Beryllium-10 contents of shergottites,nakhlites, and Chassigny

Accelerator mass spectrometry gives the following ¹⁰Be contents (dpm/kg) for the SNC meteorites: Shergotty, 13.0 ± 1.5 and 17.3 ± 2.7; Zagami, 18.6 ± 2.5 and 20.0 ± 3.2; ALHA 77005, 15 ± 3; EETA 79001A, 7.8 ± 1.1 and 6.3 ± 0.5; EETA 79001B, 8.5 ± 1.1; Nakhla, 19.7 ± 3.3; Lafayette, 18.1 ± 2.5; Governador Valadares, 25.6 ± 3.6; Chassigny, 20.5 ± 3.1. The ¹⁰Be contents of the NC meteorites indicate that significant accumulation of cosmogenic nuclides occurred in decimeter rather than planetary-size bodies. The agreement of the ³He, ²¹Ne, and ¹⁰Be exposure ages of the shergottites also supports small-body irradiation. A long terrestrial age for EETA 79001 appears unlikely.     

The carbon and nitrogen isotopic composition of ureilites: Implications for their genesis

Fourteen ureilites were analyzed for stable C isotopic composition using stepped combustion. The δ¹³C values over the temperature range 500 to 1000°C are fairly constant for any particular meteorite although there are differences between samples. The similarity in combustion temperatures of pure diamond (600–1000δC) and pure graphite (600–800°C) makes it difficult to ascertain the relative proportions of either component within each sample. However, the constant δ¹³C values observed over the range 500 to 1000°C strongly suggests that ureilite diamond and graphite have the same isotopic composition. This would seem to confirm that the diamond in ureilites formed from the graphite during a process, presumably an impact event, which did not fractionate C isotopes.There is a variation in C isotopic composition of graphite/diamond intergrowths among ureilites, which is not continuous—the samples fall into two groups, with δ¹³C values clustered around ?10%. and ?2%. PDB. These groups are also distinguishable on the basis of the Fe content of their olivines, which may reflect the existence of more than one ureilite parent body. The brecciated ureilite North Haig has a δ¹³C value of ?6.5%. and it is thus possible that this sample contains components from mixed parent materials.Nitrogen abundance and stable isotope measurements were made on five samples using stepped combustion analysis. Nitrogen concentrations range from 25 to 150 ppm and $\text{C}\text{N}$ ratios are substantially less than for carbonaceous chondrites. Variation in N isotopic composition is wide and there is evidence of different ratios in diamond/graphite, silicate and metal.     

Spectroscopic evidence for two achondrite parent bodies: asteroids 349 Dembowska and 4 Vesta

A Fourier spectrometer was used to obtain IR spectra of asteroids 349 Dembowska and 4 Vesta (0.8–2.5μm; 25cm ^?1 resolution). The spectrum of Dembowska shows olivine and pyroxene (Fs_{24 ± 8}) with an olivine/pyroxene abundance ratio greater than 2, and possibly as high as 10. This is probably an unsampled achondritic composition, similar to the unique achondrite ALHA 77005. Similar olivine-rich compositions have also been proposed for the mantles of basaltic achondrite parent bodies. Dembowska's mineralogy therefore appears related in some way to the achondrites. but a direct link cannot as yet be made. Our IR observations of Vesta have more complete spectral coverage than those first obtained by Larson and Fink (1975b). Pyroxene (Fs_{50 ± 5}) and plagioclase feldspar are both seen, with a pyroxene/feldspar abundance ratio between 1.5 and 2.0. Vesta's mineralogy is consistent with a mixture of eucrites and howardites on its surface. Time-resolved observations over one-half of the rotation period show no spectral variations at the 2% level, indicating compositional homogeneity. Both 349 Dembowska and 4 Vesta can be considered as candidates for the parent bodies of igneous meteorites.     

湖南沃溪金-锑-钨矿床成因的稀土元素地球化学证据

沃溪金-锑-钨矿床的稀土元素地球化学组成良好地反映了成矿作用的条件和过程,并为示踪矿床成因提供了有用的信息.以流体包裹体为代表的成矿溶液,以较高的稀土总量、显著的轻稀土富集和缺乏明显的铕异常为特征,代表了一种通过在碎屑沉积物柱中循环而萃取矿质的演化的海水热液.矿石相对于成矿流体(母液)富集重稀土而轻微亏损铕,反映了矿石沉淀过程中来自于海水的稀土元素掺合.同一矿层内由下往上,重稀土相对富集的程度逐渐增大而稀土总量则逐渐降低,表明随着热液化学沉淀作用的进行,海水掺合的影响逐渐增强.矿石的稀土元素组成,无论在分布模式还是在轻重稀土之间的分馏程度上,均与其他许多 Sedex型多金属矿床十分相似,暗示了这些矿床具有相似的成因机制.稀土元素地球化学特征支持矿床同生沉积成因的观点.     

Geochemical evidence for the genesis of the Sarical-Yavu hematite mineralizations (Sivas,Central Turkey)

The hematite mineralization under investigation is located 11 km NE of Sarical (Yavu) village of the Yildizeli town (Sivas Province) in central Turkey. The region is within the Central Anatolian Thrust Zone and is comprised of metamorphic units, ophiolitic rocks and overlying Tertiary volcanic and volcanosedimentary rocks in addition to Neogene terrestrial deposits. The mineralization occurs as lenticular or bedded bodies and is composed chiefly of hematite and a lesser amount of goethite. Quartz, calcite, and dolomite are the gangue minerals. Kaolinite and zeolite are the common alteration products. In this study, geochemical and mineralogical investigations were carried out using an X-ray diffractometer analysis (XRD), Raman spectroscopy, and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDX). The V/(V + Ni) ratio increases in parallel to the detritic contribution, and when this ratio is less than 0.60, deposition conditions are said to be anoxic. Considering the V/(V + Ni) ratio and Ce_anom values of the studied samples, we suggest that the environment is both oxic and anoxic in character. The overall assessment of the field observations, mineral paragenesis, major, trace, and rare earth element (REE) data indicates that the Sarical hematite mineralization is of a hydrothermal-sedimentary type.     

苏鲁造山带胶南区段片麻岩原岩的成因:地球化学及Nd同位素证据

出露于苏鲁造山带胶南区段的片麻岩主要包括三种类型黑云斜长片麻岩、二长花岗片麻岩和A型花岗质片麻岩,地球化学判别结果表明它们的原岩均为火成岩.不同类型片麻岩的地球化学特征各不相同,反映它们的原岩具有不同的形成与演化历史.其中黑云斜长片麻岩以负Eu异常弱,甚至出现明显的正异常(δEu=0.60～1.45)、强的Ba正异常、无明显的Sr异常为特征而明显不同于其它类型的片麻岩.其地球化学的总体特征类似于扬子克拉通北缘新元古代双峰式火山岩的酸性端元;与黑云斜长片麻岩类似,二长花岗片麻岩中轻、重稀土元素之间的分馏程度也较强(LaN/YbN=6.3～17.2),但以强的负Eu异常(δEu=0.27～0.54)、强的Sr负异常和弱的Ba负异常为特征明显不同于黑云斜长片麻岩;A型花岗片麻岩的稀土模式及蛛网图的形态类似于二长花岗片麻岩,所不同的是前者Nb、Ta负异常相对较弱.区内几种类型片麻岩的εNd(t)值差别不大,反映它们的原岩之间可能有一定的成因联系.它们的Nd同位素模式年龄TDM都集中在2.0Ga左右,表明它们的源区主要为早元古代的地壳物质,且岩性比较均一.推测二长花岗片麻岩与黑云斜长片麻岩的原岩是同源异相,而A型花岗岩是在大规模Ⅰ型花岗质岩浆形成后,由脱水的紫苏辉石质残留下地壳物质再次发生部分熔融形成的.     

The nature and origin of ureilites

Ureilites are carbon-bearing olivine-pigeonite achondrites which constitute a unique achondrite type. We performed a comprehensive mineralogical and petrological study on eight ureilites (Kenna, Novo Urei, Goalpara, Haverö, Dingo Pup Donga, Dyalpur, North Haig, and ALHA77257) the results of which were used to construct a hypothesis for the origin of ureilites. This hypothesis suggests that ureilites are primarily olivine-pigeonite cumulates which crystallized from a silicate liquid that also contained suspended solid carbon phases, mainly graphite. This carbon caused reduction of the melt and influenced ureilite mineral compositions. Carbonaceous material was trapped by settling cumulus mafic silicates along with other intercumulus material to form the carbon-rich ‘veins’ common to ureilites. Petrofabric analyses show that mafic silicates are oriented in lineated and foliated patterns characteristic of cumulate rocks, specifically adcumulates. Strain state of silicates suggests that ureilites were deformed subsequent to lithification by mild tectonic stress as well as by moderate to severe shock. The latter event caused the formation of micron-sized diamonds and lonsdaleite from graphite and carbon-induced reduction of silicate grain margins during temporal shock-heating.     

Classification of and elemental fractionation among ureilites

Concentrations of Ni, Zn, Ga, Ge, Cd, In, Ir and Au in five ureilites can be combined with petrographie evidence to yield a well-defined suite extending from Goalpara (heavily shocked, low Ir concentration, low Ir/Ni ratio) through Haverö, Dyalpur, Novo-Urei to Kenna (moderately shocked, high Ir concentration, high Ir/Ni ratio). Arguments are presented indicating that this suite represents the sampling of a vertical section within the ureilitic parent body. The large range in Ir/Ni and Ir/Au ratios indicates greater efficiency of extraction of primitive, refractory metal in the Goalpara region than in the Kenna region, and implies that higher maximum temperatures prevailed in the former during the production of ureilitic ultramafic silicates by a partial melting process.

A major impact event injected a deposit of C-rich material into the ultramafic silicates. This C-rich material had a moderately high content of metal; there is no direct evidence that it contained volatiles other than the rare gases. High Ca contents of the ferromagnesian minerals indicate that the ultramafics were hot at the time the injection occurred; the zoning of these mineral grains also indicates high temperatures (ca. 1400 K) and low pressures (S 10atm) such that reaction between C and Fe₂SiO₄ could occur, but that cooling occurred too quickly to allow complete equilibration. The ureilitic C-rich material appears to represent an important type of primitive material.

Two siderophile-rich components are necessary to explain the relative siderophile trends in ureilites. We interpret the high-Ir component to be a refractory nebular condensate or residue that was retained during the partial melting event. The low-Ir component, which roughly resembles E4 chondrite siderophiles, is attributed to metal injected together with the vein material.     

Noble gases in ureilites: cosmogenic, radiogenic, and trapped components

Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio ³⁸Ar/³⁶Ar showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of ⁴⁰Ar is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported ³⁶Ar excesses due to possible decay of extinct ³⁶Cl in the Efremovka meteorite.Trapped ²²Ne in the range of (1.4-2.5) × 10⁻⁸ cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. ²¹Ne-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309)All ureilites may have started with nearly the same initial elemental ratio (¹³²Xe/³⁶Ar)₀, established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio (¹³²Xe/³⁶Ar)₀ is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with Δ¹⁷O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magmaThe elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ionThe similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites—for both elemental and isotopic compositions—strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio ¹²⁹Xe/¹³²Xe (1.042 ± 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 ± 0.002), as a consequence of in situ decay of ¹²⁹I, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature.     

Chemical composition of tourmaline from the Yunlong tin deposit, Yunnan, China: implications for ore genesis and mineral exploration

Summary ?The Yunlong tin deposit is located in the northern part of the Lancangjiang metamorphic zone of the Sanjiang Tethys orogen series in western Yunan province of China. It consists of vein-type cassiterite ores, which are mainly hosted in migmatites of Caledonian age. Abundant tourmaline is associated with the ores, quartz–tourmaline veins and barren migmatized gneiss and migmatites. A detailed electron microprobe study has been carried out to document the chemical compositions of tourmaline from this deposit. The results exhibit a systematic compositional change that might be used as tracer for ore genesis and in prospecting for tin mineralization. Tourmalines from the ore bodies are dravite with Fe/(Fe + Mg) ratios of 0.09 ∼ 0.31 and Ca/(Ca + Na) ratios of 0.03 ∼ 0.40. These tourmalines are also rich in chromium (up to 0.74 wt% Cr₂O₃) and tin (up to 0.42 wt% Sn). In contrast, tourmalines from the barren migmatites are mostly schorl with Fe/(Fe + Mg) ratios of 0.38 ∼ 0.94 and Ca/(Ca + Na) ratios of 0.00 ∼ 0.14. Tourmalines from quartz–tourmaline veins that occur between ore bodies and the migmatites show intermediate compositions, i.e., Fe/(Fe + Mg) = 0.09 ∼ 0.59, Ca/(Ca + Na) = 0.01 ∼ 0.22. It is suggested that the Mg-rich nature of the tourmaline can be used as an exploration tool in this region to target tin mineralization, because the tourmalines show increasing Mg contents and are more dravitic when approaching the ore bodies. It is likely that the formation of the Yunlong tin deposit was related to migmatitic-hydrothermal processes. The high Mg and Cr contents in tourmalines from the ore bodies were probably derived from the local meta-sedimentary and meta-volcanic rocks of the Precambian Chongshan Group rather than from the granites in the region. Received December 28, 2000; revised version accepted January 25, 2002     

Experimental constraints on the parental liquid of the Chassigny meteorite: A possible link between the Chassigny meteorite and a Martian Gusev basalt

Most magmas proposed as parental to the Martian SNC meteorites are high in iron and low in alumina. Yet, experiments at low pressures on such liquids have not produced the cumulate or melt-inclusion assemblages seen in the chassignite meteorites. Therefore, elevated pressure experiments under anhydrous and hydrous (water-undersaturated) conditions were conducted on a high-Fe, low-Al liquid proposed to be parental to the Chassigny meteorite. These experiments failed to produce the most magnesian cumulate phases, as well as the olivine hosted kaersutite-bearing melt-inclusion assemblage, of the chassignites. These results suggest that the parental liquid to the chassignite meteorites is both more magnesium and aluminum-rich than the previously considered composition (A^∗; Johnson et al., 1991). The proposed composition is similar to the Martian Adirondack class Gusev basalt Humphrey and suggests a link between the Chassigny meteorite and rocks on the surface of Mars.     

Fluid inclusion and stable isotope evidence for the genesis of quartz-scheelite veins, Metaggitsi area, central Chalkidiki Peninsula, N. Greece

Scheelite mineralization accompanied by muscovite and albite, and traces of Mo-stolzite and stolzite occurs in epigenetic quartz vein systems hosted by two-mica gneissic schists, and locally amphibolites, of the Paleozoic or older Vertiskos Formation, in the Metaggitsi area, central Chalkidiki, N Greece. Three types of primary fluid inclusions coexist in quartz and scheelite: type 1, the most abundant, consists of mixed H₂O-CO₂ inclusions with highly variable (20–90 vol.%) CO₂ contents and salinities between 0.2 and 8.3 equivalent weight % NaCl. Densities range from 0.79 to 0.99 g/cc; type 1 inclusions contain also traces (<2 mol%) of CH₄. Type 2 inclusions are nearly 100 vol.% liquid CO_2, with traces of CH₄, and densities between 0.75 and 0.88 g/cc. Type 3 inclusions, the least abundant, contain an aqueous liquid of low salinity (0.5 to 8.5 equivalent weight% NaCl) with 10–30 vol.% H₂O gas infrequently containing also small amounts of CO₂ (<2 mol%); densities range from 0.72 to 0.99 g/cc. The wide range of coexisting fluid inclusion compositions is interpreted as a result of fluid immiscibility during entrapment. Immiscibility is documented by the partitioning of CH₄ and CO₂, into gas-rich (CO₂-rich) type 1 inclusions, and the conformity of end-member compositions trapped in type 1 inclusions to chemical equilibrium fractionation at the minimum measured homogenization temperatures, and calculated homogenization pressures. Minimum measured homogenization temperatures of aqueous and gas-rich type 1 inclusions of 220°–250 °C, either to the H₂O, or to the CO₂ phase, is considered the best estimate of temperature of formation of the veins, and temperature of scheelite deposition. Corresponding fluid pressures were between 1.2 and 2.6 kbar. Oxygen fugacities during mineralization varied from 10⁻³⁵ to 10⁻³¹ bar and were slightly above the synthetic Ni-NiO buffer values. The fluid inclusion data combined with δ¹⁸O water values of 3 to 6 per mil (SMOW) and δ¹³C CO₂₋ fluid of −1.2 to +4.3 per mil (PDB), together with geologic data, indicate generation of mineralizing fluids primarily by late- to post-metamorphic devolatilization reactions. Received: 8 April 1997 / Accepted: 8 July 1997     

Metasomatism in mantle xenoliths from Gees,West Eifel,Germany: evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment

 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO₂ when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO₂), characterized by high Al₂O₃ (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO₂ and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO₂ is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995     

地球早期地幔化学不均一性及其成因机制

早期地球是地球科学前沿研究方向之一,涉及地-月形成、核幔分异、原始大气圈和水圈的形成等关键科学问题。地幔是硅酸盐地球的主要组成部分,也是地球上最大的化学储库,其早期演化为揭示早期地球增生、核-幔分异、壳-幔分异等重大地质事件提供重要制约。近年来,地球早期地幔不均一性逐渐被认知,本文在总结早期地幔不均一性的稀有气体同位素、钕同位素和钨同位素等证据基础上,探讨了早期地幔不均一性形成动力学机制,并指出发展高精度同位素分析技术,结合地球物理和实验岩石学,揭示核幔边界结构、核幔物质交换过程是深入研究早期地幔不均一的重要发展方向。

     

The Granites gold deposits, Northern Territory, Australia: evidence for an early syn-tectonic ore genesis

The ore deposits of The Granites goldfield are shear-hosted within Palaeoproterozoic amphibolite facies metasedimentary rocks in the Tanami Region, Northern Territory, Australia. The ore bodies are located within a 5- to 35-m thick sequence of steeply dipping unit of metamorphosed iron-rich metasedimentary rocks. Deformation at The Granites was complex and is characterized by five successive deformation phases (D_1–5). Shear veins (central and oblique) are the dominant type of vein geometry, with minor development of extensional veins and reverse-fault related veins. Four generations of syn-tectonic veins, corresponding to D₁, D₃, D₄, and D₅, have been recognized and are comprised of quartz, quartz-carbonate, calc-silicate, and calcite. In addition, two generations of disseminated sulfide–arsenide mineralization, dominated by pyrrhotite, arsenopyrite, and loellingite, with minor pyrite, chalcopyrite and rare marcasite, formed syn-D₁ and syn- to post-D₃. Textural and structural evidence indicates deposition of gold was contemporaneous with the syn-D₁ veins and sulfide–arsenide mineralization. Four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide–arsenide assemblages. The first phase (D₁) was responsible for transport and deposition of the majority of the gold. Minor remobilization and deposition of gold occurred during the D₃ and D₄ phases. Little is known about the nature of the D₁ ore fluid, although a relatively low sulfur content is indicated by the assemblage pyrrhotite–arsenopyrite–loellingite+rare pyrite. The growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during D₁ indicates a high temperature for the fluid. The D₃ hydrothermal phase coincided with peak metamorphism. D₄ fluids were hypersaline, high temperature, CO₂-poor, and H₂S-poor. Editorial handling: L. Meinert     

宝坛地区透闪石化镁铁质岩石成因的地质地球化学证据

宝坛地区呈岩脉状或岩席状侵入于四堡群变质地层之中的透闪石化镁铁质岩石包括堆积的超镁铁质岩和分异的闪长岩,其岩石类型为透闪石化辉石岩、辉橄岵、辉长岵、辉绿岵及闪长岵等。该镁铁质－闪长质岩石以富集轻稀土和大离子新石元素、亏损高场强元素（Ｎｂ、Ｔａ）为特征;除堆积成因的超镁铁质岩石外,透闪石化镁铁质岩石及其分异的闪长岵的ＭｇＯ为４．４５％￣７．９６％,是镁铁质岩浆经结晶分异（辉石、橄榄石）作用的产物。     

Chemical compositions of sulfides in the porphyry Cu ores,Yangla Cu deposit,Yunnan, China: implication for ore genesis

Acta Geochimica - The Yangla Cu deposit is the largest ore deposit in the Jinshajiang polymetallic metallogenic belt, northwest Yunnan, China. There is no consensus on the genesis of the ore...     

Alkali metasomatism as a process for trondhjemite genesis: evidence from Aspromonte Unit, north-eastern Peloritani, Sicily

Summary Rocks of trondhjemitic composition are widespread in the North-Eastern Peloritani Belt within the Aspromonte Unit, a Hercynian medium- to high-grade metamorphic complex intruded by late-Hercynian peraluminous granites and later affected by MP/LT Alpine metamorphism. Among these trondhjemitic bodies, the Pizzo Bottino trondhjemites form one of the largest, outcropping over about 6km² and up to 400m thick. These rocks display concordant to discordant relationships with associated metamorphic rocks and are often cut by late-Hercynian leucogranitic dykes. The field, petrographic and geochemical features of these trondhjemites are consistent with an igneous origin. Petrographic and geochemical evidences suggest that the trondhjemitic character of the Pizzo Bottino rocks is due to an alkali metasomatism process involving cationic exchange of Na and Ca for K and consequent replacement of K-feldspar by oligoclase in the original granitoids. The major and trace element contents of the Pizzo Bottino trondhjemites are in fact comparable to those of the peraluminous late-Hercynian granitoids from the southern Calabrian-Peloritani Arc (CPA), when the elements directly involved in the alkali metasomatism process (Na, Ca, K, Sr, Ba, Rb) are not considered. The behaviour of REE elements, plus Th and U, also seems to be partially controlled by metasomatic processes, because their abundances vary with the K/Na ratio. Metasomatism seems to be the only viable mechanism involved in the genesis of the Pizzo Bottino trondhjemites. Other trondhjemite generation processes such as fractionation from basaltic parents and partial melting of metabasaltic or metasedimentary sources are ruled out on geological, petrographic and isotopic (Sr, Nd) grounds. Lastly, regional considerations place the metasomatic event during the late Hercynian, after the emplacement of the original granitoids and preceding the intrusion of the leucogranitic dykes, which are not affected by metasomatism.     

Two populations of carbonate in ALH84001: geochemical evidence for discrimination and genesis

We present major and trace-element, oxygen isotope, textural, and structural data for carbonates and related phases in the SNC meteorite ALH84001. These data document the existence of at least two distinct carbonate populations: one composed of finely zoned, chemically and isotopically heterogeneous concretions of magnesio-siderite with distinct white magnesite rims, and a second composed of relatively homogeneous, isotopically and compositionally simple domains of ankeritic carbonate and intimately intergrown glass and fine-grained pyroxene. We suggest on the basis of textural evidence and geochemical systematics that the first population consists of low-temperature aqueous precipitates, and the second is produced by shock melting of the first. Values of δ¹⁸O and Sr/Ca ratios are correlated with one another in magnesio-siderite concretions; the trend formed by these data is consistent with the predicted relationship for inorganic precipitation of carbonate from a solution of constant composition between temperatures of ∼190°C (for concretion cores) to 20°C (for magnesite-rich concretion rims). Given the assumptions inherent in this temperature estimate, the aqueous fluid parental to carbonate concretions is constrained to have a δ¹⁸O of −5‰ VSMOW (significantly mass fractionated compared with expected juvenile martian volatiles) and minor-element abundances broadly similar to terrestrial seawater.