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The volcanic rocks of the Chon Aike Silicic Large Igneous Province (CASP) are recognized as magmas dominantly produced by crustal anatexis. Investigating the zircon of the CASP provides an opportunity to gain further insight into geochemical and isotopic differences of the potential magmatic sources (i.e., crust versus mantle), to identify crustal reservoirs that contributed to the felsic magmas during anatexis, and to quantify the contributions of the respective sources. We present a combined zircon oxygen and hafnium isotope and trace element dataset for 16 volcanic units of the two youngest volcanic phases in Patagonia, dated here with LA-ICP-MS U–Pb geochronology at ca. 148–153 Ma (El Quemado Complex, EQC) and ca. 159 Ma (western Chon Aike Formation, WCA). The EQC zircon have¹⁸O-enriched values (δ¹⁸O from 7 to 9.5‰) with correspondingly negative initial εHf values (− 2.0 to − 8.0). The WCA zircon have δ¹⁸O values between 6 and 7‰ and εHf values ranging between − 4.0 and + 1.5. Binary δ¹⁸O-εHf mixing models require an average of 70 and 60% melt derived from partial melting of isotopically distinct metasedimentary basements for the EQC and WCA, respectively. Zircon trace element compositions are consistent with anatexis of sedimentary protoliths derived from LIL-depleted upper continental crustal sources. The overlap between a high heat flux environment (i.e., widespread extension and lithospheric thinning) during supercontinental breakup and a fertile metasedimentary crust was key in producing voluminous felsic volcanism via anatexis following the injection and emplacement of basaltic magmas into the lower crust.

 

The Jurassic Chon Aike Silicic Large Igneous Province (Patagonia and the Antarctic Peninsula) is dominated by voluminous, crust-derived magmas (235,000 km3) that erupted as predominately explosive silicic material over ~40 m.y. In this study, we combine petrological descriptions and bulk-rock major- and trace-element compositions with quartz oxygen-isotope measurements from multiple silicic units (primarily ignimbrites and some rhyolitic flows) from two of the five silicic formations in Patagonia. We have identified that quartz oxygen-isotope values are high (>9‰–12‰). Quartz phenocrysts analyzed by secondary ion mass spectroscopy (SIMS) are also homogeneous at the microscale with no measurable change in isotope value with respect to internal and often complex zoning textures. The ubiquity of widespread high δ18O rhyolites and their trace-element compositions support their origin from melting of a metasedimentary source with a similarly high δ18O value. Mass balance calculations require that an average of >75% melt derived from partial melting of the dominant basement lithology is needed to explain the isotopic and chemical composition of the rhyolites. The ideal P-T environment was identified by thermodynamic models for fluid-absent melting of graywackes at 900 °C and 5 kbar. Regional-scale crustal melting occurred during a widespread, high heat-flux environment within an extensional setting during the break- up of the Gondwanan supercontinent. The overlap of a unique tectonic and igneous environment, combined with a fertile crust dominated by graywacke and pelitic compositions in southern Patagonia, generated large volumes of some of the highest δ18O silicic magmas documented in the geologic record. 

We present a novel method for the measurement of lithium isotopes in garnet utilising glass reference materials and secondary ion mass spectrometry (SIMS). Measured lithium isotopic compositions of natural garnets are heterogeneous, making them unreliable reference materials forin situdetermination. However, SIMS lithium isotope measurements of glasses derived from these natural garnets are isotopically identical to their parent garnets and more homogeneous, demonstrating that they can be used as reliable reference materials. To characterise the composition dependence of instrumental mass fractionation (IMF), oxide and silicate powders were used to synthesise custom‐made glass reference materials (CGRMs) with garnet‐equivalent compositions. Results for six CGRMs measured by SIMS show a significant linear relationship between IMF and FeO and MnO contents. Corrections for this compositional IMF result in changes of up to 12‰ within the compositional range explored. Uncertainty in IMF‐corrected SIMS analyses with a 20 μm spot is in the range of 2.5 to 4.5‰, depending on the garnet composition and reference materials used. The method forin situlithium isotope measurement in garnet by SIMS presented here is highly adaptable, valid across a range of Al‐rich garnet compositions, and yields spatial resolution and precision necessary to address a range of geological applications.

 

Six tourmaline samples were investigated as potential reference materials (RMs) for boron isotope measurement by secondary ion mass spectrometry (SIMS). The tourmaline samples are chemically homogeneous and cover a compositional range of tourmaline supergroup minerals (primarily Fe, Mg and Li end‐members). Additionally, they have homogeneous boron delta values with intermediate precision values during SIMS analyses of less than 0.6‰ (2s). These samples were compared with four established tourmaline RMs, that is, schorl IAEA‐B‐4 and three Harvard tourmalines (schorl HS#112566, dravite HS#108796 and elbaite HS#98144). They were re‐evaluated for their major element and boron delta values using the same measurement procedure as the new tourmaline samples investigated. A discrepancy of about 1.5‰ in δ¹¹B was found between the previously published reference values for established RMs and the values determined in this study. Significant instrumental mass fractionation (IMF) of up to 8‰ in δ¹¹B was observed for schorl–dravite–elbaite solid solutions during SIMS analysis. Using the new reference values determined in this study, the IMF of the ten tourmaline samples can be modelled by a linear combination of the chemical parameters FeO + MnO, SiO₂and F. The new tourmaline RMs, together with the four established RMs, extend the boron isotope analysis of tourmaline towards the Mg‐ and Al‐rich compositional range. Consequently, thein situboron isotope ratio of many natural tourmalines can now be determined with an uncertainty of less than 0.8‰ (2s).