How to reconstruct the geometry of a Middle Triassic feeding system: clues from clinopyroxene textures in lava flows from Cima Pape (Southern Alps, Italy)

No abstract availabl

Low water content of the Cenozoic lithospheric mantle beneath the eastern part of the North China Craton

Nominally anhydrous minerals in 46 peridotite xenoliths hosted by Cenozoic basalts from five localities (Fangshan, Penglai, Qixia, Changle, and Hebi) of the eastern part of the North China Craton (NCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The water contents (H2O wt %) of clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol) range from 27 to 223 ppm, 8 to 94 ppm, and ∼0 ppm, respectively. On the basis of (1) the homogenous H2O content within single pyroxene grains and (2) the equilibrium partitioning of H2O between cpx and opx, it is suggested that the pyroxenes largely preserve theH2Ocontent of their mantle source, although possible H loss during xenolith ascent cannot be excluded for ol. The recalculated whole‐rock H2O contents, using mineral modes and assuming a partition coefficient of 10 for water between cpx and ol, range from 6 to 56 ppm (average of 23 ± 13 ppm). In combination with previously reported data, the recalculated whole‐rock water contents of peridotite xenoliths (105 samples from 9 localities) hosted by Cenozoic basalts from the eastern part of the NCC range from 6 to 85 ppm (average of 25 ± 18 ppm). The Cenozoic lithospheric mantle of the eastern part of the NCC is therefore characterized by a low water content compared to continental lithospheric mantle worldwide represented by typical cratonic and off‐cratonic peridotites (normally 40–180 ppm, with average values of 119 ± 54 ppm and 78 ± 45, respectively) and to oceanic mantle values (>50 ppm) inferred from MORB and OIB. Peridotite xenoliths have low‐to‐moderate spinel Fe3+/SFe (0.02–0.34) and whole rock DFMQ values (from −4.2 to 2.2, normally between −2.5 and 1.5), which are not correlated with pyroxene H2O contents. Therefore, the low water contents cannot have resulted from oxidation of the mantle xenoliths and may have been caused instead by heating from an upwelling asthenosphere flow that acted in concert with NCC lithospheric thinning during the late Mesozoic to early Cenozoic. If so, the present eastern NCC lithospheric mantle represents essentially relict ancient lithospheric mantle after the thinning event, rather than newly accreted and cooled asthenospheric mantle

3-D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle: III. Thermochemical tomography in the Western-Central U.S.

Acknowledgments We are indebted to F. Darbyshire and J. von Hunen for useful comments on earlier versions of this work. This manuscript benefited from thorough and constructive reviews by W. Levandowski and an anonymous reviewer. We also thank J. Connolly, M. Sambridge, B. Kennett, S. Lebedev, B. Shan, U. Faul, and M. Qashqai for insightful discussions about, and contributions to, some of the concepts presented in this paper. The work of J.C.A. has been supported by two Australian Research Council Discovery grants (DP120102372 and DP110104145). Seismic data are from the IRIS DMS. D.L.S. acknowledges support from NSF grant EAR-135866. This is contribution 848 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 1106 in the GEMOC Key Centre (http://www.gemoc.mq.edu.au).Peer reviewedPublisher PD

2021 Tajogaite eruption records infiltration of crustal fluids within the upper mantle beneath La Palma, Canary Islands

The 2021 Tajogaite eruption at La Palma has represented a unique opportunity to investigate the characteristics of the mantle source feeding modern volcanism in the Canary Islands. With the aim of track the fingerprint of carbon in the local oceanic lithosphere-asthenosphere system, we report the isotopic composition of CO2 (δ13C values versus Vienna Pee Dee Belemnite) in olivine- and clinopyroxene-hosted fluid inclusions (FI) from the 2021 Tajogaite lavas and from lavas/ultramafic xenoliths (olivine-clinopyroxenites, clinopyroxenites, dunites and harzburgites) from the nearby 1677 San Antonio eruption cone/lavas, in an attempt to characterize the origin and evolution of carbon within the local mantle source. Our results indicate that the 2021 and 1677 lavas exhibit δ13C values ranging from −4.94‰ to −2.71‰ and CO2/3He ratios from 3.37 to 6.14 × 109. Ultramafic xenoliths fall in a comparable range of values despite showing higher CO2 concentrations. Our δ13C values fall within the range of carbon isotope results previously reported for the Dos Aguas cold spring located in the Taburiente Caldera (northern La Palma), suggesting an apparent carbon isotope homogeneity at the scale of the entire island. The (relatively narrow) δ13C vs. CO2/3He ratio range of La Palma samples is interpreted to reflect either i) variable extents of open-system degassing of a common mantle endmember having δ13C of ∼1.7‰, or ii) mixing between depleted mantle-like carbon (−6‰ < δ13C < −4‰) and crustal carbon (δ13C = 0‰) endmembers. Both models testify a crustal carbon component recycled in the local mantle. This component, also detected in mantle xenoliths from the neighboring island of El Hierro and the easternmost Lanzarote, indicates a regional characteristic of the mantle beneath the Canary Islands, interpreted as a result of infiltration of carbon-rich melts during past metasomatic events in the local mantle

U and Th content in the Central Apennines continental crust: a contribution to the determination of the geo-neutrinos flux at LNGS

The regional contribution to the geo-neutrino signal at Gran Sasso National Laboratory (LNGS) was determined based on a detailed geological, geochemical and geophysical study of the region. U and Th abundances of more than 50 samples representative of the main lithotypes belonging to the Mesozoic and Cenozoic sedimentary cover were analyzed. Sedimentary rocks were grouped into four main "Reservoirs" based on similar paleogeographic conditions and mineralogy. Basement rocks do not outcrop in the area. Thus U and Th in the Upper and Lower Crust of Valsugana and Ivrea-Verbano areas were analyzed. Based on geological and geophysical properties, relative abundances of the various reservoirs were calculated and used to obtain the weighted U and Th abundances for each of the three geological layers (Sedimentary Cover, Upper and Lower Crust). Using the available seismic profile as well as the stratigraphic records from a number of exploration wells, a 3D modelling was developed over an area of 2^{\circ}x2^{\circ} down to the Moho depth, for a total volume of about 1.2x10^6 km^3. This model allowed us to determine the volume of the various geological layers and eventually integrate the Th and U contents of the whole crust beneath LNGS. On this base the local contribution to the geo-neutrino flux (S) was calculated and added to the contribution given by the rest of the world, yielding a Refined Reference Model prediction for the geo-neutrino signal in the Borexino detector at LNGS: S(U) = (28.7 \pm 3.9) TNU and S(Th) = (7.5 \pm 1.0) TNU. An excess over the total flux of about 4 TNU was previously obtained by Mantovani et al. (2004) who calculated, based on general worldwide assumptions, a signal of 40.5 TNU. The considerable thickness of the sedimentary rocks, almost predominantly represented by U- and Th- poor carbonatic rocks in the area near LNGS, is responsible for this difference.Comment: 45 pages, 5 figures, 12 tables; accepted for publication in GC

Relatively oxidized conditions for diamond formation at Udachnaya (Siberia)

Thanks to the physical strength of diamonds and their relatively unreactive chemical nature, their mineral inclusions may remain exceptionally preserved from alteration processes and chemical exchanges with surrounding minerals, fluids and/or melts following diamond formation. Cr-bearing spinels are relatively common inclusions found in peridotitic diamonds and important oxybarometers providing information about the oxygen fugacity (fO2) of their source mantle rocks. Here, we investigated a magnesiochromite-olivine touching pair in a diamond from the Udachnaya kimberlite (Siberia) by in situ single-crystal X-ray diffraction and energy-domain synchrotron Mossbauer spectroscopy, aiming to constrain the physical-chemical conditions of diamond formation and to explore the redox state of this portion of the Siberian craton when the diamond was formed. The P-T-fO(2) entrapment conditions of the inclusion pair, determined by thermo- and oxybarometric analyses, are similar to 5.7(0.4) GPa and similar to 1015(50) ? (although entrapment at higher T and re-equilibration during subsequent mantle storage are also possible) and fO(2) near the enstatite-magnesite-olivine-diamond (EMOD) buffer. The determined fO(2) is similar to, or slightly more oxidized than, those of xenoliths from Udachnaya, but whilst the xenoliths last equilibrated with the surrounding mantle just prior to their entrainment in the kimberlite at similar to 360 Ma, the last equilibration of the inclusion pair is much older, occurring at 3.5-3.1, similar to 2 or similar to 1.8 Ga before final encapsulation in its host diamond. Hence, the similarity between xenoliths and inclusion fO(2) values indicates that the modern redox state of this portion of the Siberian lithosphere was likely attained relatively early after its formation and may have persisted for billions of years after diamond formation, at least at the local scale. Moreover, the oxygen fugacity determination for the inclusion pair provides direct evidence of diamond formation near the EMOD buffer and is consistent with recent models suggesting relatively oxidized, water-rich CHO fluids as the most likely parents for lithospheric diamonds

La sequenza archeologica di Barbiano – Caserma dei vigili del fuoco (Bolzano) : evidenze di sistemazioni dei versanti durante il neolitico

Il sito di Barbiano \ue8 stato occupato in varie fasi dalla Preistoria all\u2019Et\ue0 Romana Le prime evidenze insediative risalgono all\u2019inizio del Neolitico medio, circa 7000 anni BP. L\u2019occupazione venne preceduta dalla creazione di una ampia piattaforma, un terrazzo, realizzato asportando i depositi fluvioglaciali e la sovrastante copertura pedologica. La piattaforma venne completata con il riporto su tutta la superficie di alcuni decimetri di sabbia. Si tratta di una delle pi\uf9 antiche evidenze di significative modificazioni antropiche del paesaggio in area alpina. Questi importanti lavori suggeriscono inoltre che l\u2019ampia area insediativa fosse abitata per la maggior parte dell\u2019anno. L\u2019insediamento marca dunque la transizione dalle pi\uf9 antiche attivit\ue0 agricole basate sulla pratica dello "slash and burn" a quelle stanziali. I livelli neolitici vennero coperti da sedimenti alluvionali e colluviali deposti come conseguenza dell\u2019attivazione di processi di degradazione dei versanti innescati dalla deforestazione. Durante l\u2019Et\ue0 Romana il torrente locale era delimitato da argini artificiali ed una serie di strutture murarie vennero edificate per delimitare e proteggere aree insediative che per\uf2 si sviluppano al di fuori dell\u2019area scavata. I sedimenti grossolani all\u2019interno del canale suggeriscono che limitati processi erosivi ancora interessavano il versante. I livelli romani sono stati successivamente sepolti da una sottile coltre colluviale che suggeriscono come i versanti sovrastanti, oggigiorno largamente terrazzati, non sono pi\uf9 stati interessati da processi erosivi che sono invece noti in altre aree della regione.The archaeological succession of Barbiano-Firehouse (Bozen): evidence of slope terracing during the Neolithic Barbiano archaeological site was occupied at various stages from the Prehistory to the Roman era. The earliest occupation of the area, seems to have occurred at the beginning of the Middle Neolithic, around 7 ka bP. It was preceded by the excavation of a large almost flat platform, a terrace, realised with the stripping of fluvioglacial deposits and the overlying soil cover. The platform was later covered with a few decimetre-thick layer of sand. This is one of the earliest evidence of important anthropogenic topographic and landscape modification in the Alps. These important works suggest that the large occupation surface was probably settled for most of the year following the transition from the nomadic agriculture and the "slash and burn" practices usually associated with the Neolithic . The Neolithic settlement was lately buried under colluvial and alluvial sediments that indicate slope degradation processes most probably triggered by deforestation. In Roman Times the local stream was delimited by artificial levee and a series of walls were built to delimitate and protect larger structures that extended beyond the excavated area. The coarse sediments that fill the channel bear witness that limited erosional processes still affected on the slope. The Roman structures were buried under shallow colluvial deposits suggesting that the overlying slope, nowadays largely terraced, did not experienced anymore erosive processes that are known to have occurred in nearby areas