Heavy oxygen recycled into the lithospheric mantle

Magmas in volcanic arcs have geochemical and isotopic signatures that can be related to mantle metasomatism due to fluids and melts released by the down-going oceanic crust and overlying sediments, which modify the chemistry and mineralogy of the mantle wedge. However, the effectiveness of subduction-related metasomatic processes is difficult to evaluate because the composition of arc magmas is often overprinted by interactions with crustal lithologies occurring during magma ascent and emplacement. Here, we show unequivocal evidence for recycling of continental crust components into the mantle. Veined peridotite xenoliths sampled from Tallante monogenetic volcanoes in the Betic Cordillera (southern Spain) provide insights for mantle domains that reacted with Si-rich melts derived by partial melting of subducted crustal material. Felsic veins crosscutting peridotite and the surrounding orthopyroxene-rich metasomatic aureoles show the highest 18O/16O ratios measured to date in upper mantle assemblages worldwide. The anomalously high oxygen isotope compositions, coupled with very high 87Sr/86Sr values, imply the continental crust origin of the injected melts. Isotopic anomalies are progressively attenuated in peridotite away from the veins, showing 18O isotope variations well correlated with the amount of newly formed orthopyroxene. Diffusion may also affect the isotope ratios of mantle rocks undergoing crustal metasomatism due to the relaxation of 18O isotope anomalies to normal mantle values through time. Overall, the data define an O isotope “benchmark” allowing discrimination between mantle sources that attained re-equilibration after metasomatism (>5 Myr) and those affected by more recent subduction-derived enrichment processes

Subduction-related hybridization of the lithospheric mantle revealed by trace element and Sr-Nd-Pb isotopic data in composite xenoliths from Tallante (Betic Cordillera, Spain)

Ultramafic xenoliths are rarely found at convergent plate margins. A notable exception is in the Betic Cordillera of southern Spain, where the eruption of xenolith-bearing alkaline basalts during the Pliocene post-dated the Cenozoic phase of plate convergence and subduction-related magmatism. Mantle xenoliths of the monogenetic volcano of Tallante display extreme compositional heterogeneities, plausibly related to multiple tectono-magmatic episodes that affected the area. This study focuses on two peculiar composite mantle xenolith samples from Tallante, where mantle peridotite is crosscut by felsic veins of different size and mineralogy, including quartz, orthopyroxene, and plagioclase. The veins are separated from the peridotite matrix by an orthopyroxene-rich reaction zone, indicating that the causative agents were alkali-rich hydrous silica-oversaturated melts, which were likely related to recycling of subducted continental crust components. The present study reports new and detailed major and trace elements and Sr-Nd-Pb analyses of the minerals in the composite Tallante xenoliths that confirm the continental crust derivation of the metasomatic melts, and clarifies the mode in which subduction-related components are transferred to the mantle wedge in orogenic areas. The particular REE patterns of the studied minerals, as well as the variation of the isotopic ratios between the different zones of the composite xenoliths, reveal a complex metasomatic process. The distribution of the different elements, and their isotope ratios, in the studied xenoliths are controlled by the mineral phases stabilised by the interaction between the percolating melts and the peridotitic country rock. The persistence of marked isotopic heterogeneities and the lack of re-equilibration suggest that metasomatism of the sub-continental lithospheric mantle occurred shortly before the xenolith exhumation. In this scenario, the studied xenoliths and the metasomatic processes that affected them may be representative of the mantle sources of mafic potassic to ultrapotassic magmas occurring in post-collisional tectonic settings

High H2O Content in Pyroxenes of Residual Mantle Peridotites at a Mid Atlantic Ridge Segment

Global correlations of mid-ocean-ridges basalt chemistry, axial depth and crustal thickness have been ascribed to mantle temperature variations affecting degree of melting. However, mantle H2O content and elemental composition may also play a role. How H2O is distributed in the oceanic upper mantle remains poorly constrained. We tackled this problem by determining the H2O content of orthopyroxenes (opx) and clinopyroxenes (cpx) of peridotites from a continuous lithospheric section created during 26 Ma at a 11°N Mid-Atlantic Ridge segment, and exposed along the Vema Transform. The H2O content of opx ranges from 119 ppm to 383 ppm; that of cpx from 407 ppm to 1072 ppm. We found anomalous H2O-enriched peridotites with their H2O content not correlating inversely with their degree of melting, although H2O is assumed to be incompatible during melting. Inverse correlation of H2O with Ce, another highly incompatible component, suggests post-melting H2O enrichment. We attribute a major role to post-melting temperature-dependent diffusion of hydrogen occurring above the melting region, where water-rich melt flows faster than residual peridotites through dunitic conduits cross-cutting the uprising mantle. Accordingly, estimates of the H2O content of the MORB mantle source based on H2O in abyssal peridotites can be affected by strong uncertainties

Quartz-bearing rhyolitic melts in the Earth’s mantle

The occurrence of rhyolite melts in the mantle has been predicted by high pressure-high temperature experiments but never observed in nature. Here we report natural quartz-bearing rhyolitic melt inclusions and interstitial glass within peridotite xenoliths. The oxygen isotope composition of quartz crystals shows the unequivocal continental crustal derivation of these melts, which approximate the minimum composition in the quartz-albite-orthoclase system. Thermodynamic modelling suggests rhyolite was originated from partial melting of near-anhydrous garnet-bearing metapelites at temperatures ~1000 °C and interacted with peridotite at pressure ~1 GPa. Reaction of rhyolite with olivine converted lherzolite rocks into orthopyroxene-domains and orthopyroxene + plagioclase veins. The recognition of rhyolitic melts in the mantle provides direct evidence for element cycling through earth's reservoirs, accommodated by dehydration and melting of crustal material, brought into the mantle by subduction, chemically modifying the mantle source, and ultimately returning to surface by arc magmatism

Oxygen stable isotope analyses on ameghinomya antiqua shells: A promising tool for palaeoenvironmental reconstruction along the quaternary patagonian Argentina coast?

There are only a few data concerning the Quaternary climate fluctuations in the marine environments of the Atlantic Patagonian coast. In this regard, the aragonitic shell of the Ameghinomya antiqua bivalve offers the possibility to study the climate variability and the seasonal cycles of sea water temperature in the region at different geological times. We compared oxygen isotopic profiles along the shell-growth axis on four well-preserved A. antiqua bivalves collected from marine coastal deposits of Marine Isotope Stage (MIS) 7, MIS 5, Holocene (7.284 ±140 yr cal BP), and from the present-day active beach of Bahia Bustamante (Patagonia Argentina). Shell ontogeny was determined through the annual growth lines recorded along the external region of the shell, and was also verified by cross-section analyses. The fossil bivalves were around 15 years old, while the Present day shell was 10 years old. When the modern climatic data available are compared, the higher δ18Oshell values represent the cold season, while the lower δ18Oshell values indicate the warm season. The δ18Oshell ranges indicate different environmental conditions and seasonal temperature variations between specimens. By assuming, a constant δ18Ow calculated with the limited environmental data available, superficial seawater temperatures are estimated from the specimens. These paleotemperatures may overestimate water temperatures, and shell formation seems to occur with an offset from expected oxygen isotopic equilibrium with the water in which they lived. Meanwhile, A. antiqua shells are suitable bioarchives for the comparison of seasonal patterns throughout the Quaternary, thus constituting another proxy for the evaluation of paleoclimatic and paleoenvironmental changes in the Patagonia region.Fil: Boretto, Gabriella Margherita. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Consoloni, Ilaria. Università degli Studi di Pisa; ItaliaFil: Morán, Ariana Gisela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Regattieri, Eleonora. Consiglio Nazionale delle Ricerche; ItaliaFil: Gordillo, Sandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Fucks, Enrique Eduardo. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; ArgentinaFil: Zanchetta, Giovanni. Università degli Studi di Pisa; ItaliaFil: Dallai, Luigi. Istituto di Geoscienze e Georisorse; Itali

A 19 to 17 Ma amagmatic extension event at the Mid-Atlantic Ridge: Ultramafic mylonites from the Vema Lithospheric Section

A >300 km long lithospheric section (Vema Lithospheric Section or VLS) is exposed south of the Vema transform at 11°N in the Atlantic. It is oriented along a seafloor spreading flow line and represents ∼26 Ma of accretion at a single 80 km long segment (EMAR) of the Mid-Atlantic Ridge. The basal part of the VLS exposes a mantle unit made mostly of relatively undeformed coarse-grained/porphyroclastic peridotites that were sampled at close intervals. Strongly deformed mylonitic peridotites were found at 14 contiguous sites within a ∼80 km stretch (∼4.7 Ma interval); they are dominant in a time interval of 1.4 Ma, from crustal ages of 16.8 to 18.2 Ma (mylonitic stretch). Some of the mylonites are "dry," showing anhydrous high-T deformation, but most contain amphibole. The mylonitic peridotites tend to be less depleted than the porphyroclastic peridotites on the basis of mineral major and trace elements composition, suggesting that the mylonites parent was a subridge mantle that underwent a relatively low degree of melting. The Sr, Nd, and O isotopic composition of the amphiboles is MORB-like and suggests either that seawater did not contribute to their isotopic signature or that their isotopic ratios re-equilibrated during fluid circulation in the upper mantle. Four 40Ar/39Ar ages, on three amphiboles separated from the peridotites, are close to crustal ages predicted from magnetic anomalies, confirming that the amphiboles formed close to ridge axis. We propose that crustal accretion at the EMAR segment has been mostly symmetrical for the 26 Ma of its recorded history, except for the ∼1.4 Ma interval of prevalent ultramafic mylonites (mylonitic stretch) that may record a period of quasi-amagmatic asymmetric accretion of oceanic lithosphere close to the ridge–Vema transform intersection, possibly with development of detachment faults. This interval may correspond to a thermal minimum of the subridge upwelling mantle, marking the transition from a period of decreasing to one of increasing mantle melting below the EMAR segment

Stable oxygen and carbon isotope composition of holocene mytilidae from the camarones coast (Chubut, argentina): Palaeoceanographic implications

The stable isotope composition of living and of Holocene Mytilidae shells was measured in the area of Camarones (Chubut, Argentina). The most striking results were the high δ18 O values measured in samples older than ca. 6.1 cal ka BP. In the younger samples, the δ18 O values remained substantially stable and similar to those of living specimens. Analysis of the data revealed the possibility for this isotopic shift to be driven mainly by changes in temperature probably accompanied by minor changes in salinity, suggesting cooler seawater before 6.1 cal ka BP, with a maximum possible temperature shift of ca. 5◦ C. A possible explanation of this change can be related to a northward position of the confluence zone of the Falkland and Brazilian currents. This is consistent with the data obtained in marine cores, which indicate a northerly position of the confluence in the first half of the Holocene. Our data are also in line with the changes in wind strength and position of the Southern Westerlies Wind, as reconstructed in terrestrial proxies from the Southernmost Patagonia region.Fil: Boretto, Gabriella Margherita. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Zanchetta, Giovanni. Università degli Studi di Pisa; Italia. Istituto di Geologia Ambientale e Geoingegneria; Italia. Consiglio Nazionale delle Ricerche; ItaliaFil: Consoloni, Ilaria. Università degli Studi di Pisa; ItaliaFil: Baneschi, Ilaria. Istituto di Geoscienze e Georisorse; Italia. Consiglio Nazionale delle Ricerche; ItaliaFil: Guidi, Massimo. Istituto di Geoscienze e Georisorse; Italia. Consiglio Nazionale delle Ricerche; ItaliaFil: Isola, Ilaria. Istituto Nazione di Geofisica e Vulcanologia; ItaliaFil: Bini, Monica. Consiglio Nazionale delle Ricerche; Italia. Università degli Studi di Pisa; ItaliaFil: Ragaini, Luca. Università degli Studi di Pisa; ItaliaFil: Terrasi, Filippo. Seconda Universita Degli Studi Di Napoli; ItaliaFil: Regattieri, Eleonora. Istituto di Geoscienze e Georisorse; ItaliaFil: Dallai, Luigi. Istituto di Geoscienze e Georisorse; Italia. Istituto Nazione di Geofisica e Vulcanologia; Italia. Università degli studi di Roma "La Sapienza"; Itali

THE “NEW” TOURIST-ENVIRONMENTAL MAP OF THE SALSE DI NIRANO REGIONAL RESERVE (MODENA APENNINES, NORTHERN ITALY)

La Riserva Naturale Regionale delle Salse di Nirano, visitata annualmente da circa 50.000 persone, ricade in un territorio di bassa collina (quote fra i 140 e i 308 m) al margine dell'Appennino modenese a circa 30 km da Modena. Le salse sono delle emissioni di fango freddo prodotte dalla risalita in superficie di acqua salata e fangosa frammista ad idrocarburi principalmente gassosi (metano) ed, in piccola parte, liquidi (petrolio) lungo faglie e fratture del terreno. La nuova Carta Turistico-Ambientale è una carta pieghevole, tascabile, stampata fronte/retro, con note illustrative sintetiche in italiano e in inglese, i cui elementi caratterizzanti sono una Carta Geoturistica e un'immagine ad effetto 3D del territorio della Riserva realizzati in ambiente ESRI ArcGIS. La Carta Geoturistica coniuga la rappresentazione dei più evidenti aspetti geomorfologici che possono essere osservati e riconosciuti anche da persone non esperte con l'indicazione delle informazioni turistiche fondamentali. L'immagine tridimensionale, elaborata sovrapponendo ortofoto aeree al modello digitale del terreno, permette di apprezzare la morfologia della Riserva e delle aree circostanti. Gli altri contenuti della Carta Tu ristico-Ambientale sono: i) testi esplicativi che descrivono, oltre ai prodotti appena citati, la riserva e il fenomeno delle salse, i percorsi escursionistici e didattici, fauna, flora e vegetazione, il Centro visite Cà Tassi e l'Ecomuseo Cà Rossa; ii) fotografie degli aspetti ambientali più significativi; iii) informazioni logistiche sul Centro Visite Cà Tassi, l'Ecomuseo Cà Rossa e sui punti di ristoro ed alloggio; iv) indicazioni sintetiche sulle attrattive turistiche dei dintorni. Si tratta di una "nuovo" documento turistico in quanto una precedente Carta Turistico-Ambientale è stata pubblicata all'inizio del 2004 e la realizzazione di una nuova Carta si è resa necessaria in quanto negli ultimi anni la Riserva è stata soggetta a diversi cambiamenti sia nel paesaggio che nelle strutture. La Carta Turistico-Ambientale testimonia come la ricerca scientifica possa efficacemente contribuire alla realizzazione di documenti per il settore del Turismo.The Natural Reserve of Salse di Nirano, which has about 50.000 visitors per year, is located in a low hill territory (elevation from 140 m to 308 m a.s.l.) of the Modena Apennine margin about 30 km from Modena. The salse are emissions of cold mud due to the ascent to the surface of salty and muddy waters mixed with gaseous (methane) and, in minor part, fluid hydrocarbons (petroleum veils) along faults and fractures. The new Tourist – Environmental map is a foldable, pocket-size, front/rear printed map with explanatory notes in Italian and English; the characterising documents are a Geotourist map and a 3D image of the Reserve which have been implemented by means of ESRI ArcGIS. The Geotourist map combines the most evident geomorphological features – which can be observed and recognised even by non-experts – with fundamental tourist information. The 3D image, computed overlapping aerial orthophotos to a Digital Terrain Model, allows to appreciate the morphology of the Reserve and its surroundings. The other contents of the Tourist – Environmental map are: i) explanatory notes describing synthetically, beside the above mentioned products, the Reserve and the salse phenomenon, the excursion and educational trails, fauna, flora and vegetation, Cà Tassi Visitor Centre and Cà Rossa Ecomuseum; ii) photographs of the most significant environmental sites; iii) logistic information on Cà Tassi Visitor Centre and and Cà Rossa Ecomuseum, on refreshment and overnight stay sites; iv) short information on tourist attractions in the nearby. This is a new Tourist – Environmental map because a previous one was edited at the beginning of 2004; the implementation of this new map was necessary as in the last years several changes occurred in the landscape as well as in the structures of the Reserve. The Tourist – Environmental map witnesses how scientific research can effectively contribute to the creation of maps utilisable in the tourism field