High pressure melting of eclogite and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction

In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) garnet peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both eclogites and ultramafic rocks share a common high pressure (HP) peak at 2.8\u202fGPa and 750\u202f\ub0C and post-peak static equilibration at 0.8\u20131.0\u202fGPa and 850\u202f\ub0C. Garnet peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO2, K2O, CO2 and H2O. Peridotites also display LREE fractionation (La/Nd\u202f=\u202f2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. At Borgo, retrogressed garnet peridotites show low strain domains characterised by garnet compositional layering, cut by a subsequent low-pressure (LP) chlorite foliation, in direct contact with migmatised eclogites. Kfs\u202f+\u202fPl\u202f+\u202fQz\u202f+\u202fCpx interstitial pocket aggregates and Cpx\u202f+\u202fKfs thin films around symplectites after omphacite parallel to the Zo\u202f+\u202fOmp\u202f+\u202fGrt foliation in the eclogites suggest that they underwent partial melting at HP. The contact between garnet peridotites and eclogites is marked by a tremolitite layer. The same rock also occurs as layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, flowing in the boudin necks. This clearly indicates that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl\u202f+\u202fTc\u202f+\u202fChl\u202f+\u202fTr pseudomorphs after garnet, both crystallised in a static regime postdating the boudins formation, suggesting that they derive from a garnet-bearing precursor. Tremolitites have Mg#\u202f>\u202f0.90 and Al2O3\u202f=\u202f2.75\u202fwt% pointing to ultramafic compositions but also show enrichments in SiO2, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we performed a thermodynamic modelling at fixed P\u202f=\u202f3\u202fGPa and T\u202f=\u202f750\u202f\ub0C to model the chemical interaction between the garnet peridotite and the eclogite-derived melt. Our results show that this interaction produces an Opx\u202f+\u202fCpx\u202f+\u202fGrt assemblage plus Amp\u202f+\u202fPhl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous garnet websterite with amphibole and phlogopite. Both peridotites and tremolitites also show a selective enrichment in LILE recorded by amphiboles in the spinel stability field, indicating that a fluid-assisted metasomatic event occurred at LP conditions, leading to the formation of a chlorite foliation post-dating the garnet layering in peridotites, and the retrogression of Grt-websterites in tremolitites. The Monte Duria area is a unique terrane where we can observe syn-deformation eclogite-derived melt interacting with garnet peridotite at HP, proxy of subduction environments

Early cretaceous plume–ridge interaction recorded in the band-e-zeyarat ophiolite (North Makran, Iran): New constraints from petrological, mineral chemistry, and geochronological data

The North Makran domain (southeast Iran) is part of the Makran accretionary wedge and consists of an imbricate stack of continental and Neo-Tethyan oceanic tectonic units. Among these, the Band-e-Zeyarat ophiolite consists of (from bottom to top): ultramafic cumulates, layered gabbros, isotropic gabbros, a sheeted dyke complex, and a volcanic sequence. Sheeted dykes and volcanic rocks are mainly represented by basalts and minor andesites and rhyolites showing either normal-type (N) or enriched-type (E) mid-ocean ridge basalt affinities (MORB). These conclusions are also supported by mineral chemistry data. In addition, E-MORBs can be subdivided in distinct subtypes based on slightly different but significant light rare earth elements, Th, Nb, TiO2, and Ta contents. These chemical differences point out for different partial melting conditions of their mantle sources, in terms of source composition, partial melting degrees, and melting depths. U-Pb geochronological data on zircons from intrusive rocks gave ages ranging from 122 to 129 Ma. We suggest that the Band-e-Zeyarat ophiolite represents an Early Cretaceous chemical composite oceanic crust formed in a mid-ocean ridge setting by partial melting of a depleted suboceanic mantle variably metasomatized by plume-type components. This ophiolite records, therefore, an Early Cretaceous plume–ridge interaction in the Makran Neo-Tethys

Water Masses Variability in Inner Kongsfjorden (Svalbard) During 2010–2020

Kongsfjorden is an Arctic fjord located in the Svalbard archipelago. Its hydrography is influenced by the warm and saline Atlantic Water (AW) in the West Spitsbergen Current and the cold and fresh Polar Water circulating on the shelf. We assess the so-called atlantification of Kongsfjorden in the 2010–2020 decade by inspecting modifications in water properties and water masses variability through moored data and summer CTD surveys. Atlantification in this fjord has emerged as an increasing temperature and salinity, resulting from enhanced advection of Atlantic waters from the West Spitsbergen Current. The water column in inner Kongsfjorden warmed by 0.13°C/yr at 35 m and 0.06°C/yr at 85 m depth from 2010 to 2020, while salinity increased by 0.3 PSU. Depth-averaged temperatures have increased by 0.26°C/yr in the warmest months of the year, whereas they appear relatively stable in the coldest months. Both temperature and salinity present a linear regression change point in January 2017, with latter years featuring decreasing values. Highly diluted AW is found at the beginning of the decade, which give way to more and more pure AW in latter years, culminating in extensive intrusions in 2016 and 2017 determining the warmest and saltiest conditions over the decade in inner Kongsfjorden. Observations in the 2010–2020 decade confirm that Kongsfjorden has transitioned to an Atlantic-type fjord, featuring depleted sea ice conditions and rather regular shallow intrusions of AW in summer and frequently also in winter. Although single intrusions of AW are associated with dynamical events on the shelf, we found that the long-term temperature evolution in the inner Kongsfjord is consistent with the meridional temperature transport of the West Spitsbergen Current. The AW current flowing northward from lower latitudes along the western Svalbard archipelago thus has profoundly driven local conditions in the inner fjord in this decade

Garnets from Val d’Ala Rodingites, Piedmont, Italy: An Investigation of Their Gemological, Spectroscopic and Crystal Chemical Properties

In Val d\u2019Ala (Piedmont,Western Alps, Italy), the more interesting rocks for the mineralogical research are represented by rodingites (rich in mineralized veins and fractures) associated with serpentinites in the eclogitized oceanic crust of Piemonte Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, garnets are the most appreciated as mineral specimens and, in less degree despite their vivid and rich colors, for their potential as gem-quality materials. This study provides a complete gemological characterization of five faceted samples and others new information by means of Synchrotron X-ray computed micro-tomography imaging gem features. Electron-probe microanalysis (EMPA) and laser ablation\u2013inductively coupled plasma\u2013mass spectrometry (LA\u2013ICP\u2013MS) established that the chemical composition of garnets from different localities, resulted both close to pure andradite, enriched in light rare earth elements (LREE) with a positive Eu anomaly, and grossular-andradite solid solution (grandite), enriched in heavy rare earth elements (HREE). X-ray powder diffraction analyses indicate the possible coexistence of almost pure grossular and andradite. A spectroscopic approach, commonly used with gem-like material, by Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, completes the characterization of the samples. The new data on the textural and geochemical features of the grandite and andradite garnets suggest local growth processes under various chemical and oxidation conditions of metasomatic and metamorphic fluids interacting with the host-rocks. Garnets represent long-lasting mineral records of the complex geological history of the Val d\u2019Ala rodingitic dikes during their oceanic- and subduction-related metamorphic evolution

Influence of distributary channels on sediment and organic matter supply in event-dominated coastal margins: the Po prodelta as a study case

From November 2008 through May 2009, the Po river (Italy) experienced several floods exceeding 5000 m<sup>3</sup> s<sup>−1</sup>. This long series of events ended with a large flood in early May 2009 (~8000 m<sup>3</sup> s<sup>−1</sup>). An event-response sampling was carried out in the Po prodelta in April–May 2009 to characterize the preservation of this series of floods in the sediment record and to describe the event-supply and deposition of riverborne particulate material during the May 2009 flood. The water sampling was carried out early in the event under conditions of moderate river flow (~5000 m<sup>3</sup> s<sup>−1</sup>) and 24 h later during the peak discharge (~8000 m<sup>3</sup> s<sup>−1</sup>). Sediment cores were collected in the prodelta before and after the peak flood. At each station, profiles of conductivity, transmittance, and fluorescence were acquired. Surface and bottom waters were sampled to collect sediments in suspension. In addition, a few days before the May 2009 event, suspended sediments were collected at Pontelagoscuro gauging station, ~90 km upstream from the coast. Biogeochemical compositions and sedimentological characteristics of suspended and sediment samples were investigated using bulk and biomarker analyses. Furthermore, <sup>7</sup>Be and radiographs were used to analyze the internal stratigraphy of sediment cores. <br><br> During moderate flow, the water column did not show evidence of plume penetration. Stations re-occupied 24 h later exhibited marked physical and biogeochemical changes during the peak flood. However, the concentration of terrestrially-derived material in surface waters was still less than expected. These results suggested that, since material enters the Adriatic as buoyancy-driven flow with a reduced transport capacity, settling and flocculation processes result in trapping a significant fraction of land-derived material in shallow sediments and/or within distributary channels. <br><br> Although numerous discharge peaks occurred from November 2008 through April 2009 (4000–6000 m<sup>3</sup> s<sup>−1</sup>), sediment cores collected in late April 2009 showed lack of event-strata preservation and reduced <sup>7</sup>Be penetrations. This suggested that only a small fraction of the sediment supply during ordinary events reaches the deepest region of the prodelta (12–20 m water depth). As a result, these event-strata have a thickness not sufficient to be preserved in the sediment record because of post-depositional processes that destroy the flood signal. <br><br> Stations in the northern and central prodelta were re-occupied after the peak of the May 2009 flood. Based on <sup>7</sup>Be and radiographs, we estimated event layers of 17 and 6 cm thickness, respectively. Selective trapping of coarse material occurred in the central prodelta likely because of the geomorphologic setting of the central outlet characterized by an estuary-like mouth. Despite these settling processes, lignin-based parameters indicated that the composition of the terrigenous OC was fairly homogenous throughout the network of channels and between size-fractions