Annually laminated sediments of Lago Grande di Monticchio, southern Italy, have for long proven a unique palaeoenvironmental archive that facilitated the preservation of numerous (n = 340) distal volcanic ash (tephra) layers from central-southern Italian volcanoes for the last 133 kyrs. This study focuses on the establishment of a tephrochronological model for the last c. 30,000 years for a newly retrieved sediment sequence, MON16, and tests volcanic source correlations of previous studies. A total of 29 visible tephra layers in MON16 have been geochemically analysed in terms of their major element glass compositions, and out of these 21 layers were additionally characterised with respect to their trace element concentrations. Using published and newly obtained geochemical datasets of near-vent tephra deposits, most of these tephras (n = 22) could be assigned to specific eruptive events of nearby Campanian volcanoes, i.e. Somma-Vesuvius, Campi Flegrei and Ischia. Published radiometric dates of 16 of these tephra correlatives have been implemented into a Bayesian age-depth model to provide a high-precision timescale for evolving palaeoenvironmental datasets of the MON16 sequence. MON16 tephra studies in combination with new and recently published near-vent tephra geochemical datasets allowed for a review of the previously established tephrostratigraphic framework of Monticchio, leading to the revision of 25 tephra correlations and to the addition of a total of five new primary layers to the 30-ka tephra record. Lastly, this study tests the reliability of selected widespread tephras as regional synchronisation marker and discusses the qualification of the Monticchio tephra record for volcanic eruption frequency studies.

Precise and accurate sediment chronologies are essential for studies of past climate change. In the brackish-marine environment of the Baltic Sea, radiocarbon dating is complicated by varying influx of old carbon and reservoir age changes, challenging the construction of a robust age model. One approach to overcome the problem is well-dated isochronous tie-points like tephra. Here, we present for the first time four cryptotephra horizons in the interval 4500-2000 cal. a BP from sediments of the Western Gotland Basin containing tephra populations from the Hekla-4, Hekla-S, and Hekla-3 eruptions and the Glen Garry and Gr & aacute;kolla tephras from eruptions of Askja and Torfaj & ouml;kull, respectively. In combination with new and published radiocarbon ages, we quantify 14C age offsets for these snapshots in time. In homogeneous sediments, which contain old bulk organic carbon, ages reveal divergences of up to 1200 years. In time intervals not influenced by influx of old carbon, we define a regional marine reservoir age of 250 +/- 50 yr. Based on the tephra ages as tie points and applying this reservoir age we calculate a revised age model for the Western Gotland Basin with reduced dating uncertainties that shifts previous chronologies by similar to 200 years toward older ages.

The volcanoes of Nemrut and Suphan in the Eastern Anatolian Volcanic Province (EAVP) are significant sources of volcanic ash (tephra), which are found in palaeoclimatic and archaeological records in the eastern Mediterranean region. However, there is sparse glass geochemistry and little known about the eruption history of these volcanoes, limiting their full tephrochronological potential. Here, we present detailed, comprehensive singleshard major, minor and trace elem geochemistry of tephra deposits sampled at new and previously studied proximal outcrops around Lake Van and fourteen visible tephra layers (V-layers) from the ICDP Ahlat Ridge (AR) core of Lake Van spanning 130 to 30 ka. The volcanic glass from the following proximal eruption units: Lower Trachytic Pumice (LT-P), Lower Trachytic Ignimbrite (LT-I), Middle Pumices, Upper Rhyolitic Pumice and Ignimbrite (UR-PI), Upper Trachytic Ignimbrite (UT-I), and the chosen V-layers were geochemically characterised. This new glass data allows new and revised previous chrono-stratigraphic correlations between the proximal units and several V-layers. Mixed rhyolitic and trachytic glasses of V-18a correlate to the UR-P and UR-I proximal tephra units, and to previously published data from the Middle Nemrut (M-NF)-O, M-NF-I, Tatvan Ignimbrite and AP-8 units. These are all from the same caldera forming eruption of Nemrut at similar to 33 ka. Glasses of the older V-30 layer correlate to a Middle Pumice Unit and M-NF-R, and V-45 represents the 'Cekmece Formation'. The trachytic glasses of V-51 correlate to LT-P, LT-I and the M-NF-Agglutinate unit erupted from Nemrut. The distinctive basaltic glasses from V-60 correlate to an eruption of.Incekaya and V-64 is a newly identified eruption from the Suphan volcano. The older V-layers identified and analysed are from Nemrut based on comparing the new glass compositions to previously published whole-rock and glass data. This comparison indicates the V-75 pantelleritic tephra correlates to the dated AP-4 proximal unit. Each of these large eruptions are easily chemically differentiated using SiO2, FeOt, CaO and Al2O3. The integration of the proximal outcrops with the continuous, well-constrained Lake Van sedimentary medial record provides a detailed tephrostratigraphic record in a volcanic region where the proximal outcrop record is fragmented and confusing.

& Aring;sgruvanite-(Ce), ideally Ce16Ca5Al(SiO4)6(AsO3)8(CO3)2Cl3(ClF3)(OH)2, is a new mineral species (IMA-CNMNC 2025-004) from the & Aring;sgruvan Fe-skarn deposit, Norberg, V & auml;stmanland, Sweden, which is directly related to the Bastn & auml;s type of rare earth element (REE) mineralisations in the Palaeoproterozoic Bergslagen ore province. & Aring;sgruvanite-(Ce) occurs as anhedral, occasionally elongated grains up to 400 mu m. It is greyish green to nearly colourless, with a white streak and a vitreous to greasy lustre. Cleavage is distinct on {001} and less so on {100}; the mineral is brittle, and its fracture is uneven. The calculated density is 4.79(1) g cm-3. & Aring;sgruvanite-(Ce) is optically uniaxial (+), with a refractive index above 1.8; the calculated average is 1.88 (Gladstone-Dale approach). & Aring;sgruvanite-(Ce) crystallises in the trigonal system in space group P-3m1 (Z=1), with the following unit cell parameters: a=10.5728(6) & Aring; and c=15.0899(11) & Aring;. & Aring;sgruvanite-(Ce) occurs in a magnetite-REE skarn, but its formation postdates the groundmass carbonate and skarn assemblage, and it is associated with late-stage calcite, dolomite, a dollaseite-like allanite group mineral, gadolinite-(Y/Nd), and a fluorocarbonate related to bastn & auml;site-(Ce), with variable F contents. The structure was refined to R1=6.23 % for 987 reflections. It is unique and consists of two alternating layers, A and B, along the c axis. Layer A (similar to 8.4 & Aring;) has the composition [(Ce12Ca3)AlSi6(C1.50S0.50)Sigma 2.00O30(OH)2]15+. Layer B (similar to 6.7 & Aring;) corresponds to the composition [(Ce4Ca2)As83+O24Cl4F3]15-. These layers form tunnel-like features parallel to [100], which are partially occupied by Cl atoms. Spectroscopic data (infrared and micro-Raman) support the structural model.

GeoLaB (Geothermal Laboratory in the Crystalline Basement) started in the beginning of 2023 to plan and build an underground geoscientific laboratory in a fractured crystalline basement. One of the potential selected sites is the Odenwald complex (Hessen, Germany) due to its geology (fractured crystalline basement) and petrology (Tromm granite). Considerable efforts and investigations were recently implemented to evaluate, whether this site is a suitable location for the realization of GeoLaB.In the initial exploration stage, surface rock samples were collected in the Odenwald (Streitsdöll, Hammelbach and Ober-Mengelbach) area for mineral and petrological investigations. The sampling strategy aims for different structural contexts within the same lithology, e.g. a non-fractured granite, one located in the fault damage zone, and one located in the cataclastic core zone. The rock samples are macroscopically characterized by well-formed feldspar/plagioclase and mica (biotite/muscovite). Mineralogy and petrology are fundamental for investigating the composition and the occurrence of hydrothermal alteration. This influences rock properties such as porosity-permeability and also the response to applied stress.A first set of eight samples was investigated by means of X-ray powder diffraction XRD (quantitative estimation of the mineral assemblage, rock classification), electron microprobe analyzer EMP (determination of the mineral geochemistry, hydrothermal alteration and microstructures) and X-ray fluorescence XRF (whole chemistry and trace elements). The granites/granodiorites are composed of quartz, plagioclase, felspar (andesine based on the Na-Ca geochemistry), and mica (biotite and muscovite). Apatite, magnetite, rutile and monazite were detected as accessories, thus enabling geochemical dating.Three samples (Streitsdöll) show hydrothermal alterations in the form of kaolinite or clay phases with similar mineral chemistry Al2Si2O5(OH)4 at the plagioclase rims. Traces of metasomatic processes could be observed in the images acquired with the EMP. The quantitative mineral assemblage evaluation also indicated different types of plutonic rocks: granodiorite and granite based on the QAPF (Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)) diagram. A compositional variation with depth can be expected based on the mineral heterogeneity. This hypothesis will be verified by analyzing cores samples from exploration drillings planned for 2024.Besides seismic and geophysical campaigns, additional fieldwork focusing on structural geology, rock sampling, and geomechanical experiments will be conducted to develop a baseline to scientifically assess whether the Odenwald site is a suitable location to build the GeoLaB. Keywords:underground laboratory, crystalline basement, fractured granite, mineral composition variation, hydrothermal alteration.

Abstract. Åsgruvanite-(Ce), ideally Ce16Ca5Al(SiO4)6(AsO3)8(CO3)2Cl3(ClF3)(OH)2, is a new mineral species (IMA–CNMNC 2025-004) from the Åsgruvan Fe-skarn deposit, Norberg, Västmanland, Sweden, which is directly related to the Bastnäs type of rare earth element (REE) mineralisations in the Palaeoproterozoic Bergslagen ore province. Åsgruvanite-(Ce) occurs as anhedral, occasionally elongated grains up to 400 µm. It is greyish green to nearly colourless, with a white streak and a vitreous to greasy lustre. Cleavage is distinct on {001} and less so on {100}; the mineral is brittle, and its fracture is uneven. The calculated density is 4.79(1) g cm−3. Åsgruvanite-(Ce) is optically uniaxial (+), with a refractive index above 1.8; the calculated average is 1.88 (Gladstone–Dale approach). Åsgruvanite-(Ce) crystallises in the trigonal system in space group P-3m1 (Z=1), with the following unit cell parameters: a=10.5728(6) Å and c=15.0899(11) Å. Åsgruvanite-(Ce) occurs in a magnetite–REE skarn, but its formation postdates the groundmass carbonate and skarn assemblage, and it is associated with late-stage calcite, dolomite, a dollaseite-like allanite group mineral, gadolinite-(Y/Nd), and a fluorocarbonate related to bastnäsite-(Ce), with variable F contents. The structure was refined to R1=6.23 % for 987 reflections. It is unique and consists of two alternating layers, A and B, along the c axis. Layer A (∼8.4 Å) has the composition [(Ce12Ca3)AlSi6(C1.50S0.50)Σ2.00O30(OH)2]15+. Layer B (∼6.7 Å) corresponds to the composition [(Ce4Ca2)As83+O24Cl4F3]15−. These layers form tunnel-like features parallel to [100], which are partially occupied by Cl atoms. Spectroscopic data (infrared and micro-Raman) support the structural model.

The volcanoes of Nemrut and Süphan in the Eastern Anatolian Volcanic Province (EAVP) have been active throughout the late Quaternary, dispersing ash over wide areas. Numerous eruptions spanning the last interglacial to glacial period (130-30kya) are recorded in outcrops around the volcanoes and in the cores from Lake Van. A comprehensive tephrostratigraphy framework for the EAVP is an essential requirement for synchronising palaeoclimatic and archaeological records across the Mediterranean region. More information is required from the eruption deposits, including glass geochemistry on individual shards, and the stratigraphic order and precise ages for the eruptions. Here we present the findings of our detailed tephra investigation for the palaeoclimatic record of Lake Van (Turkey), as part of the TephroMed project. The lake is located close to both Nemrut and Süphan. Fourteen visible tephra layers (called V-layers) from Lake Van PALEOVAN ICDP record underwent geochemical characterisation by using major, minor (EPMA) and trace element analysis (LA-ICP-MS). New chemical analyses and 40Ar/39Ar ages of eruption deposits from proximal outcrops, provide detailed insight into the complex volcanic history of the EAVP, particularly of Nemrut. These results have revealed new volcanic origins for certain V-layers to Nemrut and Süphan, with tentative correlations between Lake Van’s fourteen V-layers to previously published dated proximal outcrops. Additionally, new glass geochemistry from dated proximal outcrops confirm the ages of the key eruption events associated with V-51 and V-18a of Lake Van. This improves the chronology of the Lake Van record and provides an understanding for the timing of complex eruptions from Nemrut and Süphan. The results are a major step forward to building a detailed tephrostratigraphic framework for the EAVP.

The magmatic-hydrothermal transition is an important but poorly-understood process in the formation of Sn-W, Nb-Ta and Li deposits associated with evolved granites and pegmatites. Theory predicts that boron isotopes will fractionate between magma and fluid, so the magmatic-hydrothermal transition may be recorded in the borosilicate mineral tourmaline, which is widespread and common in these kinds of deposits. The key information needed to interpret the tourmaline record is the B-isotope fractionation between granitic melts and the fluids derived from them but former experimental studies on B-isotope fractionation between the relevant phases are not in agreement (e.g. Kowalski and Wunder, 2018, Maner and London, 2018). This study fills this gap by an experimental, multivariant approach. We synthesized a glass of haplogranitic composition (Ab40Or25Qtz35) and produced variants of water content (0, 4 and 6 wt%), aluminum saturation (ASI 0.7, 1, 1.3) and boron concentration (2 and 5 wt%). For each composition we determined the coordination environment of B in the glass and the fractionation of B isotopes between the respective melt and aqueous fluid at near-solidus temperature. The first part of the study was the chemical characterization and analysis of B coordination in the glasses. The NMR analysis of 11B indicates that the coordination of 11B is dominantly trigonal in all glasses, but there is an increase of tetrahedral coordination with increasing boron concentration and water content. Fluid-melt fractionation experiments are ongoing and first results will be presented. References:• Kowalski, P., Wunder, B. (2018). Boron isotope fractionation among vapor–liquids–solids–melts: Experiments and atomistic modeling. In: Marschall, H., Foster, G. (eds) Boron Isotopes. Advances in Isotope Geochemistry. Springer-Verlag, Berlin-Heidelberg, volume 7, pages 33–69• Maner, J. L., London, D. (2018) Fractionation of the isotopes of boron between granitic melt and aqueous solution at 700 °C and 800 °C (200 MPa), Chemical Geology, Volume 489, Pages 16-27

We examined the influence of Al2O3 and H2O on the position of the coesite-stishovite transition by means of in situ X-ray diffraction measurements with the large-volume press at the PETRA III synchrotron in Hamburg. The position of the transition was determined by several reversal experiments and was found to be shifted almost in parallel by about 1.5 GPa to lower pressures compared to results for the pure SiO2 system reported by Ono et al. (2017). Two further reversal experiments with either additional Al2O3 or additional H2O added to SiO2 showed smaller changes compared to the results of Ono et al. (2017), indicating the effect of the coupled Al and H incorporation in coesite and stishovite on their transition. Further investigations of the solid quenched products and of products from additional multi-anvil experiments performed at the GFZ Helmholtz-Zentrum f & uuml;r Geoforschung in Potsdam were done by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), electron probe micro-analysis (EPMA), and Fourier transform infrared (FTIR) and Raman spectroscopy. Generally, the recovered samples of the in situ experiments contained less stishovite than expected from the last in situ XRD pattern before quenching. Thus, these investigations clearly show that hydrous, Al-rich stishovite that formed at high pressure (P) and temperature (T) could, at least partly, not be quenched to room conditions and transformed to coesite with unusually high (Al, H) contents. As result of this, conventional quench experiments would lead to erroneous results of the transition in the (Al, H)-bearing SiO2 system. We observed two kinds of coesite in the experiments: one relatively Al-poor coesite transformed under equilibrium conditions at P and T from stishovite over a certain time frame and an Al-richer one, sometimes pseudomorphically replacing former stishovite during the decompression process to room conditions. Within both types of coesite, nanometre-sized kyanite inclusions and relicts or remnants of stishovite were observed by TEM. These observations resemble those of Yang et al. (2007) on ophiolites with identical textures and phases and were interpreted as result of a stishovite transition back to coesite during retrograde metamorphism. Our results clearly indicate that the coesite-stishovite transition is sharp but can considerably vary in depth by the addition of Al and H to the SiO2 system. This has consequences for the assignment and interpretation of the depth variation of the seismic X discontinuity.

Robust chronologies and time equivalent tephra markers are essential to better understand spatial palaeoenvironmental response to past abrupt climatic changes. Identification of well-dated and widely dispersed volcanic ash by tephra and cryptotephra (microscopic volcanic ash) provides time synchronous tie-points and strongly reduces chronological uncertainties. Here, we present the major, minor and trace element analyses of cryptotephra shards in the Dead Sea Deep Drilling sedimentary record (DSDDP 5017-1A) matching the Campanian Ignimbrite (CI). This geochemical identification expands the known dispersal range of the CI to the southeastern Mediterranean, over 2300 km from the volcanic source. Due to the CI eruption occurring near-synchronous with North Atlantic ice surge of Heinrich Event 4 (HE4), this tephra provides insights into regional responses to large-scale climatic change in the Mediterranean. In the Dead Sea, the CI layer is associated with wetter climatic conditions. This contrasts with the contemporaneous occurrence of the CI deposition and dry conditions in the central and eastern Mediterranean suggesting a possible climate time-transgressive expansion of HE4. Our finding underscores the temporal and spatial complexity of regional climate responses and emphasises the importance of tephra as a time marker for studying large-scale climatic changes verses regional variations.

The solubility of wo center dot hlerite, ideally Na2Ca4ZrNb(Si2O7)2O3F, in peralkaline SiO2-undersaturated melts (Na2O-CaO-Al2O3-SiO2 +/- F +/- H2O) was studied at the following temperature (T) and pressure (P) conditions: 750 degrees C, 200 MPa, 850 degrees C, 100 MPa, and 1000 degrees C, 200 MPa. Wo center dot hlerite appeared to be stable up to high tem-perature, with increasing solubility, which is consistent with field observations of early wo center dot hlerite crystallization in pegmatites in the Larvik Plutonic Complex. A lovozerite-group mineral (combeite-zirsinalite-lovozerite) was frequently observed to crystallize in the experiments at 750 degrees C, and, at very high Na/Al ratios, also at 850 degrees C. Fluorcalciopyrochlore formed at 1000 degrees C in experiments where water was added. The concentrations of Zr and Nb in the quenched glasses were generally very high and found to increase strongly with temperature. Moreover, they also increase with addition of water and with increasing peralkalinity. The effect of CaF2 addition was insignificant. Based on comparison with the limited information in the available literature, the solubilities of Ca-bearing Zr and Nb minerals (wo center dot hlerite, lovozerite-group minerals, fluorcalciopyrochlore) in peralkaline silica-undersaturated melts may be lower than those of nominally Ca-free Zr and Nb minerals (zircon, baddeleyite, wadeite, columbite-(Mn)). The experimental data do not support the existence of significant Zr-F or Nb-F complexation. The high concentrations of Nb and Zr in the reacted peralkaline melts can be explained by strongly enhanced formation of alkali-Nb/Zr-silicate complexes compared to that at peraluminous conditions. These complexes are the underlying chemical reason for characteristic high-field-strength-element (HFSE) enrichment in agpaitic and hyperagpaitic rocks.

We examined the reaction phase A plus high-P clinoenstatite to forsterite plus water (Reaction R1) by means of in situ X-ray diffraction measurements with the large volume press at the synchrotron PETRA III, Hamburg. Contrary to the study of Lathe et al. (2022), in which all experiments on Reaction (R1) were performed at a water activity of 1, the reversed experiments presented in this study were performed at reduced water activity with mole fractions of about XH2O= XCO2=0.5. The intention of this investigation was to test the observation made by Perrillat et al. (2005), which was that dehydration reactions are kinetically faster at reduced than under water-saturated conditions. The position of Reaction (R1) at the reduced conditions was determined by reversal brackets at 9.1 and 9.5 GPa (630 and 700 ∘C), at 9.7 and 10.0 GPa (725 and 700 ∘C), at 9.8 and 10.2 GPa (675 and 750 ∘C), and at 10.5 GPa (675 and 740 ∘C). Additionally, we performed two offline experiments with brackets at 10.0 and 10.6 GPa (750 and 700 ∘C, respectively) that are in agreement with the results of the in situ experiments. We do not observe any “intermediate” precursor phase in our experiments. The equilibrium of Reaction (R1) is shifted by about 100 ∘C to lower temperature compared to the results under water-saturated conditions. Thus, at a water activity (aH2O) below 1 the phase A plus clinoenstatite dehydration reaction can only occur in extremely cold subduction slabs. The kinetics of Reaction (R1) dehydration at reduced water activity is slower than that determined previously by Lathe et al. (2022) under water-saturated conditions. Thus, the above-mentioned hypothesis of Perrillat et al. (2005) could not be confirmed. However, in both of our studies on Reaction (R1), the newly formed dehydration product forsterite was of nanometer size, which supports earlier experimental observations, which is that product phases of dehydration reactions are generally very fine-grained and might promote the concept that intermediate-depth earthquakes in subduction zones are initiated by mechanical instabilities from extremely fine-grained materials formed during dehydration reactions.

The Late Glacial (14 700–11 700 cal BP) is a key climate period marked by rapid but contrasted changes in the Northern Hemisphere. Indeed, regional climate differences have been evidenced during the Late Glacial in Europe and the northern Mediterranean. However, past climate patterns are still debated since temperature and precipitation changes are poorly investigated towards the lower European latitudes. Lake Matese in southern Italy is a key site in the central Mediterranean to investigate climate patterns during the Late Glacial. This study aims to reconstruct climate changes and their impacts at Matese using a multi-proxy approach including magnetic susceptibility, geochemistry (XRF core scanning), pollen data and molecular biomarkers like branched glycerol dialkyl glycerol tetraethers (brGDGTs). Paleotemperatures and paleo-precipitation patterns are quantitatively inferred from pollen assemblages (multi-method approach: modern analogue technique, weighted averaging partial least-squares regression, random forest and boosted regression trees) and brGDGT calibrations. The results are compared to a latitudinal selection of regional climate reconstructions in Italy to better understand climate processes in Europe and in the circum-Mediterranean region. A warm Bølling–Allerød and a marked cold Younger Dryas are revealed in all climate reconstructions inferred from various proxies (chironomids, ostracods, speleothems, pollen, brGDGTs), showing no latitudinal differences in terms of temperatures across Italy. During the Bølling–Allerød, no significant changes in terms of precipitation are recorded; however, a contrasted pattern is visible during the Younger Dryas. Slightly wetter conditions are recorded south of 42∘ N, whereas dry conditions are recorded north of 42∘ N. During the Younger Dryas, cold conditions can be attributed to the southward position of North Atlantic sea ice and of the polar frontal jet stream, whereas the increase in precipitation in southern Italy seems to be linked to relocation of Atlantic storm tracks into the Mediterranean, induced by the Fennoscandian ice sheet and the North European Plain. By contrast, warm conditions during the Bølling–Allerød can be linked to the northward position of North Atlantic sea ice and of the polar frontal jet stream.

AbstractFluorbritholite-(Nd), ideally Ca2Nd3(SiO4)3F, has been approved by the International Mineralogical Association (IMA2023–001) and constitutes a new member of the britholite group of the apatite supergroup. It occurs in skarn from the Malmkärra iron mine, Norberg, Västmanland (one of the Bastnäs-type deposits in Sweden), associated with calcite, dolomite, magnetite, lizardite, talc, fluorite, baryte, scheelite, gadolinite-(Nd) and other REE minerals. Fluorbritholite-(Nd) forms anhedral and small grains, rarely up to 250 μm across. They are brownish pink and transparent with a vitreous to greasy lustre. The mineral is brittle, with an uneven or subconchoidal fracture and lacks a cleavage. In thin section, the mineral is nonpleochroic, uniaxial (–). Dcalc = 4.92(1) g⋅cm−3 and ncalc = 1.795. The empirical chemical formula from electron microprobe (WDS) point analyses is (Ca1.62Nd0.97Ce0.83Y0.52Sm0.30Gd0.23Pr0.17La0.16Dy0.11Er0.03Tb0.03Ho0.01Yb0.01)Σ4.99(Si2.92P0.08As0.01)Σ3.01O12.00[O0.48F0.26(OH)0.14Cl0.10Br0.02]Σ1.00. The crystal structure of fluorbritholite-(Nd) was refined from single-crystal X-ray diffraction data to R1= 0.043 for 704 unique reflections. It belongs to the hexagonal system, space group P63/m, with unit cell parameters a = 9.5994(3), c = 6.9892(4) Å and V = 557.76(5) Å3 for Z = 2. Fluorbritholite-(Nd) and other britholite-group minerals are a major sink for neodymium in REE-bearing skarns of Bastnäs type.

The presence of magnesite (MgCO3) in the Earth’s mantle plays a fundamental role in reducing the melting point of the mantle [1] and forming carbonate‑rich melts such as kimberlites and carbonatites [2]. The melting curve of (dry) magnesite is well constrained [3, 4], but melting of magnesite in the presence of H2O, providing the basis for more complex (natural) systems, is poorly understood from some quenched experiments [5]. Also, the distribution of trace elements such as Li, Sr, Pb, and rare earth elements during melting of magnesite is poorly considered in models that evaluate the trace element budget of carbonate‑rich melts parental to kimberlites [6].Here we report, first, the H2O‑present melting curve of magnesite between 2 and 12 GPa. The melting curve of magnesite mixed with 16 wt% brucite was established by in ‑ situ X‑ray diffraction measurements using the large volume press at P61B at PETRA III (DESY). Second, we report trace element partitioning data for congruent melting of calcite and incongruent melting of magnesite producing carbonate melt and periclase between 6 and 9 GPa. Those results were obtained from quenched experiments using a rocking multi‑anvil press at the GFZ overcoming equilibrium and quenching problems in previous studies [7]. 1 Dasgupta and Hirschmann, The deep carbon cycle and melting in Earth's interior. Earth and Planetary Science Letters, 2010. 298(1-2): p. 1-13.2 Jones, Genge, and Carmody, Carbonate Melts and Carbonatites. Reviews in Mineralogy and Geochemistry, 2013. 75(1): p. 289-322.3 Solopova, Dubrovinsky, Spivak, Litvin, and Dubrovinskaia, Melting and decomposition of MgCO3 at pressures up to 84 GPa. Physics and Chemistry of Minerals, 2014. 42(1): p. 73-81.4 Müller, Koch-Müller, Rhede, Wilke, and Wirth, Melting relations in the system CaCO3-MgCO3 at 6 GPa. American Mineralogist, 2017. 102(12): p. 2440-2449.5 Ellis and Wyllie, Carbonation, hydration, and melting relations in the system MgO-H2O-CO2 at pressures up to 100 kbar. American Mineralogist, 1979. 64(1-2): p. 32-40.6 Girnis, Bulatov, Brey, Gerdes, and Höfer, Trace element partitioning between mantle minerals and silico-carbonate melts at 6–12GPa and applications to mantle metasomatism and kimberlite genesis. Lithos, 2013. 160-161: p. 183-200.7 Buob, Experiments on CaCO3-MgCO3 solid solutions at high pressure and temperature. American Mineralogist, 2006. 91(2-3): p. 435-440.

The most profound consequences of the presence of Ca–Mg carbonates (CaCO3–MgCO3) in the Earth's upper mantle may be to lower the melting temperatures of the mantle and control the melt composition. Low-degree partial melting of a carbonate-bearing mantle produces CO2-rich, silica-poor melts compositionally imposed by the melting relations of carbonates. Thus, understanding the melting relations in the CaCO3–MgCO3 system facilitates the interpretation of natural carbonate-bearing silicate systems. We report the melting relations of the CaCO3–MgCO3 system and the partition coefficient of trace elements between carbonates and carbonate melt from experiments at high pressure (6 and 9 GPa) and temperature (1300–1800 ∘C) using a rocking multi-anvil press. In the absence of water, Ca–Mg carbonates are stable along geothermal gradients typical of subducting slabs. Ca–Mg carbonates (∼ Mg0.1–0.9Ca0.9–0.1CO3) partially melt beneath mid-ocean ridges and in plume settings. Ca–Mg carbonates melt incongruently, forming periclase crystals and carbonate melt between 4 and 9 GPa. Furthermore, we show that the rare earth element (REE) signature of Group-I kimberlites, namely strong REE fractionation and depletion of heavy REE relative to the primitive mantle, is resembled by carbonate melt in equilibrium with Ca-bearing magnesite and periclase at 6 and 9 GPa. This suggests that the dolomite–magnesite join of the CaCO3–MgCO3 system might be useful to approximate the REE signature of carbonate-rich melts parental to kimberlites.

The dehydration reaction of phase A + high P clinoenstatite to forsterite + water was experimentally investigated at water-saturated conditions in the pressure range between 7.0 and 10.0 GPa by in situ reversal runs in a multi-anvil press at the synchrotron source of PETRA III in Hamburg. By using closed watertight X-ray transparent Ti capsules, its position is determined by reversal brackets at 8.3 GPa (700–760 ∘C), 8.6 GPa (700–740 ∘C), and 9.8 GPa (750–800 ∘C); thus, the equilibrium of the reaction corresponds ideally to the data reported by Wunder (1998). Optical investigations of the quenched product phases show strong grain coarsening of phase A and clinoenstatite, whereas nucleated forsterite from the breakdown of the aforementioned phases is very fine grained. This corresponds to recent experimental observations that the grain size of phases formed in hydration reactions are significantly larger than those from dehydration reactions. In addition, we performed three time-dependent in situ experiments at 9–10 GPa and 800–870 ∘C and monitored the reaction progress every 10 min to determine the kinetics of the forsterite formation from phase A + high P clinoenstatite. The growth of forsterite at these P–T conditions, already visible after 10 min, confirms the results of the bracketing experiments. However, the reaction is extremely slow, and even after more than 3 h, significant amounts of phase A and high P clinoenstatite are still present. This is in contradiction to other dehydration reactions of former experimental studies, e.g. the fast dehydration of serpentine, which completely dehydrates within 3 h, even at much lower temperatures, closely overstepping serpentine stability. Despite its reaction sluggishness, which would contradict the concept of earthquake initiation, the observed formation of nano-sized forsterite as a dehydration product may still indicate the potential of this reaction to cause mechanical instabilities and, thus, seismicity within cold subduction zones at depths of the Earth's mantle. Additionally, at depths exceeding serpentine dehydration, the phase A + high P/low P clinoenstatite breakdown to forsterite + water might induce geochemical and geophysical processes, including the formation of low-velocity zones within the overlying mantle wedge from the large amounts of fluid liberated by this water line reaction. After the breakdown of antigorite, the assemblage phase A + clinoenstatite might act as a bridge to transport water to larger depths during cold subduction, followed by the formation of other hydrous high P phases.

This study compares the performance of three analytical methods, hyperspectral imaging in the short-wave infrared range (SWIR), X-ray powder diffraction (XRD), and electron-microprobe analysis (EMPA) for the identification of minerals frequently formed during natural and technological geothermal processes. The samples from three geothermal sites in Indonesia contain a spectrum of minerals, which are characteristic for geothermal activities (alunite, jarosite, cristobalite, tridymite) and/or are involved in scaling-forming processes in geothermal plants (barite, celestine, common opal, calcite). We show and discuss the strengths and weaknesses of the individual techniques with respect to properly identifying these minerals and to approximate their relative proportions. The SWIR camera permits an unambiguous identification of only a small selection of the geothermally relevant minerals, which restricts its employment as routine tool in the context of geothermics. XRD and EPMA are more powerful techniques and deliver complementary information. In case that SWIR inactive species occur, (b) the entire spectrum of minerals present in a sample has to be unambiguously identified, (c) amorphous substances are present, (d) accurate mineral compositions are required, and (e) detailed information on minerals occurring in low abundance and grain size is warranted, the combined use of XRD and EMPA is mandatory.

Abstract We have examined the effect of composition on the Raman-active vibrational response of the Cu2(Fe x Zn1−x )SnS4 and Cu2(Mn x Zn1−x )SnS4 solid solution series at ambient conditions. Based on these results we were able to identify the phase boundaries of the respective kesterite-type and stannite-type structures adopted by these compounds as a function of composition. In the case of Cu2(Fe x Zn1−x )SnS4, our observations correlate very well with earlier reports. For the Cu2(Mn x Zn1−x )SnS4 series, on the other hand, we were able to clearly pinpoint the kesterite↔stannite transition for intermediate compositions for the first time, indicating that Raman spectroscopy can serve as an efficient method for monitoring subtle structural transitions in these systems.