ALBERT

Description: Pyroxenites embedded in peridotite are often invoked as a major cause of short-length scale isotopic heterogeneities in the upper mantle, but there has been little direct evidence. We report spatially controlled chemical and Sr-Nd isotopic compositions of pyroxenites and their host peridotites from an ophiolitic mantle sequence in the Northern Apennines, Italy, with depleted mantle compositions, representing a surface exposure of veined upper mantle, a potential source for mid-oceanic-ridge basalts (MORB). Interaction between pyroxenites and adjacent mantle rocks results in centimeter-scale chemical modifications in the host peridotites, systematically lowering their Sm/Nd ratios. Over time, this interaction causes the host peridotite at 〉0.1 m scale to acquire an isotopic heterogeneity larger than the range defined by the peridotite and pyroxenite end-members. Moreover, the 143 Nd/ 144 Nd variation of a single outcrop covers most of the global Nd isotopic variability documented in abyssal peridotites. Such pyroxenite-peridotite veined mantle domains may represent the enriched component rarely found in abyssal peridotites, but often invoked to account for the low end of 143 Nd/ 144 Nd variations in MORB.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Since the early nineteenth century, the structural high of Mt. Nerone in the Umbria-Marche-Sabina Domain (UMS – Central/Northern Apennines, Italy) attracted scholars from all over Europe due to the wealth of fossil fauna preserved in a well-exposed Mesozoic sedimentary succession. Several geo-palaeontological studies were focused on the abundant and diverse invertebrate fauna, whereas contributions dealing with Mesozoic vertebrates were to date virtually lacking. Recently, the first material referable to hybodont sharks, consisting of an articulated crushing dentition, was described from the area and referred to 〈span〉Asteracanthus〈/span〉 cf. 〈span〉A. magnus〈/span〉. In this contribution, we report the first evidence of ginglymodian actinopterygians from the Upper Jurassic of Mt. Nerone. The material is represented by seven highly tritorial isolated teeth collected from three classic fossiliferous localities of the area (i.e. Pian del Sasso, Fosso Pisciarello, I Ranchi). The general morphology of the material under study allow us to conservatively refer the teeth to the Neoginglymodi, a clade formed by Lepisosteiformes and Semionotiformes. The occurrence of durophagous organisms, to date represented by hybodont sharks and ginglymodian fishes, reveals interesting palaeoecological scenarios characterizing the pelagic carbonate platform-basin system of Mt. Nerone, which were most likely triggered by large-scale geodynamic processes. The complex submarine palaeotopography, inheritance of the Western Tethys Early Jurassic rifting, aroused the establishment of new infaunal and epifaunal communities opening up unexplored trophic niches for durophagous predators. The particular geodynamic setting of the UMS Domain, consisting of predominantly interconnected structural highs and lows, and the relative evolution of a diverse invertebrate fauna characterized by terebratulid, ostreids, limids, brachiopods, crustaceans, and gastropods, attracted both hybodontids and durophagous ginglymodians in the Tethyan realm, influencing their dispersal during the Late Jurassic.〈/span〉

Description: Pyroxenite layers embedded within peridotite represent widespread lithological mantle heterogeneities and are potential components in the mantle source of many oceanic basalts. They can be generated by a variety of magmatic and metamorphic processes. However, in most natural samples (especially in ultramafic massifs), their primary characteristics are partially or completely erased by later processes (e.g. metamorphism, metasomatism or partial melting). Here we investigate a suite of pyroxenites from the External Liguride Jurassic ophiolites (Northern Apennines, Italy). These are spinel-bearing websterites and clinopyroxenites, partially recrystallized under plagioclase-facies conditions, and occur as centimetre-scale layers parallel to the tectonite foliation of their host peridotites. The pyroxenites have bulk-rock Mg-numbers from 74 to 88 and display rather constant light rare earth element (LREE) depletion relative to middle REE (MREE) (La N /Sm N = 0·15–0·35), but variable MREE–heavy REE (HREE) fractionation, with some having markedly positive HREE slopes (Sm N /Yb N = 0·30–0·96). The HREE enrichment, coupled with high Zr and Sc contents in clinopyroxene porphyroclasts from spinel-bearing domains, provides strong evidence that garnet was present in the precursor mineral assemblages. Mass-balance calculations suggest that the pyroxenites originally contained up to about 40 vol. % garnet, indicating that they originated by segregation of melts at relatively high pressure ( P 〉 1·5 GPa). The parental melts of the pyroxenites have reacted to some extent with the host peridotite during mantle infiltration. Lack of olivine in the primary mineral assemblage and the presence of orthopyroxene-rich rims along the contact with the wall-rock peridotites suggest that the pyroxenites crystallized from silica-rich melts. These probably had REE patterns and Sr–Nd isotope compositions similar to those of enriched mid-ocean ridge basalt. We propose that the pyroxenites originated from melts derived from a hybrid eclogite-bearing peridotite source, which subsequently reacted with their host peridotite to form ‘secondary pyroxenites’. The existence of such pyroxenites has been invoked in current models of basalt petrogenesis. During later decompression, the pyroxenites experienced recrystallization at spinel-facies conditions, at 1·2–1·5 GPa and minimum temperatures of 950–1000°C, and partial re-equilibration in the low-pressure plagioclase facies. The latter event is dated by internal Sm–Nd isochrons at 178 (±8) Ma and is associated with Mesozoic exhumation during extension of the Tethys lithosphere.

Description: An abundance of methods have been developed over the years to perform the frequency analysis (FA) of extreme environmental variables. Although numerous comparisons between these methods have been implemented, no general comparison framework has been agreed upon so far. The objective of this paper is to build the foundation of a data-based comparison framework, which aims at complementing more standard comparison schemes based on Monte Carlo simulations or statistical testing. This framework is based on the following general principles: (i) emphasis is put on the predictive ability of competing FA implementations, rather than their sole descriptive ability measured by some goodness-of-fit criterion; (ii) predictive ability is quantified by means of reliability indices, describing the consistency between validation data (not used for calibration) and FA predictions; (iii) stability is also quantified, i.e. the ability of a FA implementation to yield similar estimates when calibration data change; (iv) the necessity to subject uncertainty estimates to the same scrutiny as point-estimates is recognized, and a practical approach based on the use of the predictive distribution is proposed for this purpose. This framework is then applied to a case study involving 364 gauging stations in France, where 10 FA implementations are compared. These implementations correspond to the local, regional and local-regional estimation of Gumbel and Generalized Extreme Value distributions. Results show that reliability and stability indices are able to reveal marked difference between FA implementations. Moreover, the case study also confirms that using the predictive distribution to indirectly scrutinize uncertainty estimates is a viable approach, with distinct FA implementations showing marked differences in the reliability of their uncertainty estimates. The proposed comparison framework therefore constitutes a valuable tool to compare the predictive reliability of competing FA implementations, along with the reliability of their uncertainty estimates.

Description: We present new data on mineral assemblages, mineral chemistry (major and trace element compositions), and fluid inclusions to reconstruct the magmatic to hydrothermal history of gabbroic rocks dredged in association with serpentinized mantle peridotites from the tectonically exposed Vema lithospheric section (central Atlantic). Textural relations and mineral chemistry of the samples indicate a two-stage magmatic evolution, from an early coarse-grained pyroxene–plagioclase cumulate to overprinted assemblages, including interstitial Fe–Ti oxides and titanian hornblende. Titanian hornblende in the gabbros is a product of interaction of crystal aggregates with an evolved residual melt or with a late-magmatic aqueous fluid at 800–900°C. The different contribution of residual melt and magmatic aqueous fluid is inferred from the behaviour of incompatible elements and plagioclase composition. One gabbro sample shows features indicative of a later stage hydrothermal event: formation of Fe- and Cl-rich hornblende sealing fractures in plagioclase, olivine coronas at orthopyroxene–magnetite grain boundaries, plagioclase-hosted high-salinity fluid inclusions associated with local enrichment of plagioclase in anorthite and Sr, and local augite to diopside re-equilibration. These features record the interaction of gabbro with seawater-derived fluid at temperature up to about 600°C. The high-salinity inclusions and reducing hydrothermal reactions in the gabbro indicate that the fluid phase characteristics may have been acquired through a preceding lower-temperature (〈500°C) interaction of seawater with mantle peridotite (serpentinization) followed by infiltration of the modified aqueous solution into the higher-temperature (about 600°C) gabbroic zone in a geodynamic environment of a slow-spreading ridge segment end. Compositional features of the hydrothermal fluid were: salinity 20–22 wt % (NaCl eq.), Ca/(Ca + Na) ≤ 0·12, and log f O 2 = –19·6 to –18·8.

Description: 〈span〉〈div〉ABSTRACT〈/div〉This work presents the results of a geological mapping project performed on the Narni-Amelia Ridge (Central Apennines, Italy). Fieldwork was aimed at defining the rift and post-rift architecture of the Jurassic to Cretaceous part of the Umbria-Marche-Sabina basin succession, which is characterised by Jurassic rift-related pelagic carbonate platform (PCP)/basin systems. While the effects of the Early Jurassic rifting are well documented in the regional literature, a phase of post-rift extensional tectonics in the late Early Cretaceous is a relatively under-investigated subject. This normal faulting was apparently widespread at a super-regional scale, and the Narni-Amelia Ridge carries direct and indirect evidence for this episode of revived deformation: 〈ul〉 〈li〉(i) the occurrence of limestone megabreccias (“Mt. Cosce Breccia”) resting unconformably, through an erosional surface, on a Jurassic (and Cretaceous) horst-block. Stepped unconformities are associated with “spur and groove” geometries of the escarpment, suggesting topographic backstepping due to rock-fall processes;〈/li〉 〈li〉(ii) millimetre to decametre-scale neptunian dykes and sills made of “Mt. Cosce Breccia” and of Maiolica pelagites. These fractures cut the footwall-block of Cretaceous faults, made of Calcare Massiccio Fm (Hettangian), sub-orthogonal to its masterbedding. Dykes are associated with tension gashes filled with chert (silicification of fracture zones). The steep walls of those neptunian dykes filled with Maiolica-type deposits also exhibit a thin chert band;〈/li〉 〈li〉(iii) the onlap of Aptian-Albian Marne a Fucoidi Fm on the Hettangian Calcare Massiccio Fm. This is an 〈span〉unicum〈/span〉 in the whole Umbria-Marche-Sabina Domain;〈/li〉 〈li〉(iv) the presence of mass-transport deposits (slumps and debris flows) in the uppermost part of the Maiolica Fm and lower part of the Marne a Fucoidi Fm.〈/li〉 〈/ul〉These stratigraphic “anomalies” are collectively interpreted as the products of normal faulting, which caused the retreat of the original Jurassic margins of the PCPs, rejuvenating the submarine palaeotopography. A “middle” Barremian age for this phase of syn-sedimentary extension is indicated by the nannofossil assemblage of Maiolica-type facies sampled from the “Mt. Cosce Breccia” and from the neptunian dykes.〈/span〉