Luisa Ottolini

Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway

The buoyancy and strength of sub-continental lithospheric mantle is thought to protect the oldest continental crust (cratons) from destruction by plate tectonic processes. The exact origin of the lithosphere below cratons is controversial, but seems clearly to be a residue remaining after the extraction of large amounts of melt. Models to explain highly melt-depleted but garnet-bearing rock compositions require multi-stage processes with garnet and clinopyroxene possibly of secondary origin. Here we report on orogenic peridotites (fragments of cratonic mantle incorporated into the crust during continent-continent plate collision) from Otrøy, western Norway. We show that the peridotites underwent extensive melting during upwelling from depths of 350 kilometres or more, forming a garnet-bearing cratonic root in a single melting event. These peridotites appear to be the residue after Archaean aluminium depleted komatiite magmatism.

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.

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.

Determination of hydrogen in silicates by secondary ion mass spectrometry

A quantitative procedure for the determination of hydrogen in silicates was developed for incorporation in the general procedure used with the Cameca IMS 4f ion microprobe for light and trace elements. The procedure employs a static 16O– primary beam and the ‘energy filtering’ technique. Moreover, it allows the analysis of samples placed on separate mounts with respect to standards and, in particular, of mineral grains directly in thin sections of rock. Factors influencing the hydrogen background were studied in order to ascertain the appropriate analytical conditions and to develop a specific procedure for the determination of low hydrogen contents. A low and reproducible background was obtained with simple heating and de-gassing of the sample. The background value was equivalent to 0.007–0.015% H2O and its fluctuation over a 1 week working session indicated that the detection limit was ≈0.015% H2O. The results obtained confirm that the use of high-energy ions reduces the influence of the matrix composition on the ionization of hydrogen (relative to that of Si); effects related to the structure of the sample appear to be negligible. Residual matrix composition effects are related, to a first approximation, to the Si content of the sample. As such effects could not be completely removed, two separate ion yields were used for samples with SiO2 contents in the range 27–51% and for rhyolites (≈72% SiO2). The reproducibility of the measurements was typically ≈3% and the accuracy for the former group of samples was 15% for H2O contents >0.1% and 30% for H2O contents of ≈0.05%.

SIMS matrix effects in the analysis of light elements in silicate minerals: Comparison with SREF and EMPA data

Abstract Matrix effects in secondary-ion mass spectrometric (SIMS) analysis of light elements (H, Li, Be, B, and F) have been investigated in phenacite, kornerupine, danburite, axinite, spodumene, tourmaline, hambergite, and mica, all of which were epoxy-mounted in a known crystallographic orientation relative to the primary-ion beam. As reference chemical information, we used data from electron microprobe analysis (EMPA) and from single-crystal structure-refinement (SREF) on the same crystals used for SIMS. Quantification of secondary-ion intensities into concentrations was done using Si as the reference matrix element. The results indicate that matrix effects due to crystallographic orientation are <10% relative, or below analytical uncertainty for most analyzed elements. In dioctahedral mica, there is a difference in H/Si ion yield (IY) of ~25% relative when the crystal is analyzed parallel and orthogonal to the main cleavage (which is perpendicular to the c axis). The magnitude of this effect is significant and higher than our SIMS accuracy for H in micas: ±10% relative. Among the analyzed elements, Be is affected least by matrix effects, even when present as a major element. The most significant chemical effects on SIMS analysis of H, Li, F, and B in silicates seem to be related to the Fe (+Mn) content of the matrix: the light-element IY decreases as the Fe (+Mn) content increases, as previously seen in tourmaline, axinite, and kornerupine. Silicon and Al seem to have complementary and opposite effects on IY with respect to Fe and Mn. The agreement between SIMS and SREF is close for most light elements when they are present as major constituents. The results of our study also show that analytical problems are still present for B by EMPA, and this technique may not be adequate to measure B accurately in some minerals.

The relationship between potassic, calc-alkaline and Na-alkaline magmatism in South Italy volcanoes: A melt inclusion approach ☆

The present-day tectonic setting of the Tyrrhenian Sea is dominated by the eastward migration of the Tyrrhenian^ Appenines subduction system and the existence of a contemporaneous and parallel extensional^compressional regime. This complex setting leads to the occurrence of a wide spectrum of magma-types in the South Italy volcanoes. Here, major and trace-element data for primitive melt inclusions preserved in olivine phenocrysts have been obtained in order to add constraints on the origin of the calc-alkaline magmas from the Aeolian arc (Stromboli and Vulcano islands), the potassic magmas from the Campania Province (Vesuvius and Phlegraean Fields) and the Na-alkaline magmas from Ustica Island. The approach used to determine the possible mantle sources of the trapped melts for each population of melt inclusions is based on the determination of the trace-element incompatibility sequence taken as the relative order of increasing bulk partition coefficients, which depends on the mineralogy of the source and gives direct information about minerals residual at the time of melting. Compositional similarities between the melt inclusions and their host lavas suggest that shallow-level magma contamination did not contribute significantly to the geochemical characteristics of the magma-types encountered in the region. Results of the trace-element modelling indicate that the melt inclusions from the Aeolian Islands and Campania Province volcanoes originate from mantle sources strongly affected by subduction-related metasomatic processes. Trace-element relationships of melt inclusions from Vulcano and Stromboli reflect melting of peridotitic sources that have been enriched by a slab-derived, aqueous fluid formed during dehydratation of K-free phases at shallow to intermediate depths. The negative high-field strength elements (HFSE) anomalies of these inclusions were generated in the absence of any residual phase in which HFSE might be compatible. In addition, their major element characteristics require the involvement of a clinopyroxene-rich source component in their genesis. With regard to the Vesuvius and Phlegraean Fields melt inclusions, their calculated incompatibility sequences point to a common phlogopite-bearing mantle source likely to be the result of interaction and hybridisation reactions with K2O and H2O-rich phases released from the slab at larger depths. Finally, trace

Peridotite clinopyroxene chemistry reflects mantle processes rather than continental versus oceanic settings

Abstract Comparison of 360 trace element analyses of clinopyroxenes from peridotites indicates that clinopyroxene composition cannot discriminate between suboceanic and subcontinental mantle. Clinopyroxenes from abyssal and fore-arc peridotites have the lowest incompatible element concentrations and record melting and basalt extraction as the dominant process. Clinopyroxenes from continental peridotite massifs partially overlap the composition of clinopyroxenes from abyssal peridotites and, in general, behave as a less depleted component in the chemical variation trends defined by the latter. In the peridotite massifs, subsolidus re-equilibration involving phase transitions (spinel to plagioclase facies) may cause significant element variations in clinopyroxene (e.g. Sr and Eu/Eu * decrease, REE, Ti increase). Metasomatic processes cause only comparatively minor trace element variations. Clinopyroxenes from mantle xenoliths in OIB and continental alkali basalts have the highest incompatible trace element concentrations and the largest compositional variations. They primarily record metasomatic enrichment processes, which are similar in suboceanic and subcontinental mantle. These processes induced the highest incompatible element enrichment in the clinopyroxenes from the most refractory peridotites, supporting the chromatographic nature of infiltration metasomatism. This enrichment, however, does not affect Ti, which is in the same concentration range in clinopyroxenes from both xenoliths and abyssal peridotites. The apparent Ti immobility may be due to several causes, such as reduced Ti solubility in hydrous fluids, fractionation of Ti-rich phases from percolating silicate melts, reaction with carbonatite melts formerly equilibrated with amphibole-peridotite. In general, clinopyroxene geochemistry does not allow a clear distinction between different metasomatic agents. The similarity between the geochemical characteristics of xenoliths from continental and oceanic environments supports previous results that the compositions of the pre-metasomatic lithosphere and of the asthenosphere, from where metasomatic agents derive, do not differ in the two environments.

The relationship between websterite and peridotite in the Balmuccia peridotite massif (NW Italy) as revealed by trace element variations in clinopyroxene

Clinopyroxenes from peridotite, Cr-diopside and Al-augite websterites of the Balmuccia massif have been analyzed by ion-probe for REE, Sr, Zr, Ti, V, Y, Sc and Cr. In the peridotites, the REE pattern of clinopyroxene varies from markedly LREE depleted [(Ce/Yb)n=0.03] to moderately LREE enriched [(Ce/Yb)n=1.96]. Clinopyroxenes of the Cr-Diopside suite vary from moderately LREE depleted to enriched [(Ce/Yb)n=0.53–1.36], but HREE are always depleted with respect to MREE [(Sm/Yb)n=1.19–2.06]. The clinopyroxenes of the Al-Augite dikes have higher trace element concentrations and are relatively depeleted in LREE. Two types of peridotite to websterite transitions have been documented. One type characterizes the transitions from peridotite to Cr-diopside websterite. The depletion of peridotite increases, and cpx “fertility” decreases, towards the contact; i.e. REE from Sm-Eu to Yb and Ti, V, Y, Sc and Cr in cpx define abundance trends consistent with melt extraction. LREE, Sr, and perhaps Zr, on the contrary, increase in the contact region over a distance of 0–40 cm. The variation of the latter elements and of CaO and Al2O3 from peridotite to websterite is smooth, whereas it is steep for the other elements. Alumina, HREE, Y and Sc are lower, Ca is higher, and the MREE/HREE pattern is steeper in the clinopyroxene of the websterite with respect to peridotite. In the second type of transition, as the contact is approached, there is an increase in modal cpx and decrease of the opx/cpx ratio. Cpx trace element patterns are similar in peridotite and websterite. This transition is generally at the Al-augite websterite contacts, but also was found at two Cr-diopside contacts. The wall rock depletion of the first transition type is interpreted as caused by melt extraction induced by intrusion of melts derived from deeper mantle (garnet-facies), whereas the LREE and Sr variation is interpreted in terms of diffusive redistribution of trace elements after emplacement of the dikes and after the modal clinopyroxene has been removed from the peridotite. Diffusive exchange occurred under stagnant conditions but in the presence of a partial melt or a “metamorphic fluid”, rather than under dry subsolidus conditions. There is no conclusive evidence favouring either a partial melt or an aqueous fluid as a medium for diffusive transport. However, we favour transport via a fluid phase because the presence of such a fluid is indicated by ubiquitous traces of amphibole in the peridotite. The second transition type is interpreted in terms of melt infiltration and re-equilibration of the melt with the peridotite matrix.

Geochemical and structural evolution of micas in the Rožná and Dobrá Voda pegmatites, Czech Republic

SummaryThe chemistry, structural parameters, polytypism, optical properties and Rb-Sr isotopes were examined in 11 to 60 samples of biotite, muscovite and lepidolite from the pegmatites at Rožná (the type locality of lepidolite; 323 ± 4Ma) and Dobrá Voda (306 ± 9Ma) in western Moravia. At both localities, early endocontact biotite is followed inwards by muscovite and lepidolite, which is concentrated in and around the core. At Rožná, a 1M lepidolite follows after 2M1 muscovite but all later generations of lepidolite are 2M2, close to Tri50 Ply50 and in part associated with muscovite 2M1. At Dobrá Voda, all lepidolite types are 1M and free of muscovite, and the late varieties approximate Tri30 Ply70. At both localities, a trend of increasing μHF is indicated during the progress of mica crystallization, culminating in precipitation of topaz. Polytypism of lepidolite is not correlatable with any compositional or growth feature, or their combination. Throughout the mica crystallization, Rb/Cs decreases but K/Rb becomes reversed after an initial decrease. Boron is partitioned preferentially into muscovite (up to 1.10 wt.% B2O3) but Be, Zn, Mn and Sc are enhanced in lepidolite. A slight increase in Fe, Ba and Cl in the last generation of lepidolite might be possibly due to mixing of residual pegmatite fluids with metamorphic pore solutions.ZusammenfassungIn 11 bis 60 Proben von Biotit, Muskovit und Lepidolith aus den Pegmatiten von Rožná (Typlokalität des Lepidoliths; 323 ± 9 Ma) in Westmähren wurden Chemie, Struktur-parameter, Polytypie, optische Eigenschaften und Rb-Sr-Isotopie untersucht. An beiden Lokalitäten wird früher Biotit an Endokontakten nach Innen von Muskovit und Lepidolith gefolgt, letzterer ist in und um den Kern konzentriert. In Rožná folgt 1M-Lepidolith auf 2M1-Muskovit, aber alle späteren Lepidolithgenerationen sind 2M2, nahe Tri50Ply50 und zum Teil mit 2M1-Muskovit vergesellschaftet. In Dobrá Voda sind alle Lepidolithe vom Typ 1M und frei von Muskovit, die späten Varietäten kommen Tri50Ply50 nahe. An beiden Lokalitäten ist während des Fortschreitens der Glimmerkristallisation eine Tendenz von steigendem μHF angezeigt, die in der Ausfällung von Topas ihren Höhepunkt findet. Die Polytypie des Lepidoliths kann nicht mit irgendeiner Eigenheit der Zusammensetzung oder des Wachstums korreliert worden, auch nicht mit einer Kombination von diesen. Während der ganzen Glimmerkristallisation nimmt Rb/Cs ab, aber die Tendenz von K/Rb ändert sich nach anfänglichem Abfall. Das Bor verteilt sich bevorzugt auf den Muskovit (bis zu 1.10 Gew. -% B2O3), aber die Be-, Zn-, Mn- und Sc-Gehalte sind im Lepidolith erhöht. Ein leichter Ansteig von Fe, Ba und Cl in der letzten Lepidolithgeneration könnte vielleicht durch eine Mischung von pegmatitischen Restlösungen mit metamorphen Porenlösungen verursacht sein.

Piston-cylinder experiments on H2O undersaturated Fe-bearing systems: An experimental setup approaching fO2 conditions of natural calc-alkaline magmas

Abstract In this study, we present a modified double-capsule technique to perform experiments on H2O undersaturated, Fe-bearing systems at elevated pressures and temperatures and oxygen fugacities (fO₂) relevant for natural calc-alkaline magmas. Welded shut, Fe-preconditioned Au90Pd10 capsules were placed in an outer Pt capsule that contains the same starting material. Experiments were performed at 1.0 GPa and 1200 °C using a synthetic, hydrous basalt and run with either boron nitride (BN) or MgO surrounding the welded capsules. Optimum results were obtained by using Fe-preconditioned Au90Pd10 inner capsules in combination with MgO assemblies. The application of the modified doublecapsule technique with Fe-preconditioned inner AuPd capsules reduced Fe loss to less than 3% relative, conserved H2O within the error of ion-microprobe analyses, and kept the fO₂ (QFM+1.1) within 1 log unit of the initial value constrained by the Fe2O3/FeO ratio of the starting material (QFM+0.43). These conditions are similar to estimates of fO₂ during the crystallization of natural calc-alkaline magmas.