Sonia Tonarini

Precise boron isotopic analysis of complex silicate (rock) samples using alkali carbonate fusion and ion-exchange separation

We describe a technique for determining the boron isotopic composition of low-B silicate rocks which until recently have been difficult to analyze. The method involves an alkali carbonate fusion for sample dissolution, ion-exchange chromatography for boron purification, and PTIMS analysis using Cs2BO2+. Our procedure combines several previously published methods in a unique manner to simultaneously (1) achieve state-of-the-art precision and accuracy, and (2) expedite analysis with respect to competing methods. Because isotopic fractionation corrections typically are based on concurrent analyses of the SRM-951 boric acid reference standard, these data do not provide a true measure of analytical accuracy or interlaboratory biases. In this study, analytical precision and accuracy were evaluated independently through replicate analyses of the JB-2 basalt reference standard, with completely independent chemical extractions of boron in each case. The average corrected sol11B10B ratio obtained is 4.0728 ± 0.0019 (2 σ external uncertainty), which corresponds to a δ11B value of +7.23 ± 0.47‰ This result agrees closely with data obtained by Nakamura et al. [Nakamura, E., Ishikawa, T., Birck, J.L., Allegre, C., 1992. Precise boron isotopic analysis of natural rock samples using a boron-mannitol complex. Chem. Geol. 94, 193–204.] using the HF digestion method. We strongly recommend that JB-2 be analyzed routinely in future boron isotopic studies to facilitate assessment of analytical uncertainties and interlaboratory biases.

The magmatic evolution of the late Miocene laccolith–pluton–dyke granitic complex of Elba Island, Italy

Since late Miocene time, post-collisional extension of the internal parts of the Apennine orogenic belt has led to the opening of the Tyrrhenian basin. Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Italian mainland during this time, building up the Tuscan Magmatic Province as the fold belt was progressively thinned, heated and intruded by mafic magmas. An intrusive complex was progressively built on western Elba Island by emplacement, within a stack of nappes, of multiple, shallow-level porphyritic laccoliths, a major pluton, and a final dyke swarm, all within the span from about 8 to 6.8 Ma. New geochemical and Sr–Nd isotopic investigations constrain the compositions of materials involved in the genesis of the magmas of Elba Island compared to the whole Tuscan Magmatic Province. Several distinct magma sources, in both the crust and mantle, have been identified as contributing to the Elba magmatism as it evolved from crust-, to hybrid-, to mantle-dominated. However, a restricted number of components, geochemically similar to mafic K-andesites of the Island of Capraia and crustal melts like the Cotoncello dyke at Elba, are sufficient to account for the generation by melt hybridization of the most voluminous magmas ( c. e Nd (t) −8.5, 87 Sr/ 86 Sr 0.715). Unusual magmas were emplaced at the beginning and end of the igneous activity, without contributing to the generation of these hybrid magmas. These are represented by early peraluminous melts of a different crustal origin (e Nd (t) between −9.5 and −10.0, 87 Sr/ 86 Sr variable between 0.7115 and 0.7146), and late mantle-derived magma strongly enriched in incompatible elements (e Nd (t) = −7.0, 87 Sr/ 86 Sr = 0.7114) with geochemical–isotopic characteristics intermediate between contemporaneous Capraia K-andesites and later lamproites from the Tuscan Magmatic Province. Magmas not involved in the generation of the main hybrid products are not volumetrically significant, but their occurrence emphasizes the highly variable nature of crust and mantle sources that can be activated in a short time span during post-collisional magmatism.

Subduction-like fluids in the genesis of Mt. Etna magmas: evidence from boron isotopes and fluid mobile elements

Abstract New whole-rock B, Sr, Nd isotope ratios and 87Sr/86Sr on clinopyroxenes have been collected to study the enrichment of fluid mobile elements (FMEs) observed in Mt. Etna volcanics. Etna volcano, one of the most active in the world, is located in an extremely complex tectonic context at the boundary between colliding African and European plates. The analytical work focuses on current (1974–1998) and historic (1851–1971) eruptive activity, including some key prehistoric lavas, in order to interpret the secular shift of its geochemical signature to more alkaline compositions. Boron is used as a tool to unravel the role of fluids in the genesis of magmas, revealing far-reaching consequences, beyond the case study of Mt. Etna. Small variations are observed in δ11B (−3.5 to −8.0‰), 87Sr/86Sr (0.70323–0.70370), and 143Nd/144Nd (0.51293–0.51287). Moreover, temporal evolution to higher δ11B and 87Sr/86Sr, and to lower 143Nd/144Nd, is observed in the current activity, defining a regular trend. Sr isotopic equilibrium between whole-rock and clinopyroxene pairs indicates the successive introduction of three distinct magma types into the Etna plumbing system over time; these are characterized by differing degrees of FME enrichment. In addition, certain lavas exhibit evidence for country rock assimilation, magma–fluid interaction, or magma mixing in the shallow feeding system; at times these processes apparently lowered magmatic δ11B and/or induced Sr isotopic disequilibrium between whole rock and clinopyroxene. The regular increase of δ11B values is correlated with Nb/FME and 87Sr/86Sr ratios; these correlations are consistent with simple mixing between the mantle source and aqueous fluids derived from nearby Ionian slab. The best fit of Mt. Etna data is obtained using an enriched-MORB mantle source and a fluid phase with δ11B of about −2‰ and 87Sr/86Sr of 0.708. We argue that the slab window generated by differential roll-back of subducting Ionian lithosphere (with respect to Sicily) allows the upwelling of asthenosphere from below the subduction system and provides a suitable path of rise for subduction-related fluids. The increasing geochemical signature of fluid mobile elements enrichment to Mt. Etna lavas is considered a consequence of the progressive opening of slab window through time.

Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle

Serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fluid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron isotopes are established markers of fluid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle 11 B-depleted fluids, which cannot explain 11 B enrichment of many arcs. In contrast to these crustal reservoirs, we document high δ 11 B values retained in subduction-zone Alpine serpentinites. No 11 B fractionation occurs in these rocks with progressive burial: the released 11 B-rich fluids uniquely explain the elevated δ 11 B of arc magmas. B, O-H, and Sr isotope systems indicate that serpentinization was driven by slab fluids that infiltrated the slab-mantle interface early in the subduction history.

GEOCHEMICAL AND ISOTOPIC STRUCTURE OF THE EARLY PALAEOZOIC ACTIVE MARGIN OF GONDWANA IN NORTHERN VICTORIA LAND, ANTARCTICA

Abstract The regional distribution of geochemical and isotopic compositions of granitoid rocks from a Gondwana continental margin is studied to highlight its structure and geodynamic evolution. The intrusive rocks emplaced during the early Palaeozoic Ross Orogeny in northern Victoria Land (Antarctica) constitute a high-K calc-alkaline association. The geographic patterns of isotope and geochemical data on granitoid rocks allow the distinction of two portions of the continental margin, separated by a sharp discontinuity. The portion towards the palaeo-Pacific Ocean (Oceanward Side) displays strongly regular inland increase of Sr- and decrease of Nd-isotope ratios, coupled with analogous variations in major and trace elements; on this basis we infer a NW-SE-trending margin affected by SW-directed subduction. The portion towards the East Antarctic Craton (Continentward Side) shows a similar regular variation only for Nd isotope compositions, consistent with a hypothesis of a N-S-striking margin with west-ward subduction. In the Oceanward Side, isotope and trace-element characteristics suggest that the granites were generated by extensive interaction of mantle-derived magmas with high-level crustal melts. The origin of Continentward Side intrusives is compatible with a process of interaction between mantle-derived melts and a mafic granulite lower crust. The granitoids of the two crustal sectors share the same Proterozoic Sm-Nd model ages, suggesting that they both belong to the same crustal province. We interpret this arrangement of crustal segments as due to the shift and rotation of a forearc sliver of the Gondwana margin. This movement was likely enhanced by oblique subduction under an irregular margin weakened by the presence of a magmatic arc.

Two-stage growth of laccoliths at Elba Island, Italy

At Elba Island, Italy, nine shallow-level late Miocene granite porphyry layers connected by feeder dikes built up three nested Christmas-tree laccoliths. Detailed mapping and reconstruction of tectonic history led to restoration of the original 5-km-thick sequence and determination of the dimensional parameters of each intrusive layer. The laccolith layers were emplaced at depths between 1.9 and 3.7 km, exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes. The intrusive layers are 50–700 m thick, with diameters between 1.6 and 10 km. Length to thickness relationships for individual laccolith layers show a power-law correlation that does not fit the known dimensional distribution for laccoliths, but instead fits a line with a slope typical of the theoretical vertical-inflation stage of laccolith development. This is interpreted as the first reported natural example of the occurrence of a vertical-inflation stage during laccolith growth. The dimensional data for Elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet-like bodies, while multilayer laccoliths form when coalescence fails, possibly owing to the large availability of crustal magma traps.

B/Nb and δ11B systematics in the Phlegrean Volcanic District, Italy

Abstract Boron concentration and isotopic compositions were determined in representative products from the Phlegrean Volcanic District (PVD), which includes Campi Flegrei (CF), and the islands of Procida and Ischia. The most primitive products (from Procida and Ischia islands) are moderately enriched in B (4.6–12 ppm), whereas more evolved products of CF and Ischia Island are more enriched (21–118 ppm). The content of B is positively correlated with Nb, Zr, Th, La and, generally, with all incompatible elements. δ11B values are generally lower and more uniform in samples from CF (−6.8 to − 10.6‰) compared with those from Procida (−3.6 to −8.5‰) and Ischia (−2.8 to −8.4‰) islands. Overall, B-enrichments relative to fluid-immobile elements of PVD are only slightly higher than those observed in mid-ocean ridge basalts and there is no significant correlation between δ11B and B/immobile element ratios. For CF samples including the Campanian Ignimbrite (CI) and most post-CI products, δ11B is negatively correlated with 87Sr/86Sr, with extreme compositions represented by evolved products of the CI (higher δ11B, lower 87Sr/86Sr) and less evolved shoshonite (lower δ11B, higher 87Sr/86Sr). In contrast, samples from Procida, Ischia, and pre-CI products and one young shoshonite from CF define a distinct correlation with primitive trachybasalt as the high δ11B, low 87Sr/86Sr end-member, and young shoshonites as the low δ11B, high 87Sr/86Sr end-member. The overall interpretation of the geochemical and isotopic data suggests three conclusions: (1) Geochemical and isotopic variations of post-CI products from CF can be explained by mixing/mingling between at least two distinct magmas, i.e. the CI and the young shoshonite (Minopoli 1). (2) The isotopic trend described by Procida trachybasalts, Ischia samples, pre-CI samples from CF and Pigna St. Nicola shoshonite is a mantle trend suggesting the involvement in their genesis of a distinct component depleted in B, with low δ11B and high 87Sr/86Sr. A similar component has been invoked to explain the differences between Stromboli potassic lavas and more typical calc-alkaline rocks of the Aeolian Islands. This component is consistent with ‘sediments or melts thereof’ introduced into the mantle by subduction, but depleted in B due to metamorphic reactions accompanying subduction. (3) The displacement from the mantle trend of CI and Minopoli 1 shoshonite magmas is probably due to crustal contamination processes.

Boron and oxygen isotope evidence for recycling of subducted components over the past 2.5 Gyr

Evidence for the deep recycling of surficial materials through the Earth’s mantle and their antiquity has long been sought to understand the role of subducting plates and plumes in mantle convection. Radiogenic isotope evidence for such recycling remains equivocal because the age and location of parent–daughter fractionation are not known. Conversely, while stable isotopes can provide irrefutable evidence for low-temperature fractionation, their range in most unaltered oceanic basalts is limited and the age of any variation is unconstrained. Here we show that δ18O ratios in basalts from the Azores are often lower than in pristine mantle. This, combined with increased Nb/B ratios and a large range in δ11B ratios, provides compelling evidence for the recycling of materials that had undergone fractionation near the Earth’s surface. Moreover, δ11B is negatively correlated with 187Os/188Os ratios, which extend to subchondritic values, constraining the age of the high Nb/B, 11B-enriched endmember to be more than 2.5 billion years (Gyr) old. We infer this component to be melt- and fluid-depleted lithospheric mantle from a subducted oceanic plate, whereas other Azores basalts contain a contribution from ∼3-Gyr-old melt-enriched basalt. We conclude that both components are most probably derived from an Archaean oceanic plate that was subducted, arguably into the deep mantle, where it was stored until thermal buoyancy caused it to rise beneath the Azores islands ∼3 Gyr later.

Petrology and isotope-geochemistry of San Vincenzo rhyolites (Tuscany, Italy)

Two groups of rhyolites have been recognized at San Vincenzo (Tuscany, Italy). Group A rhyolites are characterized by plagioclase, quartz, biotite, sanidine and cordierite mineral assemblages. They show constant MgO and variable CaO and Na2O contents. Initial87Sr/86Sr ratios in group A samples range between 0.71950 and 0.72535, whereas the Nd isotopic compositions are relatively constant (0.51215–0.51222). Group B rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals, and show textural, mineralogical and chemical evidence of interaction with more mafic magmas. The Sr and Nd isotopic ratios range between 0.71283–0.71542 and 0.51224–0.51227 respectively. Magmatic inclusions of variable size (1 mm to 10 cm) were found in groups B rhyolites. These inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite. They are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts. These magmatic inclusions have relatively high Sr contents (996–1529 ppm) and Sr and Nd isotope-ratios of 0.70807–0.70830 and 0.51245–0.51252 respectively.87Sr/87Sr data on minerals separated from both group A and B rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group A rhyolites and due to interaction of rhyolites with a mantle-de-rived magma in group B rhyolites. Isotopic data on whole rocks and minerals allow us to interpret the group A rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source; conversely, group B rhyolites underwent interactions with a mantle-derived magma. The crustal source as inferred from isotopic systematics would be characterized by87Sr/86Sr and143Nd/144Nd ratios close to 0.7194 and 0.51216 respectively. The sub-crustal magma would have Sr isotopic composition close to 0.7077 and a143Nd/144Nd ratio greater than or equal to 0.51252. These isotopic features are different from those reported for the parental magmas postulated for Vulsini and Alban Hills in the nearby Roman Magmatic Province, and are similar to those of the Vesuvius and Ischia magmas.

Geochemical and isotopic monitoring of Mt. Etna 1989-1993 eruptive activity: bearing on the shallow feeding system

Two flank eruptions took place at Mt. Etna during 1989–1993. The former (September–October 1989) produced lavas among the most primitive of this century while the latter (December 1991–March 1993) was unusually long lasting and the largest of the last three centuries. Alkali basalts of 1989 displayed initial enrichments of Rb and K accompanied by an higher value of radiogenic-Sr and isotopic disequilibrium between host-rock and clinopyroxene (0.70364 and 0.70356, respectively). Subsequently, within one week, K, Rb and 87Sr86Sr ratio decreased to constant values (87Sr86Sr = 0.70355). Nd isotopic ratio did not show significant variations. These features can be explained by a selective contamination process involving essentially Rb, K and radiogenic-Sr. The bulk from the 1991–1993 eruption hawaiites, were erupted between January and May 1992. In this period, lavas (here called JML) were characterised by uniform geochemical and isotopic compositions and Sr-isotopic equilibrium between whole rock and pyroxene (0.70355). In other periods (December 1991 and June 1992) we observed lower and variable Sr isotopic compositions coupled with disequilibria between lavas and their clinopyroxenes (0.70351 in the w.r. and 0.70345 in the cpx of the sample of 15 December 1991). Starting from June 1992, the products seem to reveal fractionation effects as suggested by the continuous rise in concentrations of incompatible elements. When the observed variability cannot be ascribed to fractionation effects, we are not able to discriminate mixing from selective contamination. However, lavas erupted during December 1991 evolved towards the isotope composition of JML that was never exceeded, moreover the regular behaviour of Rb, K and radiogenic-Sr of 1989 lavas was not observed during the 1991–1993 eruption. Thus, we prefer to interpret these variations as mainly due to mixing processes between an end-member of JML composition and another one having the same, or lower, isotopic composition of the clinopyroxene found in the sample of December 1991. Several petrogenetic processes are contemporaneously active at Mt. Etna; they encompass magma mixing, selective contamination and fractional crystallization. Their relative importance varies according to the original volume of each magma batch and to the status of the shallow feeding system. The complexity of these processes, acting in the short time span of a few months, claims caution about the geochemical representativity of a single lava sample for each eruption.