This is a repository copy of Calibration of the oxygen and clumped isotope thermometers for (proto-)dolomite based on synthetic and natural carbonates.

9The application of clumped isotopes ( 47 ) in carbonate minerals as a sensitive temperature proxy in 10 paleo-environments depends on a well-constrained clumped isotope fractionation for the necessary 11 step of phosphoric acid digestion of the carbonate mineral to produce CO 2 . Published estimates for 12 clumped isotope fractionations vary, and the effect of different carbonate mineralogies is still under 13 debate. Differences in the sample preparation design and sample digestion temperatures are 14 potential sources for varying acid fractionations and could be a source for discrepant  47 -15 temperature calibrations observed in different laboratories. To evaluate the clumped isotope acid 16 fractionation at 70 °C and simultaneously account for a potential cation effect we analyzed a set of 17 eight carbonate minerals (calcite, aragonite, dolomite and magnesite) that were driven to a 18 stochastic isotope distribution by heating them to temperatures of 1000 °C. Our study reveals 19 significant cation-and mineral-specific differences for the  47 acid fractionation of carbonate 20 minerals digested at 70 °C or 100 °C. The  47 acid fractionation at 70 °C for calcite is 0.197±0.002 ‰, 21 for aragonite 0.172±0.003 ‰, whereas dolomite has a significantly larger acid fractionation of 22 0.226±0.002 ‰. For magnesite digested at 100 °C we observed a  47 acid fractionation of 23 0.218±0.020 ‰. Projected to an acid digestion at 25 °C, our acid fractionation for calcite of 0.260 ‰ 24 is statistically indistinguishable from existing studies. We further show that the  47 of the calcite 25 standards ETH-1 and ETH-2 of 0.265 ‰ and 0.267 ‰, respectively, are in the range of the 26 determined acid fractionation projected to 25 °C suggesting that they have an identical and near 27 stochastic isotope distribution. The observed differences in the  47 acid fractionation between calcite 28 and aragonite ( 47 = -0.025 ‰) and between calcite and dolomite ( 47 = -0.029 ‰) does not 29 correlate with the phosphoric acid fractionation of oxygen isotopes, but rather depends on the radius 30 of the cation as well as on the mineral structure. Our results reveal that the acid fractionation of 31 dolomite at 70 °C is significantly distinct from the one of calcite, but at 90 °C the two are within error 32 © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ with M being the metal cation of the corresponding carbonate mineral (e.g. calcite and aragonite 52 (CaCO 3 ), dolomite (CaMg(CO 3 ) 2 ), magnesite (MgCO 3 ), siderite (FeCO 3 ), witherite (BaCO 3 )). Having 53 attained equilibrium this exchange reaction (Eq. 1) is thermodynamically controlled and produces an 54 excess in the doubly substituted isotopologue relative to a stochastic isotope distribution (0.5 ‰ at 0 55 °C), which decreases with increasing temperature and approaches a stochastic isotope composition 56 at 1000 °C (Schauble et al., 2006). Ghosh et al. (2006a) demonstrated the feasibility...

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Middle Triassic magmatism in the Southern Alps (northern Italy) consists of widespread volcanoclastic deposits, basaltic lava flows and intrusive complexes. Despite their importance in understanding the geodynamic evolution of the westernmost Tethys, the timing of magmatic activity and the links between the different igneous products remain poorly understood. We present a comprehensive high-precision zircon U-Pb geochronology dataset for the major intrusive complexes and several volcanic ash layers and integrate this with a high-resolution stratigraphic framework of Middle Triassic volcanosedimentary successions. The main interval of Middle Triassic magmatism lasted at least 5.07 ± 0.06 myr. Magmatic activity started with silicic eruptions between 242.653 ± 0.036 and 238.646 ± 0.037 Ma, followed by a <1 myr eruptive interval of voluminous basaltic lava flows. Coeval mafic to intermediate intrusions dated at 238.190 ± 0.055 to 238.075 ± 0.087 Ma may represent feeder and subvolcanic complexes related to the basalt flows. The youngest products are silicic tuffs from latest Ladinian to early Carnian sequences dated at 237.680 ± 0.047 and 237.579 ± 0.042 Ma. Complemented by zircon trace element data, our high-resolution temporal framework places tight constraints on the link between silicic and mafic igneous products in a complex geodynamic setting. Supplementary material: Isotope dilution thermal ionization mass spectrometry U-Pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables, sample coordinates, supplementary geochemical data, cathodoluminescence images of isotope dilution thermal ionization mass spectrometry dated zircons and supplementary field documentation are available at

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The growth and evolution of crustal-scale magmatic systems play a key role in the generation of the continental crust, the largest eruptions on Earth, and the formation of metal resources vital to our society. However, such systems are rarely exposed on the Earth’s surface, limiting our knowledge about the magmatic processes occurring throughout the crust to indirect geochemical and petrographic data obtained from the shallowest part of the system. The Hf isotopic composition of accessory zircon is widely used to quantify crust-mantle evolution and mass transfers to and within the crust. Here we combine single-grain zircon Hf isotopic analysis by LA-MC-ICP-MS with thermal modelling to one of the best-studied crustal-scale igneous systems (Sesia Magmatic System, northern Italy), to quantify the relative contribution of crustal- and mantle-derived magmas in the entire system. Zircons from the deep gabbroic units define a tight range of εHf (−2.5 ± 1.5). Granites and rhyolites overlap with this range but tail towards significantly more negative values (down to −9.5). This confirms that the entire system consists of hybrid magmas that stem from both differentiation of mantle-derived magmas and melting of the crust. Thermal modelling suggests that crustal melting and assimilation predominantly occurs during emplacement and evolution of magmas in the lower crust, although melt production is heterogeneous within the bodies both spatially and temporally. The spatial and temporal heterogeneity resolved by the thermal model is consistent with the observed Hf isotope variations within and between samples, and in agreement with published bulk-rock Sr–Nd isotopic data. On average, the crustal contribution to the entire system determined by mixing calculations based on Hf isotopic data range between 10 and 40%, even with conservative assumptions, whereas the thermal model suggests that this space- and time-averaged contribution does not exceed 20%. However, spatial and temporal variations in the crustal melt proportion (from 0 up to 80% as observed in the thermal model) may impart significant isotopic variability to different batches of magma observed on the outcrop scale, emphasizing the need to consider a magmatic system as a whole, i.e., by integrating all spatial and temporal scales, to more precisely quantify crustal growth vs. reworking.