西秦岭温泉斑岩钼矿床岩浆-热液演化

西秦岭北缘广泛出露印支期中酸性侵入岩和相关的斑岩-矽卡岩矿床。温泉矿床位于该矿带东段,是其内已探明规模最大的斑岩钼矿床。温泉矿床发育多阶段热液脉体,黄铁矿作为其中的贯通性金属硫化物,其化学组成蕴含着岩浆-热液演化及金属沉淀过程等诸多信息,对于斑岩系统模型的厘定具有重要意义。温泉矿床热液脉体时序为:钾长石-黑云母-石英脉(A脉)、石英-黄铜矿脉、石英-辉钼矿脉(B脉)和石英-绢云母-黄铁矿脉(D脉)。A脉是斑岩系统岩浆-热液演化的最早期脉体,主要矿物组合为钾长石+黑云母+石英+黄铁矿±磁铁矿±磷灰石±黄铜矿,代表了引起早期基性岩浆矿物被蚀变为黑云母的流体通道;B脉与钾长石化蚀变关系密切,围岩中斜长石斑晶大量被蚀变为钾长石;石英-辉钼矿脉切割所有早期黑云母化-钾化蚀变阶段的石英-硫化物网脉,并形成于所有斑岩侵位之后,少量黄铁矿和黄铜矿共生于辉钼矿裂隙及边部;D脉是斑岩系统岩浆-热液成矿作用的最晚期事件,其主要被黄铁矿和石英及少量黄铜矿填充,发育晚期的绢英岩化和泥化蚀变,长石多发生破坏性蚀变。四个阶段石英网脉中黄铁矿电子探针分析显示,A脉的黄铁矿中Cu、Mo和Au含量均较低,有少量的金属硫化物(黄铁矿+黄铜矿)沉淀,但通常不能形成规模矿体;石英-黄铜矿脉的黄铁矿中Cu含量明显较高,且多与高品位Cu矿体的空间产出位置相一致,可能是斑岩系统伴随钾化蚀变作用主要的铜沉淀阶段;B脉的黄铁矿中Mo含量明显较高,与高品位钼矿体空间产出关系密切,可能代表了斑岩系统钼成矿作用的主要阶段;D脉的黄铁矿中Au含量明显升高,可能代表了金在斑岩系统岩浆-热液成矿作用的最晚期事件中的沉淀。     

Intracrystalline microstructures in alkali feldspars from fluid-deficient felsic granulites: a mineral chemical and TEM study

Samples of essentially “dry” high-pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) contain up to 2-mm-large perthitic alkali feldspars with several generations of plagioclase precipitates in an orthoclase-rich host. The first generation takes the form of lenses homogeneous in size, whereas the size of a second generation of very thin albite-rich precipitates is more variable with comparatively high aspect ratios. In the vicinity of large kyanite, garnet or quartz inclusions, the first generation of plagioclase precipitates is significantly less abundant, the microstructure is coarser than in the remainder of the perthitic grain and the host is a tweed orthoclase. The first generation of precipitates formed at around 850 °C during the high-pressure stage (16–18 kbar) of metamorphism. Primary exsolution was followed by primary coarsening of the plagioclase precipitates, which still took place at high temperatures (850–700 °C). The coarsening was pronounced due to the access of fluids in the outer portions of the perthitic alkali feldspar and in more internal regions around large inclusions. The second generation of albite-rich precipitates was formed at around 570 °C. TEM investigations revealed that the interfaces between the second-generation plagioclase lamellae and the orthoclase-rich host are coherent or semi-coherent. During late evolutionary stages of the perthite, albite linings were formed at phase boundaries, and the perthitic microstructure was partially replaced by irregularly shaped precipitates of pure albite with incoherent interfaces. The albitization occurred below 400 °C and was linked to fluid infiltration in the course of deuteric alteration. Based on size-distribution analysis, it is inferred that the precipitates of the first generation were most probably formed by spinodal decomposition, whereas the precipitates of the second generation rather were formed by nucleation and growth.     

The I-Xe record of alteration in the allende CV chondrite

Complex I-Xe and mineralogical studies have been performed on four heavily-altered Allende fine-grained spinel-rich Ca, Al-rich inclusions (CAIs) and four Allende dark inclusions (DIs) showing various degrees of iron-alkali metasomatic alteration. The CAIs are largely composed of Fe-rich spinel, Al-diopside, and secondary nepheline and sodalite. The DIs consist of chondrules and Allende-like matrix composed of lath-shaped fayalitic olivine, nepheline, sodalite, and Ca, Fe-rich pyroxene ± andradite ± FeNi-sulfide nodules. Chondrule phenocrysts are extensively or completely replaced by fayalitic olivine, nepheline, and sodalite; metal nodules are replaced by FeNi-sulfides, andradite and Ca, Fe-rich pyroxenes. The chondrules and matrices are crosscut by Ca, Fe-rich pyroxene ± FeNi-sulfide ± fayalitic olivine veins. DIs are surrounded by continuous Ca-rich rims composed of andradite, wollastonite, kirschsteinite, and Ca, Fe-rich pyroxenes, whereas the outer portions of the inclusions are depleted in Ca.Three CAIs yield well-defined I-Xe isochrons with ages 3.1 ± 0.2, 3.0 ± 0.2 and 3.7 ± 0.2 Ma younger than the Shallowater internal standard (4566 ± 2 Ma). Similar release profiles suggest the same iodine carrier (most probably sodalite) for all four CAIs. The Allende DIs yield I-Xe ages from 0.8 ± 0.3 to 1.9 ± 0.2 Ma older than Shallowater. Based on the petrographic observations, we infer that the DIs experienced at least two-stage alteration. During an early stage of the alteration, which took place in an asteroidal setting, but not in the current location of the DIs, chondrule silicates were replaced by secondary fayalitic olivine, nepheline, and sodalite. Calcium lost from the chondrules was redeposited as Ca, Fe-rich pyroxene veins and Ca, Fe-rich pyroxene ± andradite nodules in the matrix. The second stage of alteration resulted in mobilization of Ca from the DIs and its re-deposition as Ca-rich rims composed of Ca, Fe-rich pyroxenes, andradite, and wollastonite, around the DIs. We interpret I-Xe ages of the DIs as time of their alteration prior incorporation into Allende. The younger I-Xe ages of the fine-grained spinel-rich CAIs may reflect hydrothermal alteration of the Allende host, which could have occurred contemporaneously with the second stage of alteration of the Allende DIs. The lack of evidence for the disturbance of I-Xe system in the Allende DIs may suggest that fluid responsible for the alteration of the Allende CAIs was in equilibrium with the I- and Xe-bearing phases of the DIs.     

Chemical alteration in the micro weathering environment within a spheroidally-weathered anorthosite boulder

A chemical and mineralogical study of the innermost rindlets of a spheroidally weathered boulder from the Wichita Mountains, Oklahoma details incipient alteration of minerals along solution channels. Alteration products in plagioclase areas are highly aluminous, whereas ferruginous residuum is commonly found in olivine alteration zones. Alteration of plagioclase along solution channels results in sharp weathering contacts as contrasted with wider zones of “limonite” bordering solution channels in olivine. In certain instances, cores of fresh olivine are surrounded by limonite indicating that diffusion may play a role in this type of alteration.Mobility of sesquioxides is evidenced by aluminum-rich debris adjacent to mafic materials in solution channels and iron-bearing material in channels cutting through plagioclase. Not resolved is whether the movement of sesquioxides in solution channels is by transport of colloidal particulates or by diffusion of dissolved species.Calcic plagioclase alters to calcite along the innermost solution channel. It is postulated that solution, exterior to the boulder and undersaturated with respect to calcite, enters the rind system via solution channels. Cut off from the CO₂ reservoir of the atmosphere by the narrow confines of this solution channel, the solution reacts with the bytownite to precipitate calcite.Apart from the occurrence of calcite, no crystalline weathering products were found in the innermost rindlet samples. A poorly crystallized illite was identified in the outermost rindlet. These findings suggest that initial alteration products on weathering rinds are amorphous and that, with time, these products attain a degree of identifiable crystallinity.     

Type C Ca, Al-rich inclusions from Allende: Evidence for multistage formation

The coarse-grained, igneous, anorthite-rich (Type C) CAIs from Allende studied (100, 160, 6-1-72, 3529-40, CG5, ABC, TS26, and 93) have diverse textures and mineralogies, suggesting complex nebular and asteroidal formation histories. CAIs 100, 160, 6-1-72, and 3529-40 consist of Al,Ti-diopside (fassaite; 13-23 wt% Al₂O₃, 2-14 wt% TiO₂), Na-bearing åkermanitic melilite (0.1-0.4 wt% Na₂O; Åk_30-75), spinel, and fine-grained (∼5-10 μm) anorthite groundmass. Most of the fassaite and melilite grains have “lacy” textures characterized by the presence of abundant rounded and prismatic inclusions of anorthite ∼5-10 μm in size. Lacy melilite is pseudomorphed to varying degrees by grossular, monticellite, and pure forsterite or wollastonite. CAI 6-1-72 contains a relict Type B CAI-like portion composed of polycrystalline gehlenitic melilite (Åk_10-40), fassaite, spinel, perovskite, and platinum-group element nuggets; the Type B-like material is overgrown by lacy melilite and fassaite. Some melilite and fassaite grains in CAIs 100 and 160 are texturally similar to those in the Type B portion of 6-1-72. CAIs ABC and TS26 contain relict chondrule fragments composed of forsteritic olivine and low-Ca pyroxene; CAI 93 is overgrown by a coarse-grained igneous rim of pigeonite, augite, and anorthitic plagioclase. These three CAIs contain very sodium-rich åkermanitic melilite (0.4-0.6 wt% Na₂O; Åk_63-74) and Cr-bearing Al,Ti-diopside (up to 1.6 wt% Cr₂O₃, 1-23 wt% Al₂O, 0.5-7 wt% TiO₂). Melilite and anorthite in the Allende Type C CAI peripheries are replaced by nepheline and sodalite, which are crosscut by andradite-bearing veins; spinel is enriched in FeO. The CAI fragment CG5 is texturally and mineralogically distinct from other Allende Type Cs. It is anorthite-poor and very rich in spinel poikilitically enclosed by Na-free gehlenitic melilite (Åk_20-30), fassaite, and anorthite; neither melilite nor pyroxene have lacy textures; secondary minerals are absent. The Al-rich chondrules 3655b-2 and 3510-7 contain aluminum-rich and ferromagnesian portions. The Al-rich portions consist of anorthitic plagioclase, Al-rich low-Ca pyroxene, and Cr-bearing spinel; the ferromagnesium portions consist of fosteritic olivine, low-Ca pyroxene, and opaque nodules.We conclude that Type C CAIs 100, 160, 6-1-72, and 3529-40 formed by melting of coarse-grained Type B-like CAIs which experienced either extensive replacement of melilite and spinel mainly by anorthite and diopside (traces of secondary Na-bearing minerals, e.g., nepheline or sodalite, might have formed as well), or addition of silica and sodium during the melting event. CG5 could have formed by melting of fine-grained spinel-melilite CAI with melilite and spinel partially replaced anorthite and diopside. CAIs ABC, 93, and TS-26 experienced melting in the chondrule-forming regions with addition of chondrule-like material, such as forsteritic olivine, low-Ca pyroxene, and high-Ca pyroxene. Anorthite-rich chondrules formed by melting of the Al-rich (Type C CAI-like) precursors mixed with ferromagnesian, Type I chondrule-like precursors. The Allende Type C CAIs and Al-rich chondrules experienced fluid-assisted thermal metamorphism, which resulted in pseudomorphic replacement of melilite and anorthite by grossular, monticellite, and forsterite (100, 160, 6-1-72, 3592-40) or by grossular, monticellite, and wollastonite (ABC, 93, TS-26). The pseudomorphic replacement was followed or accompanied by iron-alkali metasomatic alteration resulting in replacement of melilite and anorthite by nepheline and sodalite, enrichment of spinel in FeO, and precipitation of salite-hedenbergite pyroxenes, wollastonite, and andradite in fractures and pores in and around CAIs.     

Tungsten and hafnium distribution in calcium-aluminum inclusions (CAIs) from Allende and Efremovka

Recent ¹⁸²Hf-¹⁸²W age determinations on Allende Ca-, Al-rich refractory inclusions (CAIs) and on iron meteorites indicate that CAIs have initial ε¹⁸²W (−3.47 ± 0.20, 2σ) identical to that of magmatic iron meteorites after correction of cosmogenic ¹⁸²W burn-out (−3.47 ± 0.35, 2σ). Either the Allende CAIs were isotopically disturbed or the differentiation of magmatic irons (groups IIAB, IID, IIIAB, and IVB) all occurred <1 m.y. after CAI formation. To assess the extent of isotopic disturbance, we have analyzed the elemental distribution of Hf and W in two CAIs, Ef2 from Efremovka (CV3 reduced), and Golfball from Allende (CV3 oxidized). Fassaite is the sole host of Hf (10-25 ppm) and, therefore, of radiogenic W in CAIs, with ¹⁸⁰Hf/¹⁸⁴W > 10³, which is lowered by the ubiquitous presence of metal inclusions to ¹⁸⁰Hf/¹⁸⁴W > 10 in bulk fassaite. Metal alloy (Ni ∼ 50%) is the sole host of W (∼500 ppm) in Ef2, while opaque assemblages (OAs) and secondary veins are the hosts of W in Golfball. A large metal alloy grain from Ef2, EM2, has ¹⁸⁰Hf/¹⁸⁴W < 0.006. Melilite has both Hf and W below detection limits (<0.01 ppm), but the presence of numerous metallic inclusions or OAs makes melilite a carrier for W, with ¹⁸⁰Hf/¹⁸⁴W < 1 in bulk melilite. Secondary processes had little impact on the ¹⁸²Hf-¹⁸²W systematics of Ef2, but a vein cross-cutting fassaite in Golfball has >100 ppm W with no detectable Pt or S. This vein provides evidence for transport of oxidized W in the CAI. Because of the ubiquitous distribution of OAs, interpretations of the ¹⁸²Hf-¹⁸²W isochron reported for Allende CAIs include: (i) all W in the OAs was derived by alteration of CAI metal, or (ii) at least some of the W in OAs may have been equilibrated with radiogenic W during metamorphism of Allende. Since (ii) cannot be ruled out, new ¹⁸²Hf-¹⁸²W determinations on CAIs from reduced CV3 chondrites are needed to firmly establish the initial W isotopic composition of the solar system.     

Redistribution of Trace Elements During Shock-inducd Melting and Phase Tranzition of Minerals in the Suizhou l6 Chondrite

The Suizhou meteorite is an L6 chondrite. This meteorite is consisted of olivine, low-Ca pyroxene, plagioclase, FeNi metal, troilite, whitlockite, chlorapatite, chromite and ilmenite. Olivine and pyroxene grains display shock-induced mosaic texture, and most plagioclase grains were melted and transformed to maskelynite. This meteorite contains a few very thin shock-produced melt veins ranging from 20 to 100 μm in width. They are chondritic in composition and contain abundant high-pressure minerals in two assemblages. One is the coarse-grained assemblage of ringwoodite, majorite, lingunite with minor amount of tuite, xieite, the CF-phase, akimotoite and amorphized perovskite, and the fine-grained assemblage (the melt vein matrix) composed of majorite-pyrope garnet, magnesiowüstite. FeNi metal and troilite in the Suizhou shock veins were molten and occur as small intergrowth grains or veinlets filling the interstices of garnet crystals or cracks in the vein matrix. It was revealed that olivine, pyroxene and plagioclase in the Suizhou shock veins have transformed in solid state to their high-pressure polymorphs ringwoodite, majorite, and lingunite, respectively, without change in their chemical compositions.     

Experimental aqueous alteration of the Allende meteorite under oxidizing conditions: Constraints on asteroidal alteration

We have performed an experimental study of the aqueous alteration of the Allende CV3 carbonaceous chondrite under highly oxidizing conditions, in order to examine the alteration behavior of Allende’s anhydrous mineralogy. The experiments were carried out at temperatures of 100, 150, and 200 °C, for time periods between 7 and 180 days, with water/rock ratios ranging from 1:1 to 9:1. Uncrushed cubes of Allende were used so that the spatial relationships between reactant and product phases could be examined in detail. Scanning electron microscope studies show that in all the experiments, even those of short duration (7 days), soluble salts of Ca and Mg (CaSO₄, CaCO₃, and MgSO₄) precipitated on the sample surface, indicating that these elements are rapidly mobilized during alteration. In addition, iron oxides and hydroxides formed on the sample surfaces. The sulfates, carbonates, and the majority of the iron-bearing secondary minerals are randomly distributed over the surface of samples. In some instances the iron oxides and hydroxides are constrained to the boundaries of altering mineral grains. Transmission electron microscope studies show that the FeO-rich olivine in the interior of the samples has altered to form interlayered serpentine/saponite and Fe-oxyhydroxides. The degree of alteration increases significantly with increasing water/rock ratio, and to a lesser extent with increasing duration of heating. The serpentine/saponite forms both by direct replacement of the olivine in crystallographically oriented intergrowths, and by recrystallization of an amorphous Si-rich phase that precipitates in pore space between the olivine grains. The alteration assemblage bears many similarities to those found in altered carbonaceous chondrites, although in detail there are important differences, which we attribute to (a) the relatively high temperatures of our experiments and (b) comparatively short reaction times compared with the natural examples. In terms of mineral assemblage, our experiments most closely resemble alteration in the CI chondrites, although the degree of alteration of our experiments is much lower. CI chondrites contain serpentine/saponite intergrowths and veins of Ca-sulfate and Ca-carbonate as well as the Fe-oxyhydroxide, ferrihydrite. However, the phyllosilicate phases formed in our experiments are somewhat coarser-grained than the finest phyllosilicate fraction present in CI chondrites, suggesting that alteration of the CI chondrites occurred at lower temperatures. In terms of mineral assemblage, our experiments also appear to come close to matching CR chondrites, although we infer that CR alteration probably occurred at temperatures <100 °C, based on the very fine-grained size of phyllosilicates in CR matrices.     

Hydrothermal clinopyroxenes of the Skaergaard intrusion

Magmatic augites reacted with high temperature aqueous solutions to form secondary calcic pyroxenes during the subsolidus cooling of the Skaergaard intrusion. Secondary, hydrothermal clinopyroxenes replace wall rock igneous augites at the margins of veins filled with calcic amphibole. These veins are up to several millimeters wide and tens of meters in length. Hydrothermal clinopyroxenes are a ubiquitous and characteristic phase in the earliest veins throughout the Layered Series of the intrusion, and occur rarely in late veins that, in some places, crosscut the early veins. Associated secondary phases in early veins include amphiboles ranging in composition from actinolite to hornblende, together with biotite, Fe-Ti oxides and calcic plagioclase. Hydrothermal clinopyroxenes in late veins may be associated with actinolite, hornblende, biotite, magnetite and albite.Hydrothermal clinopyroxenes are depleted in Fe, Mg and minor elements, and enriched in Ca and Si relative to igneous augites in the Layered Series gabbros. Secondary vein pyroxenes are similar in composition to calcic pyroxenes from amphibolite facies metamorphic rocks. Clinopyroxene solvus thermometry suggests minimum temperatures of equilibration of between 500° and 750° C. These temperatures, combined with numerical transport models of the intrusion, suggest that vein clinopyroxenes could have formed during 20,000 to 60,000 year time intervals associated with a maximum in the fluid flux through fractures in the Layered Series.     

Relative timing of deformation and two-stage gold mineralization at the Hutti Mine, Dharwar Craton, India

Gold mineralization at Hutti is confined to a series of nine parallel, N–S to NNW–SSE trending, steeply dipping shear zones. The host rocks are amphibolites and meta-rhyolites metamorphosed at peak conditions of 660±40°C and 4±1 kbar. They are weakly foliated (S₁) and contain barren quartz extension veins. The auriferous shear zones (reefs) are typically characterized by four alteration assemblages and laminated quartz veins, which, in places, occupy the entire reef width of 2–10 m, and contain the bulk of gold mineralization. A <1.5 m wide distal chlorite-sericite (+biotite, calcite, plagioclase) alteration zone can be distinguished from a 3–5 m wide proximal biotite-plagioclase (+quartz, muscovite, calcite) alteration zone. Gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization. An inner chlorite-K-feldspar (+quartz, calcite, scheelite, tourmaline, sphene, epidote, sericite) alteration halo, which rims the laminated quartz veins, is characterized by a pyrrhotite, chalcopyrite, sphalerite, ilmenite, rutile, and gold paragenesis. The distal chlorite-sericite and proximal biotite-plagioclase alteration assemblages are developed in microlithons of the S₂–S₃ crenulation cleavage and are replaced along S₃ by the inner chlorite-K-feldspar alteration, indicating a two-stage evolution for gold mineralization. Ductile D₂ shearing, alteration, and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions (560±60°C, 2±1 kbar). The reefs were reactivated in the D₃ dextral strike-slip to oblique-slip environment by fault-valve behavior at lower greenschist facies conditions (ca. 300–350°C), which formed the auriferous laminated quartz veins. Later D₄ crosscutting veins and D₅ faults overprint the gold mineralization. The alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization, which took place either during isobaric cooling or at different levels of the Archean crust. From overlaps in the tectono-metamorphic history, it is concluded that gold mineralization occurred during two tectonic events, affecting the eastern Dharwar craton in south India between ca. 2550 – 2530 Ma: (1) The assemblage of various terranes of the eastern block, and (2) a tectono-magmatic event, which caused late- to posttectonic plutonism and a thermal perturbation. It differs, however, from the pre-peak metamorphic gold mineralization at Kolar and the single-stage mineralization at Ramagiri. Notably, greenschist facies gold mineralization occurred at Hutti 35–90 million years later than in the western Dharwar craton. Editorial handling: G. Beaudoin     

Ca-rich majorite derived from high-temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the Ries impact crater (Germany)

Shock veins up to 1.1 mm thick were found within non-porous lithic clasts from suevite breccia of the Nördlinger Ries impact structure. These veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy. In shocked amphibolites, two types of Ca-rich majorite occur within and adjacent to the veins. The first type crystallized from shock-induced melts within the veins. Si contents of these majorites suggest dynamic pressure of ~15–17 GPa, implying minimum temperatures in the range of ~2,150–2,230°C. The second type of majorite was formed adjacent to the shock veins within pargasitic hornblende. This majorite contains significant amounts of H₂O (0.7–0.9 wt%). Based on the textural setting, the shrinkage cracks and the chemical compositions of both phases, a solid-state mechanism is deduced for the hornblende to majorite phase transition. Both genetic types of Ca-rich majorite are described for the first time from a terrestrial impact crater. Along with stishovite, majorite constitutes the second silicate mineral displaying sixfold coordination of Si at Ries. Using micro-Raman spectroscopy, jadeite + coesite and jadeite + grossular were identified within local melt glasses of alkali feldspar and plagioclase composition, respectively. Stishovite aggregates, produced by solid-state reaction, along with shock-induced high-pressure melt glasses of almandine composition were also detected in shock veins of a garnet-cordierite-sillimanite restite. The quenched, homogeneous almandine glasses point to melting temperatures of more than ~2,500°C for the veins. Our findings demonstrate that terrestrial shock veins can give valuable information on shock-induced mineral transformations and transient high pressures of host rocks during a natural impact event.     

Replacement textures in CAI and implications regarding planetary metamorphism

Textural and chemical features of five coarse-grained, calcium-aluminum-rich inclusions from the Allende meteorite indicate that some of the melilite in these inclusions was formed by a secondary metamorphic event and not by primary crystallization from a melt or by a sequential nebular condensation process. These inclusions contain embayed pyroxene surrounded by melilite. Physically separated pyroxene crystals are often in optical continuity indicating that they were once part of larger single crystals that have been partly replaced by melilite. Other evidences of metamorphism include reaction textures between melilite and spinel, and metamorphic textures such as kink-band-like features, lobate sutured grain boundaries, and 120° triple-points. This type of metamorphic process requires the addition of Ca which we propose came from calcite or by introduction of a fluid phase. We believe that the most likely environment for this metamorphic process is on a small planetary body, and not in the solar nebula. The results of this study are compatible with oxygen isotopic heterogeneities within CAI, and provide a mechanism for producing lower temperature alteration phases and the rim phases found in these inclusions. We conclude that planetary processes must thus be considered in the formation history of CAI, and that it is necessary to reconsider the classification system of these objects in light of the replacement process proposed here.     

Extremely Na- and Cl-rich chondrule from the CV3 carbonaceous chondrite Allende

We report on a study of Al3509, a large Na- and Cl-rich, radially-zoned object from the oxidized CV carbonaceous chondrite Allende. Al3509 consists of fine-grained ferroan olivine, ferroan Al-diopside, nepheline, sodalite, and andradite, and is crosscut by numerous veins of nepheline, sodalite, and ferroan Al-diopside. Some poorly-characterized phases of fine-grained material are also present; these phases contain no significant H₂O. The minerals listed above are commonly found in Allende CAIs and chondrules and are attributed to late-stage iron-alkali-halogen metasomatic alteration of primary high-temperature minerals. Textural observations indicate that Al3509 is an igneous object. However, no residual crystals that might be relicts of pre-existing CAI or chondrule minerals were identified. To establish the levels of ²⁶Al and ³⁶Cl originally present, ²⁶Al-²⁶Mg and ³⁶Cl-³⁶S isotopic systematics in sodalite were investigated. Al3509 shows no evidence of radiogenic ²⁶Mg^∗, establishing an upper limit of the initial ²⁶Al/²⁷Al ratio of 3 × 10⁻⁶. All sodalite grains measured show large but variable excesses of ³⁶S, which, however, do not correlate with ³⁵Cl/³⁴S ratio. If these excesses are due to decay of ³⁶Cl, local redistribution of radiogenic ³⁶S^∗ after ³⁶Cl had decayed is required. The oxygen-isotope pattern in Al3509 is the same as found in secondary minerals resulting from iron-alkali-halogen metasomatic alteration of Allende CAIs and chondrules and in melilite and anorthite of most CAIs in Allende. The oxygen-isotope data suggest that the secondary minerals precipitated from or equilibrated with a fluid of similar oxygen-isotope composition. These observations suggest that the formation of Al3509 and alteration products in CAIs and chondrules in Allende requires a very similar fluid phase, greatly enriched in volatiles (e.g., Na and Cl) and with Δ¹⁷O ∼ −3‰. We infer that internal heating of planetesimals by ²⁶Al would efficiently transfer volatiles to their outer portions and enhance the formation of volatile-enriched minerals there. We conclude that the site for the production of Na- and Cl-rich fluids responsible for the formation of Al3509 and the alteration of the Allende CAIs and chondrules must have been on a protoplanetary body prior to incorporation into the Allende meteorite. Galactic cosmic rays cannot be the source of the inferred initial ³⁶Cl in Allende. The problem of ³⁶Cl production by solar energetic particle (SEP) bombardment and the possibility that ³⁶Cl and ⁴¹Ca might be the product of neutron capture resulting from SEP bombardment of protoplanetary surfaces are discussed. This hypothesis can be tested comparing inferred “initial” ³⁶Cl with neutron fluencies measured on the same samples and on phases showing ³⁶S^∗ by Sm and Gd isotopic measurements.     

Universal stage measurements and transmission electron microscope observations of fractured plagioclase

At low to moderate temperatures of deformation, fracturing of plagioclase is common. The mechanism of fracturing is generally thought to be either a dislocation assisted process with fractures typically exhibiting some crystallographic regularity or a process of breaking along cleavage planes without the involvement of dislocations. In this study, naturally fractured plagioclase from granodiorites and a gabbro deformed at high strain rates are examined with the transmission electron microscope (TEM) to identify structures at that scale. In addition, fracture orientations are determined with the Universal stage.Some fractures observed in thin section occur parallel to (001) but many are not so simple but are confined to the [112], [1$\text{1}$2], [101], [$\text{1}$01] zones. At the TEM scale, dislocation walls or arrays are common in plagioclase. They also occupy the [101], [$\text{1}$01], [112], [1$\text{1}$2] zones. Microcracks form when dislocations are pinned in these arrays or when a free dislocation interacts with dislocations within a dislocation wall. In this way, large-scale fractures which develop inherit their crystallographic orientation from the dislocation wall.     

Possible fluid-rock interactions on differentiated asteroids recorded in eucritic meteorites

The eucritic meteorites are basaltic rocks that originate from the upper part of the crust of some small bodies as exemplified possibly by asteroid 4-Vesta. A few eucrites appear to have been modified by different degrees of a late stage alteration process that caused significant variations in mineralogy. Three distinct alteration stages are identified: (1) Fe-enrichment along the cracks that cross cut the pyroxene crystals (“Fe-metasomatism”); secondary olivine and minute amounts of troilite are found only occasionally in cracks at this stage; (2) deposits of Fe-rich olivine (Fa_64-86) and minor amounts of troilite are frequent inside the cracks; sporadic secondary Ca-rich plagioclase (An_97-98) is associated with the fayalitic olivine; (3) at this stage, the Fe-enrichment of the pyroxene is accompanied by a marked Al-depletion; moreover, secondary Ca-rich plagioclase is more frequent and partly fills some cracks or rims of the primary plagioclase crystals. The composition of the secondary phases on one hand, the lack of incompatible trace element enrichment in the metasomatized pyroxenes on the other hand, rule out a silicate melt as the metasomatic agent. Although no hydrous phase has been yet identified in the studied samples, aqueous fluids are plausible candidates for explaining the deposits of ferroan olivine and anorthitic plagioclase inside the fractures of the studied unequilibrated eucrites.     

Stable isotope evidence for magmatic fluid input during large-scale Na–Ca alteration in the Cloncurry Fe oxide Cu–Au district, NW Queensland, Australia

Sodic–calcic alteration is common in mineralized hydrothermal systems, yet the relative importance of igneous vs. basinal fluid sources remains controversial. One of the most extensive volumes of sodic–calcic rocks occurs near Cloncurry, NW Queensland, and was formed by overlapping hydrothermal systems that were active synchronously with emplacement of mid‐crustal batholithic granitoids (c. 1.55–1.50 Ga). Altered rocks contain albite–oligoclase, actinolite, diopside, titanite and magnetite. Alteration was localized by: (A) composite veins and breccias containing crystallized magma intimately intergrown with hydrothermal precipitates; (B) intrusions that host setting A veins and breccias; and (C) extensive breccia and vein systems linked to regional fault systems. Isotope analyses of actinolites in settings A and B indicate calculated δ¹⁸O_H2O (+8.2 to +10.6‰) and variably depleted δD_H2O (?130 to ?54‰) compared with typical magmatic fluids, whereas those from setting C typically indicate calculated δ¹⁸O_H2O (+8.0 to +12.8‰) and δD_H2O (?29 to ?99‰). The lowest δD_H2O values are interpreted as representing residual fluids after significant (> 90%) open‐system magmatic degassing. Overall the stable isotope, field, geochronological and geobarometric data suggest that these sodic–calcic alteration systems were formed by the episodic incursion of magmatic fluids that underwent minor isotopic modification as a result of varying degrees of interaction with country rocks.     

Mineralogical and chemical characteristics of newer dolerite dyke around Keonjhar,Orissa: Implication for hydrothermal activity in subduction zone setting

The newer dolerite dykes around Keonjhar within the Singbhum Granite occur in NE–SW, NW–SE and NNE–SSW trends. The mafic dykes of the present study exhibit several mineralogical changes like clouding of plagioclase feldspars, bastitisation of orthopyroxene, and development of fibrous amphibole (tremolite–actinolite) from clinopyroxene, which are all considered products of hydrothermal alterations. This alteration involves addition and subtraction of certain elements. Graphical analyses with Alteration index and elemental abundances show that elements like Rb, Ba, Th, La and K have been added during the alteration process, whereas elements like Sc, Cr, Co, Ni, Si, Al, Fe, Mg and Ca have been removed. It is observed that in spite of such chemical alteration, correlation between major and trace elements, characteristic of petrogenetic process, is still preserved. This might reflect systematic Alteration (addition or subtraction) of elements without disturbing the original element to element correlation. It has also been established by earlier workers that the evolution of newer dolerite had occurred in an arc-back arc setting which may also be true for newer dolerites of the present study. This is evident from plots of pyroxene composition and whole rock composition of newer dolerite samples in different tectonic discrimination diagrams using immobile elements. The newer dolerite dykes of the Keonjhar area may thus be considered to represent an example of hydrothermal activity on mafic rocks in an arc setting.     

Crystallization sequences of Ca-Al-rich inclusions from Allende: The effects of cooling rate and maximum temperature

We have studied the crystallization sequences, mineral chemistries, and textures that develop when an average Type B Ca-Al-rich inclusion composition is cooled in air from 1275–1580° C to below 1000°C at rates between 0.5 and 1000°C/hr. Crystallization sequences, the textures of all the major phases, pyroxene chemistry, and melilite zoning patterns are functions of both the cooling rate and the temperature from which cooling begins. Determination of the order of pyroxene and plagioclase crystallization has been identified as an important goal for petrographic studies of CAIs because it can be used to set constraints on the cooling rate experienced by an individual inclusion. Overall textures plus melilite zoning patterns and pyroxene chemistry can give important clues as to whether pyroxene or plagioclase began to crystallize first. Melilite texture and chemistry appear to yield the most valuable information on the maximum temperature to which an inclusion was raised prior to cooling.Comparison of our experimental results with petrographic observations of Type B CAIs suggests that most inclusions were partially melted and then cooled at rates on the order of a few tenths to tens of degrees per hour. Maximum temperatures of about 1400°C appear most likely for intermediate Type B Allende inclusions. Our results do not support the suggestion that the textures observed in these inclusions formed by crystallization of supercooled, metastable melt droplets condensed from nebular gas. The slow cooling rates we infer for CAIs are difficult to reconcile with models for their origin that imply simple radiative cooling of individual molten or partially molten droplets in a cold, low density environment. On the other hand, cooling rates of the nebular cloud are believed to have been much slower than those we have inferred for Type B CAIs. Scenarios that could be reconciled with the thermal history that we have inferred include drag heating of particles falling through nebular gas, heating by intense radiation (e.g., via flares) from the early sun, heating in nebular shock fronts, or other thermal heterogeneities in the early nebula allowing time scales for cooling (and heating) of CAIs much shorter than those for the nebular cloud as a whole. Successful models for the origin of Type B CAIs must account for the fact that most Type B CAIs cooled relatively slowly from a partially molten state.     

Textural evolution and metamorphism of pillow basalts from the Franciscan Complex,western Marin County,California

Petrogenesis of Franciscan pillow basalts from the Franciscan Complex of western Marin County California entails both dynamic crystallization of tholeiitic magma and subsequent low-temperature metamorphism. Brittle deformation during tectonic emplacement of pillow basalts into a chert greywacke terrain is manifested by the shearing of interpillow matrix and polishing of pillow rims, but the igneous textures within pillows are well preserved.The cooling history of pillow basalts can be understood through analysis of morphologic variations of primary olivine and plagioclase from rim to core of the pillow. Crystal sizes and plagioclase dendrite spacings are consisted with a cooling rate which generally decreases inward. Some pillows show a marked asymmetry in plagioclase and olivine morphology suggesting lower cooling rates caused by asymmetric cooling of the pillows. Olivine morphologies, primarily hopper and chain forms, are consistent with cooling rates of 2–10 °C/h for pillow cores and 50–75 °C/h for pillow rims.Low temperature hydrothermal alteration has produced secondary minerals indicative of zeolite facies conditions. Pillow matrix is either chloritic or zeolitic (in part laumontized). Pillow rims display incomplete replacement of calcic palagonite by pumpellyite (Fe₂O₃=9–21 wt%), prehnite (Fe₂O₃=5–7 wt%), sphene and quartz. Metamorphism of pillow interiors, manifested by: (1) veins of quartz, pumpellyite, calcite, or harmotome (BaO=15 wt%); (2) amygdules containing analcime, chlorite or quartz; and (3) replacement of olivine by pumpellyite or smectite/illite, of plagioclase by albite (An3)+sericite, and of glassy groundmass by fine-grained chlorite. Primary augite (Wo3₃₉En₁₃Fs₄₈) was not altered. The described paragenesis may be attributed to oceanfloor and/or Franciscan-type metamorphism.