Boron isotopic constraints on the source of Hawaiian shield lavas

Boron isotopic compositions of lavas from three representative Hawaiian shield volcanoes (Kilauea, Mauna Loa, and Koolau) were analyzed by thermal ionization mass spectrometry. The boron isotopic composition of each sample was analyzed twice, once with and once without acid leaching to evaluate the effect of posteruptive boron contamination. Our acid-leaching procedure dissolved glass, olivine, secondary zeolite, and adsorbed boron; this dissolved boron was completely removed from the residue, which was comprised of plagioclase, pyroxenes, and newly formed amorphous silica. We confirmed that an appropriate acid-leaching process can eliminate adsorbed and incorporated boron contamination from all submarine samples without modifying the original ¹¹B/¹⁰B ratio. On the other hand, when the sample was weathered, i.e., the olivine had an iddingsite rim, ¹¹B/¹⁰B of the acid-resistant minerals are also modified, thus it is impossible to get the preeruptive ¹¹B/¹⁰B value from the weathered samples. Through this elimination and evaluation procedure of posteruptive contamination, preeruptive δ¹¹B values for the shield lavas are −4.5 to −5.4‰ for Koolau (N = 8), −3.6 to −4.6‰ for Kilauea (N = 11), and −3.0 to −3.8‰ for Mauna Loa (N = 6).Historical Kilauea lavas show a systematic temporal trend for B content and Nb/B coupled with other radiogenic isotopic ratios and trace element ratios, at constant δ¹¹B, indicating little or no assimilation of crustal materials in these lavas. Uncorrelated B content and δ¹¹B in Koolau and Mauna Loa lavas may also indicate little or no effect of crustal assimilation in these lavas. The source of KEA-component (identical to the so-called Kea end member in Hawaiian lavas) of the Hawaiian source mantle, represented by Kilauea, should be derived from lower part of subducted oceanic crust or refractory peridotite in the recycled subducted slab. The systematic trend from Kilauea to Koolau—decreasing δ¹¹B coupled with decreasing εNd as well as increasing ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb—is consistent with involvement of subducted sediment components in the EMK(enriched Makapuu)-component, represented by Makapuu-stage of Koolau lavas.     

Stable isotopic compositions of carbonates from the Mesoproterozoic Bangemall Group, northwestern Australia

Marine carbonate rocks from the Mesoproterozoic Bangemall Group of northwestern Australia show little deviation (+/-1.3%) in whole-rock delta 13C(carb)-values about a mean of -0.5%. This narrow range persists despite close sampling (every 10-20 m) through long sections (up to 2500 m) that are geographically widespread (up to 250 km apart), over many depositional environments (supralittoral to outer shelf), sediment sources (stromatolitic bioherms to detrital calcilutites) and rock types (pure limestones to dolomitic shales). The only major excursions from the norm seem related to unusual environmental or post-depositional processes, as they are correlated with large enrichments (to -3%) or depletions (to -16%) in 18O. Relatively heavy delta 13C-values, up to +2.5%, occur in a single bed of brecciated ferruginous dolostone at a single locality; these abnormal values may result from local evaporitic conditions. Limey and shaley nodular dolostones have delta 13C-values as low as -4.3%, probably caused by remineralization of organic matter during late and patchy dolomitization. Most notably, sharp negative excursions in delta 13C, up to -8.4%, occur in bleached kerogen-free rocks with mineral assemblages of dolomite + quartz + calcite +/- tremolite + talc, reflecting isotopic re-equilibration in thick metamorphic aureoles around dolerite intrusions. General environmental variations are minor, with delta 13C-values of peritidal facies tending to be slightly positive whereas those of subtidal facies are slightly negative. There are no strong secular trends, but subtle fluctuations within the range -2 to +l% can be correlated along the northwestern margin of the basin. This resembles the pattern seen in other Mesoproterozoic successions, but is markedly unlike the heavy background (> +5%) and extreme variations (up to l0%) in delta 13C evident in Neoproterozoic successions of similar thickness and environmental setting. Hence, in contrast to the Neoproterozoic, the global rate of organic carbon burial was probably fairly constant during deposition of the Bangemall Group, and perhaps generally during the Mesoproterozoic, as was the redox state of the atmosphere and hydrosphere.     

Stable isotopic compositions of Recent freshwater cyanobacterial carbonates from the British Isles: local and regional environmental controls

Recent (<50 years old) freshwater cyanobacterial carbonates from diverse environments (streams, lakes, waterfalls) throughout Britain and Ireland were analysed for their stable carbon and oxygen isotope compositions. The mean δ¹⁸O value of ?5–9‰ PDB for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central Britain (?7–5‰SMOW) assuming a mean water temperature of 9°C. The mean δ¹⁸O of lake data, ?4–5‰ PDB, is statistically different, reflecting the effects of residence time and/or variations in the oxygen isotopic composition of rainfall. Carbon isotopes have wide variations in both fluviatile and lake data sets (+ 3 to ?12‰ PDB). These variations are principally controlled in the fluviatile samples by contribution of isotopically light ‘soil zone’ carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution. Lake samples have the heaviest carbon isotope values, reflecting a trend toward isotopic equilibrium between atmospheric CO₂ and aqueous HCO^?₃. We infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information, although the effects of stabilization and diagenesis on primary δ¹⁸O values will need careful consideration. Primary carbon isotope compositions should be well preserved, although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate.     

Tungsten in Hawaiian picrites: A compositional model for the sources of Hawaiian lavas

Concentrations of tungsten (W) and uranium (U), which represent two of the most highly incompatible elements during mantle melting, have been measured in a suite of Hawaiian picrites and primitive tholeiites from nine main-stage shield volcanoes. Tungsten abundances in the parental melts are estimated from correlations between sample W abundances and MgO contents, and/or by olivine correction calculations. From these parental melt determinations, along with independent estimates for the degree of partial melting at each volcanic center, we extrapolate the W content of the mantle sources for each shield volcano. The mantle sources of Hualalai, Mauna Loa, Kohala, Kilauea, Mauna Kea, Koolau and Loihi contain 9 ± 2 (2σ), 11 ± 5, 10 ± 4, 12 ± 4, 10 ± 5, 8 ± 7 and 11 ± 5 ng/g, respectively. When combined, the mean Hawaiian source has an average of 10 ± 3 ng/g W, which is three-times as enriched as the Depleted MORB Mantle (DMM; 3.0 ± 2.3 ng/g).The relatively high abundances of W in the mantle sources that contribute to Hawaiian lavas may be explained as a consequence of the recycling of W-rich oceanic crust and sediment into a depleted mantle source, such as the depleted MORB mantle (DMM). However, this scenario requires varying proportions of recycled materials with different mean ages to account for the diversity of radiogenic isotope compositions observed between Kea- and Loa-trend volcanoes. Alternatively, the modeled W enrichments may also reflect a primary source component that is less depleted in incompatible trace elements than the DMM. Such a source would not necessarily require the addition of recycled materials, although the presence of some recycled crust is permitted within our model parameters and likely accounts for some of the isotopic variations between volcanic centers.The physical admixture of ?0.5 wt.% outer core material with the Hawaiian source region would not be resolvable via W source abundances or W/U ratios; however, W isotopes may provide a more sensitive to this mixing process. Recent W isotopic studies show no indication of core-mantle interaction, indicating that either such a process does not occur, or that mechanisms other than physical mixing may operate at the core-mantle boundary.     

四川盆地寒武系含盐盆地演化及其找钾意义:来自碳氧同位素的证据

四川盆地寒武系岩盐分布广泛,面积约20万km2,主要分布于川东南、滇东北、渝东北、黔西北和鄂西北等地。含盐层位主要为中下寒武统(寒武系第二统与第三统)。不同地区的含盐层位不同,根据含盐盆地的分布与含盐系地层,将四川盆地大致划分为两个膏盐聚集区(次盐盆地):(1)川东南次盐盆地;(2)渝东北次盐盆地。川东南次盐盆地含盐层位为下寒武统清虚洞组(寒武系第二统第四阶中上部);渝东北次盐盆地含盐层位为中寒武统覃家庙组(寒武系第三统)。前人对四川盆地在岩相古地理与找钾水化学等方面进行了较多研究,但缺少寒武系钻孔样品的碳酸盐岩碳氧同位素分析其含盐盆地的演化研究。所以本文首次选取四川盆地寒武系3个具有代表性的钻孔岩屑样品(丁山1井、临7井与建深1井),对碳酸盐岩碳氧同位素组成进行对比分析,认为四川盆地寒武系碳酸盐岩δ13 C负漂移多与海退及生物灭绝事件有关,δ13 C的正漂移则多受海侵事件影响。四川盆地寒武纪海进、海退交替较为频繁,有利于组成良好的生储盖组合而形成油气藏。四川盆地下寒武统龙王庙阶(第四阶中上部)在川东南地区环境相对封闭,蒸发作用强烈,形成以江津—泸州为沉积中心的膏盐岩沉积区。中寒武世建深1井碳酸盐岩样品的δ13 C相对临7井较高,盐度也相对较高。结合四川盆地岩相古地理研究,认为从早寒武世至中寒武世古海水的浓缩方向有由川东南次盆地向渝东北次盆地演化的趋势,该认识对该区寒武系成盐找钾提供重要的依据,对成油气条件研究也有一定的参考价值。     

Neodymium isotope systematics of the mantle beneath the Baltic shield: Evidence for depleted mantle evolution since the Archaean

More than 300 published and unpublished Nd isotopic analyses of mantle derived rocks from the Baltic shield have been compiled. The rocks range in age from Archaean to Phanerozoic. Within any given age-interval, the mantle derived rocks range in ε_Nd(t) from depleted mantle values at or above the growth curves of the global depleted mantle reservoirs of DePaolo (1981) and DePaolo et al. (1991) to negative values. Initial neodymium isotopic compositions below the De Paolo curve are best explained by interaction between depleted mantle derived magmas and local crustal contaminants. The data now available lend no support to the existence of isolated, less depleted or undepleted mantle domains beneath the Baltic Shield, as was suggested by Mearns et al. (1986) and Valbracht (1991a, b).     

Stable isotopic compositions of waters in the karst environments of China: Climatic implications

A total of 117 water samples, including cave water, ground water, spring water and river water, collected from the monsoonal area of China have been analyzed for their H- and O-isotope composition. Overall, a δ¹⁸O–δD correlation is observed of δD = −4.45 + 6.6δ¹⁸O (R² = 0.90) and a significant evaporation effect observed for the southern sites. Average δ¹⁸O and δD site values generally correspond to those of precipitation in nearby cities, with correlations of δD = 2.18 + 7.23δ¹⁸O (R² = 0.95) for the sample sites and δD = 11.05 + 7.95δ¹⁸O (R² = 0.95) for the cities. The effects of rainfall amount and temperature on precipitation δ¹⁸O were calculated using a simplified theoretical model derived from the Rayleigh distillation equation, which demonstrated that the sign of δ¹⁸O_pvs. T correlation is dependent on precipitation intensity. The mean δ¹⁸O value of cave waters exhibit decreasing trends with increasing latitude and reveal a spatial pattern of positive correlation with annual mean temperature and precipitation, mainly reflecting isotopic fractionations in the moisture source traveling from the ocean side to the inland continent. This spatial pattern implies that the δ¹⁸O values recorded in the proxy climate records derived from speleothems might be influenced by shifts in monsoon boundary during the past, especially between glacial and interglacial intervals.     

Observations of Li isotopic variations in the Trinity Ophiolite: Evidence for isotopic fractionation by diffusion during mantle melting

The Trinity peridotite (northern CA) contains numerous lithologic sequences consisting of dunite to harzburgite to spinel lherzolite to plagioclase lherzolite. Previous workers have documented geochemical gradients in these sequences consistent with melt-rock reaction processes occurring around dunites, interpreted to reflect conduits for melt ascent. We have undertaken a study of Li isotope compositions of clinopyroxene and some olivine within these sequences using ion probe techniques to test the hypothesis that the geochemical gradients are related to diffusive fluxing of alkali elements into or away from the melt conduit.Results show large variations in ⁷Li/⁶Li occurring in a consistent pattern across three transects from dunite to plagioclase lherzolite within the Trinity peridotite. Specifically, measurements of average δ⁷Li for single thin sections along the traverse reveal a low in δ⁷Li in the harzburgite adjacent to the dunite returning to higher values farther from the dunite with a typical offset of ∼10 per mil in the low δ⁷Li trough. This pattern is consistent with a process whereby Li isotopes are fractionated during diffusion through a melt either from the dunite conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit. The patterns in ⁷Li/⁶Li occur over a length scale similar to the decrease in REE concentration in these same samples. Explaining both the trace element and Li isotopic gradients requires a combined process of alkali diffusion and melt extraction.We develop a numerical model and examine several scenarios of the combined diffusion-extraction process. Using experimentally constrained values for the change in Li diffusion coefficient with isotope mass, large changes in δ⁷Li as a function of distance can be created in year to decade timescales. The addition of the melt extraction term allows larger Li concentration gradients to be developed and thus produces larger isotopic fractionations than diffusion only models. The extraction aspect of the model can also account for the observed decrease in rare earth element concentrations across the transects.     

Carbon and oxygen isotopic composition of carbonates in lavas and ejectites from the Alban Hills,Italy

Carbon and oxygen isotopic analyses have been carried out on carbonates from lavas, ejectites and sedimentary formations in the region of the Alban Hills.The calcite occurring in the lavas, both in veins and cavities and dispersed in the groundmass shows within each flow a fairly uniform isotopic composition not different from that normally observed in sedimentary carbonates, except in the case of one particular flow, where unusually low ¹³C values were recorded. The latter are discussed in terms of a possible contribution of organic carbon or of isotopically light carbonates, the presence of which in the Alban Hills area had been previously recorded.The ejectites examined comprise both limestone and dolostone blocks of various degree of metamorphism and materials of uncertain origin, some of which containing carbon and oxygen of isotopic composition wholly different from that of all carbonates analysed in this work, approaching the range observed in some carbonatites. The isotopic data and the geochemical features of the latter materials are discussed in terms of thermal metamorphism of limestones and of a possible syntexis of evaporite materials.The ¹⁸O and ¹³C values of certain marine limestones from major Mesozoic sedimentary formations in the region are also reported.     

Lithium isotopic composition of Central American Volcanic Arc lavas: implications for modification of subarc mantle by slab-derived fluids

Li contents and isotopic compositions were determined for a suite of well-characterized basaltic lavas from the Central American Volcanic Arc (CAVA). Variable Li/Y (0.2–0.5), Li/Sc (0.1–0.4), and δ⁶Li values (+2.6 to −7.7‰) attest to significant compositional heterogeneity in the subarc mantle. Within specific arc segments, these parameters correlate strongly with each other and with a number of other constituents (e.g., K, Rb, Ba, B/La, ¹⁰Be/⁹Be, ⁸⁷Sr/⁸⁶Sr, U/Ce, and ²³⁰Th/²³²Th, among others); these correlations are particularly strong for Nicaragua samples. Coupling of this particular set of constituents is best explained in terms of addition of ‘subduction components' to the subarc mantle. Moreover, their selective enrichment with respect to relatively fluid-immobile incompatible elements signifies the dominance of fluid vs. silicate melt transport of slab components to the subarc mantle. Several interesting nuances are revealed by the Li data. First, although Li and B are strongly correlated in both Costa Rica and Nicaragua, there are systematic along-strike variations in Li/B that are consistent with these elements having different ‘fluid release patterns' from subducted slab segments. For example, Li/B is highest in Costa Rica where auxiliary evidence indicates higher subduction zone temperatures; apparently B is preferentially depleted and Li retained in the slab under warmer conditions. The same relations are reflected in Li/¹⁰Be and other subduction tracer systematics, all of which point to larger subduction contributions below Nicaragua. Yet, even Nicaragua lavas vary widely in levels of subduction enrichment. High-Ti basalts from Nejapa are the least enriched and have the highest δ⁶Li (1.4 to 2.6‰); these values are greater than in fresh MORB (ca. −4‰) and are not easily explained by additions of subducted Li because most oceanic crustal rocks and marine sediments have lower δ⁶Li than MORB (with typical values between −8 and −20‰). Thus, it appears the Nejapa data may be representative of isotopically light mantle domains. Relatively light δ⁶Li values in an undepleted spinel lherzolite (+11.3‰) from Zabargad Is. (Red Sea) and in primitive backarc basalts (−1.6 to −0.5‰) from Lau Basin support this conclusion. Considering representative fluid and mantle endmember compositions, the CAVA data are consistent with limited (up to a few percent) additions of slab-derived fluids to a heterogeneous mantle containing variably depleted and enriched domains to form the respective magma sources. In our view, the subarc mantle is heterogeneous on a small scale, but some arc sectors clearly received greater slab inputs than others.     

Lead isotopic compositions in fault gouges and their parent rocks: implications for ancient fault activities

This is the first study on Pb stable isotopes in fault gouges and their parent rocks. We analyzed the composition of Pb isotopes and contents of U and Pb in 10 pairs of fault gouges and their parent rocks collected along several active faults in central Japan. Thorium-232-²⁰⁸Pb ages of two fault systems were determined as pre-Tertiary, which are consistent with the data from KAr ages and geological considerations.Naturally, the²³⁵U²⁰⁷Pb system is of little use for dating because the magnitude of difference in²⁰⁷Pb/²⁰⁴Pb between gouges and parent rocks is too small. It is found that the²⁰⁶Pb/²⁰⁴Pb can indicate the contribution of²⁰⁶Pb resulting from excess supplies of²²⁶Ra and²²²Rn along the fault. The excess²⁰⁶Pb accumulation rate corresponds to the average²²²Rn concentration in soil gas or groundwater through geological time since the gouge formation. A comparison of Quaternary fault activity and estimated Tertiary activity reveals the characteristics of each fault system.     

Lithium isotopic composition of Central American volcanic arc lavas: implications for modification of subarc mantle by slab-derived fluids: correction

Our previously reported lithium isotope data for the least enriched members of the Central American Volcanic Arc were in error due to problems in ion extraction from high MgO rocks. This paper presents reanalyses of the entire suite of the CAVA lavas after a systematic investigation of the elution behavior of Li as a function of the rock composition. The most significant correction pertains to the mantle end members of the Nicaragua and Costa Rica series, which now display MORB-like δ⁶Li values (−4.5‰) and not much lighter (+1‰ to +3‰) as previously reported. Also revised is the composition of a peridotite from Zabargad Island (Red Sea), considered to represent undepleted upper mantle, whose corrected δ⁶Li also resembles MORB. These new data therefore remove the speculation that Earth's primitive mantle has extremely light Li isotopic composition and that the mantle beneath CAVA contains such isotopically light domains. The δ⁶Li range for the arc segment from Costa Rica to Guatemala has now been reset to −4.5‰ to −6.4‰. Despite the narrower range, Li isotopic values remain well correlated with other subduction-related properties, including LILE and fluid-mobile elements. Model calculations show that the isotopic compositions of the lavas are consistent with small additions of slab-derived fluids to the enriched and depleted domains of the subarc mantle.     

西伯利亚雅库特金伯利岩中四颗古老锆石巨晶的微量元素和Hf同位素特征——对区分金伯利岩中锆石巨晶的启示

金伯利岩中的锆石按照颗粒大小可以分为细粒锆石（一般小于200μm）和巨晶锆石（一般大于500μm）。前人的研究结果显示在金伯利岩中粒径较大的巨晶锆石的U-Pb体系在高温的地幔中一直保持着开放状态,直到寄主金伯利岩浆的喷发才使地幔锆石的U-Pb体系封闭,因此这些巨晶锆石是确定金伯利岩年龄的重要矿物之一。然而,近年来的研究表明,金伯利岩中还存在一些时代远老于金伯利岩年龄的锆石,也具有较大的粒径（以下称古老锆石巨晶）。它们的存在无疑影响了利用锆石U-Pb方法确定金伯利岩年龄的准确性。本文以西伯利亚雅库特（Yakutia）金伯利岩省中的四颗古老锆石巨晶为研究对象,通过形态学、年代学、微量元素和Hf同位素组成,讨论古老锆石巨晶的来源。同时,我们统计和对比了全球多个金伯利岩中能够确定金伯利岩年龄的锆石和古老锆石巨晶的形态学、U、Th含量和微量元素组成、Hf-O同位素等特征。研究结果显示,金伯利岩中的古老锆石巨晶的t_DM年龄和O同位素组成与可以用来确定金伯利岩年龄的锆石巨晶具有明显的差别。这些手段在未来的研究中可以用来区分可确定金伯利岩年龄的锆石巨晶和古老的锆石巨晶。

     

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of ¹⁶O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most ¹⁶O-rich (Δ¹⁷O = −25‰ to −15‰), followed by Al,Ti-dioside (Δ¹⁷O = −20‰ to −5‰) and anorthite (Δ¹⁷O = −15‰ to 0‰). Melilite is the most ¹⁶O-depleted primary mineral (Δ¹⁷O = −5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a ¹⁶O-rich (Δ¹⁷O  −20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a ¹⁶O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating ¹⁶O-poor chondrule fragments. The observation that the pyroxene is ¹⁶O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a ¹⁶O-rich solid in a ¹⁶O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is ¹⁶O-enriched (Δ¹⁷O = −10‰ to −6‰) relative to melilite (Δ¹⁷O = −5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism.     

Geochemistry of Quaternary lavas from NE Sulawesi: transfer of subduction components into the mantle wedge

Major and trace element, and Sr-Nd isotope compositions were determined for Quaternary volcanic rocks from NE Sulawesi (the Sangihe are), Indonesia, in order to examine the origin of across-arc variation in lava and magma source chemistry. The arc is formed in an intraoceanic tectonic setting and is not associated with a backarc basin, thereby minimizing possible contributions from non-arc geochemical reservoirs. The geochemistry of these arc lavas is likely to provide essential information about the chemical characteristics of subduction components. All incompatible elements, except Pb, increase away from the volcancic front. Major element data for Mg-rich lavas together with available experimental data, suggest that primary magmas are produced at higher pressured by smaller degrees of partial melting beneath the backarc-side volcanoes. Rb/K and Ba/Pb are higher, and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd are lower in backarc-side lavas. These variations may be attributed to generation of hydrous fluids in the downdragged hydrous peridotite layer at the base of the mantle wedge through the following reactions: decompositions of pargasitic amphibole to form phlogopite and breakdown of phlogopite to crystallize K-richterite, beneath the volcanic front and the backarc-side volcanoes, respectively.     

The PT-phase relations of an MgO-rich Hawaiian tholeiite: the compositions of primary Hawaiian tholeiites

The PT-phase relations of a Hawaiian tholeiite with 18.2% MgO has olivine–orthopyroxene multiple saturation at 20.5 kbar and 1,550°C. This pressure is less than the pressure at the lithosphere/asthenosphere transition, and it is suggested that tholeiites with this and lesser MgO contents are fractionated. Assuming a harzburgitic residuum it is shown that Hawaiian primary tholeiites contain about 23% MgO, and are generated at 36±5 kbar and 1,680±50°C. This pressure is equivalent to a depth of 112 km, which is consistent with the thickness of the lithosphere and thermal plume modeling. The minimal MgO content of primary Hawaiian tholeiites is suggested as 19% MgO.     

Low δO in the Icelandic mantle and its origins: Evidence from Reykjanes Ridge and Icelandic lavas

Oxygen isotope ratios have been determined using laser fluorination techniques on olivine and plagioclase phenocrysts and bulk glasses from the Reykjanes Ridge and Iceland. δ¹⁸O in Reykjanes Ridge olivines shows hyperbolic correlations with Sr-Nd-Pb isotope ratios that terminate at δ¹⁸O = +4.5‰ at compositions almost identical to those of moderately enriched lavas on the Reykjanes Peninsula, Iceland. Samples with low δ¹⁸O show no indication of contamination by oceanic crust such as elevated Cl/K, and are too deep to have been influenced by meteoric water hydrothermal systems. They cannot represent Icelandic melts contaminated in the crust and transferred laterally along the ridge since fissure systems are strongly oblique to the ridge axis. It follows that Icelandic mantle advected along the ridge has low δ¹⁸O. The hyperbolic ¹⁴³Nd/¹⁴⁴Nd-δ¹⁸O correlation appears to be more strongly curved than magma mixing trajectories and suggests that melt fractions are ∼4.5× greater and source Nd contents ∼9× greater in the mantle at 63°N compared with that at 60°N. Primitive lavas from the Reykjanes Peninsula show linear correlations between olivine δ¹⁸O and ¹⁴³Nd/¹⁴⁴Nd or ²⁰⁶Pb/²⁰⁴Pb, extending to δ¹⁸O of +4.3‰ at ¹⁴³Nd/¹⁴⁴Nd close to the lowest ratios observed in Icelandic magmas. These correlations cannot be produced by melt mixing or crustal contamination because these would yield strongly hyperbolic trajectories. Lower δ¹⁸O seen in more evolved samples from the Eastern Rift Zone may reflect crustal contamination, though there is some evidence of a mantle source with lower δ¹⁸O in eastern Iceland. It is very difficult to explain the low δ¹⁸O of enriched Icelandic mantle sources on current understanding of mantle and crustal oxygen isotopes. There is no obvious reason why such low-δ¹⁸O sources should not contribute to other ocean islands. No oceanic crustal lithologies exist that could produce the low-δ¹⁸O enriched sources by recycling into the mantle, and there is no evidence for changes in δ¹⁸O of ophiolite suites with time, nor of changes during high-P metamorphism. Low δ¹⁸O appears to be associated with high ³He/⁴He, and we speculate that this signature may be characteristic of the host mantle into which ocean crust was recycled.     

Sm–Nd isotopic compositions as a proxy for magmatic processes during the Neoproterozoic of the southern Brazilian shield

Combined analyses of Nd isotopes from a wide range of Neoarchaean–Cretaceous igneous rocks provides a proxy to study magmatic processes and the evolution of the lithosphere. The main igneous associations include the Neoproterozoic granitoids from the southern Brazilian shield, which were formed during two tectonothermal events of the Brasiliano cycle: the São Gabriel accretionary orogeny (900–700 Ma) and the Dom Feliciano collisional orogeny (660–550 Ma). Rocks related to the formation of the São Gabriel arc (900–700 Ma) mainly have a depleted juvenile signature. For the Neoproterozoic collisional event, the petrogenetic discussion focuses on two old crustal segments and three types of mantle components. However, no depleted juvenile material was involved in the formation of the Dom Feliciano collisional belt (800–550 Ma), which implies an ensialic environment for the Dom Feliciano orogeny. In the western Neoproterozoic foreland, records of a Neoarchaean lower crust predominate, whereas a Paleoproterozoic crust does in the eastern Dom Feliciano belt. The western foreland includes two amalgamated geotectonic domains, the São Gabriel arc and Taquarembó block. In the collisional belt, the old crust was intensely reworked during the São Gabriel event. In addition to the Neoproterozoic subduction-processed subcontinental lithosphere (São Gariel arc), we recognize two old enriched mantle components, which also are identified in the Paleoproterozoic intraplate tholeiites from Uruguay and the Cretaceous potassic suites from eastern Paraguay. One end member displays the prominent influence of Trans-Amazonian (2.3–2.0 Ga) or older subduction events, whereas the other can be interpreted as a reenrichment of the first during the latest Trans-Amazonian collisional or younger events. This reenriched mantle is documented in late Neoproterozoic suites from the western foreland (605–550 Ma) and younger suites from the eastern collisional belt (600–580 Ma). The other enriched mantle component with an old subduction signature, however, appears only in older rocks of the collisional belt (800–600 Ma). The participation of the subduction-related Brasiliano mantle as an end member of binary mixing occurred in some early Neoproterozoic suites (605–580 Ma) from the western foreland, but the contribution of the Neoarchaean lower crust increased near the late igneous event (575–550 Ma).     

Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions

Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa_<2), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (Åk_<15). ∼10% of AOAs contain low-Ca pyroxene (Fs_1-3Wo_1-5) that is in corrosion relationship with forsterite and is found in three major textural occurrences: (i) thin (<15 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) 5-10-μm-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and (iii) shells of variable thickness (up to 70 μm), commonly with abundant tiny (3-5 μm) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%-20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally ¹⁶O-rich isotopic compositions (Δ¹⁷O < −20‰). Low-Ca pyroxenes of the textural occurrences (i) and (ii) are ¹⁶O-enriched (Δ¹⁷O < −20‰), whereas those of (iii) are ¹⁶O-depleted (Δ¹⁷O = −6‰ to −4‰). One of the extensively melted (>50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is ¹⁶O-rich (Δ¹⁷O ∼ −22‰), whereas the neighboring plagioclase mesostasis is ¹⁶O-depleted (Δ¹⁷O ∼ −11‰).We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, spinel, and ±melilite) formed in an ¹⁶O-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted ¹⁶O-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into chondrules. The original ¹⁶O-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains.