the fluid characteristics associated with convergent plate margin processes and propose that the early aqueous fluids probably associated with the eclogites were consumed during the formation of the retrograde hydrous mineral assemblages, whereas the fluid regime of the high-pressure and ultrahigh-temperature granulites was mostly CO.sub.2-dominated. The tectonic setting of the rocks along a collisional suture marking the trace along which crustal blocks were welded through subduction--collision process is in favor of a model involving the derivation of CO.sub.2 from sub-lithospheric sources such as a carbonated tectosphere invaded by hot asthenosphere, or underplated mafic magmas. Author Affiliation: (1) Faculty of Science, Kochi University, Akebono-cho, Kochi, 780-8520, Japan (2) Department of Earth and Atmospheric Sciences, Center for Environmental Sciences, Saint Louis University, St. Louis, MO, 63108, USA (3) Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki, 305-8572, Japan (4) G2R(UMR 7566), Nancy Universite and CNRS, BP70239, 54506, Vandoeuvre-les-Nancy Cedex, France Article History: Registration Date: 16/07/2009 Received Date: 22/10/2008 Accepted Date: 16/07/2009 Online Date: 30/07/2009 Article note: Communicated by J.L.R. Touret. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0431-4) contains supplementary material, which is available to authorized users.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2009.11.002 Byline: Neil S. Davies, Martin R. Gibling Keywords: Palaeozoic; Ordovician; Silurian; vegetation; fluvial; biogeomorphology Abstract: In present-day alluvial environments, the impact of vegetation on sedimentological processes and deposits is well known. A vegetated catchment may decrease sediment yield, sediment erodibility, Hortonian overland flow, aeolian winnowing of fines, the proportion of sediment transported as bedload, and may increase bank stability, infiltration into substrates, and bed roughness. Vegetation also promotes the production of chemically-weathered clays and soils and the adoption of a meandering style. It is generally understood that, prior to the evolution of terrestrial vegetation during the Early Palaeozoic, ancient alluvial systems were markedly different from modern systems, with many systems adopting a 'sheet-braided' style. This understanding has previously informed the interpretations of many Precambrian pre-vegetation alluvial successions, but there has been relatively little work regarding Early Palaeozoic alluvial successions laid down prior to and during the initial colonization of the Earth's surface by plants. A comprehensive review of 144 Cambrian to Devonian alluvial successions documented in published literature was combined with original field data from 34 alluvial successions across Europe and North America. The study was designed to identify changes in alluvial style during the period that vegetation was evolving and first colonizing alluvial environments. An increase in mudrock proportion and sandstone maturity is apparent, along with a decrease in overall sand grain size through the Early Palaeozoic. These trends suggest that primitive vegetation cover promoted the production and preservation of muds from the mid Ordovician onwards and increased the residence time of sand-grade sediment in alluvial systems. The compilation also enables the first stratigraphic occurrence of certain vegetation-dependent sedimentary features to be pinpointed and related to the evolution of specific palaeobotanical adaptations. The first markedly heterolithic alluvial sequences appeared at about the same time as the most primitive terrestrial vegetation in the Ordovician, and prolific pedogenic calcite, charcoal and bioturbated floodplain fines first appeared in the rock record at about the same time as vascular-plant macrofossils became abundant in the late Silurian. Lateral accretion sets in channel deposits appeared near the Silurian-Devonian boundary, at or shortly before the appearance of underground rooting systems, and become progressively more abundant in the record during the Devonian, implying a major expansion of meandering rivers as rooted plants stabilized river banks. Coals become abundant after the development of plant arborescence. The analysis suggests that the evolution of embryophytes had a profound effect on fluvial processes and deposits, and this period of landscape evolution must be considered amongst the most significant environmental and geomorphological changes in Earth history, with profound consequences for all aspects of the Earth system. Author Affiliation: Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1 Article History: Received 29 July 2009; Accepted 4 November 2009

Byline: Lili Xu (1), Jorg Renner (2), Marco Herwegh (3), Brian Evans (1) Keywords: Rheology; Carbonates; Constitutive laws; Solute strengthening; Normal grain growth Abstract: We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2--30 h) at 850degC and 300 MPa confining pressure. The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 um, increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5--17.0 mol% Mg, respectively. We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus. The deformation tests took place at between 700 and 800degC and at strain rates between 10.sup.-6 and 10.sup.-3 s.sup.-1. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress, (n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical composition will be an important element to consider when solid substitution can occur during natural deformation. Author Affiliation: (1) Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Mass. Avenue, Cambridge, MA, 02139, USA (2) Ruhr-University Bochum, 44780, Bochum, Germany (3) Institute for Geology, University Bern, 3012, Bern, Switzerland Article History: Registration Date: 15/08/2008 Received Date: 18/10/2007 Accepted Date: 15/08/2008 Online Date: 07/09/2008 Article note: Communicated by J. Blundy.

Byline: N. V. Chalapathi Rao (1), Rajesh K. Srivastava (1) Keywords: Petrology; Geochemistry; Kimberlite; Diamond; India Abstract: The petrology and geochemistry of some new occurrences of Mesoproterozoic diamondiferous hypabyssal-facies kimberlites from the Chigicherla, Wajrakarur-Lattavaram and Kalyandurg clusters of the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are reported. The kimberlites contain two generations of olivine, and multiple groundmass phases including phlogopite, spinel, calcite, dolomite, apatite, perovskite, apatite and rare titanite, and xenocrysts of eclogitic garnet and picro-ilmenite. Since many of the silicate minerals in these kimberlites have been subjected to carbonisation and alteration, the compositions of the groundmass oxide minerals play a crucial role in their characterisation and in understanding melt compositions. While there is no evidence for significant crustal contamination in these kimberlites, some limited effects of ilmenite entrainment are evident in samples from the Kalyandurg cluster. Geochemical studies reveal that the WKF kimberlites are less differentiated and more primitive than those from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton. Highly fractionated (La/Yb = 108--145) chondrite-normalised distribution patterns with La abundances of 500--1,000 x chondrite and low heavy rare earth elements (HREE) abundances of 5--10 x chondrite are characteristic of these rocks. Metasomatism by percolating melts from the convecting mantle, rather than by subduction-related processes, is inferred to have occurred in their source regions based on incompatible element signatures. While the majority of the Eastern Dharwar craton kimberlites are similar to the Group I kimberlites of southern Africa in terms of petrology, geochemistry and Sr--Nd isotope systematics, others show the geochemical traits of Group II kimberlites or an overlap between Group I and II kimberlites. Rare earth element (REE)-based semi-quantitative forward modelling of batch melting of southern African Group I and II kimberlite source compositions involving a metasomatised garnet lherzolite and very low degrees of partial melting demonstrate that (1) WKF and NKF kimberlites display a relatively far greater range in the degree of melting than those from the on-craton occurrences from southern Africa and are similar to that of world-wide melilitites, (2) different degrees of partial melting of a common source cannot account for the genesis of all the EDC kimberlites, (3) multiple and highly heterogeneous kimberlite sources involve in the sub-continental lithospheric mantle (SCLM) in the Eastern Dharwar craton and (4) WKF and NKF kimberlites generation is a resultant of complex interplay between the heterogeneous sources and their different degrees of partial melting. These observations are consistent with the recent results obtained from inversion modelling of REE concentrations from EDC kimberlites in that both the forward as wells as inverse melting models necessitate a dominantly lithospheric, and not asthenospheric, mantle source regions. The invading metasomatic (enriching) melts percolating from the convecting (asthenosphere) mantle impart an OIB-like isotopic signature to the final melt products. Author Affiliation: (1) Igneous Petrology Laboratory, Department of Geology, Banaras Hindu University, Varanasi, 221005, India Article History: Registration Date: 25/07/2008 Received Date: 17/04/2008 Accepted Date: 24/07/2008 Online Date: 07/08/2008 Article note: Communicated by T.L. Grove.

Byline: John R. Bowman (1), John W. Valley (2), Noriko T. Kita (2) Abstract: Oxygen isotope ratios have been measured by ion microprobe and millimeter-scale dental drill along detailed sampling traverses across the boundary between periclase-bearing ([delt[a].sup.18]O = 11.8[per thousand]) and periclase-free ([delt[a].sup.18]O = 17.2[per thousand]) marble layers in the periclase (Per) zone of the Alta Stock aureole, Utah. These data define a steep, coherent gradient in [delt[a].sup.18]O that is displaced a short distance (~4 cm) into the periclase-free (Cal + Fo) layer. SEM and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty +-0.27[per thousand], 2SD). More poorly polished (pitted), texturally retrograde 'turbid'-looking calcite has lower and more variable [delt[a].sup.18]O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Despite significant lowering of the [delt[a].sup.18]O values in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in [delt[a].sup.18]O. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion, 30,000--62,000 years at 575--600degC, exceed significantly the timescale (~1,250 years) estimated for the prograde development of the [delt[a].sup.18]O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains. Thus, the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization. Author Affiliation: (1) Department of Geology and Geophysics, University of Utah, Salt Lake City, UT, 84112, USA (2) Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI, 53706, USA Article History: Registration Date: 20/06/2008 Received Date: 26/01/2008 Accepted Date: 20/06/2008 Online Date: 20/08/2008 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-008-0321-1) contains supplementary material, which is available to authorized users.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.jenvman.2007.03.037 Byline: Sandow Mark Yidana (a), Duke Ophori (a), Bruce Banoeng-Yakubo (b) Keywords: Afram plains; Principal component analysis; Hierarchical cluster analysis; PHREEQC; Inverse modeling Abstract: Inverse geochemical modeling from PHREEQC, and multivariate statistical methods were jointly used to define the genetic origin of chemical parameters of groundwater from the Voltaian aquifers in the Afram Plains area. The study finds, from hierarchical cluster analysis that there are two main hydrochemical facies namely the calcium-sodium-chloride-bicarbonate waters and the magnesium-potassium-sulfate-nitrate waters in the northern and southern sections, respectively, of the Afram Plains area. This facies differentiation is confirmed by the distribution of the SO.sub.4.sup.2-/Cl.sup.- ratio, which associates groundwater from the northern and southern sections to areas influenced by contact with evaporites and seawater, respectively. Principal component analysis (PCA) with varimax rotation using the Kaiser criterion identifies four principal sources of variation in the hydrochemistry. Mineral saturation indices calculated from both major ions and trace elements, indicate saturation -- supersaturation with respect to calcite, aragonite, k-mica, chlorite, rhodochrosite, kaolinite, sepiolite, and talc, and undersaturation with respect to albite, anorthite, and gypsum in the area. Inverse geochemical modeling along groundwater flowpaths indicates the dissolution of albite, anorthite and gypsum and the precipitation of kaolinite, k-mica, talc, and quartz. Both the PCA and inverse geochemical modeling identify the incongruent weathering of feldspars as the principal factors controlling the hydrochemistry in the Afram Plains area. General phase transfer equations have been developed to characterize the geochemical evolution of groundwater in the area. A very good relationship has been established between calcite and aragonite saturation indices in the Afram Plains area, with R.sup.2=1.00. Author Affiliation: (a) Department of Earth and Environmental Studies, Montclair State University, NJ 07043, USA (b) Department of Geology, University of Ghana, P.O. Box LG 58, Legon, Ghana Article History: Received 28 September 2006; Revised 9 March 2007; Accepted 25 March 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2006.06.001 Byline: Abigail M. Smith (a), Marcus M. Key (b), Dennis P. Gordon (c) Keywords: bryozoans; carbonate mineralogy; biomineralization; calcite; aragonite Abstract: Skeletal carbonate mineralogy of 1183 specimens of marine bryozoans from the literature was examined for phylogenetic patterns in order to elucidate the effects of bryozoan mineralogy on geochemical and paleoenvironmental analysis. Colonies are composed of calcite (66% of specimens), aragonite (17% of specimens) or various mixtures of the two (17% specimens) (phylum mean=72.9 wt.% calcite, n =1051). When calcite is present, it ranges from 0.0 to 13.7 wt.% MgCO.sub.3 (mean=5.0 wt.% MgCO.sub.3, n =873). Most (61%) calcitic specimens are formed of intermediate-Mg calcite (4 to 8 wt.% MgCO.sub.3), others (28%) of low-Mg calcite (0 to 4 wt.% MgCO.sub.3), and few of high-Mg calcite (>8 wt.% MgCO.sub.3). The phylum occupies at least 63% of the theoretical mineralogical 'space' available to biomineralisation. Most of this variation occurs in the class Gymnolaemata, order Cheilostomata, suborder Neocheilostomata. Fossil and Recent stenolaemate taxa are generally low- to intermediate-Mg calcite (mean=99.7 wt.% calcite, 2.6 wt.% MgCO.sub.3, 17% of available biomineral space). Variability among families is related in a general way to first appearance datum: families younger than 100 Ma display greater mineralogical complexity than older ones. The cheilostome infraorder Flustrina includes unusual free-living aragonitic families, dual-calcite skeletons (mainly low-Mg calcite, but with secondary high-Mg calcite), and some genera with considerable mineralogical variability. Families (e.g., Membraniporidae and Phidoloporidae) and species (e.g., Schizoporella unicornis) with the highest degree of variability have potential for environmental correlations with mineralogy, paleoenvironmental interpretation, and possibly molecular investigation for potential cryptic species. Stenolaemate families, genera and species with low variability, on the other hand, are well-suited for geochemical work such as stable isotope analysis. Variability in the skeletal mineralogy of bryozoans suggests that they may be useful in geochemical, phylogenetic, and paleoenvironmental studies, with careful choice of study material. Author Affiliation: (a) Department of Marine Science, University of Otago, P. O. Box 56, Dunedin, New Zealand (b) Department of Geology, Dickinson College, P. O. Box 1773, Carlisle, PA 17013, USA (c) National Institute for Water and Atmospheric Research, P. O. Box 14-901, Wellington, New Zealand Article History: Received 28 July 2005; Accepted 1 June 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2005.08.003 Byline: Ian J. Fairchild (a), Claire L. Smith (a), Andy Baker (a), Lisa Fuller (a), Christoph Spotl (b), Dave Mattey (c), Frank McDermott (d), E.I.M.F. (e) Keywords: Speleothems; Stable isotopes; Trace elements; Palaeoclimate; Time series analysis; Karst Abstract: Speleothems are primarily studied in order to generate archives of climatic change and results have led to significant advances in identifying and dating major shifts in the climate system. However, the climatological meaning of many speleothem records cannot be interpreted unequivocally; this is particularly so for more subtle shifts and shorter time periods, but the use of multiple proxies and improving understanding of formation mechanisms offers a clear way forward. An explicit description of speleothem records as time series draws attention to the nature and importance of the signal filtering processes by which the weather, the seasons, and longer-term climatic and other environmental fluctuations become encoded in speleothems. We distinguish five sources of variation that influence speleothem geochemistry, i.e. atmospheric, vegetation/soil, karstic aquifer, primary speleothem crystal growth and secondary alteration, and give specific examples of their influence. The direct role of climate diminishes progressively through these five factors. We identify and review a number of processes identified in recent and current work that bear significantly on the conventional interpretation of speleothem records, for example: (1) speleothem geochemistry can vary seasonally and hence a research need is to establish the proportion of growth attributable to different seasons and whether this varies over time; (2) whereas there has traditionally been a focus on monthly mean [delta].sup.18O data of atmospheric moisture, current work emphasizes the importance of understanding the synoptic processes that lead to characteristic isotope signals, since changing relative abundance of different weather types might control their variation on the longer-term; (3) the ecosystem and soil zone overlying the cave fundamentally imprint the carbon and trace element signals and can show characteristic variations with time; (4) new modelling on aquifer plumbing allows quantification of the effects of aquifer mixing; (5) recent work has emphasized the importance and seasonal variability of CO.sub.2-degassing leading to calcite precipitation upflow of a depositional site on carbon isotope and trace element composition of speleothems; (6) although much is known about the chemical partitioning between water and stalagmites, variability in relation to crystal growth mechanisms and kinetics is a research frontier; (7) aragonite is susceptible to conversion to calcite with major loss of chemical information, but the controls on the rate of this process are obscure. Analytical factors are critical in generating high-resolution speleothem records. A variety of methods of trace element analysis is available, but standardization is a common problem with the most rapid methods. New stable isotope data on Irish stalagmite CC3 compares rapid laser-ablation techniques with the conventional analysis of micromilled powders and ion microprobe methods. A high degree of comparability between techniques for [delta].sup.18O is found on the millimeter to centimeter scale, but a previously described high-amplitude oxygen isotope excursion around 8.3 ka is identified as an analytical artefact related to fractionation of the laser-analysis associated with sample cracking. High-frequency variability of not less than 0.5a[degrees] may be an inherent feature of speleothem [delta].sup.18O records. Author Affiliation: (a) School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (b) Institut fur Geologie und Palaontologie, Leopold-Franzens-Universitat Innsbruck, Innrain 52, 6020 Innsbruck, Austria (c) Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK (d) UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland (e) Edinburgh Ion Microprobe Facility, Institute of Earth Science, School of Geosciences, University of Edinburgh EH9 3JW, UK Article History: Received 20 May 2005; Accepted 5 August 2005

The Waikite Hot Springs emit waters (most >90 [degrees] C) with a pH of 7.2-8.9, high Si[O.sub.2] (150-170mg [l.sup.-1]) concentrations, but low Ca ( Keywords: New Zealand, hot springs, calcite, crystal growth.

We have reconstructed the compositional evolution of the silicate and carbonate melt, and various crystalline phases in the subvolcanic reservoir of Kerimasi Volcano in the East African Rift. Trace element concentrations of silicate and carbonate melt inclusions trapped in nepheline, apatite and magnetite from plutonic afrikandite (clinopyroxene-nepheline-perovskite-magnetite-melilite rock) and calciocarbonatite (calcite-apatite-magnetite-perovskite-monticellite-phlogopite rock) show that liquid immiscibility occurred during the generation of carbonatite magmas from a C[O.sub.2]-rich melilite-nephelinite magma formed at relatively high temperatures (1,100 °C). This carbonatite magma is notably more calcic and less alkaline than that occurring at Oldoinyo Lengai. The CaO-rich (32-41 wt%) nature and alkali-'poor' (at least 7-10 wt% [Na.sub.2]O + [K.sub.2]O) nature of these high-temperature (>1,000 °C) carbonate melts result from strong partitioning of Ca (relative to Mg, Fe and Mn) in the immiscible carbonate and the CaO-rich nature (12-17 wt%) of its silicate parent (e.g., melilite-nephelinite). Evolution of the Kerimasi carbonate magma can result in the formation of natrocarbonatite melts with similar composition to those of Oldoinyo Lengai, but with pronounced depletion in REE and HFSE elements. We suggest that this compositional difference results from the different initial parental magmas, e.g., melilite-nephelinite at Kerimasi and a nephelinite at Oldoinyo Lengai. The difference in parental magma composition led to a significant difference in the fractionating mineral phase assemblage and the element partitioning systematics upon silicate-carbonate melt immiscibility. LA-ICP-MS analysis of coeval silicate and carbonate melt inclusions provides an opportunity to infer carbonate melt/silicate melt partition coefficients for a wide range of elements. These data show that Li, Na, Pb, Ca, Sr, Ba, B, all REE (except Sc), U, V, Nb, Ta, P, Mo, W and S are partitioned into the carbonate melt, whereas Mg, Mn, Fe, Co, Cu, Zn, Al, Sc, Ti, Hf and Zr are partitioned into the silicate melt. Potassium and Rb show no preferential partitioning. Kerimasi melt inclusions show that the immiscible calcic carbonate melt is strongly enriched in Sr, Ba, Pb, LREE, P, W, Mo and S relative to other trace elements. Comparison of our data with experimental results indicates that preferential partitioning of oxidized sulfur (as S[O.sub.4.sup.2-]), Ca and P (as P[O.sub.4.sup.3-]) into the carbonate melt may promote the partitioning of Nb, Ta, Pb and all REE, excluding Sc, into this phase. Therefore, it is suggested that P and S enrichment in calcic carbonate magmas promotes the genesis of REE-rich carbonatites by liquid immiscibility. Our study shows that changes in the partition coefficients of elements between minerals and the coexisting melts along the liquid line of descent are rather significant at Kerimasi. This is why, in addition to the REE, Nb, Ta and Zr are also enriched in Kerimasi calciocarbonatites. We consider significant amounts of apatite and perovskite precipitated from melilite-nephelinite-derived carbonate melt as igneous minerals can have high LREE, Nb and Zr contents relative to other carbonatite minerals. Keywords Liquid immiscibility * Melt inclusions * Element partitioning * Kerimasi * LA-ICP-MS, Introduction The study of primary melt inclusions is a powerful tool to reconstruct the compositional evolution of various melt phases in magmas, but has rarely been utilized in carbonate-rich systems [...]

Multiple calcrete horizons previously considered representative of true subaerial exposures may, in fact, be false penetrative calcretes related to only one subaerial exposure event. Individual subaerial exposure periods can produce a true surficial calcrete plus a series of penetrative calcretes. Although penetrative calcrete horizons are not laterally traceable over long distances, they are very similar to surficial calcretes, are commonly found at sequence and/or lithologic boundaries (permeability anomalies), and can be easily misinterpreted in core borings as representing individual exposure events. Recent penetrative calcretes are common on elevated limestone ridges of the Caicos Islands where roots penetrate downward, seeking ground-water moisture. In glacial times, penetrative calcretes may have been present throughout carbonate platforms, as ground-water levels were much lower. Penetrative and surficial calcretes can be differentiated by the lower concentrations of insoluble Al and Fe in penetrative calcretes.

Compositional zoning of trace elements is observed in time-equivalent regions within 1014 growth sectors of natural and synthetic calcite single crustals, and has been called 'intrasectoral zoning' to distinguish it from conventional sectoral zoning. Differential interference contrast microscopy shows that {1014} faces grow by the spiral mechanism. Cathodoluminescence microscopy and election probe microanalyses reveal that trace element partitioning differences on {1014} faces correspond exactly to cyrstallographically controlled orientations of growth steps. Effective partitioning of Mg and Mn varies up to a factor of four between time-correlative growth steps spreading in different directions. Effective Sr partitioning varies by approximately 20% and is inversely correlated with enrichment trends of Mg and Mn. Preferential incorporation of different aqueous species at different growth steps may provide an explanation for morphological and chemical variation in natural calcites.

Analysis of magmatic and sedimentary rocks of several large igneous provinces has demonstrated that the release of gas during plutonic-metamorphic processes may be linked to global climate change and mass extinctions. Aguablanca, one of the largest Cu-Ni-PGE deposits in Europe, formed during the Variscan orogeny when a mafic magma intruded limestones and shales, creating a contact aureole composed of marble, skarn and hornfels. Our petrological and geochemical investigation of the aureole provides evidence that a combination of the two processes led to the formation of the ore deposit: The assimilation of terrigenous sediments supplied S to the magma while the assimilation of carbonates changed the oxygen fugacity and decreased the solubility of sulfur in the magma. The metamorphic assemblages in the contact aureole are directly related to heterogeneity of the protolith and particularly to the original proportions of calcite and clay. We modeled carbon dioxide degassing during contact metamorphism and showed that pure limestone is relatively unproductive because of its high reaction temperature. The presence of clay, however, leads to the formation of calc-silicates and significantly enhances C[O.sub.2] degassing. Our estimations suggest that degassing of the Aguablanca contact aureole released about 74.8 Mt of C[O.sub.2], a relatively low volume that we attribute to the composition of the host rock, mainly a pure limestone. A far larger volume of carbon dioxide was emitted by the contact metamorphism of dolostones in the contact aureole of Panzhihua (part of Emeishan large igneous province, SW China). We propose that the level of emission of carbon dioxide depends strongly on the nature of the protolith and has to be considered when predicting environmental impact during the emplacement of large igneous provinces. Keywords Contact aureole * Thermal metamorphism * Carbonates * Limestones * Carbon dioxide emission, Introduction Metamorphic fluid flow is a component of global geochemical cycles, some of which, like the C cycle, have implications for the behavior of Earth's climate. Kerrick and Caldeira (1993, [...]

Reaction rims of dolomite (CaMg[[C[O.sub.3]].sub.2]) were produced by solid-state reactions at the contacts of oriented calcite (CaC[O.sub.3]) and magnesite (MgC[O.sub.3]) single crystals at 400 MPa pressure, 750-850 °C temperature, and 3-146 h annealing time to determine the reaction kinetics. The dolomite reaction rims show two different microstructural domains. Elongated palisades of dolomite grew perpendicular into the MgC[O.sub.3] interface with length ranging from about 6 to 41 µm. At thesametime, a5-71 µm wide rim of equiaxed granular dolomite grew at the contact with CaC[O.sub.3]. Platinum markers showed that the original interface is located at the boundary between the granular and palisade-forming dolomite. In addition to dolomite, a 12-80 µm thick magnesio-calcite layer formed between the dolomite reaction rims and the calcite single crystals. All reaction products show at least an axiotactic crystallographic relationship with respect to calcite reactant, while full topotaxy to calcite prevails within the granular dolomite and magnesio-calcite. Dolomite grains frequently exhibit growth twins characterized by a rotation of 180° around one of the [1120] equivalent axis. From mass balance considerations, it is inferred that the reaction rim of dolomite grew by counter diffusion of MgO and CaO. Assuming an Arrhenius-type temperature dependence, activation energies for diffusion of CaO and MgO are [E.sub.a] (CaO) = 192 ± 54 kJ/mol and [E.sub.a] (MgO) = 198 ± 44 kJ/mol, respectively. Keywords Rim growth, Carbonates, Diffusion, Reaction kinetics, Diffusion-controlled reaction, Introduction Mineral reactions in the solid state are ubiquitous in the Earth's crust and mantle. Unless deformed or overprinted, the product phases of such solid-state reactions are commonly located along [...]

Episyenite is a quartz-depleted vuggy rock resulting from hydrothermal alteration of granitic rocks. This is the first report of its existence in an island arc, which is identified in a deep drill core of the Toki Cretaceous granite distributed in central Japan. In order to understand the petrographical features of the episyenite, neutron porosity measurement, geochemical analysis, microscopic observation, and X-ray computed tomography scanning were carried out. The results show remarkably high porosity (35.4 %) due to interconnecting vugs and the removal of quartz, plagioclase, and biotite. The Rb-Sr isotopic results and the paragenetic sequence of secondary minerals in the vugs suggest that the hydrothermal alteration process can be divided into an episyenitization stage and a later hydrothermal stage. At the episyenitization stage (70.6 ± 3.1 Ma) ca. 6 million years after the emplacement of the unaltered granite (76.3 ± 1.5 Ma), dissolution of quartz, biotite, and plagioclase occurred and was followed by the precipitation of albite, vermicular chlorite, and platy calcite. The episyenitization is considered as a local alteration of the Toki granite in an isotopically closed system. At the later hydrothermal stage, illite and secondary quartz precipitated from circulating meteoric-derived water in the dissolution vugs. Superimposing alteration at the later hydrothermal stage is limited, which results in the preservation of the episyenite in an almost primitive condition. Keywords Episyenite * Granite * Hydrothermal alteration * Fluid-rock interaction * Orogenic belt, Introduction Alteration of granitic rocks is a result of fluid-rock interaction after solidification. It is significant to characterize the features of hydrothermal alteration of granitic rocks not only for understanding [...]