Nd, Pb, and Sr isotope composition of juvenile magmatism in the Mesozoic large magmatic province of northern Chile (18–27°S): indications for a uniform subarc mantle

The Jurassic to Early Cretaceous magmatic arc of the Andes in northern Chile was a site of major additions of juvenile magmas from the subarc mantle to the continental crust. The combined effect of extension and a near stationary position of the Jurassic to lower Cretaceous arc favoured the emplacement and preservation of juvenile magmatic rocks on a large vertical and horizontal scale. Chemical and Sr, Nd, and Pb isotopic compositions of mainly mafic to intermediate volcanic and intrusive rock units coherently indicate the generation of the magmas in a subduction regime and the dominance of a depleted subarc mantle source over contributions of the ambient Palaeozoic crust. The isotopic composition of the Jurassic (²⁰⁶Pb/²⁰⁴Pb: ∼ 18.2; ²⁰⁷Pb/²⁰⁴Pb: ∼ 15.55; ¹⁴³Nd/¹⁴⁴Nd: ∼ 0.51277; ⁸⁷Sr/⁸⁶Sr: ∼ 0.703–0.704) and Present (²⁰⁶Pb/²⁰⁴Pb: ∼ 18.5; ²⁰⁷Pb/²⁰⁴Pb: ∼ 15.57; 143Nd/144Nd: ∼ 0.51288; ⁸⁷Sr/⁸⁶Sr: ∼ 0.703–0.704) depleted subarc mantle beneath the Central and Southern Andes (18°–40°S) was likely uniform over the entire region. Small differences of isotope ratios between Jurassic and Cenozoic to Recent of subarc mantle-derived could be explained by radiogenic growth in a still uniform mantle source.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Compilation of radiogenic isotope data in Mexico and their petrogenetic implications

Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos Plate were also compiled. In the MVB the isotopic ratios range as follows:⁸⁷Sr/⁸⁶Sr 0.703003-0.70841;¹⁴³Nd/¹⁴⁴Nd 0.512496-0.513098;²⁰⁶Pb/²⁰⁴Pb 18.567-19.580;²⁰⁷Pb/²⁰⁴Pb 15.466-15.647;²⁰⁸Pb/²⁰⁴Pb 38.065-38.632. The SMO shows a large variation in⁸⁷Sr/⁸⁶Sr ranging from ∼0.7033 to 0.71387.¹⁴³Nd/¹⁴⁴Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows:²⁰⁶Pb/²⁰⁴Pb 18.060-18.860;²⁰⁷Pb/²⁰⁴Pb 15.558-15.636;²⁰⁸Pb/²⁰⁴Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261 for Nd). Pb isotopes for PacOc show the following range:²⁰⁶Pb/²⁰⁴Pb 18.049-19.910;²⁰⁷Pb/²⁰⁴⁷Pb 15.425-15.734;²⁰⁸Pb/²⁰⁴Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc. Most samples with reported Sr and Nd isotopic data are spread within and around the “mantle array”. The SMO seems to have been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data in all Mexican magmatic provinces and of Nd isotopes in BC, AP, SMS, and SMOr.     

Sr and Nd isotope data of apatite,calcite and dolomite as indicators of source,and the relationships of phoscorites and carbonatites from the Kovdor massif,Kola peninsula,Russia

A detailed Sr−Nd isotopic study of primary apatite, calcite and dolomite from phoscorites and carbonatites of the Kovdor massif (380 Ma), Kola peninsula, Russia, reveals a complicated evolutionary history. At least six types of phoscorites and five types of carbonatite have been identified from Kovdor by previous investigators based on relative ages and their major and accessory minerals. Isotopic data from apatite define at least two distinct groups of phoscorite and carbonatite. Apatite from the earlier phoscorites and carbonatites (group 1) are characterized by relatively low⁸⁷Sr/⁸⁶Sr (0.70330–0.70349) and¹⁴³Nd/¹⁴⁴Nd initial ratios (0.51230–0.51240) with F=2.01–2.23 wt%, Sr=2185–2975 ppm, Nd=275–660 ppm and Sm=31.7–96.2 ppm. Apatite from the second group has higher⁸⁷Sr/⁸⁶Sr (0.70350–0.70363) and¹⁴³Nd/¹⁴⁴Nd initial ratios (0.51240–0.51247) and higher F (2.63–3.16 wt%), Sr (4790–7500 ppm), Nd (457–1074 ppm) and Sm (68.7–147.6 ppm) contents. This group corresponds to the later phoscorites and carbonatites. One apatite sample from a carbonatite from the earlier group fits into neither of the two groups and is characterized by the highest initial⁸⁷Sr/⁸⁶Sr (0.70385) and lowest¹⁴³Nd/¹⁴⁴Nd (0.51229) of any of the apatites. Within both groups initial⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios show negative correlations. Strontium isotope data from coexisting calcite and dolomite support the findings from the apatite study. The Sr and Nd isotopic similarities between carbonatites and phoscorites indicate a genetic relationship between the two rock types. Wide variations in Sr and Nd isotopic composition within some of the earlier carbonatites indicate several distinct intrusive phases. Oxygen isotopic data from calcite and dolomite (δ¹⁸O=+7.2 to +7.7‰ SMOW) indicate the absence of any low-temerature secondary processes in phoscorites and carbonatites, and are consistent with a mantle origin for their parental melts. Apatite data from both groups of phoscorite plot in the depleted quadrant of an ε_Nd versus ε_Sr diagram. Data for the earlier group lie along the Kola Carbonatite Line (KCL) as defined by Kramm (1993) and data from the later group plot above the KCL. The evolution of the phoscorites and carbonatites cannot be explained by simple magmatic differentiation assuming closed system conditions. The Sr−Nd data can best be explained by the mixing of three components. Two of these are similar to the end-members that define the Kola Carbonatite Line and these were involved in the genesis of the early phoscorites and carbonatites. An additional component is needed to explain the isotopic characteristics of the later group. Our study shows that apatite from rocks of different mineralogy and age is ideal for placing constraints on mantle sources and for monitoring the Sr−Nd evolution of carbonatites. Editorial responsibility: W. Schreyer     

Nd and Sr isotopic studies on cenozoic mafic lavas from West Antarctica: Another source for continental alkali basalts

The Nd and Sr isotopic ratios on a suite of continental alkali basalts from Marie Byrd Land, West Antarctica, define a change in the source over the range of K/Ar dates between 1 and 28 m.y. ago. The ⁸⁷Sr/⁸⁶Sr isotopic ratios (0.7026 to 0.7031) are unusually low for continental alkali basalts, although the corresponding ¹⁴³Nd/¹⁴⁴Nd ratios (0.51283 to 0.51299) are similar to previously reported values. On a ⁸⁷Sr/⁸⁶Sr vs. ¹⁴³Nd/¹⁴⁴Nd diagram, they define a trend on the low ⁸⁷Sr/⁸⁶Sr side of the “mantle array”, which has a slope steeper than the mantle array. An explanation for the light rare earth elements (LREE) enrichment of the alkali basalts, with high ¹⁴³Nd/¹⁴⁴Nd ratios and low ⁸⁷Sr/⁸⁶Sr ratios, is suggested by a model which modifies the source region with a mantle-derived, CO₂-enriched metasomatic fluid.     

Particularités de la contamination crustale des phonolites : exemple du Velay oriental (Massif central)

Sr and Nd isotopic compositions of one trachyte, eight phonolites and five basalts have been measured. The isotopic characteristics of the trachyte can be explained by a combined assimilation–fractional crystallization process within an upper crustal magmatic chamber. Some phonolites display isotopic signatures identical to basalts, suggesting that they have been protected against any crustal assimilation during their formation. Some others have low Sr contents, whereas they are enriched in radiogenic Sr (0.70451<⁸⁷Sr/⁸⁶Sr_i<0.71192), and display basaltic ¹⁴³Nd/¹⁴⁴Nd ratios. Both observations could be explained by very strong alkali feldspar fractionation and by subsequent very low assimilation of surrounding rocks (between 0.3 and 4%) during intrusion. To cite this article: J.-M. Dautria et al., C. R. Geoscience 336 (2004).     

Western Kamchatka-Koryak continental-margin volcanogenic belt: Age,composition, and sources

The isotope-geochemical study of the Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed a lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones. In the Western Kamchatka segment, magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher ⁸⁷Sr/⁸⁶Sr (0.70429–0.70564) and lower ¹⁴³Nd/¹⁴⁴Nd(ɛ_Nd(T) = 0.06–2.9) ratios in the volcanic rocks from the Central Koryak segment presumably reflect the contribution of enriched mantle source; the high positive ɛ_Nd(T) and low ⁸⁷Sr/⁸⁶Sr ratios in the magmatic rocks from the Northern Koryak segment area indicate their derivation from isotopically depleted mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly different contamination histories of the Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different thermal regimes: the higher heat flow beneath Kamchatka led to the crustal melting and contamination of mantle suprasubduction magmas by crustal melts. The cessation of suprasubduction volcanism in the Western Kamchatka segment of the continentalmargin belt was possibly related to the accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in the Koryak segment stopped due to the closure of the Ukelayat basin in the Oligocene time.     

Multistage melt/fluid-peridotite interactions in the refertilized lithospheric mantle beneath the North China Craton: constraints from the Li–Sr–Nd isotopic disequilibrium between minerals of peridotite xenoliths

Elemental and Li–Sr–Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing models for Li isotopic fractionation in mantle peridotites and constrain the melt/fluid-peridotite interaction in the lithospheric mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx and olivine (<4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral δ⁷Li vary greatly, with olivine (+0.7 to +5.4‰) being isotopically heavier than coexisting opx (−4.4 to −25.9‰) and cpx (−3.3 to −21.4‰) in most samples. The δ⁷Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances, likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine δ⁷Li of −3.0 and −7.9‰, indicating the existence of low δ⁷Li domains in the mantle, which could be transient and generated by meter-scale diffusion of Li during melt/fluid-peridotite interaction. The ¹⁴³Nd/¹⁴⁴Nd (0.5123–0.5139) and ⁸⁷Sr/⁸⁶Sr (0.7018–0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in ⁸⁷Sr/⁸⁶Sr and slightly higher in ¹⁴³Nd/¹⁴⁴Nd than coexisting opx, implying an intermineral Sr–Nd isotopic disequilibrium. This is observed more apparently in peridotites having low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd ratios than in those with high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd, suggesting that a relatively recent interaction existed between an ancient metasomatized lithospheric mantle and asthenospheric melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the lithospheric mantle represented by the peridotites has been heterogeneously refertilized by multistage melt/fluid-peridotite interactions.     

Strontium and neodymium isotopic study of the western Mogollon-Datil volcanic region,New Mexico,USA

The Sr- and Nd-isotopic compositions of large mid-Cenozoic caldera-forming eruptions, and related rocks, from the western portion of the Mogollon-Datil volcanic field have been determined. The average initial ⁸⁷Sr/⁸⁶Sr ratios of 27 samples from felsic flows range from 0.70629 to 0.72872; however, all but two flows are 0.71337 or less. Ten analyses of intermediate and mafic rocks showed a tendency towards lower initial ⁸⁷Sr/⁸⁶Sr ranging from 0.70363 to 0.70968. Initial ¹⁴³Nd/¹⁴⁴Nd ratios of II felsic and intermediate rocks range from 0.51216 to 0.51231. Two basalts analyzed for ¹⁴³Nd/¹⁴⁴Nd have ratios of 0.51250 and 0.51291. During the course of the volcanic activity from 34 Ma to the present, there was a shift towards lower initial ⁸⁷Sr/⁸⁶Sr ratios, and lower SiO₂ contents. A number of models of crustal melting, fractionation, mixing, and assimilation and fractional crystallization (AFC), using a variety of possible endmembers, were tested, to see if they could explain the isotopic and geochemical characteristics of the Mogollon-Datil volcanic rocks. The best fit was an AFC model using two components, one a mantle-sourced primary magma, with isotopic ratios of the Kilbourne Hole, N. M., basanite, and the other an upper crust with average continental isotopic ratios, and Sr and Nd abundances similar to the Texas Canyon pluton of Arizona.     

Sr and Nd Isotopic Characteristics and Magma Genesis of Mesozoic Volcanic Rocks Along the Coastal Region of Southeastern China

The Mesozoic volcanic rocks in the coastal region of southeastern China were superimposed on some different basement tectonic elements. The volcanic rocks developed in these different basement tectonic elements have great differences in Sr and Nd isotopic compositions. The rocks in western Zhejiang and northeastern Jiangxi Provinces which belong to the Lower Yangtze subplate have lower initial 87Sr/ 86Sr ratios, but are higher in initial Nd isotopic ratios. The initial 143Nd / 144Nd values of the volcanic rocks developed in the Cathaysian subplate increase clearly from early to late in time, and from the core of the Wuyishan uplift coastwards constantly, but the initial 87Sr/86Sr values tend to decrease. The isotopic characteristics and their spatial variations in Mesozoic volcanic rocks in the study region are, to a great extent, manifestations of the isotopic characteristics in basement metamorphic complexes, and the generation of the Mesozoic acid magma in this region is attributed to the recycling o     

Enriched mantle – Dupal signature in the genesis of the Jurassic Ferrar tholeiites from Prince Albert Mountains (Victoria Land, Antarctica)

The major, trace (including rare earth) element abundances, and Sr-Nd-Pb isotopic compositions, have been analysed for andesitic basalt and andesitic sills and lavas of the Jurassic Ferrar Magmatic Province, Prince Albert Mountains, Antarctica. The typical “crustal signature” of the Ferrar magmatism, characterized by relatively high SiO₂, LREE and LILE contents in these samples, is associated with high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd. Systematic correlations of major and trace elements indicate that fractional crystallization was important. However, increases in incompatible elements are positively correlated with initial ⁸⁷Sr/⁸⁶Sr, suggestive of crustal assimilation processes. The observed correlations between initial ⁸⁷Sr/⁸⁶Sr and LREE enrichments have been modelled by an AFC process, starting from the least evolved sample and assuming the compositions of the orthogranulites of Victoria Land as contaminants. The REE patterns of the least evolved Ferrar rocks approach those of E-type MORB, differing only by higher LREE/IREE. The enrichment in LREE, accompanying high initial ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁴Pb and low ¹⁴³Nd/¹⁴⁴Nd compared with E-type MORB, can be explained by interaction of “primary Ferrar basalt” with crystalline basement. We propose a petrological model whereby Ferrar magmas were generated through high degrees of melting of an E-type MORB mantle source, and subsequently these “primary” melts underwent AFC processes inheriting a crustal signature. The Sr-Nd-Pb isotopic compositions required by the AFC model for the primary Ferrar basalt are similar to those of the Dupal signature of the oceanic basalts of the Southern Hemisphere (Hart 1984). Transantarctic Mountains would have been located inside the Dupal anomaly in pre-Gondwana dispersion times. Received: 21 April 1998 / Accepted: 25 January 1999     

Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas,north Victoria Land,Antarctica

Isotopic (Nd and Sr) and chemical compositions of the 177 Ma Kirkpatrick Basalt and Ferrar Dolerite from north Victoria Land, Antarctica, are examined in order to address the role of crustal assimilation and the characteristics of their mantle source. Results for the Scarab Peak chemical type (SPCT) that constitutes the flow unit capping the lava sequence [Mg-number, Mg/(Mg+Fe⁺²)=24, MgO=2.4%, SiO₂=57.1%, initial⁸⁷Sr/⁸⁶Sr=0.7087–0.7097, (ε_Nd=−4.3) conform previous reports that attribute variations in the concentrations of the more mobile elements and calculated initial⁸⁷Sr/⁸⁶Sr to mid-Cretaceous alteration and elevated δ¹⁸O to low-temperature interaction with meteoric water. The underlying lavas and the sills that are of the Mt. Fazio chemical type (MFCT) display a much wider range of both chemical and isotopic compositions (Mg-number=40–65, MgO=3.7 7.5%; SiO₂=52.6–58.3%, initial⁸⁷Sr/⁸⁶Sr=0.7087–0.7117, ε_Nd=−5.6 to −4.8). The effects of rock alteration on apparent initial⁸⁷Sr/⁸⁶Sr are demonstrated by large differences between the initial ratio of mineral separates or leached fractions and whole rocks. Cretaceous alteration produced Rb and Sr redistribution within the lava sequence that results in erroneous calculated initial⁸⁷Sr/⁸⁶Sr ratios. These effects are responsible for the large initial⁸⁷Sr/⁸⁶Sr variations previousl7 proposed which, combined with the large range in whole-rock δ¹⁸O, were purported to show very large degrees of crustal assimilation. The variations in ε_Nd are restricted and indicate much smaller degrees of assimilation. The least altered of the MFCT rocks show good chemical and isotopic correlations that can be integrated into a model involving fractionation of pyroxene and plagioclase coupled with assimilation of material similar to early Paleozoic basement. The lower⁸⁷Sr/⁸⁶Sr and higher ε_Nd of the SPCT suggest that they were derived by extensive fractionation of a more primitive, less contaminated, precursor of the MFCT. The most isotopically primitive Ferrar rocks from the region still have a high initial⁸⁷Sr/⁸⁶Sr and low initial¹⁴³Nd/¹⁴⁴Nd; this may reflect either earlier assimilation or an enriched source. The chemical and isotopic similarities, as well as the close geographic correspondence of the Ferrar Group to granitoids produced during the early Paleozoic Ross Orogeny suggest that in either case Ross-type material may have been involved in the development of the enriched isotopic signature. Editorial responsibility: I. Parsons     

Hafnium isotope results from mid-ocean ridges and Kerguelen

¹⁷⁶Hf/¹⁷⁷Hf ratios are presented for oceanic volcanics representing both extremes of the range of mantle HfNdSr isotopic variation. Hf from critical mid-ocean ridgebasalts shows that ¹⁷⁶Hf/¹⁷⁷Hf does indeed have a greater variability than ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr in the depleted mantle. This extra variation is essentially of a random nature, and can perhaps be understood in terms of known Rb/SrSm/NdLu/Hf fractionation relationships. At the other extreme of mantle isotopic composition, ¹⁷⁶Hf/¹⁷⁷Hf ratios for igneous rocks from the Indian Ocean island of Kerguelen show a closely similar variation to published ¹⁴³Nd/¹⁴⁴Nd ratios for the same samples. Comparison of HfNdSr c relationships for Tristan da Cunha, Kerguelen and Samoa reveals divergences in the mantle array for ocean island magma sources, and perhaps suggests that these irregularities are largely the result of an extra component of ⁸⁷Sr/⁸⁶Sr variation.     

Petrogenesis of the Laguna del Maule volcanic complex,Chile (36° S)

From 33°–42° S in central-south Chile, there are numerous volcanoes which form part of the Andean magmatic arc caused by subduction of the Nazca plate beneath western South America. The <0.3 m.y. old Laguna del Maule volcanic complex at 36° S is in a transition region between volcanoes at 33°–34° S formed dominantly of hornblende-bearing andesite and volcanoes south of 37° S dominantly composed of basalt and basaltic andesite. The Laguna del Maule complex ranges in composition from basalt (0.3 m.y.) to rhyolite (post-glacial). Although there is abundant evidence for magma mixing, basalt and rhyolite have similar Sr and Nd isotopic ratios, thereby requiring that the mixing members had the same isotopic ratios (⁸⁷Sr/ ⁸⁶Sr 0.70419 and ¹⁴³Nd/¹⁴⁴Nd 0.51274). In contrast, dacitic dikes and a volcanic neck which also have evidence for magma mixing are isotopically distinct. Major and trace element abundances are consistent with a genetic relationship between the basalt and rhyolite, either by low-pressure, plagioclase-dominated, fractional crystallization or by partial melting of a plagioclase-rich assemblage. There is no evidence that the rhyolites contain more of a crustal component than the associated basic volcanics.     

Petrogenesis of late stage magmatism at Hold with Hope, East Greenland

The Myggbukta caldera complex and a swarm of basic dykes constitute the latest Tertiary magmatism in the Hold with Hope region, East Greenland. The Sr and Nd isotope ratios of these rocks show coherent variations which extend to high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd values and require a contribution from continental lithosphere. Broad correlations with major element differentiation indices suggest that the continental component was incorporated during magmatic differentiation thereby favouring a crustal contamination process. Trace element concentrations are strongly correlated with isotopic compositions but display ranges for many incompatible elements which extend beyond likely crustal contaminant compositions. This is readily modelled by AFC processes in which the dominant cause of trace element enrichment is the concentration effect of fractional crystallisation rather than the composition of the contaminant. The simplest such models still require unrealistically high degrees of fractional crystallisation to explain the ten-fold enrichment of some trace elements. This can be overcome if the primary magmas entering the crust already had highly variable trace element compositions. Such variability is readily achieved if melts from different parts of the melting column escape without thorough homogenization. An AFC model which incorporates variability in parental magma composition is then able to simulate the range of compositions observed at Hold with Hope. This carries the implication that the variations observed are more readily attributed to changes in uncontaminated parental magma than to variations in the composition or amount of contaminant. Received: 5 March 1998 / Accepted: 16 June 1998     

Petrological and geochemical characteristics of Plio-Pleistocene Volcanics from Ponza Island (Tyrrhenian Sea, Italy)

Summary Volcanic rocks on Ponza Island (Tyrrhenian Sea, central Italy) consist of Pliocene submarine rhyolites and Pleistocene subaerial trachyte and comendite lavas. Chemical variations and the homogeneous Sr and Nd isotopic signatures within the analyzed Pliocene rocks are ascribed to crystal fractionation. The absolute isotopic values, however, indicate the important role of a crustal component in the origin of these magmas. The very high-silica rocks were probably derived from a superimposed mechanism which may have been connected to the ascent of hydrothermal magmatic fluids. Compositional and ⁸⁷Sr/⁸⁶Sr variations at constant ¹⁴³Nd/¹⁴⁴Nd values in the Pleistocene rocks are likely due to fractionation of the observed phenocryst assemblage, possibly coupled with minor crustal interaction. These processes, however, cannot account for the extreme enrichment of many incompatible trace elements in the comendites. Some evidence suggests the influence of a halogen- and/or CO₂-rich volatile phase. Received February 17, 2000; revised version accepted November 29, 2000     

Geochemistry of lamprophyres from the Western Alps,Italy: implications for the origin of an enriched isotopic component in the Italian mantle

Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected ⁸⁷Sr/⁸⁶Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: ²⁰⁶Pb/²⁰⁴Pb = 18.669–18.895, ²⁰⁷Pb/²⁰⁴Pb = 15.605–15.689 and ²⁰⁸Pb/²⁰⁴Pb = 38.224–39.134. ⁸⁷Sr/⁸⁶Sr ratios show a negative correlation with ¹⁴³Nd/¹⁴⁴Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.     

Isotopic and Geochemical Investigation of the Chilwa Island Carbonatite Complex, Malawi: Evidence for a Depleted Mantle Source Region, Liquid Immiscibility, and Open-System Behaviour

Initial Nd, Pb, and Sr isotopic data from carbonatites and associatedintrusive silica-undersaturated rocks from the early Jurassic,Chilwa Island complex, located in southern Malawi, central Africa,suggest melt derivation from a Rb/Sr- and Nd/Sm-depleted butTh/Pb- and U/Pb-enriched mantle source. Initial ¹⁴³Nd/¹⁴⁴Nd(0.51265–0.51270) isotope ratios from the Chilwa Islandcarbonatites are relatively constant, but their initial ⁸⁷Sr/⁸⁶Sr(0.70319–0.70361) ratios are variable. The ¹⁸O_smow (9.53–14.15%0)and ¹³C_PDB (–3.27 to –1.50%0) isotope ratios ofthe carbonates are enriched relative to the range of mantlevalues, and there is a negative correlation between ¹⁸O andSr isotope ratios. The variations in Sr, C, and O isotopic ratiosfrom the carbonatites suggest secondary processes, such as interactionwith meteoric groundwater during late-stage carbonatite activity.The initial ¹⁴³Nd/¹⁴⁴Nd (0.51246 0.51269) and initial ⁸⁷Sr/⁸⁶Sr(0.70344–0.70383) isotope ratios from the intrusive silicaterocks are more variable, and the Sr more radiogenic than thosefrom the carbonatites. Most of the Pb isotope data from Chilwa Island plot to the rightof the geochron and close to the oceanic regression line definedby MORBs and OIBs. Initial Pb isotopic ratios from both carbonatites(²⁰⁷Pb/²⁰⁴Pb 15.63–15.71; ²⁰⁶Pb/²⁰⁴Pb 19.13–19.78)and silicate rocks (²⁰⁷Pb/²⁰⁴Pb 15.61–15.72; ²⁰⁶Pb/²⁰⁴Pb18.18–20.12) show pronounced variations, and form twogroups in Pb-Pb plots. The isotopic variations shown by Nd, Pb, and Sr for the ChilwaIsland carbonatites and intrusive silicates suggest that thesemelts underwent different evolutionary histories. The chemicaldata, including isotopic ratios, from the carbonatites and olivinenephelinites are consistent with magmatic differentiation ofa carbonated-nephelinite magma. A model is proposed in whichdifferentiation of the carbonatite magma was accompanied byfenitization (metasomatic alteration) of the country rocks bycarbonatite-derived fluids, and subsequent alteration of thecarbonatite by hydrothermal activity. The chemical and isotopicdata from the non-nephelinitic intrusive silicate rocks reveala more complex evolutionary history, involving either selectivebinary mixing of lower-crustal granulites and a nephelinitemagma, or incremental batch melting of a depleted source andsubsequent crustal contamination.     

Calbuco Volcano and minor eruptive centers distributed along the Liquiñe-Ofqui Fault Zone,Chile (41°–42° S): contrasting origin of andesitic and basaltic magma in the Southern Volcanic Zone of the Andes

Calbuco volcano is a Late Pleistocene-Holocene composite stratovolcano located at 41°20 S, in the southern region of the Southern Volcanic Zone of the Andes (SSVZ; 37°–46° S). In contrast to basalt and basaltic andesite, which are the dominant lava types on the volcanic front from 37° to 42° S, Calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths. They have SiO₂ contents in the 55–60% range, and have comparatively low K₂O, Rb, Ba, Th and LREF abundances relative to other SSVZ centers. Incompatible element abundance ratios are similar to those of most SSVZ volcanics, but ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd are respectively higher and lower than those of adjacent volcanic centers. Basalts from nearby Osorno stratovolcano, 25 km to the northeast, are similar to other basaltic SSVZ volcanoes. However, basalts from several minor eruptive centers (MEC), located east of Calbuco and Osorno volcano along the Liquiñe-Ofqui fault zone (LOFZ), are enriched in Ba, Nb, Th and LREE, and have higher La/Yb and lower Ba/La, K/La and Rb/La. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd in MEC basalts are respectively lower and higher than those of Osorno and Calbuco lavas. We suggest that MEC basalts were produced by lower extents of mantle melting than basalts from Osorno and other SSVZ stratovolcanoes, probably as a result of lower water content in the source of MEC basalts. Calbuco andesites formed from basaltic parents similar to Osorno basalts, by moderate pressure crystallization of a hornblende-bearing assemblage accompanied by crustal assimilation. Hornblende stability in the Calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks, which are also an appropriate endmember for isotopic trends, together with magma storage at mid-crustal depths. The unique characteristics of Calbuco volcano, i.e. the stability of hornblende at andesitic SiO₂ contents, low ¹⁴³Nd/¹⁴⁴Nd and high ⁸⁷Sr/⁸⁶Sr, and abundant crustal xenoliths, provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the SSVZ.     

Baseline determination of the atmospheric Pb,Sr and Nd isotopic compositions in the Rhine valley,Vosges mountains (France) and the Central Swiss Alps

The Pb, Sr and Nd isotopic compositions of biomonitors (lichen, moss, bark) and soil litter from different regions in the Rhine valley, as well as of <0.45 μm particles separated out of ice of the Rhône and Oberaar glaciers and lichens from the Swiss Central Alps, have been determined in order to deduce the natural baseline of the atmospheric isotopic compositions of these regions, which are suggested to be close to the isotopic compositions of the corresponding basement rocks or soils at the same sites. ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr isotope ratios are positively correlated. Most polluted samples from traffic-rich urban environments have the least radiogenic Pb and Sr isotopic compositions with ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr ratios of 1.11 and 0.7094, respectively. These ratios are very different from those of the atmospheric baseline for the Vosges mountains and the Rhine valley (²⁰⁶Pb/²⁰⁷Pb: 1.158–1.167; ⁸⁷Sr/⁸⁶Sr: 0.719–0.725; ε_Nd: −7.5 to −10.1). However, this study indicates that the baseline of the atmospheric natural Pb and Sr isotopic compositions is affected by anthropogenic (traffic, industrial and urban) emissions even in remote areas. Lichen samples from below the Rhône and Oberaar glaciers reflect the baseline composition close to the Grimsel pass in the Central Swiss Alps (⁸⁷Sr/⁸⁶Sr: 0.714 − 0.716; ε_Nd: −3.6 to −8.1). The ¹⁴³Nd/¹⁴⁴Nd isotope ratios are highly variable (8ε units) and it is suggested that the variation of the ¹⁴³Nd/¹⁴⁴Nd is controlled by wet deposition and aerosols originating from the regional natural and industrial urban environments and from more distant regions like the Sahara in North Africa. The least anthropogenetically affected samples collected in remote areas have isotopic compositions closest to those of the corresponding granitoid basement rocks.     

利用套柱法快速分离提纯Sr和Nd元素

样品放射性成因Sr-Nd同位素比值受控于源区初始同位素组成、放射性元素母体与子体相对丰度,以及衰变时间等因素。它们具有极强的示踪能力,因而在地质学领域有广泛的应用。传统的Sr-Nd同位素分析使用的是阳离子树脂,提纯Nd元素时往往涉及有机试剂以及调节pH值等操作,其分析效率较低。近年来特效树脂的出现使得分离这些元素变得简单,但是受硫酸根等因素影响,特效树脂使用次数有限。为了提高分析效率,缩短分析时间,本文开发了一种套柱法,该方法结合阳离子树脂和特效树脂,实现了Sr-Nd元素的快速分离,并且能延长特效树脂的使用寿命。实验采用阳离子树脂、Sr特效树脂和LN稀土特效树脂对玄武岩BCR-2标样进行了分析。Sr-Nd回收率均90%,BCR-2玄武岩~(87)Sr/~(86)Sr比值为0.705016±0.000016(n=36,1SD),~(143)Nd/~(144)Nd比值为0.512624±0.000012(n=39,1SD),与前人TIMS法获得的结果吻合(~(87)Sr/~(86)Sr:0.705000～0.705023;~(143)Nd/~(144)Nd:0.512630～0.512650)。最终分离提纯的溶液中~(85)Rb/~(86)Sr值小于0.01,~(147)Sm/~(144)Nd值小于0.001,表明该方法可以高效分离Rb-Sr和Sm-Nd,实现Sr、Nd同位素的准确分析。