Francesca Castorina

Late Quaternary sea-level changes along the Tyrrhenian coast near Orbetello (Tuscany, central Italy): Palaeoenvironmental reconstruction using ostracods

Abstract A multidisciplinary study has been carried out on a 51 m long core, drilled on coastal lowlands near the Albegna River (Tuscany, Central Italy). Both sedimentological and faunal analyses (ostracods and molluscs) indicate a coastal lagoon environment repeatedly influenced both by the rising Holocene sea and the Albegna River alluvial inflow. δ13C, δ18O, Mg/Ca, Sr/Ca and 87Sr/86Sr analyses were performed on Cyprideis torosa shells in order to detail the influence of Albegna River freshwater on the coastal lagoon water. The analytical data illustrate the difficulties of using geochemical proxies in marginal marine settings. Four radiocarbon dates provide a chronostratigraphy for the early Holocene part of the core. The data are used to reconstruct Late Pleistocene to early Holocene palaeoenvironments along this part of the coast and show the development of freshwater environments followed by a coastal brackish lagoon formed in response to the initial Holocene rapid rise in relative sea level. An attempt is made to relate sediments and age data from the core to the relative sea-level curve for the Tyrrhenian Sea.

Is there a mantle plume below Italy

Some of the most diverse igneous rocks found on Earth occur along the length of Italy and in many of the islands in the southeastern Tyrrhenian Sea, all the result of Cenozoic magmatism. Magmas extremely rich in alkalis, particularly potassium, and many undersaturated with respect to silica, were erupted, as well as others of calc-alkalic affinity (see legend in Figure 1). Their origin has been the subject of heated debate, and there is still no general consensus about how they formed. Most attribute them to subduction-related processes (see Beccaluva et al. [2004] for a review); others consider them to be the result of within-plate magmatism [e.g., Vollmer, 1976; Lauecchia and Stoppa, 1996]. Still others consider magmatism the result of a deep, mantle upwelling within a slab window coupled with mixing between isotopically different reservoirs [Gasperini et al., 2002].

A Hercynian suture zone in Sardinia: geological and geochemical evidence

AbstractThe Hereynian basement of Northern Sardinia consists of two main teetonometamorphic complexes : the Southern Paleozoic Low to Medium Grade Metamorphic Complex (LMGMC) and the Northern (Precaimbrian ?) High Grade Metamorphic Complex (HGMC). These complexes are separateli bu the “Posada-Asinara Line” which is a several kilometer-wide mylonitie belt consisting of micaschist. parag-neiss and quartzite that cuts across all Northern Sardinia and contains scattered hodies of amphibolites with granulile and eclogite facies relies.Major element composition of ampbibolites indicates they were derived from metamorphosed basalts of tholciitie affinity. REE and the relative abundances of HKS elements strongly suggest an oceanic basaltie prololith (N- and T-typc MORB). A whole-rock Sm-Nd isochron for the amphibolite protoliths yields an age of ea. 960 Ma.The Northern Sardinia basement can he interpreted as a continental collisional zone with the “Posada-Asinara Line” representing a suture zone squeezed between ...

An enriched mantle source for Italy’s melilitite-carbonatite association as inferred by its Nd-Sr isotope signature

Abstract New Sr-Nd isotope data were obtained from Late Pleistocene carbonatite-kamafugite associations from the Umbria-Latium Ultra-Alkaline District of Italy (ULUD) with the aim of constraining their origin and possible mantle source(s). This is relevant to the origin and evolution of ultrapotassic (K/Na ≫ 2) and associated rocks generally, notably the occurrences from Ugandan kamafugites,Western Australian lamproites and South African orangeites. The selected ULUD samples yielded 87Sr/86Sr and 143Nd/144Nd ranging from 0.7100 to 0.7112 and from 0.5119 to 0.5121 respectively, similar to cratonic potassic volcanic rocks with higher Rb/Sr and lower Sm/Nd ratios than Bulk Earth. Silicate and carbonate fractions separated from melilitite are in isotopic equilibrium, supporting the view that they are cogenetic. The ULUD carbonatites yielded the highest radiogenic Sr so far reported for carbonatites. In contrast, sedimentary limestones from ULUD basement formations are lower in radiogenic Sr, i.e. 87Sr/86Sr = 0.70745−0.70735. The variation trend of ULUD isotopic compositions is similar to that reported for Ugandan kamafugites and Western Australian lamproites and overlaps the values for South African orangeites in the ƐSr-ƐNd diagram. A poor correlation between Sr/Nd and 87Sr/86Sr ratios in ULUD rocks is inconsistent with a mantle source generated by subduction-driven processes, while the negligible Sr and LREE in sedimentary limestones from the ULUD region fail to account for a hypothetical limestone assimilation process. The Nd model ages of 1.5−1.9 Ga have been inferred for a possible metasomatic event, allowing further radiogenic evolution of the source, a process which may have occurred in isolation until eruption time. While the origin of this component remains speculative, the Sr-Nd isotope trend is consistent with a simple mixing process involving an OIB-type mantle and a component with low ƐNd and high ƐSr.

Geochemical characteristics of Cretaceous carbonatites from Angola

Abstract The Early Cretaceous (138–130 Ma) carbonatites and associated alkaline rocks of Angola belong to the Parana-Angola-Etendeka Province and occur as ring complexes and other central-type intrusions along northeast trending tectonic lineaments, parallel to the trend of coeval Namibian alkaline complexes. Most of the Angolan carbonatite-alkaline bodies are located along the apical part of the Mocamedes Arch, a structure representing the African counterpart of the Ponta Grossa Arch in southern Brazil, where several alkaline-carbonatite complexes were also emplaced in the Early Cretaceous. Geochemical and isotopic (C, 0, Sr and Nd) characteristics determined for five carbonatitic occurrences indicate that: (1) the overall geochemical composition, including the OC isotopes, is within the range of the Early and Late Cretaceous Brazilian occurrences from the Parana Basin; (2) the La versus La Yb relationships are consistent with the exsolution of COi2-rich melts from trachyphonolitic magmas; and (3) the 143 Nd 144 Nd and 87 Sr 86 Sr initial ratios are similar to the initial isotopic ratios (129 Ma) of alkaline complexes in northwest Namibia. In contrast, the Lupongola carbonatites have a distinctly different 143 Nd 144 Nd initial ratio, suggesting a different source. The Angolan carbonatites have SrNd isotopic compositions ranging from bulk earth to time-integrated depleted sources. Since those from eastern Paraguay (at the western fringe of the Parana-Angola-Etendeka Province) and Brazil appear to be related to mantle-derived melts with time-integrated enriched or B.E. isotopic characteristics, it is concluded that the carbonatites of the Parana-Angola-Etendeka Province have compositionally distinct mantle sources. Such mantle heterogeneity is attributed to ‘metasomatic processes’, which would have occurred at ca 0.6–0.7 Ga (Angola, northwest Namibia and Brazil) and ca 1.8 Ga (eastern Paraguay), as suggested by Nd-model ages.

Calcite in fractures in a volcanic environment (Vulcano Island, Italy): contribution of geochronological and isotopic studies to volcanotectonics

Abstract The southern and southeastern rim of the present caldera (La Fossa caldera) at Vulcano Island (Aeolian Islands, Italy) is crosscut by a network of fractures filled with calcite and minor chalcedony. Fluid inclusion studies indicate that both mineral phases were deposited from hydrothermal solutions. Several calcite samples were analyzed and their U, Sr, Nd, O and C isotopic composition was determined. U Th geochronological data suggest two discrete depositional episodes: one at 50 ka (southern rim) and the other at 25 ka (southeastern rim). Oxygen isotopic compositions, 87 Sr 86 Sr and 234 U 238 U ratios exclude that seawater was involved significantly in the hydrothermal system, in agreement with fluid inclusion studies which did not document the presence of NaCl. The Nd and Sr isotopic composition of calcite reflects the composition of the host rocks. The oxygen isotopic composition excludes a post-depositional interaction with meteoric water and suggests a depositional temperature consistent with that obtained from the fluid inclusion study (160–170 °C), assuming equilibrium conditions and a fluid isotopic composition close to present-day steam emissions. The positive correlation between U content and C isotopic composition indicates a rapid change in CO 2 pressure during calcite deposition. This observation is consistent with a rapid fluid ascent, likely connected to two phases of collapse of the La Fossa caldera. The ages of the calcites which fill the fractures at the La Fossa caldera indicate the activation time of the fracture systems which, in turn, are connected to the above mentioned phases of the caldera collapse.

The role of fractional crystallisation, crustal melting and magma mixing in the petrogenesis of rhyolites and mafic inclusion-bearing dacites from the Monte Arci volcanic complex (Sardinia, Italy)

Abstract Rhyolitic lavas and mafic inclusion-bearing dacites (MIBD) form the dominant products of the Monte Arci volcanic complex, one of the most active sites of volcanic activity in Sardinia during the Pliocene. The massif is composed of four distinct eruptive episodes (Phase 1: rhyolites; Phase 2: dacites and andesites; Phase 3: quartz-normative trachytes; Phase 4: mafic lavas ranging from subalkaline to mildly alkaline). Monte Arci magmatism has been characterized by open-system behaviour, with both mantle and crustal contributions and magma mixing. Although the mafic products are restricted to the latest stage of activity, mafic inclusions are quite common in many rhyolites and dacites. The mineral assemblages of the inclusions are dominated by plagioclase + orthopyroxene ± augite, with minor olivine, FeTi oxides and variable amounts of residual trapped liquid giving rise to a fine-grained groundmass. They represent blobs of magma entrained in a partly molten state and provide evidence of a basaltic contribution to the petrogenesis of their enclosing lavas, both as parental magmas or as a source of heat for partial melting of crustal rocks. Rhyolites are metaluminous to slightly peraluminous rocks with a wide range of SiO2 contents (67–75%). They fall into two groups: (1) Mafic inclusion-bearing rhyolites (MIBR). These have initial 87 Sr 86 Sr ratios between 0.70526 and 0.70897 and commonly show relatively high values of Ti, Mg, Fe, Ca, Sr and compatible trace elements (Ni, Cr, V); cordierite-bearing, highly restitic, crustal xenoliths are rare. (2) Inclusion-free rhyolites (IFR). They include both porphyritic lavas (IFR1) and obsidians (IFR2, IFR3), which share low values of Ti, Fe, Mg, Ca, Sr and are strongly depleted in compatible trace elements. IFR1–2 have a marked LREE/HREE fractionation and a narrow range of 87 Sr 86 Sr ratios (0.70885–0.70972), whereas IFR3 have larger Sr isotopic ratios (0.71529–0.71553), low Th, Zr, Hf, LREE contents and low LREE/HREE. Nd isotopic composition is quite uniform for all rhyolite types, with relatively low 143 Nd 144 Nd ratios (0.51216–0.51228, ϵNd ranging between −9.3 and −6.9). MIBR appear to be the product of fractionation of subalkaline (tholeiitic) magmas + assimilation of a crustal component moderately enriched in 87Sr and/or with low Sr content. IFR1–2 may be derived from partial melting of moderately 87Sr-rich crustal lithologies with retention of garnet and plagioclase (+K-feldspar) in the restite, although AFC processes (followed by effective separation from the basalts) cannot be excluded. Obsidians IFR3 need a crustal source with higher 87 Sr 86 Sr ; their peculiar geochemical features most probably reflect the complex behaviour during partial melting of accessory minerals carrying REE, Th, Zr, Hf. Among the eruptive products of Phase 2, mafic inclusion-bearing dacites (MIBD) provide evidence of basalt-rhyolite mixing: (a) coexistence of glasses with SiO2 content ranging from 64 to 72 wt.%; (b) a phenocryst assemblage including phases of basaltic and rhyolitic provenance; and (c) abundant crystal-rich inclusions related to a basaltic intrusion. The extrusion of these lavas has been preceeded by an explosive eruption of rhyolitic tuffs ( 87 Sr 86 Sr initial ratio =0.71097 ). The most satisfactory petrogenetic model for MIBD includes: (1) differentiation of subalkaline (tholeiitic) magma to a silica-poor and Ti-Fe-P rich liquid; (2) partial melting of wall-rocks yielding a rhyolitic magma segregated at the top of the basaltic reservoir; and (3) explosive eruption of the uppermost portion of the magma chamber followed by mixing of rhyolite with the underlying part of the system to form a hybrid dacite.

Sea-level and climate forcing of the Sr isotope composition of late Miocene Mediterranean marine basins

Sr isotope records from marginal marine basins track the mixing between seawater and local continental runoff, potentially recording the effects of sea level, tectonic, and climate forcing in marine fossils and sediments. Our 110 new 87Sr/86Sr analyses on oyster and foraminifera samples from six late Miocene stratigraphic sections in southern Turkey, Crete, and Sicily show that 87Sr/86Sr fell below global seawater values in the basins several million years before the Messinian Salinity Crisis, coinciding with tectonic uplift and basin shallowing. 87Sr/86Sr from more centrally located basins (away from the Mediterranean coast) drop below global seawater values only during the Messinian Salinity Crisis. In addition to this general trend, 55 new 87Sr/86Sr analyses from the astronomically tuned Lower Evaporites in the central Apennines (Italy) allow us to explore the effect of glacio-eustatic sea level and precipitation changes on 87Sr/86Sr. Most variation in our data can be explained by changes in sea level, with greatest negative excursions from global seawater values occurring during relative sea level lowstands, which generally coincided with arid conditions in the Mediterranean realm. We suggest that this greater sensitivity to lowered sea level compared with higher runoff could relate to the inverse relationship between Sr concentration and river discharge. Variations in the residence time of groundwater within the karst terrain of the circum-Mediterranean region during arid and wet phases may help to explain the single (robust) occurrence of a negative excursion during a sea level highstand, but this explanation remains speculative without more detailed paleoclimatic data for the region.

Transition from mildly-tholeiitic to calc-alkaline suite: the case of Chichontepec volcanic centre, El Salvador, Central America

Abstract The Chichontepec volcano is a Plio-Pleistocene composite volcano that erupted lavas ranging from high-alumina basalts to dacites. It experienced a caldera-forming paroxysmal eruption during the early Pleistocene. Pre-caldera lavas are mildly tholeiitic and they evolved mainly by low pressure crystal fractionation, notwithstanding the fact that most mafic lavas (low-MgO high-alumina basalts) retain traces of polybaric evolution. Conversely, post-caldera lavas, which are mainly pyroxene andesites, are clearly calc-alkaline, having evolved by open-system crystal fractionation. Sr–Nd isotopic data and trace elements characteristics indicate that the same mantle source was involved in the petrogenesis of these series. Modelling the AFC process showed that it did not play any role in the petrogenesis of these rocks; a crystal fractionation model is considered to be more relevant. A slight variation in the fractionating assemblage could have caused the transition from an early mildly tholeiitic trend to a late calc-alkaline one. Mineralogical evidence, mass-balance calculations and elemental chemistry support this hypothesis, assuming that the greater amount of pyroxene on the liquidus is at the expense of plagioclase; this would have prevented the trend in iron enrichment.

First Strontium Isotope Evidence of Mobility in the Neolithic of Southern France

AbstractThis study presents the first 87Sr/ 86Sr isotope results obtained on Neolithic humans from Southern France. These analyses aimed at exploring patterns of mobility in the Languedoc and Garonne areas, at sites dated to the Middle Neolithic (c. 4500–3500 cal bc). Strontium isotope analysis of tooth enamel and bone are a useful geochemical tracer to investigate the origin and residential mobility of ancient people. Strontium isotope ratios (87Sr/ 86Sr) of material from five sites located in two geographic areas were measured. Results obtained do not support our hypothesis of mobility for a number of individuals considered as outsiders in a previous study. On the other hand, the difference recorded between teeth and bone suggests mobility of other individuals between childhood and the last years of adult life. This preliminary study highlights the importance of combining multi-isotope analysis to discuss human subsistence economy and mobility.