ALBERT

Notes: We studied interaction of molten silicon at the meniscus surface with CO and CO2 gas added to an inert argon atmosphere during the growth of edge-defined film-fed growth polycrystalline silicon nonagons. Both gases caused a significant increase in carbon content of the nonagons. However, most carbon remained trapped in the layer close to the surface and made a SiC-like contribution to the infrared spectrum. In addition, CO2 gas caused enhanced surface oxidation (with respect to CO gas).

Notes: Abstract Mg-gabbros from East Ligurian ophiolites (Northern Apennines, Italy) display a high-temperature/low-pressure recrystallization localized along ductile shear zones. In deformed gabbros, the igneous diopside is recrystallized into granoblastic aggregates of neoblastic diopside and minor red-brown amphibole. The latter displays a pargasitic composition, with high amounts of AlIV Na(A) and Ti (± 1.8, 0.7 and 0.4 atoms per formula unit, respectively). Major element composition of neoblastic minerals highlight equilibration temperature conditions in the range 800–950° C. Red-brown Ti-pargasite also occurs as a minor interstitial constituent, presumably growing from a residual trapped liquid, in the differentiated lithologies (Fe-Ti-diorites) of the plutonic ophiolitic complex.By means of ion microprobe (SIMS technique), rare earth (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) and selected trace elements (Sr, Y, Cr, V, Sc, Zr, Ti) have been analysed in igneous and neoblastic diopside, as well as in Ti-pargasites. Ti-pargasites have also been analysed for F and Cl, and compared with the halogen composition of the amphiboles, mainly hornblendes to actinolites, which are related to the subsequent low-temperature brittle evolution.Neoblastic Ti-pargasite from deformed Mg-gabbros bears close compositional similarities with igneous Ti-pargasite from undeformed Fe-Ti-diorites, whereas it is geochemically distinct from the amphiboles post-dating the ductile event. In particular, Ti-pargasites have relatively high contents of F, REE, Y, Zr and Sr, which are not consistent with crystallization in the presence of seawater-derived hydrothermal fluids.High-grade recrystallization probably developed in the presence of volatile-rich igneous fluids, either trapped between the cumulus minerals or injected into the ductile shear zones from outside the local system. An alternative hypothesis comprises the absence of fluid phase and the development of ductile shear zones in Ti-pargasite-rich layers. The petrological features of deformed gabbros recovered from present-day slow-spreading ridges and fracture zones bear close similarities with the investigated ophiolitic metagabbros. In East Ligurian ophiolites, high-grade ductile shear zones have been related to the initial stages of the uplift of the gabbro-peridotite complex to the sea-floor.

Notes: The Bouvet Triple Junction (BTJ) region in the South Atlantic, where the African, South American and Antarctica plates meet, is affected by several topographic/melting anomalies. Causes of these anomalies were investigated through a study of mantle-derived serpentinized peridotites sampled from three sites in the BTJ region: (1) the Inner Corner High at the intersection of the America Antarctic Ridge (AAR) with the Conrad transform; (2) the south wall of the Bouvet transform (South West Indian Ridge, SWIR); and (3) the eastern Bouvet SWIR Transform Intersection. The degree of melting undergone by these rocks was estimated from relic mineral major- and trace-element composition. Geochemical profiles from residual peridotites and associated basalts show a 〉 1000-km-wide melting anomaly centred on the Bouvet and Spiess topographic anomalies.

Notes: Different doses of iron ions were implanted in Czochralski grown single-crystal silicon samples and subsequently annealed at 1000 °C for 2 h in a dry nitrogen atmosphere. The behavior of the implanted iron and of oxygen already present in the material was monitored. It was found that the existence of the remaining structural disorder after the annealing treatment plays a dominant role in iron and oxygen segregation into the disordered region. This confirms the theory which predicts that the structural disorder and related strain fields are dominant mechanisms for gettering of metallic ions.

Notes: Rare earth elements were determined by secondary ion mass spectrometry for 13 silicate rock samples, ranging from ultramafic-mafic to acidic compositions, 2 feldspar and 1 biotite separates. As a whole, the investigated samples are characterized by matrices and rare earth elements spectra covering most geological applications. The present data are compared with reference values. The advantage of using secondary ion mass spectrometry as a fundamental tool for trace element detection in bulk samples in the few ppm-ppb region is demonstrated.

Notes: Summary The 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.

Notes: Abstract  This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20  °C) and similar volatile abundances. The H2O, S and Cl contents are relatively high, whereas magmatic CO2 concentrations are very low, probably due to CO2 loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K2O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs.

Notes: Abstract 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.

Notes: Abstract Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clinopyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a epx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i.e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly fractionated LREE (CeN/SmN∼0.5) and HREE (GdN/ YbN∼1.5) patterns and large variations in the REE composition (up to 6 times from microphenocryst cores to interstitial clinopyroxenes). Interstitial clinopyroxenes in IL basalts are similar to the microphenocrysts from the most primitive EL basalts. By contrast, IL microphenocrysts are characterized by greater LREE (CeN/SmN ∼0.3) and lesser HREE (GdN/YbN〈1.2) fractionation. The comparison of trace element variations in wholerocks and clinopyroxenes clearly shows that the olivine and plagioclase portion of the fractionation sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does not provide reliable chemical information to constrain the composition and the generation of the parental magmas. Unfortunately, most clinopyroxenes are characterized by complex zoning, probably caused by disequilibrium partitioning during crystal growth as a result of kinetic factors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core compositions of microphenocrystic clinopyroxenes. Modelling of (Nd/Yb)N and Ti/Zr in the parental magmas, as deduced from the clinopyroxene compositions, indicates thata EL and IL basalts do not represent products of different mantle source composition. Rather, they were generated by varying degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyroxene with REE patterns similar to those shown by IL suboceanic type peridotites. Accordingly, these latter are interpreted as refractory residua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual clinopyroxenes from the EL subcontinental type peridotites are not consistent with low degrees of fractional melting in agreement with the current interpretation that EL peridotites are unrelated to the MORB magmatism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudation mechanism during the early stages of oceanic rifting.