Melt generation and trace element fractionation of intermediate arc magma from Andaman subduction zone

The submarine volcanoes, located in the southern part of Andaman Sea, north eastern Indian Ocean, result from the subduction of the Indo-Australian Plate beneath the Southeast Asian Plate and represent one of the less studied submarine volcanism among the global arc systems. The present study provides new petrological and geochemical data for the recovered rocks from the submarine volcanoes and documents the petrogenetic evolution of Andaman arc system. Geochemical attributes classify the studied samples as basaltic andesite, andesite, dacite to rhyodacite reflecting sub-alkaline, intermediate to acidic composition of the magma. Petrographic studies of the basaltic andesites and andesites show plagioclase [An₃₈-An₅₇ in basaltic andesites; An₂₇-An₂₈ in andesites] and clinopyroxene as dominant phenocrystal phase in a cryptocrystalline groundmass. Plagioclase (An₂₅-An₄₅) marks the principal phenocrystal phase in dacite with sub-ordinate proportion of biotite and amphibole of both primary and secondary origin along with minor amount of K-feldspar. The submarine volcanic rocks from Andaman arc system exhibit pronounced LILE, LREE enrichments and HFSE (negative Nb, Ta and Ti anomalies), MREE and HREE depletion thereby endorsing the influence of subduction zone processes in their genesis. Elevated abundances of Th with relatively higher LREE/HFSE than LILE/HFSE, LILE/LREE suggest significant contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids towards mantle wedge metasomatism thereby modifying the sub-arc mantle. Partial melting curves calculated using the non-modal batch melting equation suggest primary magma generated due to ~31–35 % degree of partial melting of spinel lherzolite mantle beneath the arc system. Fractional crystallization model suggests fractionation of 45 % plagioclase, 40 % clinopyroxene, 5–10 % amphibole and 5–10 % biotite which is consistent with the petrographic observations. Further, the assimilation-fractional-crystallization (AFC) model for the studied rocks indicates nominal crustal contamination. Therefore, this study infers that the melt evolution history for the Andaman arc volcanic rocks can be translated in terms of (i) generation of precursor magma by ~31–35 % partial melting of a spinel lherzolite mantle wedge, metasomatized predominantly by subducted slab sediments and (ii) the parent magma generation was ensued by fractionation dominated melt differentiation with nominal input from arc crust.     

The water and trace element contents of melt inclusions across an active subduction zone

Water concentrations of olivine-hosted melt inclusions show no consistent variation across the northern part of the Central American subduction zone in southeastern Guatemala. Magmatic water contents remain moderately high (~2 wt%) throughout the back-arc region. Melt inclusions from some of these back-arc basalts also have notably high CO₂ contents (>900 ppm CO₂). The B and B/Ce ratios of melt inclusions systematically decline across the arc, the first parameters to exhibit systematic changes across southeastern Guatemala. It appears, therefore, that dehydration-driven, flux-melting persists across the arc, although decompression melting is of approximately equal importance in the back-arc region. Dehydration of the slab/wedge region is regarded as semi-continuous down-dip, to depths of at least 175–200 km. Moderate water contents are maintained by stepwise dehydration reactions, while truly incompatible fluid mobile elements are progressively stripped from the Cocos plate. The notably high CO₂ contents of some back-arc basalts may indicate increasing devolatilization of subducted carbonate sediments with slab depth. The moderate H₂O contents of back-arc basaltic magmas has likely contributed to their early fractionation of clinopyroxene around the Moho.Editorial responsibility: T.L. Grove     

湘南中生代钨锡大规模成矿控制因素：锆石年龄和微量元素的启示

以湘南地区震旦系、泥盆系、石炭系和白垩系碎屑岩中碎屑锆石及印支期—燕山期花岗岩中岩浆锆石为对象,通过锆石U-Pb年龄和微量元素地球化学研究,探讨湘南地区中生代钨锡大规模成矿的控制因素。锆石年龄结果表明,碎屑锆石年龄缺乏2.0 Ga左右的谱峰,显示湘南地区震旦纪和晚古生代沉积盆地物源具有华夏陆块的亲缘性,不具扬子陆块亲缘性,可以代表研究区华夏地块岩浆活动的记录;碎屑锆石年龄峰值记录了华夏陆块2.5 Ga、1.3~1.1 Ga、950 Ma、800 Ma、650 Ma和430 Ma六期前中生代构造-岩浆事件,对应早前寒武纪陆壳形成、Grenville碰撞造山运动及Rodinia超大陆的聚合-裂解过程。锆石微量元素分析结果表明,同一构造-岩浆事件锆石的δEu、LREE/HREE、Y/Ho、Nb/Ta值变化幅度均较大,LREE/HREE和Nb/Ta值不随时代变新而变小(反而有变大的趋势),表明岩浆分异程度与时代无关。但是,锆石∑REE、Y、Th、U、Nb和Ta含量,以及LREE/HREE和Nb/Ta值随时代变新而增高,表明岩浆源区的成分成熟度逐渐增高。湘南地区钨锡矿化规模随着时代变新而增大,在中生代达到顶峰,这一趋势与地壳成熟度的不断增高密切相关。锆石微量元素特征可作为示踪钨锡成矿规模的重要指标。     

Major, trace element and isotope geochemistry (Sr-Nd-Pb) of interplinian magmas from Mt. Somma-Vesuvius (Southern Italy)

Summary Major, trace element and isotopic (Sr, Nd, Pb) data are reported for representative samples of interplinian (Protohistoric, Ancient Historic and Medieval Formations) activity of Mt. Somma-Vesuvius volcano during the last 3500 years. Tephra and lavas exhibit significant major, trace element and isotopic variations. Integration of these data with those obtained by previous studies on the older Somma suites and on the latest activity, allows to better trace a complete petrological and geochemical evolution of the Mt. Somma-Vesuvius magmatism. Three main groups of rocks are recognized. A first group is older than 12.000 yrs, and includes effusive-explosive activity of Mt. Somma. The second group (8000–2700 yrs B.P.) includes the products emitted by the Ottaviano (8000 yrs. B.P.) and Avellino (3550 yrs B.P.) plinian eruptions and the interplinian activity associated with the Protohistoric Formation. Ancient Historic Formation (79–472 A.D.), Medieval Formation (472–1139 A.D.) and Recent interplinian activity (1631–1944 A.D.) belong to the third group of activity (79–1944 A.D.). The three groups of rocks display distinct positive trends of alkalis vs. silica, which become increasingly steeper with age. In the first group there is an increase in silica and alkalis with time, whereas an opposite tendency is observed in the two younger groups. Systematic variations are also evident among the incompatible (Pb, Zr, Hf, Ta, Th, U, Nb, Rb, Cs, Ba) and compatible elements (Sr, Co, Cr). REE document variable degrees of fractionation, with recent activity displaying higher La/Yb ratios than Medieval and Ancient Historic products with the same degree of evolution. N-MORB normalized multi-element diagrams for interplinian rocks show enrichment in Rb, Th, Nb, Zr and Sm (> ^*10 N-MORB). Sr isotope ratios are variable, with Protohistoric rocks displaying ⁸⁷Sr/⁸⁶Sr =  0.70711–0.70810, Ancient Historic ⁸⁷Sr/⁸⁶Sr = 0.70665–0.70729, and Medieval ⁸⁷Sr/⁸⁶Sr = 0.70685–0.70803. Neodymium isotopic compositions in the interplinian rocks show a tendency to become slightly more radiogenic with age, from the Protohistoric (¹⁴³Nd/¹⁴⁴Nd = 0.51240–0.51247) to Ancient Historic (¹⁴³Nd/¹⁴⁴Nd = 0.51245–0.51251). Medieval interplinian activity (¹⁴³Nd/¹⁴⁴Nd: 0.51250–0.51241) lacks meaningful internal trends. All the interplinian rocks have virtually homogeneous compositions of ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb in acid-leached residues (²⁰⁷Pb/²⁰⁴Pb ∼15.633 to 15.687, ²⁰⁸Pb/²⁰⁴Pb ∼38.947 to 39.181). Values of ²⁰⁶Pb/²⁰⁴Pb are very distinctive, however, and discriminate among the three interplinian cycles of activity (Protohistoric: 18.929–18.971, Ancient Historic: 19.018–19.088, Medieval: 18.964–19.053). Compositional trends of major, trace element and isotopic compositions clearly demonstrate strong temporal variations of the magma types feeding the Somma-Vesuvius activity. These different trends are unlikely to be related only to low pressure evolutionary processes, and reveal variations of parental melt composition. Geochemical data suggest a three component mixing scheme for the interplinian activity. These involve HIMU-type and DMM-type mantle and Calabrian-type lower crust. Interaction between these components has taken place in the source; however, additional quantitative constraints must be acquired in order to better discriminate between magma characteristics inherited from the sources and those acquired during shallow level evolution. Received May 5, 2000; revised version accepted June 19, 2001     

Insights into the origin of primitive silica-undersaturated arc magmas of Aoba volcano (Vanuatu arc)

Aoba picrites in Vanuatu arc (Southwestern Pacific) offer the opportunity to address the question of the origin of Si-undersaturated arc magmas, through the geochemical study of their olivine-hosted melt inclusions. These latter delineate a differentiation trend of calc-alkaline silica-undersaturated basalts, with typical trace-element patterns of arc magmas. The most primitive melt inclusions, preserved in olivines with Fo ≥ 89, have normative nepheline compositions with CaO/Al₂O₃ > 0.8, but belong to three distinct populations differing in their enrichment or depletion in LILE, Cl, and alkalis (Rb, K). The dominant population is characterized by medium-LILE concentrations (La/Yb ~ 7–8) and represents the parental magma of the Aoba lavas. The two others (La/Yb ~ 20 and 2) are either significantly enriched or extremely depleted in LILE, Cl, and alkalis. This compositional variability of primitive magma batches requires the multi-stage mixing between melts generated by partial melting of both peridotite and clinopyroxene-rich lithologies. Medium-LILE magma derives from the mixing between peridotite- and clinopyroxenite-derived melts, whereas the high- and low-LILE melts involve amphibole-bearing and amphibole-free clinopyroxenite sources, respectively.     

西藏西部与班公湖特提斯洋盆俯冲相关的火成岩年代学和地球化学

对青藏高原西北部班公湖缝合带开展了野外地质调查,初步查明区内缝合带至少包含日土和狮泉河-改则两条蛇绿岩带。在两条蛇绿岩带北侧发现各有两期岛弧型岩浆岩发育,且形成时间严格对应。岩石地球化学分析表明,班公湖缝合带岛弧型岩浆岩的共同特征是富集大离子不相容元素Rb、Th、K和Pb;强烈亏损高场强元素Nb、Ta和Ti;Ba在微量元素蛛网图中总是相对亏损,这些特征说明班公湖地区存在两条俯冲带。从演化序列看,俯冲初期岩石属中钾钙碱性系列,之后岛弧岩浆作用向高钾钙碱性系列演变。锆石U—PbLA—ICPMS定年结果表明,北面的日土俯冲带洋壳俯冲从辉长岩墙开始,时代为（165．5±1．9）Ma（MSWD=1．16）,在159Ma时岛弧岩浆作用规模增大,形成小型的花岗岩基;南面的狮泉河-改则俯冲带一开始俯冲（（166．4±2．0）Ma,MSWD=3．0）就有较大规模的石英闪长岩体侵入,之后岩浆作用减弱,到159．4Ma时只有一些小体积的花岗斑岩和闪长玢岩侵入。根据岛弧岩浆作用规模,认为班公湖中特提斯洋盆的俯冲一开始是以狮泉河俯冲带为主,之后狮泉河俯冲带的俯冲作用逐渐减弱。到晚侏罗世初（159Ma）北面的日土俯冲带成为洋壳俯冲的主体。鉴于两条岛弧火成岩带在空间配置上都位于由基性-超基性岩构成的蛇绿岩带北侧,地球化学上显示陆缘弧特征,因此,认为班公湖中特提斯洋盆应该是在中侏罗世晚期（约166Ma）沿日土和狮泉河两条俯冲带同时向北俯冲,构造属性上可能不是一个统一的大洋,而是包含了多个局限性洋盆。     

Young basalts of the central Washington Cascades, flux melting of the mantle, and trace element signatures of primary arc magmas

Basaltic lavas from the Three Sisters and Dalles Lakes were erupted from two isolated vents in the central Washington Cascades at 370–400 ka and 2.2 Ma, respectively, and have distinct trace element compositions that exemplify an important and poorly understood feature of arc basalts. The Three Sisters lavas are calc-alkaline basalts (CAB) with trace element compositions typical of most arc magmas: high ratios of large-ion-lithophile to high-field-strength elements (LILE/HFSE), and strong negative Nb and Ta anomalies. In contrast, the Dalles Lakes lavas have relatively low LILE/HFSE and no Nb or Ta anomalies, similar to ocean-island basalts (OIB). Nearly all Washington Cascade basalts with high to moderate incompatible element concentrations show this CAB or OIB-like compositional distinction, and there is pronounced divergence between the two magma types with a large compositional gap between them. We show that this trace element distinction can be easily explained by a simple model of flux-melting of the mantle wedge by a fluid-rich subduction component (SC), in which the degree of melting (F) of the peridotite source is correlated with the amount of SC added to it. Distinctive CAB and OIB-like trace element compositions are best explained by a flux-melting model in which dF/dSC decreases with increasing F, consistent with isenthalpic (heat-balanced) melting. In the context of this model, CAB trace element signatures simply reflect large degrees of melting of strongly SC-fluxed peridotite along relatively low dF/dSC melting trends, consistent with derivation from relatively cold mantle. Under other conditions (i.e., small degrees of melting or large degrees of melting of weakly SC-fluxed peridotite [high dF/dSC]), either OIB- or MORB (mid-ocean ridge basalt)-like compositions are produced. Trace element and isotopic compositions of Washington Cascade basalts are easily modeled by a correlation between SC and F across a range of mantle temperatures. This implies that the dominant cause of arc magmatism in this region is flux melting of the mantle wedge. Received: 2 March 1999 / Accepted: 18 August 1999     

锆石微量元素地球化学对硅质火山岩浆系统的制约

大型硅质火山作用(喷发体积约102～104 km3)的岩浆系统是地壳尺度的,经历了复杂的起源、运移、存储、补给和喷发等过程.揭示岩浆从起源到喷发过程中的结晶分异、堆晶、晶体-熔体分离、地壳混染、岩浆补给、晶粥活化等岩浆作用的细节是认识硅质火山岩浆系统演化的关键.锆石中Th、U、Ti、Hf和REE等微量元素的含量和系统变...     

Petrology and trace element geochemistry of the Papuan Ultramafic Belt

New petrologic and geochemical data are presented for a suite of rocks from the Papuan Ultramafic Belt (PUB), Papua New Guinea. Tectonite harzburgites at the base of the ophiolite have extremely refractory, uniform mineralogy, and are exceptionally depleted in lithophile elements. These features are consistent with the proposed origin of these rocks as depleted upper mantle, residual after extraction of a basaltic melt. The tectonite peridotites are overlain by a thick sequence of layered ultramafic and mafic cumulates containing olivine, orthopyroxene, clinopyroxene and plagioclase as the major cumulus phases. Early cumulates are characterized by magnesian olivine Mg₉₀, orthopyroxene Mg₉₀ and calcic plagioclase An₈₆, and exhibit cryptic variation towards more iron-rich and sodic compositions. Abundances of incompatible elements in the cumulates are extremely low which, together with the nature of the cumulus phases, points to a magnesian olivine-poor tholeiite or magnesian quartz tholeiite parent magma(s) strongly depleted in incompatible elements. Highly fractionated iron-rich products of this parent magma type are represented by the LREE-depleted lavas in the overlying basalt sequence which, although resembling the most depleted mid-ocean ridge basalts (MORB) in terms of their low abundances of incompatible elements, have higher abundances of transition metals and lower abundances of Ti, HREE and other high valence cations compared to common MORB of similar Mg/(Mg+Fe) ratio.Eocene tonalites intruding the PUB are genetically unrelated to the ophiolites, and appear to be related to the Ti-poor high-Mg andesites of Cape Vogel and similar andesites and dacites at the northern end of the PUB. These rocks are considered to represent the early stages of island-arc magmatism associated with a northeastward-dipping subduction zone in the early Eocene immediately prior to emplacement of the PUB.     

Isotope and trace element geochemistry of Colorado Plateau volcanics

Basalts from the San Francisco Peaks and North Rim of Grand Canyon, nephelinites from the Hopi Buttes and Navajo minettes (Colorado Plateau) have been analyzed for trace element contents and Sr, Nd, Pb isotope compositions. The ages increase eastward from the Quaternary (basalt) to 5 Ma (nephelinite) and 30 Ma (minette) as does the depth of melt generation inferred from xenolith mineralogy and major element geochemistry.

The three rock types present an enrichment of incompatible elements (although minettes present negative concentration spikes for Nb, Zr, Ti, Ba, Sr) relative to other magma types. The chondrite-normalized Ce/Yb ratio changes from 8–22 (basalt) to 25–30 (nephelinite) and 33–60 (minette) and reflects small degrees of partial melting of a mantle source with a garnet/clinopyroxene ratio increasing with depth. The negative Eu anomaly present in minette, the low Sr/Nd and high Pb/Ce suggest the presence of a recycled continental crust component in their mantle source.

The ⁸⁷Sr/⁸⁶Sr ratio varies from 0.7032-0.7045 (basalt and nephelinite) to 0.7052-0.7071 (minette), while ε_Nd is remarkably more constant at +0.8 to +3.7 (nephelinite) and −2.6 to +2.2 (basalt and minette). Good linear correlations are observed in both ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams with basalt being the least and nephelinite the most radiogenic and indicate a 2.3 ±0.1 Ga age and a Th/U of 3.4.

Three lithospheric source components are indicated: a) an OIB-type depleted mantle source, b) an end-member with unradiogenic Sr, Nd and Pb for basalt and nephelinite and c) a recycled crustal component for minette.     

Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago

New lead, strontium and helium isotopic data, together with trace element concentrations, have been determined for basalts from the Cape Verde archipelago (Central Atlantic). Isotopic and chemical variations are observed at the scale of the archipelago and lead to the definition of two distinct groupings, in keeping with earlier studies. The Northern Islands (Santo Antão, São Vicente, São Nicolau and Sal) present Pb isotopic compositions below the Northern Hemisphere Reference Line (NHRL) (cf. Hart, 1984), unradiogenic Sr and relatively primitive ⁴He/³He ratios. In contrast, the Southern Islands (Fogo and Santiago) display Pb isotopes above the NHRL, moderately radiogenic Sr and MORB-like helium signatures. We propose that the dichotomy between the Northern and Southern Islands results from the presence of three isotopically distinct components in the source of the Cape Verde basalts: (1) recycled ∼1.6-Ga oceanic crust (high ²⁰⁶Pb/²⁰⁴Pb, low ⁸⁷Sr/⁸⁶Sr and high ⁴He/³He); (2) lower mantle material (high ³He); and (3) subcontinental lithosphere (low ²⁰⁶Pb/²⁰⁴Pb, high ⁸⁷Sr/⁸⁶Sr and moderately radiogenic ⁴He/³He ratios). The signature of the Northern Islands reflects mixing between recycled oceanic crust and lower mantle, to which small proportions of entrained depleted material from the local upper mantle are added. Basalts from the Southern Islands, however, require the addition of an enriched component thought to be subcontinental lithospheric material instead of depleted mantle. The subcontinental lithosphere may stem from delamination and subsequent incorporation into the Cape Verde plume, or may be remnant from delamination just before the opening of the Central Atlantic. Basalts from São Nicolau reflect the interaction with an additional component, which is identified as oceanic crustal material.     

Application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis

Carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium. This wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid-rock interaction. Careful consideration of the uncertainties associated with trace element partitioning, sample heterogeneity and fluid-rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis, goundwater evolution, ancient seawater chemistry and isotope stratigraphy. A principal uncertainty involved in the application of Sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of Sr between mineral and solution. This variation may be a function of precipitation rate, mineral stoichiometry, crystal growth mechanism, fluid composition and temperature. Calcite and dolomite in ancient limestones commonly have significantly lower Sr concentrations (20–70 p.p.m.) than would be expected from published trace element distribution coefficient values and Sr/Ca ratios of most modern sedimentary pore waters. This discrepancy probably reflects the uncertainties associated with determining distribution coefficient values. As techniques improve for the analytical measurement and theoretical modelling of Sr concentration and isotopic variations, the petrological analysis of carbonate samples becomes increasingly important. The presence of even small percentages of non-carbonate phases with high Rb concentrations and high ⁸⁷ Sr⁸⁶ Sr values, such as clay minerals, can have significant effects on the measured ⁸⁷ Sr/⁸⁶ Sr values of carbonate rocks, due to the decay of ⁸⁷Rb to ⁸⁷ Sr. For example, a Permian marine limestone with 50 p.p.m. Sr and 1 p.p.m. Rb will have a present-day ⁸⁷ Sr/⁸⁶ Sr value that is >2 × 10^?4 higher than its original value. This difference is an order of magnitude greater than the analytical uncertainty, and illustrates the importance of assessing the need for and accuracy of such corrections. A quantitative evaluation of the effects of water-rock interaction on Sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers. Geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water-rock interaction, including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals. Quantitative modelling may also be used to construct diagnostic fluid-rock interaction trends that are independent of distribution coefficient values, and to distinguish between mixing of mineral end-members and fluid-rock interaction.     

Geochemistry of eclogites of the Tso Morari complex,Ladakh, NW Himalayas:Insights into trace element behavior during subduction and exhumation

Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs(Rb, Th, K except Ba) with LREE enrichments((La/Lu)n = 1.28-5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of(La/Yb)n(2.1-9.4) and Ce/Yb(6.2-16.2) along with Ti/Y(435-735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams(viz. Ti-Zr-Y and Nb-Zr-Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts(WPB)to enriched MORB(E-MORB) indicating that the eclogite protoliths originated from basaltic magmas.Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB(E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities(viz. Zr, Nb, La/Yb, La/Gd,La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.     

Metabasalts from the Troodos Massif,Cyprus: Genetic implication deduced from petrography and trace element geochemistry

A wide variety of data support the contention that the Troodos massif, Cyprus is a fragment of oceanic lithosphere formed at a constructive margin some 85 m.y. ago. However, Troodos rocks differ significantly in major and trace element content and mineral parageneses from those formed at present day active constructive margins. It is here argued that the geochemical and petrographic characteristics of the massif are compatible with its formation at the axis of a slow spreading ridge within a small, marginal ocean basin.     

西藏松多榴辉岩的矿物原位微量元素地球化学研究

对松多榴辉岩中单矿物进行的LA-ICP-MS原位微区微量元素分析研究结果表明,石榴石主要富集中、重稀土元素和Y,同时具有高丰度的Sc、V、Cr和Co等元素;绿辉石中的微量元素以中稀土元素、Sr、Sc、V、Cr、Co、Ni和Ti为主,含有一定量的Zr、Hf等。石榴石、绿辉石、角闪石和绿帘石中均显示轻稀土元素亏损的特点,表明在退变质过程中没有发生明显的富轻稀土元素的外来流体交代作用,因而其微量元素矿物地球化学的某些特点不同于苏鲁地区的榴辉岩。石榴石变斑晶中某些元素(如Ti、Zr)的分带性暗示了榴辉岩在紧随峰期变质之后的折返过程中发生了降压增温过程。榴辉岩主要变质矿物中微量元素的分配显然受到矿物主量元素的分配所控制,如MgO在石榴石和绿辉石之间的分配对Ni、Co、Ti分配的控制以及CaO的分配对Sr、Y、REE分配的控制等。退变质过程中矿物的形成或分解以及物理化学条件的改变都可以引起矿物间微量元素的重新分配。由绿辉石退变质而形成的角闪石,较之原先的绿辉石,其微量元素配分曲线总体特征会发生变化,但元素总体丰度相近,某些元素特点相似,又反映了绿辉石和角闪石之间的成生联系。金红石是Ti、Nb、Ta、Zr、Hf的主要赋存矿物,而与之共生的绿帘石所表现出来的高场强元素的亏损特征表明了金红石的存在所带来的影响。     

Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region,Atlantic Ocean

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb${}^{206}\text{Pb}{}^{204}\text{Pb}$ up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and${}^{143}\text{Nd}{}^{144}\text{Nd}$ increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas.Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of “normal” depleted MORB.     

Experimental geochemistry of Pu and Sm and the thermodynamics of trace element partitioning

The partitioning of Pu and Sm between diopside/liquid and whitlockite/liquid has been investigated experimentally to evaluate the geochemical coherence of Pu and the light REEs. ${}^{\mathrm{Pu}}\text{D}{}^{\mathrm{Sm}}\text{D}$ is 2̃ for both diopsidic pyroxene and whitlockite. This small amount of fractionation would be decreased further if Pu were compared to Ce or Nd. Our experimental results thus validate the suggestion that Pu behaves as a LREE during igneous processes in reducing environments.Our data and the data of Ray et al. (1983) indicate that temperature rather than melt composition is the most important control on elemental partitioning. This is true even though we demonstrate that additions of only 1–2 wt.% of P₂O₅ to the diopside-anorthite-albite system change ^PuD_cpx by a factor of two. Our data suggest that P₂O₅ in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides.     

Mesoproterozoic differential metasomatism in subcontinental lithospheric mantle of Central Indian Tectonic zone: Evidence from major and trace element geochemistry of padhar mafic-ultramafic complex

Metasomatism above subduction zone is an important process capable of producing a heterogeneous mantle wedge with locally varying characters. This, in turn, leads to variation in character of the mafic and intermediate rocks produced through partial melting of lithospheric mantle. The Padhar mafic-ultramafic complex is situated in western part of the Betul belt, Central India, shows enrichment in Rb, Ba, Th and Pb and depletion in Nb, Hf and Zr. Major and trace element data along with REE, confirms the presence of a metasomatized mantle above a Mesoproterozoic subduction zone. Differential petrological evolution, corresponding to the different magma batches, as reflected in almost all of the binary element / oxide variation diagrams, testifies small scale metasomatic heterogeneity in the underlying suprasubduction zone mantle wedge.     

Genesis,evolution and tectonic significance of K-rich volcanics from the Alban Hills (Roman comagmatic region) as inferred from trace element geochemistry

Trace element data are reported in 21 lava samples from the Alban Hills, one of the most important volcanic complexes of the Roman comagmatic region. The samples consist mostly of tephritic leucitites with minor phonolitic tephrites and tephritic phonolites emplaced during two distinct phases of activity, separated by a caldera collapse. The ferromagnesian element contents are variable (Ni=93-26 ppm; Co=37-20 ppm; Cr=359-5 ppm; Sc=35-6 ppm) and tend to have higher values in the post-caldera rocks. Rb, Cs, Th, Sr, and LREE are extremely enriched in all the samples analyzed, with the pre-caldera rocks displaying a lower content of Rb and Cs and a higher abundance of Th, light REE and La/Yb ratio. Ta and Hf are not so high and are more enriched in the pre-caldera samples. Sr displays comparable values in the two groups of rocks. The trace element variation indicates that the rocks from the Alban Hills represent two distinct series of liquids formed by crystal/liquid fractionation processes starting from two parental magmas. The genesis of the primary melts is hypothesized as due to a low degree of partial melting of a mantle peridotite enriched in incompatible elements. All of the studied samples have distribution patterns of incompatible elements normalized against a hypothetical primordial mantle composition, which are similar to that displayed by the aeolian calc-alkaline and leucite-tephritic products and distinctively different from those of typical K-rich volcanics from an intraplate rift environment. This strongly supports the hypothesis that there is a close genetic connection between Roman magmatism and subductionrelated processes.     

Constraining input and output fluxes of the southern-central Chile subduction zone: water,chlorine and sulfur

In this paper, we constrain the input and output fluxes of H₂O, Cl and S into the southern-central Chilean subduction zone (31°S–46°S). We determine the input flux by calculating the amounts of water, chlorine and sulfur that are carried into the subduction zone in subducted sediments, igneous crust and hydrated lithospheric mantle. The applied models take into account that latitudinal variations in the subducting Nazca plate impact the crustal porosity and the degree of upper mantle serpentinization and thus water storage in the crust and mantle. In another step, we constrain the output fluxes of the subduction zone both to the subcontinental lithospheric mantle and to the atmosphere–geosphere–ocean by the combined use of gas flux determinations at the volcanic arc, volume calculations of volcanic rocks and the combination of mineralogical and geothermal models of the subduction zone. The calculations indicate that about 68 Tg/m/Ma of water enters the subduction zone, as averaged over its total length of 1,480 km. The volcanic output on the other hand accounts for 2 Tg/m/Ma or 3 % of that input. We presume that a large fraction of the volatiles that are captured within the subducting sediments (which accounts for roughly one-third of the input) are cycled back into the ocean through the forearc. This assumption is however questioned by the present lack of evidence for major venting systems of the submarine forearc. The largest part of the water that is carried into the subduction zone in the crust and hydrated mantle (accounting for two-thirds of the input) appears to be transported beyond the volcanic arc.