Anhedral Shape Research Articles

Bi-Ag-Sulfosalts and Sulfoarsenides in the Ruwai Zn-Pb-Ag Skarn Deposit, Central Borneo, Indonesia

The Ruwai skarn deposit is located in the Schwaner Mountain complex within the central Borneo gold belt and is currently considered the largest Zn skarn deposit in Indonesia. The deposit has been known to host Zn-Pb-Ag mineralization in the form of massive sulfide ore bodies; however, the occurrence of Ag-bearing minerals has not been identified yet. This study documents the mineralogical characteristics of several Bi-Ag sulfosalts and sulfoarsenides, as well as their chemical compositions. Ten Bi-Ag sulfosalts were identified, including native bismuth, tetrahedrite, cossalite, tsumoite, bismuthinite, joseite-B, Bi6Te2S, Bi-Pb-Te-S, Bi-Ag-S, and Bi-Te-Ag. Three sulfoarsenides were identified, including arsenopyrite, glaucodot, and alloclasite. The occurrence of Bi-Ag sulfosalts is typically associated with massive sulfide mineralization, although tsumoite can also be found associated with massive magnetite. In terms of sulfoarsenides, both arsenopyrite and glaucodot are associated with massive sulfide mineralization, whereas alloclasite is associated with massive magnetite mineralization. The Bi-bearing minerals are characterized by irregular, bleb-like texture or patch morphology, and occur either as free grains or inclusions within sulfides, such as galena or pyrite. Tetrahedrite typically has an anhedral shape with a rim or atoll texture surrounding sphalerite or galena. In contrast, sulfoarsenides are typically found as euhedral–subhedral grains where glaucodot typically is rimmed by arsenopyrite. Both Bi-Ag sulfosalt and sulfoarsenides were formed during the retrograde stage under high oxidation and a low sulfidation state condition. The ore-forming temperature based on arsenopyrite geothermometry ranges from 428 °C to 493 °C.

Geochemical and Fe-isotope characteristics of the largest Mesozoic skarn deposit in China: Implications for the mechanism of Fe skarn formation

Skarn deposits are major repositories of Fe and are the principal sources for iron mining in China. However, their mechanism of formation remains debated. Here we investigate the Xishimen (XSM) Fe deposit, located in Wu’an, China. This deposit is a well-known Mesozoic large skarn Fe deposits in the Han-Xing Fe metallogenic belt. Here we report the various features of this Fe deposit that suggest a clear magmatic affinity. We present data on the mineral assemblages, major and trace element data and Fe isotope analysis of magnetite from the XSM Fe deposit. Our results identify four distinct types of magnetite (Type I, II, III and IV). Magnetite type I is characterized by anhedral shape with vesicles in the ores, and is associated with diopside (Di), tremolite (Tr), ±garnet (Gt) and/or augite (Aug), together with phlogopite. Magnetite type I shows high content of TiO2 (0.25–1.21 wt%) and Al2O3 + MnO (0.24–0.72 wt%). These features suggest that magnetite type I is of magmatic origin. Magnetite type IV displays euhedral shape, and is associated with calcite, apatite and pyrite, and is enriched in SiO2, CaO, Sr, Ba, Rb, Nb, and total REE, and depleted in TiO2. These features suggest that magnetite type IV is of hydrothermal origin. The features of magnetite types II and III fall in between magnetite I and magnetite IV. The δ56Fe values are also distinct for the four types of magnetites: δ56Fe is the highest for type I magnetite (average 0.085‰); and the lowest for magnetite type IV (average 0.016‰). Our results also show that the δ56Fe value tends to be higher under high temperature environment and in lower part of the main orebody. Statistical analysis shows a negative correlation between the δ56Fe values and depth.With a view to constrain the origin of the skarn type Fe deposit, we present a conceptual model based on the geochemical and isotope data. We envisage distinct phases during the Fe ore genesis, including: (a) the formation of an iron-rich magma of dioritic composition which reacted with the carbonate wall rocks; (b) the iron-rich magma sinks into the bottom of the magma chamber with high density; (c) volatiles from deep magmatic chamber evolve and interact with the surrounding carbonate rocks; and (d) due to fluid overpressure in the chamber, the iron bearing melt and fluids migrate along structural conduits and precipitate the Fe ore. Because of the difference in temperature, pressure, and oxygen fugacity, the composition of magnetite varied along the fluid/melt conduits. Thus, magnetite type I (high Ti Mt) was mainly distributed in the lower part of the conduit, and magnetite IV (high Si Mt) was mainly concentrated in the upper part during progressive cooling in the conduit. Fe isotope fractionation occurred during the evolution of the Fe-bearing melt/fluid in response to changes in temperature and oxygen fugacity.

Role of fluids in Fe–Ti–P mineralization of the Proterozoic Damiao anorthosite complex, China: Insights from baddeleyite–zircon relationships in ore and altered anorthosite

The Damiao Fe–Ti–P deposit offers a rare opportunity for studying late-stage Fe–Ti ore-forming processes in Proterozoic anorthosites. The orebodies are hosted in anorthosite and commonly show chlorite-dominated alteration in the contact zone on both sides, but the nature and origin of fluids in Fe-Ti-P mineralization remains contentious. Baddeleyite is a common accessory mineral in anorthosites and Fe–Ti–P orebodies, and typically occurs as blebs and lamellae in primary ilmenite reflecting decreasing solubility of Zr in ilmenite during slow cooling and consequent exsolution of ZrO2. Two types of zircon are identified in the Fe–Ti–P orebodies and altered anorthosite at Damiao, and both are related to hydrothermal replacement of baddeleyite by Si-rich fluids. The type-I zircon shows subhedral to anhedral shapes with variable sizes (5–50 μm) in Fe–Ti–P orebodies, and coexists with magnetite–rutile symplectite formed by ilmenite breakdown. In contrast, the type-II zircon typically occurs as tiny aggregates in chlorite–quartz–titanite replacement fronts of altered anorthosite, indicative of a hydrothermal origin. The type-I zircon yielded an age of 1739 ± 16 Ma, similar to the age of baddeleyite previously reported for the orebodies. Formation of the type-I zircon is related to the replacement of baddeleyite in the presence of Si-enriched hydrothermal fluids evolved from magma. Ti-in-zircon geothermometry indicates a fluid temperature of >700 °C for the formation of the type-I zircon. However, homogenization temperature of fluid inclusions in co-precipitated apatite suggests that the type-II zircon in altered anorthosite may have formed by later hydrothermal fluids at temperature of ~350 °C. Our results indicate that the Fe–Ti–P mineralization of the Damiao anorthosite complex involved hydrous melts and magmatic–hydrothermal processes, with the Fe–Ti oxides being formed at the magmatic stage and apatite at the hydrothermal stage.

Morphological Analysis of Archetypal Calcite Cement

Archetypal cements from their deposition to the time they are sampled preserve a record of subsurface fluid evolution that can cover many millions of years. Their morphological development after nucleating on fissure walls is simulated by graphical models showing maturation from isolated to competitive to parallel growth. Graphical models are not available for the cementation of pores that are more complex and diverse. The reconstruction of cement growth in pores is accomplished here using multiple cathodoluminescent (CL) growth zones. Sediment pores are filled in two stages separated by a cementation threshold associated with a rapid drop in permeability. Pre-threshold cement is centripetal ordered, regularly distributed, and nurtured by a megapore network. Post-threshold cement is disordered, sporadically distributed, and nurtured by labile micropore connections. The crystallographic form of euhedral crystals at cement's growing front has environmental significance while the anhedral shape of aggregate crystals it leaves behind is controlled by geometrical selection. Growth of seeded cement is divided into epitaxial and mantle stages; syntaxial when these two are not identified. The epitaxial stage begins with a seed at multiple points then coalesces, simplifies, and morphs towards the Wulff or equilibrium surface. The extent of epitaxial growth is correlated with the size of crystals forming pore walls. Increments of mantle growth can be initiated over the whole surface of a pre-existing crystal, or can be restricted to its edges and/or corners. Restricted seeding is often linked with changes of crystal habit resulting in the reorientation of the crystal's fastest growth direction and the direction of maturation. Some impingement intercrystalline boundaries have offset growth zones explained by dissolution; such a self-cannibalization process is new. The existing explanation that some enfacial junctions are due to pauses in growth is confirmed by the arrangement of growth zones around triple points, but a new type is recorded formed from continuously growing crystals whose intercrystalline boundaries become modified. The presence of the cement's internal growth surfaces allow its development to be reconstructed, providing the foundation for further cement studies; however, one example is presented where the disproportionate filling of pores in adjacent millimeter-thick layers is obscure because its crystals lack internal growth surfaces.

Constraining the origin of the Mesozoic Xishimen Fe deposit in the Taihang Mountain Range, North China using geochemistry of magnetite

The Xishimen (XSM) iron (Fe) deposit located in the Taihang Mountain Range in North China is one of the largest known Mesozoic skarn Fe deposits in China. Recent research suggests that the Fe deposit exhibits some magmatic features. Here, we report new major and trace element, mineral assemblage and Fe isotope data of magnetite from the XSM Fe deposit in the Han-Xing Fe metallogenic belt, which allows for the identification of four distinct types of magnetite (Type I, II, III and IV). Magnetite type I is characterized by anhedral shape with vesicles in the ores, and consists of minerals diopside (Di), tremolite (Tr), ±garnet (Gt) and/or augite (Aug), spreading over diopside, tremolite, phlogopite and/or Aug as an interstitial mineral, which can dissolve or melt pre-existing minerals (such as Di and Tr). Magnetite type I also contains high content of TiO2 (0.25–1.21 wt.%) and Al2O3 + MnO (0.24–0.72 wt.%). These features suggest that magnetite type I is of magmatic origin. Magnetite type IV is marked by euhedral shape, consists of Cc, Ap and/or Py, enriched in SiO2, CaO, Sr, Ba, Rb, Nb, and total REE, and depleted in TiO2. These properties suggest that magnetite type IV is of hydrothermal origin. The features of magnetite types II and III fall in between magnetite I and magnetite IV. The δ56Fe value also differs for the four types of magnetite: δ56Fe is the highest for type I magnetite (average 0.085‰); and the lowest for magnetite type IV (average 0.016‰). We also find that δ56Fe value tends to be higher under high temperature environment and in lower part of the main orebody. Statistical analysis shows that δ56Fe and altitudes are negatively correlated.In order to constrain the origin of the XSM Fe deposit, we present a new conceptual model based on our new geochemistry data. This model suggests a stepwise pattern of Fe ore genesis, including (1) “melt-fluid bearing Fe” forms in the deep magma chamber; (2) “melt-fluid bearing Fe” rises along the magmatic conduit due to fluid overpressure in the deep magma chamber; (3) changes in ambient temperature, pressure, and oxygen fugacity cause the different composition of magnetite along the magmatic conduit; and (4) Fe isotope fractionation occurs during the rising and settlement of “melt-fluid bearing Fe” in response to changes in temperature and oxygen fugacity. This model explains the magmatic characteristics of magnetite formed under high temperature in the lower part of the conduit, and the hydrothermal characteristics of magnetite occurred under low temperature in the upper part of the conduit.

Rutile alteration and authigenic growth in metasandstones of the Moeda Formation, Minas Gerais, Brazil – A result of Transamazonian fluid–rock interaction

Microstructures, trace elements and U–Pb ages of rutile grains are reported from two metasandstone samples of the Moeda Formation, which is the basal unit of the Minas Supergroup in the Quadrilátero Ferrífero, Minas Gerais, Brazil. The Moeda metasandstones, deposited between 2.68 and 2.65 Ga, are intercalated with Witwatersrand-like, pyritiferous and auriferous metaconglomerate layers. Rutile from both samples shows unusually high contents of Th (1–142 ppm) and rare-earth elements (ΣREE = 1–132 ppm), with chondrite-normalized middle REE humps, and reveals post-depositional U–Pb ages of ≤2.25 Ga; only three grains yielded significantly older ages of up to 2.72 Ga, pointing to a detrital origin. The combined data set suggests that rutile formation and alteration at 2.25 Ga was caused by distinct fluid-mediated processes, which erased nearly all provenance information. Detrital, rounded rutile grains in sample A were altered by a coupled dissolution–reprecipitation process (type 1), resulting in highly porous grains. These show variable contents of rutile-compatible and -incompatible trace elements: Th (1–48 ppm), Zr (25–796 ppm), U (12–167 ppm), Pb (9–52 ppm), Cr (900–12,878 ppm), W (143–3939 ppm), Nb (1148–6168 ppm), Sb (6–63 ppm), as well as variable ratios of Zr/Hf, Y/Ho and Th/U. Rutile grains in sample B are mostly authigenic (type 2), and formed after dissolution of pre-existing detrital rutile grains and subsequent transport and homogenization of trace elements. This interpretation is supported by the anhedral shape of some grains and the finding of numerous, randomly oriented muscovite laths. The authigenic population has significantly lower, and less variable contents of trace elements compared to type-1 rutile: Th (0.5–16 ppm), Zr (21–67 ppm), U (5.8–24 ppm), Pb (6-18 ppm), Cr (1379–3153 ppm), W (9.4–70 ppm), Nb (921–3058 ppm), Sb (3.3–10 ppm), and distinctively lower ratios of Zr/Hf (7.9) and Th/U (0.2), but variable Y/Ho ratios. Significant contents of Th and REE are explained by co-precipitation with TiO2 aggregates prior to structural ripening. The close association of rutile with muscovite, the rutile MREEN humps and the results of Zr-in-rutile thermometry suggest that rutile formation and alteration in the investigated sandstones was mediated by K(Na)–Si–P-bearing aqueous fluids at temperatures of about 490 °C. The rutile MREEN humps are interpreted to result from the tetrad effect during leaching of spatially associated detrital zircon grains. Uranium–Pb ages of ≤2.25 Ga indicate that the fluid overprint started at the onset of the Transamazonian orogeny, perhaps related to the opening of the Sabará foreland basin.

Diamonds, Super‐Reduced and Crustal Minerals in Chromitites of the Hegenshan and Sartohay Ophiolites, Central Asian Orogenic Belt, China

The Central Asian Orogenic Belt (CAOB) is a huge tectonic mélange that lies between the North China Craton and the Siberian Block. It is composed of multiple orogenic belts, continental fragments, magmatic and metamorphic rocks, suture zones and discontinuous ophiolite belts. Although the Hegenshan and Sartohay ophiolites are separated by nearly 3000 km and lie in completely different parts of the CAOB, they are remarkably similar in many respects. Both are composed mainly of serpentinized peridotite and dunite, with minor gabbro and sparse basalt. They both host significant podiform chromitites that consist of high‐Al, refractory magnesiochromite with Cr#s [100Cr/(Cr+Al)] averaging >60. The Sartohay ophiolite has a zircon U‐Pb age of ca. 300 Ma and has been intruded by granitic plutons of similar age, resulting in intense hydrothermal activity and the formation of gold‐bearing listwanites. The age of the Hegenshan is not firmly established but is thought to have formed in the Carboniferous.Like many other ophiolites that we have investigated in other orogenic belts, the chromitites in these two bodies have abundant diamonds, as well as numerous super‐reduced and crustal minerals. The diamonds are mostly, colorless to pale yellow, 200–300 μm across and have euhedral to anhedral shapes. They all have low carbon isotopes (δ14C = −18 to −29) and some have visible inclusions. These are accompanied by numerous super‐reduced minerals such as moissanite, native elements (Fe, Cr, Si, Al, Mn), and alloys (e.g., Ni‐Mn‐Fe, Ni‐Fe‐Al, Ni‐Mn‐Co, Cr‐Ni‐Fe, Cr‐Fe, Cr‐Fe‐Mn), as well as a wide range of oxides, sulfides and silicates. Grains of zircon are abundant in the chromitites of both ophiolites and range in age from Precambrian to Cretaceous, reflecting both incorporation of old zircons and modification of grains by hydrothermal alteration.Our investigation confirms that high‐Al, refractory chromitites in these two ophiolites have the same range of exotic minerals as high‐Cr metallurgical chromitites such as those in the Luobusa ophiolite of Tibet. These collections of exotic minerals in ophiolitic chromitites indicate complex, multi‐stage recycling of oceanic and continental crustal material at least to the mantle transition zone, followed by uprise and emplacement of the peridotites into relatively shallow ophiolites.

Olivine dissolution in molten silicates: An experimental study with application to chondrule formation

AbstractMg‐rich olivine is a ubiquitous phase in type I porphyritic chondrules in various classes of chondritic meteorites. The anhedral shape of olivine grains, their size distribution, as well as their poikilitic textures within low‐Ca pyroxene suggest that olivines suffer dissolution during chondrule formation. Owing to a set of high‐temperature experiments (1450–1540 °C) we determined the kinetics of resorption of forsterite in molten silicates, using for the first time X‐ray microtomography. Results indicate that forsterite dissolution in chondrule‐like melts is a very fast process with rates that range from ~5 μm min−1 to ~22 μm min−1. Forsterite dissolution strongly depends on the melt composition, with rates decreasing with increasing the magnesium and/or the silica content of the melt. An empirical model based on forsterite saturation and viscosity of the starting melt composition successfully reproduces the forsteritic olivine dissolution rates as a function of temperature and composition for both our experiments and those of the literature. Application of our results to chondrules could explain the textures of zoned type I chondrules during their formation by gas‐melt interaction. We show that the olivine/liquid ratio on one hand and the silica entrance from the gas phase (SiOg) into the chondrule melt on the other hand, have counteracting effects on the Mg‐rich olivine dissolution behavior. Silica entrance would favor dissolution by maintaining disequilibrium between olivine and melt. Hence, this would explain the preferential dissolution of olivine as well as the preferential abundances of pyroxene at the margins of chondrules. Incipient dissolution would also occur in the silica‐poorer melt of chondrule core but should be followed by crystallization of new olivine (overgrowth and/or newly grown crystals). While explaining textures and grain size distributions of olivines, as well as the centripetal distribution of low‐Ca pyroxene in porphyritic chondrules, this scenario could also be consistent with the diverse chemical, isotopic, and thermal conditions recorded by olivines in a given chondrule.

Thermoluminescence age of quartz xenocrysts in basaltic lava from Oninomi monogenetic volcano, northern Kyushu, Japan

Abstract We determined the eruption age of basaltic rocks by application of thermoluminescence (TL) method, which is often used for TL dating, to quartz. Mafic magma only rarely includes quartz because of their mutual disequilibration. The basaltic lavas reported herein include quartz as xenocrysts, as corroborated by their rounded or anhedral shape. The basaltic lava used for this study is from the Oninomi monogenetic volcano in northern Kyushu, Japan. The volcano eruption was estimated as occurring 7.3–29 ka because the lava exists between two widespread tephras: Aira-Tanzawa ash (26–29 ka) and Kikai-Akahoya ash (7.3 ka). We succeed-ed in collecting ca. 200 mg of quartz by decomposition of 30 kg of the lava samples. TL measurements for the lava indicate the eruption age as 15.8 ± 2.5 ka, which is fairly consistent with the stratigraphical estimation. Although the TL method has played a considerable part in constraining the timescale of Quaternary events, its application has been limited to silicic samples. The present result demonstrates the availability of quartz for dating even of mafic rock.

A comparison of properties of clay minerals in isalteritic and in degraded facies

AbstractThe mineralogical, geochemical and micromorphological features of an isalteritic clay facies, which originated from weathering of an anorthosite, were compared to those of clay facies derived from the degradation of a bauxite developed from the same rock. The isalteritic clay was formed by the hydrolytic alteration of plagioclase, whereas the degraded clays were formed by decomposition of gibbsite and neoformation of kaolinite. This resilification process resulted from the reintroduction of silica via the oscillation of the phreatic level and/or the decomposition of organic matter on the surface. The degradation process was gradual and yielded two different facies: (a) degraded clays with almost total decomposition of gibbsite, and (b) degraded clays with gibbsite nodules. Morphologically, the isalteritic clays differ from the degraded clays because they contain larger hexagonal and pseudo-hexagonal crystals. The degraded clays have more irregular crystal shapes, ranging from laths to anhedral shapes.

Garnet-bearing granite from the Třebíč pluton, Bohemian Massif (Czech Republic)

Garnet occurs as a significant mineral constituent of felsic garnet-biotite granite in the southern edge of the Třebic pluton. Two textural groups of garnet were recognized on the basis of their shape and relationship to biotite. Group I garnets are 1.5–2.5 mm, euhedral grains which have no reaction relationship with biotite. They are zoned having high XMn at the rims and are considered as magmatic. Group II garnets form grain aggregates up to 2.5 cm in size, with anhedral shape of individual grains. The individual garnet II grains are usually rimmed by biotite and have no compositional zoning. The core of group I garnets and group II garnets contains 67–80 mol% of almandine, 5–19 mol% of pyrope, 7–17 mol% of spessartine and 2–4 mol% of grossular. Biotite occurs in two generations; both are magnesian siderophyllites with Fe/(Fe + Mg) = 0.50–0.69. The matrix biotite in granites (biotite I) has high Ti content (0.09–0.31 apfu) and Fe/(Fe + Mg) ratio between 0.50 and 0.59. Biotite II forms reaction rims around garnet, is poor in Ti (0.00–0.06 apfu) and has a Fe/(Fe + Mg) ratio between 0.61 and 0.69. The textural relationship between biotite and garnet indicates that garnet reacted with granitic melt to form Ti-poor biotite and a new granitic melt, depleted in Ti and Mg and enriched in Fe and Al. In contrast to the host durbachites (hornblende-biotite melagranites), which originated by mixing of crustal melts and upper mantle melts, the origin of garnet-bearing granites is related to partial melting of the aluminium-rich metamorphic series of the Moldanubian Zone.

GEOCHEMICAL SIGNATURE OF THE QUARTZ DIORITE VEIN IN MANTLE PERIDOTITE XENOLITH FROM TALLANTE, SE SPAIN: LASER-ABLATION ICP-MS ANALYSIS

We found slab-derived quartz diorite veins in mantle peridotite xenolith from Tallante, SE Spain, and discussed their origin and importance (Arai et al. 2003; Shimizu et al. 2004). These studies, however, did not include “in-situ” trace-element analyses of constituent minerals in the quartz diorite vein. Here, we report the trace-element compositions of minerals in the quartz diorite veins in the mantle xenolith obtained by laser ablation ICP-MS, and examine the origin of Tallante quartz diorite vein. The Tallante quartz diorite vein is up to 8 mm in thickness, and secondarily cuts the host mantle lherzolite. Orthopyroxene intervenes between the quartz diorite and host lherzolite. The quartz diorite vein is composed mainly of plagioclase and quartz, rarely with orthopyroxene, clinopyroxene, amphibole, phlogopite, apatite, rutile, zircon, monazite, thorite and glass. Rutile, zircon and apatite are 10 to 50 µm in size and show subhedral to anhedral shapes. Monazite and thorite are less than 15µm and show highly anhedral (skeletal) shapes. Glass occurs along grain boundaries. The orthopyroxene wall sometimes contains traces of clinopyroxene, rutile, phlogopite and apatite. Bulk trace-element pattern of the Tallante quartz diorite shows enrichment in LREE and depletion in HREE (La/Yb ratio = 37.2). It also shows low Yb content (= 0.06 ppm) and depletion in HFSE, Rb, Ba, Sr and Eu. These features are similar to slab-derived arc magmas such as adakite (Shimizu et al. 2004). Trace-element compositions of minerals in the Tallante quartz diorite vein were measured using the laser ablation (193 nm ArF excimer: MicroLas GeoLas Q-plus)-inductively coupled plasma mass spectrometry (Agilent 7500S) (LAICP- MS) at the Incubation Business Laboratory Center of Kanazawa University (Ishida et al., 2004). Analytical details and quality of data are shown in Morishita et al. (2005). Each analysis was performed by ablating 50 ~ 100 µm diameter spot. Orthopyroxene in the quartz diorite vein was too small to analyze, and so the analysis was taken from the orthopyroxene wall. Clinopyroxene has high concentrations of REE. Chondrite- normalized REE patterns show enrichment in LREE and depletion in HREE with a negative spike on Eu (Fig. 1a). Amphibole is similar in trace-element character to clinopyroxene, although the former shows slightly higher LREE (= La and Ce) and weaker Eu spike than the latter (Fig. 1a). The REE pattern of plagioclase shows a strong negative slope from LREE through MREE to HREE (Ho, Er, Tm, Yb and Lu) of which contents were below the detection limits, except for a strong positive Eu spike (Fig. 1b). Most orthopyroxene grains show the LREE depletion typical to mantle orthopyroxene, while some show enrichment in LREE (Fig. 1b). This LREE-enriched pattern for orthopyroxene was also reported by Beccaluva et al. (2004). Phlogopite has high concentrations of Rb, Ba, Nb and Ta (not shown). Apatite has high concentrations of REE, and the chondrite-normalized REE pattern exhibits strong enrichment in LREE relative to HREE with a negative spike in Eu (Fig. 1c). Glass has lower concentrations of REE than apatite, although they have the similar REE pattern (Fig. 1c). The geochemical characteristics of the Tallante quartz diorite vein suggest that the involved melt had arc signature, and was of adakite affinity. There is, however, a discrepancy between the Tallante quartz diorite and adakite; the former shows negative anomalies for Rb, Ba, Sr and Eu. Furthermore, the composition of melt in equilibrium with the clinopyroxene is not consistent with the bulk quartz diorite composition (not shown). This can be explained by “in-situ“ crystallization of the melt as a nearly closed system, giving rise to the concentration of REE, except for Eu, to clinopyroxene. The negative Rb, Ba, Sr and Eu anomalies can be explained by fractionation of some amounts of hydrous minerals (phlogopite and amphibole) and plagioclase in the deeper part. Co-precipitation of plagioclase resulted in distinct negative Eu anomalies in other minerals. In conclusion, the Tallante quartz diorite vein was derived from an evolved adakite melt, and individual minerals were precipitated within a stagnant melt pocket filling the cracks.

Oscillatory zoning in eclogitic garnet and amphibole, Northern Serpentinite Melange, Cuba: a record of tectonic instability during subduction?

AbstractExotic blocks of eclogite from distant localities along the Northern Serpentinite Melange of Cuba have comparable P–T histories that include high‐pressure prograde sections (450–600 °C, >15 kbar) associated with subduction of oceanic lithosphere, and retrograde sections within the albite–epidote amphibolite facies (<500 °C, <10 kbar) related to melange uplift. 40Ar/39Ar and Rb/Sr cooling ages (118–103 Ma) of one of the blocks indicate pre‐Aptian subduction and Aptian–Albian uplift. Detailed X‐ray imaging and profiling further reveals that minerals in these eclogite blocks (notably garnet and amphibole) display subtle but well defined oscillatory zoning that developed along the prograde trajectory of the rocks, previous to attainment of peak eclogitic conditions. The chemistry (e.g. coupled changes of Mg# and Mn in garnet, and of Si, Ti, Al and Na in amphibole) and geometry (euhedral to anhedral shapes) of the oscillations can be interpreted in terms of subtle fluctuations in P–T during the general prograde subduction‐related metamorphic path. A (near‐) equilibrium model is presented for the formation of oscillations at near peak conditions by means of recurrent dissolution‐growth reaction processes. This model for near‐peak conditions, and the chemical signatures of earlier oscillations (notably in amphibole), suggest that episodes of retrogression (upward movement?) affected parts of the subducting slab. It is proposed that these retrograde episodes record the tectonic rupture of the subducting slab and, probably, of the upper plate mantle, either due to the intrinsic dynamic behaviour of subduction systems or to the effects of the plate‐tectonic rearrangement of the Caribbean region during the Early Cretaceous.

TEM Investigation of U6+ and Re7+ Reduction by Desulfovibrio desulfuricans, a Sulfate-Reducing Bacterium

Uranium and its fission product Tc in aerobic environment will be in the forms of UO{sub 2}{sup 2+} and TcO{sub 4}{sup {minus}}. Reduced forms of tetravalent U and Tc are sparingly soluble. As determined by transmission electron microscopy, the reduction of uranyl acetate by immobilized cells of Desulfovibrio desulfuricans results in the production of black uraninite nanocrystals precipitated outside the cell. Some nanocrystals are associated with outer membranes of the cell as revealed from cross sections of these metabolic active sulfate-reducing bacteria. The nanocrystals have an average diameter of 5 nm and have anhedral shape. The reduction of Re{sup 7+} by cells of Desulfovibrio desulfuricans is fast in media containing H{sub 2} an electron donor, and slow in media containing lactic acid. It is proposed that the cytochrome in these cells has an important role in the reduction of uranyl and Re{sup 7+} is (a chemical analogue for Tc{sup 7+}) through transferring an electron from molecular hydrogen or lactic acid to the oxyions of UO{sub 2}{sup 2+} and TcO{sub 4}{sup {minus}}.

Ettringite solubility and geochemistry of the Ca(OH) 2–Al 2(SO 4) 3–H 2O system at 1 atm pressure and 298 K

The solubility and weathering reactions of ettringite, (Ca 6Al 2(SO 4) 3(OH) 12·26H 2O), were used to study the geochemical equilibria of the Ca(OH) 2–Al 2(SO 4) 3–H 2O system at environmental pH conditions. Ettringite is a stable mineral above a pH of 10.7 and dissolved congruently with a log K sp of −111.6 (±0.8). Between pH 10.7 and 9.5, ettringite underwent incongruent dissolution to gypsum and Al-hydroxides and controlled Ca 2+, Al 3+, and SO 4 2− activities. At near neutral pH, Al-hydroxy sulfates precipitated in addition to gypsum and Al-hydroxide. These Al-hydroxy sulfate phases exhibited prismatic and anhedral shapes and had variable Al/S ratios. In addition, some new poorly crystalline Ca–Al-hydroxy sulfate phases were identified in microscopic studies when the pH was acidic (pH∼5). The activities of Ca 2+, Al 3+, and SO 4 2− suggest that the geochemistry of the Ca(OH) 2–Al 2(SO 4) 3–H 2O system in the pH range of 7 to 10 is simple and its component Ca(OH) 2–SO 3–H 2O and Al 2(SO 4) 3–H 2O systems behave independently of each other. The precipitation of Al-hydroxy sulfates below pH 7.0 significantly influenced Ca 2+ and SO 4 2− activities. This effect was pronounced when Ca–Al-hydroxy sulfate phases started precipitating (pH<5.0). The lack of thermodynamic data on the newly identified Al, and Ca–Al-hydroxy sulfates makes it difficult to interpret the geochemistry of Ca(OH) 2–Al 2(SO 4) 3–H 2O system for pH≤5.0. Reaction path calculations conducted using the EQ6 computer code predicted ion activities close to the experimental values above pH 5.0. The observed differences between thermodynamic modelling and actual experimental data below this pH can be explained by the formation of Al–/Ca–Al-hydroxy sulfate phases in the system, as detected by electron microscopy and X-ray elemental analysis. These reactions are relevant and useful to the prediction of Al, and Ca geochemistry in natural systems.

Marcasite inversion and the petrographic determination of pyrite ancestry

Pyrite formed by the inversion of marcasite or by oxidative leaching of pyrrhotite can be recognized in polished sections by the following characteristics, compiled from synthetic pyrite after marcasite and natural samples of pyrite after marcasite and after pyrrhotite. Pyrite formed by inversion from marcasite is characterized by approximately 2 modal percent pore space, no by-products filling the pores, optical anisotropy (dark green to dark red), anhedral shape, and polycrystalline daughter pyrite domains in two orientations relative to the marcasite parent crystal. Pyrite or marcasite formed by oxidative dissolution (as opposed to leaching) of pyrrhotite exhibits up to 32 modal percent pore space, gaps between the pyrite and parent pyrrhotite, pores and gaps filled with by-product siderite, magnetite, hisingerite, or other Fe-bearing phase, optically isotropic character (for pyrite), development of crystal faces, and random orientation relative to the parent pyrrhotite. Marcasite formed by oxidative leaching of Fe (as opposed to complete dissolution) from pyrrhotite also exhibits a large amount of pore space but has preferred orientation relative to the parent pyrrhotite.Pyrite after marcasite may be used as an indicator of pH < 5 and T < ca 240 degrees C during deposition of the parent marcasite. Marcasite precipitated following dissolution of pyrrhotite is also indicative of those conditions during alteration. Marcasite formed by oxidative leaching of pyrrhotite cannot be used as evidence of those conditions, however.

Conversion of tetrabasic lead sulfate to lead dioxide in lead/acid battery plates

The formation of cured lead/acid battery plates containing a high level (∼ 70 wt.%) of tetrabasic lead sulfate (4PbO·PbSO 44BS) has been studied under both cyclic voltammetric and constant-potential conditions. The resulting phase composition is determined by an X-ray diffraction technique, while the morphology is examined by both optical and scanning-electron microscopy. The cyclic voltammogram for a 4BS cured plate displays only one anodic peak in the ‘PbO 2’ region. X-ray diffraction phase-analysis confirms that this is due to the conversion of PbSO 4 to the β polymorph of PbO 2. Thus, the long-held belief that 4BS promotes the formation of α-PbO 2 is shown to be incorrect. The formation process commences at the grid surface and then spreads into the cured material, preferentially via the surface of the 4BS crystalline network. The product consists of a large number of fine β-PbO 2 particles gathered into porous agglomerates that retain the overall geometry of the 4BS network. The dioxide morphology can vary widely — from laminar to anhedral shapes — and is determined largely by the value of the applied potential. Thus, the industrial practice of conducting formation under constant-current (as opposed to constant-potential) conditions may result in plates with inconsistent physicochemical characteristics.