Riccardo Vannucci

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Growth of early continental crust controlled by melting of amphibolite in subduction zones

It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process.

Nb and Ta incorporation and fractionation in titanian pargasite and kaersutite: crystal–chemical constraints and implications for natural systems

New partition coefficients between liquid and pargasitic/kaersutitic amphiboles (Amph/LDNb,Ta) experimentally determined for Nb and Ta at upper-mantle conditions, combined with single-crystal structure refinement of the synthesised amphiboles, show that Amph/LDNb,Ta are strongly dependent on the structure and composition of both amphibole and melt. The correlation of the Amph/LDNb,Ta with the amphibole oxy-component is explained by the ordering of Nb and Ta at the M1 site, contributing with the fraction of Ti at M1 to locally balance the O3O2−↔O3(OH)− substitution. In our set of dehydrogenated amphiboles, variations in the SiO2 content of the melt from 41.5 to 54.6 correspond to a six-fold increase of the Amph/LDNb,Ta, in which Amph/LDNb varies from 0.14 to 0.71 and Amph/LDTa from 0.11 to 0.54. Partition coefficients for Nb and Ta abruptly increase in Ti-depleted compositions (Amph/LDNb up to 1.63 and Amph/LDTa to 1.00). The ratio of DNb to DTa (Amph/LDNb/Ta) varies from 0.71 to 1.63, and is a function of the M1 site dimension, which in turn depends on its Fe, Mg and Ti contents. The observed variations can be explained by assuming that the ionic radius of Nb is (∼0.01–0.02 A) larger than that of Ta, contrary to the common assumption that they are both equal to 0.64 A. We calibrated a simplified model for the prediction of Amph/LDNb/Ta values shown to be negatively related mainly to mg# [Mg/(Mg+Fe)] and to Ti content. High-mg# amphiboles have Amph/LDNb/Ta close to unity, so the low Nb/Ta found in convergent margin volcanics and in the continental crust cannot be explained by the involvement of amphibole in the mantle wedge. Amphibole in the subducting slab may have lower mg# and consequently high Nb/Ta values, and thus may give rise to subchondritic Nb/Ta values in coexisting melts. Nb/La is also negatively correlated with mg#, and strongly increases in Ti-depleted compositions.

Trace-element incorporation in titanite: constraints from experimentally determined solid/liquid partition coefficients

Titanite/liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite-saturated liquids equilibrated at 1.5 GPa and 850 °C starting from lamproitic compositions. The high compatibility observed for REE, HFSE, Sr, V and Sc, and the strong incompatibility observed for actinides, large ion lithophile and light elements are discussed in terms of available crystal-chemical mechanisms for incorporation and crystal-structure control. The exchange vectors Na1Ca−1 and Al1Ti−1 allow local charge balance to be achieved after incorporation of REE3+ and R5+ at the Ca and Ti site, respectively. The significant amounts of H measured are also relevant in this regard [the exchange mechanism being (OH)1O−1 at the O1 site]. The incorporation of U4+ and Th4+ at the Ca site is scarce, and is likely balanced by that of Mg2+ at the Ti site; both these substitutions are responsible for strong structural strain. Titanite can thus be considered an important repository for REE and HFSE in metamorphic and igneous rocks, and its role must be accounted for when modelling trace-element residence during metamorphic reactions and late magmatic crystallisation history. Due to the measured differences in compatibility, titanite crystallisation increases the values of Nb/Ta and LREE/HREE ratios in residual liquids. The similar compatibility of U and Pb makes titanite suitable for U–Pb geochronology of igneous rocks only after common Pb correction. Finally, this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal-chemical constraints, and that titanite-based waste forms should contain high amounts of Na+ and Mg2+.

Incompatible element-rich fluids released by antigorite breakdown in deeply subducted mantle

We present first trace element analyses of the fluid produced during breakdown of antigorite serpentine, a major dehydration reaction occurring at depth within subducting oceanic plates. Microinclusions filled with crystals+aqueous liquid are disseminated within olivine and orthopyroxene grown at pressures and temperatures beyond the stability field of antigorite. Despite hydrogen loss and significant major element changes that have affected the analyzed inclusions, their trace element composition still reflects characteristics of the subduction fluid released during serpentinite dehydration. The fluid is enriched in incompatible elements indicating either (1) interaction with fluids derived from crustal slab components, or (2) dehydration of altered (serpentinized) oceanic mantle previously enriched in incompatible elements. Several features of the analyzed fluid+mineral inclusions (high Pb/Th, Pb/U and Pb/Ce) are in agreement with available experimental work, as well as with the geochemical signatures of most arc lavas and of several ocean island basalt mantle sources. The trace element patterns of the fluid+mineral inclusions do not display relative enrichment in large ion lithophile elements compared to high field strength elements, thus suggesting that the latter elements may become soluble in natural subduction fluids. fl 2001 Elsevier Science B.V. All rights reserved.

Chemistry and origin of trapped melts in ophioiitic peridotites

Melt impregnation and peridotite-melt interaction are ubiquitous phenomena in the oceanic-type lithospheric mantle. Nevertheless, the nature of the infiltrating melts is still poorly understood. We performed detailed textural and chemical investigations (by means of electron and ion microprobe) on: (1) impregnated plagioclase-bearing ophiolitic peridotites from the Internal Ligurides (Northern Apennine, Italy) and Mt. Maggiore (Corsica), and (2) olivine cumulates (consisting of 85 vol% olivine plus interstitial plagioclase and rare poikilitic clinopyroxene) from magmatic pods intruded within the Mt. Maggiore peridotites. Field evidence indicates that such cumulates crystallized from the melts, which impregnated the surrounding peridotites. Melt impregnation in the peridotites is verified by the occurrence of peculiar microstructures: (1) plagioclase blebs and/or veins confined along grain boundaries or crosscutting mantle minerals; (2) partial dissolution of mantle clinopyroxene and replacement by orthopyroxene and plagioclase aggregates, which indicate disequilibrium between melts and host peridotites. Interaction with melts also produces chemical modifications in mantle clinopyroxenes, i.e., Ti, M(middle)- to H(heavy)-REEs (and, to a lesser extent, Zr, Y, and Sc) enrichment coupled to depletion in Al. Minerals crystallized from the melts have depleted Geochemical signature: plagioclases are highly Anorthitic (An88An94 in the peridotites; An81An85 in the cumulates), and show extremely low Sr (<26 ppm) and LREE (< 1 × Cl; CeNSMN = 0.13−0.50) concentrations. Interstitial clinopyroxenes in the cumulates are characterized by high Mg values (90.6–91.7): their REE spectra show significant LREE depletion (CeNYbN = 0.027−0.039), high M- to H-REE concentrations (15–30 × C1) and pronounced negative EuN anomalies. Geochemical modeling indicates that the impregnating liquids probably consisted of unmixed depleted melt increments produced by 6–7% fractional melting. The results of this study therefore suggest that the impregnating melts originated at deeper mantle levels and presumably represented the last melt increments of a fractional melting process. There is thus a higher probability that they will remain incorporated in the upper mantle. They subsequently ascended, partly crystallized as cumulate pods, and interacted with the studied peridotites dissolving mantle clinopyroxene and precipitating orthopyroxene. Such a process has been increasingly invoked in studies of melt/rock reaction zones from ophiolitic peridotites.

Rare earth element redistribution during high-pressure–low-temperature metamorphism in ophiolitic Fe-gabbros (Liguria, northwestern Italy): Implications for light REE mobility in subduction zones

To unravel the rare earth element (REE) redistribution during high-pressure–low-temperature metamorphism, we have analyzed by ion microprobe all the minerals from representative Fe-gabbros from Ligurian metaophiolites (northwestern Italy). Contrary to what is observed for fresh Fe-gabbros, the clinopyroxene contribution to the whole-rock REE inventory of blueschists and eclogitized Fe-gabbros is minor or negligible. In both blueschists and eclogites, REE are redistributed among newly formed minerals. In blueschists, lawsonite is the major REE carrier and concentrates almost all the light REE (LREE) in the rock, whereas titanite plays an important role for middle and heavy REE (MREE, HREE). In eclogites, LREE and HREE mainly reside in accessory allanite and garnet, respectively. The development of blueschist and eclogite facies mineral assemblages in mafic oceanic crust, due to subduction to 35–65 km depths, was not accompanied by release of significant amounts of LREE to the overlying mantle wedge. Owing to the stability of lawsonite and epidote at ultra-high-pressure conditions, subduction of mafic rocks is considered an effective mechanism to bring LREE to great depth within the mantle.

Subsolidus reactions monitored by trace element partitioning: the spinel- to plagioclase-facies transition in mantle peridotites

Mantle peridotites of the External Liguride (EL) Units (Northern Apennines) mainly consist of fertile spinel-lherzolites partially recrystallized to plagioclase-facies assemblages, and are consequently appropriate to investigate the interphase element partitioning related to the transition from spinel- to plagioclase-facies stability field. Evidence for the development of the plagioclase-facies assemblage is mainly given by: (1) large exsolution lamellae of orthopyroxene and plagioclase within spinel-facies clinopyroxene; (2) plagioclase rims around spinel; (3) granoblastic domains made up of olivine+plagioclase±clino-and orthopyroxene. In situ major and trace [REE (rare-earth elements), Ti, Sc, V, Cr, Sr, Y, Zr and Ba] element mineral analyses have been performed, by electron and ion probe, on selected samples which show the progressive development of the plagioclase-bearing assemblage. The main compositional variations observed during the change from spinel- to plagioclase-facies minerals are as follows: (1) clinopyroxenes decrease in Al, Na, Sr, Eu/Eu* and increase in Y, V, Sc, Cr, Zr and Ti; (2) amphiboles decrease in Eu/Eu*, Sr, Ba and increase in Zr and V; (3) spinels decrease in Al and increase in Cr and Ti. The most striking feature is the decoupling in the behaviour of similarly incompatible elements (D about 0.1) in clinopyroxene, e.g. Sr decrease is mirrored by Zr increase. Massbalance calculations indicate that the trace element interphase redistribution documented in the EL peridotites occurred in a closed system and in response to the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. The observed element partitioning reveals, moreover, that subsolidus re-equilibration processes in the upper mantle produce HFSE (high-field-strength element)/REE fractionation in minerals, which must be evaluated for a reliable determination of mineral-melt distribution coefficients. The results of this study furnish evidence for subsolidus metamorphic evolution during decompression, without concomitant partial melting processes. This is consistent with the interpretation that the EL peridotites represent subcontinental lithospheric mantle emplaced at the surface in response to lithospheric thinning and tectonic denudation mechanisms related to the Triassic-Jurassic rifting of the Ligure-Piedmontese basin.

Partitioning of REE, Y, Sr, Zr and Ti between clinopyroxene and silicate melts in the mantle under La Palma (Canary Islands): implications for the nature of the metasomatic agents

Abstract Ion microprobe analyses of clinopyroxenes in equilibrium with glasses (SiO2 from 42 to 69 wt%) formed as the result of AFC-type reactions between infiltrating basaltic melt and peridotite wall-rock in the upper mantle under La Palma (Canary Islands) reveal trace element signatures usually ascribed to carbonatite metasomatism (i.e. strong REE-enrichment and Zr,Ti-depletion). Cpx/glass partition coefficients for REE, Y and Sr progressively increase with increasing SiO2 in response to liquid composition/structure effects, approaching unity for La, Ce and Sr, and exceeding unity for the other REE in the most silicic glasses. Partition coefficients for Zr and Ti remain constant or decrease, probably as the combined effect of melt composition/structure and difficulty of charge-balancing 4+ cations in the crystal. Incorporation of the higher-charged HFSE in the cpx lattice requires complex coupled substitutions (e.g. M1Zr+1TAl+2M1R2+−1TSi−2 or M2Na+1M1Zr+1TAl+1M2Ca−1M1R2+−1TSi−1). Our results demonstrate that REE-enrichment and REE/HFSE fractionation of clinopyroxene do not necessarily reflect these characteristics in the metasomatic agent and that cpx/basaltD cannot confidently be used to infer the geochemical nature of equilibrium melts under upper mantle conditions. Moreover, we provide evidence that the strong REE-enrichment and Zr,Ti-depletion of some clinopyroxene from La Palma xenoliths is a feature commonly observed in clinopyroxenes from various mantle occurrences often interpreted as reflecting chemical interaction with metasomatic fluids and melts of different composition (either alkaline or carbonatitic) and provenance (from either the enriched asthenospheric mantle or the subducted slab). Therefore, claims of carbonatite metasomatism as opposed to silicate melt infiltration based on trace element signatures of investigated clinopyroxene should be regarded with caution unless the role of liquid composition/structure and of crystal chemical control is investigated.

Infiltration metasomatism at Lherz as monitored by systematic ion-microprobe investigations close to a hornblendite vein

Abstract Systematic investigations (electron- and ion-microprobe, and X-ray structure-refinement) of pyroxenes and amphiboles close to a cm-wide hornblendite vein have been carried out on a composite peridotite sample from the Lherz massif with the aim of constraining the processes of melt infiltration in the subcontinental mantle. Vein amphiboles have light rare earth element (LREE)-enriched patterns with the maximum at Nd and Sm, and positive Ba, Nb, Sr, and Ti anomalies. The liquid flowing in the vein had an alkaline geochemical signature (i.e., large-ion lithophile element (LILE)-enriched without significant high field strength element (HFSE) anomalies). Modal metasomatism, represented by crystallization of amphibole and consumption of clinopyroxene, occurred in a 2-cm thick layer of wall-rock. Newly formed amphiboles, i.e., those in the vein and in the modally metasomatised layer, have a lower [6] Al disorder than the pre-existing disseminated amphiboles, which formed during an older metasomatic event. Cryptic metasomatism is recorded beyond the modally altered layer in the cores of clinopyroxenes and in amphiboles. Moving away from the vein towards the peridotite, amphiboles and pyroxenes display systematic compositional variations, and approach equilibrium with the wall-rock within a few cm. Porphyroclastic clinopyroxenes record chromatographic separation of elements: less incompatible elements reach equilibrium with the wall-rock lherzolite at shorter distances than the more incompatible ones. The inverse correlation between the depth of the metasomatic effect and partition coefficients in porphyroclastic clinopyroxenes indicates that different concentration fronts developed for different elements during liquid/rock interaction, suggesting that liquid percolation along grain boundaries was responsible for clinopyroxene metasomatism. As in the clinopyroxene, abundances of incompatible elements in the amphibole decrease in the peridotite with distance from the vein. However, a less marked separation of elements is recorded by the amphiboles. Elements that are markedly more compatible in amphibole than in other minerals of a lherzolite, such as K, Ba, and Rb, have abrupt gradients which are confined to the modally-metasomatised region. The lack of metasomatic enrichment in these elements in the farther, disseminated amphiboles, indicates that amphibole crystallization was synchronous with melt infiltration and acted as a buffer for such elements. Micro-analytical investigations indicate that the geochemical effects associated with small-scale liquid infiltration can be successfully described by models which take into account both the chromatographic fractionation of elements and the influence of mineral-assemblage variations.