Controls on ostracod valve geochemistry, Part 1: Variations of environmental parameters in ostracod (micro-)habitats

The variations of environmental conditions (T°, pH, δ¹³C_DIC, [DIC], δ¹⁸O, Mg/Ca, and Sr/Ca) of ostracod habitats were examined to determine the controls of environmental parameters on the chemical and isotopic composition of ostracod valves. Results of a one-year monitoring of environmental parameters at five sites, with depths of between 2 and 70 m, in Lake Geneva indicate that in littoral to sub-littoral zones (2, 5, and 13 m), the chemical composition of bottom water varies seasonally in concert with changes in temperature and photosynthetic activity. An increase of temperature and photosynthetic activity leads to an increase in δ¹³C values of DIC and to precipitation of authigenic calcite, which results in a concomitant increase of Mg/Ca and Sr/Ca ratios of water. In deeper sites (33 and 70 m), the composition of bottom water remains constant throughout the year and isotopic values and trace element contents are similar to those of deep water within the lake. The chemical composition of interstitial pore water also does not reflect seasonal variations but is controlled by calcite dissolution, aerobic respiration, anaerobic respiration with reduction of sulphate and/or nitrate, and methanogenesis that may occur in the sediment pores. Relative influence of each of these factors on the pore water geochemistry depends on sediment thickness and texture, oxygen content in bottom as well as pore water. Variations of chemical compositions of the ostracod valves of this study vary according to the specific ecology of the ostracod species analysed, that is its life-cycle and its (micro-)habitat. Littoral species have compositions that are related to the seasonal variations of temperature, δ¹³C values of DIC, and of Mg/Ca and Sr/Ca ratios of water. In contrast, the compositions of profundal species are largely controlled by variations of pore fluids along sediment depth profiles according to the specific depth preference of the species. The control on the geochemistry of sub-littoral species is a combination of controls for the littoral and profundal species as well as the specific ecology of the species.     

大别-苏鲁造山带大理岩碳氧同位素地球化学研究

大别-苏鲁造山带的大理岩分布非常广泛,在三叠纪的陆-陆碰撞中,经受了不同程度的变质作用。本文对分布在大别地区北淮阳、苏家河、桐柏、宿松和苏鲁地区张八岭、五莲和坪上等地区的浅变质大理岩进行了碳氧同位素分析。结果发现,除了张八岭大理岩δ~(13)C值出现明显正异常(1‰～8‰)外,其他地区大理岩的δ~(13)C值均处于0±2‰(PDB)范围之内。所有地区大理岩的δ~(18)O值都有不同程度的降低,最低的δ~(18)O值达到4.5‰(SMOW)。与前人对大别-苏鲁超高压大理岩碳氧同位素研究结果相对比,我们发现大理岩δ~(13)C值分布与岩石是否经过超高压没有联系,而主要反映了其原岩沉积的时代和环境,并且其特征可以与发生在新元古代的冰川事件相关联。区域性的δ~(18)O值降低则说明,大部分岩石都经过了流体交换,并且流体的主要成分是水,含碳很少或者不舍碳,因此流体的来源是大气降水,可能与新元古代冰川溶融有关。     

白云鄂博碳酸岩墙碳氧同位素地球化学

对内蒙古白云鄂博 REE- Fe- Nb矿床周围碳酸岩墙中共存的方解石和白云石进行了 C和 O同位素分析。结果表明，方解石和白云石的δ 13C值变化范围一致，均为－ 3.5‰～－ 7.3‰，落在正常地幔δ 13C值范围 (－ 5‰± 2‰ )内；而它们的δ 18O值可分为两组，第Ⅰ 组为 9.5‰～ 18.0‰，第Ⅱ 组为 20.6‰～ 22.6‰，均远大于正常地幔δ 18O值范围 (5.7‰± 1.0‰ )。第Ⅰ 组低δ 18O值样品中共存白云石与方解石之间的 C和 O同位素分馏均为负值，因此处于热力学不平衡状态，指示它们自形成后受到过后期热液蚀变，与先前的岩石学观察一致。相反，第Ⅱ 组高δ 18O值样品中白云石与方解石之间的 C和 O同位素分馏均为正值，处于热力学平衡状态，指示它们自形成后未受到后期热液蚀变，因此可能沉淀于晚期低温高δ 18O值流体。第Ⅰ 组碳酸岩墙中白云石的 C和 O同位素组成不呈线性分布，指示碳酸岩浆并非由幔源碳酸盐与沉积碳酸盐混合形成。应用水-岩交换模型计算得到，第Ⅰ 组碳酸岩在侵位后经历了碳酸岩浆期后热液的不均一蚀变，蚀变温度约在 220～ 800℃之间，蚀变流体的 CO2/H2O比值较小 (1/500)，但水 /岩比值变化较大 (10～ 400)。由于低温下方解石与热液之间的碳氧同位素交换速率大于白云石，导致这部分碳酸     

Carbon and oxygen isotope compositions of Lower-Middle Ordovician carbonate rocks in the northwestern Russian platform

Carbon and oxygen isotope compositions of Lower-Middle Ordovician carbonate rocks in the northwestern Russian Platform (eastern Ladoga Klint, Lynna River, and Babino quarry sections) are considered. In the studied section interval, average δ¹³C and δ¹⁸O values are 0 ± 0.5 and ?5 ± 0.5‰ (V-PDB), respectively. Two closely-spaced negative carbon isotope excursions with the amplitude of 2‰ are established near the Lower-Middle Ordovician boundary (between the Floian and Dapingian stages). The lower part of the Darriwilian Stage is marked by the gradual decrease in δ¹³C values to 1‰. Excursions of δ¹³C do not correlate with δ¹⁸O variations and can be considered as primary. The carbon isotope event defined at the Lower-Middle Ordovician boundary is traceable at the interregional level and represents a promising stratigraphic reference level. It may likely be explained by decrease in the relative rate of organic matter burial due to sea level fall and expansion of well-aerated shallow-water basins with a low primary production of phytoplankton.     

湖北徐家山锑矿床方解石C、O、Sr同位素地球化学

湖北通山的徐家山锑矿床产在上震旦统陡山沱组和灯影组的海相碳酸盐岩中.对该矿床中成矿前和成矿期方解石进行了较系统的C、O、Sr同位素地球化学研究,结果表明,成矿前方解石具相对较高的δ13CPDB(-0.7‰～ 2.0‰)、δ18OSMOW( 18.6‰～ 19.6‰)和Sr含量(2645-8174 μg/g,平均5656μg/g),及较低的87Sr/86Sr比值(0.7096～0.7097);而成矿期方解石具相对较低的δ13CPDB(-3.9‰～-2.1‰)、δ18OSMOW( 11.5‰～ 15.3‰)、Sr含量(785～2563μg/g,平均1571μg/g),和较高的87Sr/86Sr比值(O.7109～0.7154,平均0.7141).分析认为,成矿前方解石的C、O和绝大部分Sr来源于赋矿围岩--震旦系海相碳酸盐岩;而成矿流体为富H2CO3的溶液,它来自或流经富放射成因87Sr的下伏基底碎屑岩--中元古界冷家溪群浅变质岩;该流体与围岩发生水.岩反应导致成矿期方解石和辉锑矿的沉淀.     

Carbon and oxygen isotope compositions of the carbonate facies in the Vindhyan Supergroup, central India

The Vindhyan sedimentary succession in central India spans a wide time bracket from the Paleoproterozoic to the Neoproterozoic period. Chronostratigraphic significance of stable carbon and oxygen isotope ratios of the carbonate phase in Vindhyan sediments has been discussed in some recent studies. However, the subtle controls of facies variation, depositional setting and post-depositional diagenesis on stable isotope compositions are not yet clearly understood. The Vindhyan Supergroup hosts four carbonate units, exhibiting a wide variability in depositional processes and paleogeography. A detailed facies-specific carbon and oxygen isotope study of the carbonate units was undertaken by us to investigate the effect of these processes and to identify the least altered isotope values. It is seen that both carbon and oxygen isotope compositions have been affected by early meteoric water diagenesis. The effect of diagenetic alteration is, however, more pronounced in case of oxygen isotopes than carbon isotopes. Stable isotope compositions remained insensitive to facies only when sediments accumulated in a shallow shelf setting without being exposed. Major alteration of original isotope ratios was observed in case of shallow marine carbonates, which became exposed to meteoric fluids during early diagenetic stage. Duration of exposure possibly determined the magnitude of alteration and shift from the original values. Moreover, dolomitization is found to be accompanied by appreciable alteration of isotope compositions in some of the carbonates. The present study suggests that variations in sediment depositional settings, in particular the possibility of subaerial exposure, need to be considered while extracting chronostratigraphic significance from δ¹³C data.     

Distinguishing magmatic from subsolidus epidote: laser probe oxygen isotope compositions

Epidote occurs in four textural varieties in the Mount Lowe intrusion of southern California: euhedra, anhedra, intergrowths and cross-cutting veins. Of these, conspicuous elongate euhedra, which range up to 3 cm in length, meet most of the established textural criteria for magmatic epidote. Equant anhedral grains, which are ˜5 mm in diameter, are texturally ambiguous although rare allanite cores are suggestive of a magmatic origin. Epidote intergrowths with hornblende and biotite also meet certain textural criteria for crystallization from a magma, notably, euhedral faces against biotite. Finally, late-stage veins of epidote cross-cut all phases in the rock and are likely subsolidus. Oxygen isotopic compositions of these four textural varieties of epidote determined using the laser probe indicate that the use of textural criteria alone in establishing epidote parageneses can be misleading. Intracrystalline δ¹⁸O variations in the euhedra document both magmatic and subsolidus compositions. Oxygen isotope compositions are bimodal averaging 5.36 ± 0.13‰ (n = 11) and 4.66 ± 0.23‰ (n = 21). These data combined with plagioclase and zircon δ¹⁸O values are interpreted to indicate that magmatic oxygen isotopic compositions have been preserved where epidote δ¹⁸O is greater than 5‰. Significant portions of each euhedral crystal have been affected by subsolidus exchange and are characterized by epidote δ¹⁸O values less than 5‰. Intracrystalline δ¹⁸O values of epidote anhedra range from 4.50 to 6.08‰ and thus also document both magmatic and subsolidus compositions. Subsolidus exchange is volumetrically less significant in the anhedra than in the euhedra. Values of δ¹⁸O for intergrowths and late-stage cross-cutting veins of epidote average 3.81 ± 0.22‰ and document clearly subsolidus growth. The data confirm that in the Mount Lowe intrusion, large euhedra of epidote are magmatic in origin, consistent with textural evidence. The data also indicate that equant anhedra of epidote are also magmatic in origin and thus the absence of good crystal faces does not necessarily indicate subsolidus growth. The subsolidus origin of intergrowths of epidote with euhedral faces against biotite indicates that well developed crystal faces do not require growth from a magma. Finally, the subsolidus origin of the vein epidote is consistent with textural evidence. The increased resolution available with laser-based oxygen isotope measurements offers an important test for documenting whether epidote is of magmatic or subsolidus origin. Given the barometric significance of magmatic epidote, oxygen isotope compositions can be used to aid in the interpretation of epidote-bearing plutons. Received: 9 April 1996 / Accepted: 3 August 1996     

湖南香花岭锡多金属矿床C、O、Sr同位素地球化学

对湖南香花岭锡多金属矿床成矿早、晚两期方解石进行了系统的C、O、Sr同位素研究。结果表明,成矿早期方解石具有相对较低的δ13CPDB（-5.4‰ ~ -1.4‰）、δ18OSMOW（+6.1‰ ~ +13.9‰）和较高的87Sr/86Sr值（0.7101 ~ 0.7230）;而成矿晚期方解石则具有相对较高的δ13CPDB（+0.2‰ ~ +0.6‰）、δ18OSMOW（+19.4‰ ~ +21.5‰）和较低的87Sr/86Sr值（0.7101）。成矿早期方解石为岩浆热液与海相碳酸盐岩相互作用的产物,成矿流体中的碳源于岩浆碳和海相碳酸盐岩;模拟计算结果显示,成矿早期方解石为成矿溶液发生0.05-0.1摩尔分数的 CO2去气作用的产物。而成矿晚期方解石主要为矿区碳酸盐岩围岩低温淋滤的产物,流体中的碳主要源于矿区碳酸盐岩围岩。成矿流体中Sr源于矿区花岗岩和碳酸盐岩,从成矿早期到晚期,岩浆来源的Sr逐渐减少,而碳酸盐岩围岩提供的Sr比例不断增大。     

苏州A型花岗岩氢氧同位素地球化学研究

对苏州Ａ型花岗岩氢氧同位素组成进行了系统深入的研究,其全岩δ１８Ｏ值为＋３．５‰～＋９．２‰,全岩δＤ值在－８１‰～－５９‰之间变化。主要造岩矿物对保持氧同位素平衡分馏的样品,其Ｄ亏损主要受单阶段岩浆去气机理的制约。部分全岩样品表现出不同程度Ｄ－１８Ｏ同步亏损,这种亏损要受岩浆期后固相线下与外来渗透大气降水之间进行同位素交换机理的制约。石英δ１８Ｏ值基本正常,石英与碱性长石之间氧同位素不平衡分馏特征表明,苏州Ａ型花岗岩整体上起源于亏损１８Ｏ源区物质通过地球动力学再循环产生低δ１８Ｏ岩浆的可能性不大。根据氢氧同位素实测值和理论模型计算结果,推测苏州Ａ型花岗岩浆δＤ和δ１８Ｏ初始值分别为－５０±５‰和７．５±１．０‰,这排除了岩浆起源于曾经历过化学风化循环的地壳上部岩石的可能性。     

Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget

Carbon isotope values of 260 Precambrian limestones and dolomites (most of them being substantially unaltered) have yielded an overall mean of $\text{δ}{}^{13}\text{C}\text{= +0.4 ± 2.7‰}$ vs. PDB; the corresponding oxygen values average at $\text{δ}{}^{13}\text{O = +20.0 ± 4.2‰}$ vs. SMOW. Like the overall mean, the δ ¹³C values furnished by individual carbonate occurrences are, as a rule, fairly “modern” and almost constant as from the very beginning of the sedimentary record. A remarkable exception are the “heavy” dolomites of the Middle Precambrian Lomagundi Group, Rhodesia, with $\text{δ}{}^{13}\text{C}\text{= +9.4 ± 2.0‰}$ vs. PDB. As a result of our measurements, the sporadic occurrence in the geological past of anomalously heavy carbonates seems to be established.The approximate constancy around zero per mill of the δ ¹³C values of marine carbonates through geologic time would imply a corresponding constancy of the relative proportion of organic carbon in the total sedimentary carbon reservoir since about 3.3 · 10⁹ y ago (with C_org/C_total ? 0.2). Utilizing this ratio and current models for the accumulation of the sedimentary mass as a function of time, we get a reasonable approximation for the absolute quantity of organic carbon buried in sediments and, accordingly, of photosynthetic oxygen released. Within the constraints of our model (based on a terrestrial degassing constant λ = 1.16 · 10^?9 y^?1) close to 80% of the amount of oxygen contained in the present oxygen budget should have been released prior to 3 · 10⁹ y ago. Since geological evidence indicates an O₂-deficient environment during the Early and most parts of the Middle Precambrian, there is reason to believe that the distribution of this oxygen between the “bound” and the “molecular” reservoir was different from that of today (with effective O₂-consuming reactions bringing about an instantaneous transfer to the crust of any molecular oxygen released). Accordingly, the amount of C_org in the ancient sedimentary reservoir as derived from our isotope data is just a measure of the gross amount of photosynthetic oxygen produced, withholding any information as to how this oxygen was partitioned between the principal geochemical reservoirs. As a whole, the carbon isotope data accrued provide evidence of an extremely early origin of life on Earth since the impact of organic carbon on the geochemical carbon cycle can be traced back to almost 3.5 · 10⁹y.     

Carbon,oxygen, and sulfur isotope compositions and model of the Silurian rock formation in northwestern Belarus

The paper describes the results of study of the Silurian clayey–carbonate rocks ranging from the Telychian Stage (Llandovery) to the Gorstian Stage (Ludlow) recovered by the Borehole Davtyuny 3k in northwestern Belarus. Rocks of the Sheinwoodian Stage demonstrate a positive excursion of δ¹³C with amplitude of 4.7‰, marking the Ireviken biotic event recorded in the global chemostratigraphic curve. Values of δ¹⁸O for the carbonate material in the studied section (25.5–29.2‰ SMOW) are close to those for Silurian rocks from the Baltic region, Scandinavia, Ukraine, Poland, and Canada. The whole section contains postsedimentary gypsum as nodules and the infilling of fissures and fenestrae. Values of δ³⁴S in gypsum (21.3–26.7‰ CDT) are close to those for the Silurian rocks on the Phanerozoic isotope plot. The formation of gypsum was related to a partial development of the supralittoral environment over the sublittoral and littoral clayey–carbonate substrate. The seawater accumulated in lowlands of the supralittoral plain after storms was intensely concentrated during arid conditions and accumulated in the clayey–carbonate sediment. The subsequent underground evaporation promoted the formation of gypsum as nodules in the unlithified sediments and the infilling of fissures and fenestrae in the lithified rocks.     

Carbon,oxygen and strontium isotope geochemistry of Proterozoic carbonate rocks of the Vindhyan Basin,central India

Carbon, oxygen and strontium isotope compositions of carbonate rocks of the Proterozoic Vindhyan Supergroup, central India suggest that they can be correlated with the isotope evolution curves of marine carbonates during the latter Proterozoic. The carbonate rocks of the Lower Vindhyan Supergroup from eastern Son Valley and central Vindhyan sections show δ¹³C values of ∼0‰ (V-PDB) and those from Rajasthan section are enriched up to +2.8‰. In contrast, the carbonate rocks of the Upper Vindhyan succession record both positive and negative shifts in δ¹³C compositions. In the central Vindhyan section, the carbonates exhibit positive δ¹³C values up to +5.7‰ and those from Rajasthan show negative values down to –5.2‰. The δ¹⁸O values of most of the carbonate rocks from the Vindhyan Supergroup show a narrow range between –10 and –5‰ (V-PDB) and are similar to the ‘best preserved’ ¹⁸O compositions of the Proterozoic carbonate rocks. In the central Vindhyan and eastern Son Valley sections, carbonates from the Lower Vindhyan exhibit best-preserved ⁸⁷Sr/⁸⁶Sr compositions of 0.7059±6, which are lower compared to those from Rajasthan (0.7068±4). The carbonates with positive δ¹³C values from Upper Vindhyan are characterized by lower ⁸⁷Sr/⁸⁶Sr values (0.7068±2) than those with negative δ¹³C values (0.7082±6). A comparison of C and Sr isotope data of carbonate rocks of the Vindhyan Supergroup with isotope evolution curves of the latter Proterozoic along with available geochronological data suggest that the Lower Vindhyan sediments were deposited during the Mesoproterozoic Eon and those from the Upper Vindhyan represent a Neoproterozoic interval of deposition.     

Petrology and oxygen isotope compositions of chondrules in E3 chondrites

Chondrules in E3 chondrites differ from those in other chondrite groups. Many contain near-pure endmember enstatite (Fs_<1). Some contain Si-bearing FeNi metal, Cr-bearing troilite, and, in some cases Mg, Mn- and Ca-sulfides. Olivine and more FeO-rich pyroxene grains are present but much less common than in ordinary or carbonaceous chondrite chondrules. In some cases, the FeO-rich grains contain dusty inclusions of metal. The oxygen three-isotope ratios (δ¹⁸O, δ¹⁷O) of olivine and pyroxene in chondrules from E3 chondrites, which are measured using a multi-collection SIMS, show a wide range of values. Most enstatite data plots on the terrestrial fractionation (TF) line near whole rock values and some plot near the ordinary chondrite region on the 3-isotope diagram. Pyroxene with higher FeO contents (∼2-10 wt.% FeO) generally plots on the TF line similar to enstatite, suggesting it formed locally in the EC (enstatite chondrite) region and that oxidation/reduction conditions varied within the E3 chondrite chondrule-forming region. Olivine shows a wide range of correlated δ¹⁸O and δ¹⁷O values and data from two olivine-bearing chondrules form a slope ∼1 mixing line, which is approximately parallel to but distinct from the CCAM (carbonaceous chondrite anhydrous mixing) line. We refer to this as the ECM (enstatite chondrite mixing) line but it also may coincide with a line defined by chondrules from Acfer 094 referred to as the PCM (Primitive Chondrite Mineral) line (Ushikubo et al., 2011). The range of O isotope compositions and mixing behavior in E3 chondrules is similar to that in O and C chondrite groups, indicating similar chondrule-forming processes, solid-gas mixing and possibly similar ¹⁶O-rich precursors solids. However, E3 chondrules formed in a distinct oxygen reservoir.Internal oxygen isotope heterogeneity was found among minerals from some of the chondrules in E3 chondrites suggesting incomplete melting of the chondrules, survival of minerals from previous generations of chondrules, and chondrule recycling. Olivine, possibly a relict grain, in one chondrule has an R chondrite-like oxygen isotope composition and may indicate limited mixing of materials from other reservoirs. Calcium-aluminum-rich inclusions (CAIs) in E3 chondrites have petrologic characteristics and oxygen isotope ratios similar to those in other chondrite groups. However, chondrules from E3 chondrites differ markedly from those in other chondrite groups. From this we conclude that chondrule formation was a local event but CAIs may have all formed in one distinct place and time and were later redistributed to the various chondrule-forming and parent body accretion regions. This also implies that transport mechanisms were less active at the time of and following chondrule formation.     

云南金顶铅锌矿碳、氧、硫同位素地球化学特征及其地质意义

本文对金顶铅锌多金属矿床矿石中硫化物硫同位素及碳酸盐胶结物中碳、氧同位素进行了研究分析,结果表明:成矿流体中的CO2与沉积有机物和海相碳酸盐岩有关,源自碳酸盐岩热解作用和去碳作用以及沉积有机物的氧化作用和脱羧基作用。矿床的矿石矿物组成反映了成矿环境为高氧逸度环境。硫同位素特征研究表明:矿床中石膏矿体中的硫可能来自三叠系古大洋硫酸盐矿物。而金属硫化物矿物中的硫具有多源性,其主要来源为生物或细菌还原地层中膏盐(石膏、硬石膏、有机质等)的还原硫、壳源硫和有机硫。同位素研究表明金顶铅锌多金属矿的成矿物质应源自地壳。     

Rare sulfur and triple oxygen isotope geochemistry of volcanogenic sulfate aerosols

We present analyses of stable isotopic ratios ¹⁷O/¹⁶O, ¹⁸O/¹⁶O, ³⁴S/³²S, and ³³S/³²S, ³⁶S/³²S in sulfate leached from volcanic ash of a series of well known, large and small volcanic eruptions. We consider eruptions of Mt. St. Helens (Washington, 1980, ∼1 km³), Mt. Spurr (Alaska, 1953, <1 km³), Gjalp (Iceland, 1996, 1998, <1 km³), Pinatubo (Phillipines, 1991, 10 km³), Bishop tuff (Long Valley, California, 0.76 Ma, 750 km³), Lower Bandelier tuff (Toledo Caldera, New Mexico, 1.61 Ma, 600 km³), and Lava Creek and Huckleberry Ridge tuffs (Yellowstone, Wyoming, 0.64 Ma, 1000 km³ and 2.04 Ma 2500 km³, respectively). This list covers much of the diversity of sizes and the character of silicic volcanic eruptions. Particular emphasis is paid to the Lava Creek tuff for which we present wide geographic sample coverage.This global dataset spans a significant range in δ³⁴S, δ¹⁸O, and Δ¹⁷O of sulfate (29‰, 30‰, and 3.3‰, respectively) with oxygen isotopes recording mass-independent (Δ¹⁷O > 0.2‰) and sulfur isotopes exhibiting mass-dependent behavior. Products of large eruptions account for most of‘ these isotopic ranges. Sulfate with Δ¹⁷O > 0.2‰ is present as 1-10 μm gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions. Sediments that embed ash layers do not contain sulfate or contain little sulfate with Δ¹⁷O near 0‰, suggesting that the observed sulfate in ash is of volcanic origin.Mass-dependent fractionation of sulfur isotopic ratios suggests that sulfate-forming reactions did not involve photolysis of SO₂, like that inferred for pre-2.3 Ga sulfates from Archean sediments or Antarctic ice-core sulfate associated with few dated eruptions. Even though the sulfate sulfur isotopic compositions reflect mass-dependent processes, the products of caldera-forming eruptions display a large δ³⁴S range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0.515 and 1.90 for ³³S/³²S vs. ³⁴S/³²S and ³⁶S/³²S vs. ³⁴S/³²S, respectively. The data presented here are consistent with modification of a chemical mass-dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic SO₂ with OH and/or a Rayleigh processes involving a residual Rayleigh reactant—volcanic SO₂ gas, rather than a Rayleigh product. These results may also imply at least two removal pathways for SO₂ in volcanic plumes.Above-zero Δ¹⁷O values and their positive correlation with δ¹⁸O in sulfate can be explained by oxidation by high-δ¹⁸O and high-Δ¹⁷O compounds such as ozone and radicals such as OH that result from ozone break down. Large caldera-forming eruptions have the highest Δ¹⁷O values, and the largest range of δ¹⁸O, which can be explained by stratospheric reaction with ozone-derived OH radicals. These results suggest that massive eruptions are capable of causing a temporary depletion of the ozone layer. Such depletion may be many times that of the measured 3-8% depletion following 1991 Pinatubo eruption, if the amount of sulfur dioxide released scales with the amount of ozone depletion.     

Carbon and oxygen stable isotope compositions of late Pleistocene mammal teeth from dolines of Ajoie (Northwestern Switzerland)

Fossils of megaherbivores from eight late Pleistocene ¹⁴C- and OSL-dated doline infillings of Ajoie (NW Switzerland) were discovered along the Transjurane highway in the Swiss Jura. Carbon and oxygen analyses of enamel were performed on forty-six teeth of large mammals (Equus germanicus, Mammuthus primigenius, Coelodonta antiquitatis, and Bison priscus), coming from one doline in Boncourt (~ 80 ka, marine oxygen isotope stage MIS5a) and seven in Courtedoux (51–27 ka, late MIS3), in order to reconstruct the paleoclimatic and paleoenvironmental conditions of the region. Similar enamel δ¹³C values for both periods, ranging from − 14.5 to − 9.2‰, indicate that the megaherbivores lived in a C₃ plant-dominated environment. Enamel δ¹⁸O_PO4 values range from 10.9 to 16.3‰ with a mean of 13.5 ± 1.0‰ (n = 46). Mean air temperatures (MATs) were inferred using species-specific δ¹⁸O_PO4–δ¹⁸O_H2O-calibrations for modern mammals and a present-day precipitation δ¹⁸O_H2O-MAT relation for Switzerland. Similar average MATs of 6.6 ± 3.6°C for the deposits dated to ~ 80 ka and 6.5 ± 3.3°C for those dated to the interval 51–27 ka were estimated. This suggests that these mammals in the Ajoie area lived in mild periods of the late Pleistocene with MATs only about 2.5°C lower than modern-day temperatures.     

Genetic relations among basic lavas and ultramafic nodules: Evidence from oxygen isotope compositions

¹⁸O values of unaltered basic lavas range from 4.9 to 8.3 but different types of basalts are usually restricted to narrow and distinct ranges of isotopic composition. The average ¹⁸O values for Hawaiian tholeiites, mid-ocean ridge tholeiites, and alkali basalts are 5.4, 5.7, and 6.2 permil, respectively. Potassic lavas and andesites tend to be more ¹⁸O rich with ¹⁸O values between 6.0 and 8.0 permil. The differences among the oxygen isotopic compositions of most of these lavas can be attributed to partial melting of isotopically distinct sources. The oxygen isotope compositions of the sources may be a function of prior melting events which produce ¹⁸O-depleted partial melts and ¹⁸O-enriched residues as a consequence of relatively large isotopic fractionations that exist at high temperatures. It is proposed that lavas with relatively low ¹⁸O values are derived from primitive, ¹⁸O-depleted sources whereas ¹⁸O-rich basalts are produced from refractory sources that have already produced partial melts. High temperature fractionations among silicate liquids and coexisting minerals can be used in conjunction with the oxygen isotope compositions of ultramafic nodules to place constraints on the genetic relations between some nodules and different types of basic lavas.     

Carbon and oxygen isotope fractionation in scleractinian corals: a review

The present theories on the fractionation of stable isotopes in scleractinian corals are critically discussed in the light of data available on primary productivity, respiration and stable isotope chemistry. These data support a model of fractionation in which the carbon and oxygen isotopes are decoupled. Calcification occurs from a reservoir of carbon dioxide derived from both organic and inorganic sources. Photosynthesis preferentially fixes¹³C and thereby leaves behind¹³C. Increases in the rate of photosynthesis therefore also enrich the carbon isotope ratio of the skeleton. From theoretical considerations, photosynthesis has little effect on the oxygen isotope ratio of the skeleton, a fact confirmed by available data. The process of respiration adds depleted carbon and oxygen to the calcification reservoirs. The varying correlations between carbon and oxygen isotopes seen in hermatypic corals are caused by changes in the relationship between photosynthesis and respiration at different geographical localities. The isotopic compositions in the skeletons of non-zooxanthellate corals, which show a consistent positive correlation, can also be explained by the above scenario.     

云南白秧坪银铜多金属矿集区成矿流体的稳定同位素地球化学研究

白秧坪银铜多金属矿集区位于滇西兰坪中-新生代沉积盆地中北部,由东矿带(上三叠统碳酸盐岩建造内的铅锌银铜矿床)和西矿带(下白垩统碎屑岩建造内的银铜钴铅锌矿床)两部分组成.本研究对该矿集区东、西矿带不同矿段、不同矿化类型矿石样品进行了硫-碳-氧同位素的研究.硫同位素研究表明,东矿带硫主要为地层硫,西矿带热液硫为沉积地层硫、有机硫及深源硫或地幔硫的混合.碳同位素显示,东矿带碳酸盐矿物δ13CPDB值为-3.0‰～+3.1‰,接近于海相碳酸盐,明显区别于其他各类地质体,暗示成矿流体的碳应来自碳酸盐岩;西矿带各矿段的δ13CpDB值变化范围小,除白秧坪少量样品外,其余均为负值(-5.1‰～-1.5‰),表明该区热液流体中碳的来源复杂,存在有机碳、地壳碳酸盐的碳及深源(地幔)碳.综合分析表明,西矿带成矿流体是一种混入深源流体的盆地热卤水,形成了下白垩统碎屑岩建造内的银铜钴铅锌矿床;东矿带成矿流体则是源于大气降水的盆地热卤水,形成了上三叠统碳酸盐岩建造内的铅锌银铜矿床.     

大别山东南部高压—超高压变质岩的氢氧同位素地球化学

本文报道了大别山东南部安徽石马地区柯石英榴辉岩组合和黄镇地区石英榴辉岩组合的全岩及单矿物的氢氧同位素特征。石马榴辉岩的δ18O值在1.0‰～7.4‰之间,云母δD值为-76‰～-61‰;黄镇榴辉岩的全岩δ18O值为-1.1‰～-2.1‰,云母δD值为-89‰～-77‰。矿物之间的氢氧同位素分馏既有处于平衡状态者,也有处于不平衡状态者,反映这些岩石除继承变质前母体的18O亏损特征外,还经历了显著的同位素退化交换作用。石马和黄镇榴辉岩的矿物对氧同位素温度均给出两个峰值380～430℃与绿片岩相变质条件一致,600～640℃与高角闪岩相变质条件一致;给不出峰变质温度(750～800℃)是由于岩石经历了退变质流体的强烈改造作用。大别东南部高压-超高压榴辉岩的南部边界为石英榴辉岩与花岗片麻岩的界线,但氧同位素研究指示两者为原位接触关系。