Carbonatitic dykes surrounded by K-Na-fenites were discovered in the Pelagonian Zone in Greece. Their carbonate portions have an isotopic mantle signature of δ13C and δ18O ranging from −5.18 to −5.56 (‰ vs. VPDB) and from 10.68 to 11.59 (‰ vs. VSMOW) respectively, whereas their mafic silicate portions have high Nb, Ta and ɛNd values, typical of alkaline basalts. Textural relationships hint at a cogenetic intrusion of silicate and carbonate liquids that according to antithetic REE profiles segregated at shallow depths (<0.6 GPa) from a parental melt sourced deeper in the mantle. Fenites bear similar REE abundances to mafic rocks but with high Rb-Ba and low Nb-Ta values. SHRIMP II U-Pb analyses of magmatic zircon cores (δ18O = 7.21–7.51) from a carbonate-bearing syenitic amphibolite yielded a Permian intrusion age at 278 ± 2 Ma, considerably older than a Cretaceous (118 ± 4 Ma) greenschist overprint obtained from metamorphic zircon rims (δ18O = 6.78–7.02). From 300 to 175 Ma the ɛNd of the Pelagonian magmatism rose irregularly to more primitive values attesting to a higher increment of asthenosphere-derived melts. In this context, the carbonatite formed within a transtensional regime of an intra-Pangaea dextral transform fault that signalled the forthcoming penetrating breakoff of the supercontinent, manifested in the Permo-Triassic.
The oxygen isotope (δ18O) analysis of carbonate fossils is widely applied for palaeoceanographic analysis, whereas that of siliceous fossils is only limited partly due to technical constraints and uncertain fractionation factors. Here we used a secondary ion mass spectrometer (SIMS) for δ18O of radiolarian silica, precipitated inside radiolarian molds in Mesozoic radiolarites from Japan, Italy, Switzerland and Romania in order to examine its potential for palaeoceanographic proxy. 507 measurements of the isotopic oxygen signature relative to the Vienna Standard Mean Ocean Water (δ18OVSMOW) of 53 chert samples range between 19.8 to 35.3 ‰ overlapping with that of modern and Cenozoic radiolarian tests in the equatorial Pacific. Relatively large intra-chert variability supports that δ18O of the Mesozoic radiolarian tests are not perfectly homogenized within a chert bed during the diagenetic segregation. The temporal changes in the δ18O values of radiolarians (δ18Oradiolarians) show an Early-Middle Triassic slight positive excursion, a Late Triassic high plateau, an Early Jurassic negative excursion with up to 8 ‰ , a Middle Jurassic slight positive excursion, and a few light values for the Cretaceous despite of their low resolution. A comparison of δ18O between radiolarian molds, conodont apatite, and the low magnesium calcium shells show overall similar secular variations during the Triassic, but different trends was observed during the Early Jurassic. Because our data is low-resolution, further cross check of δ18Oradiolarians is necessary to use as a proxy for paleoceanography
Significance The increasingly popular notion that steady-state magma chambers are highly crystallized, and thus only capable of erupting during brief (<1 ka) reheatings, implies that melt detection beneath volcanoes warns of imminent eruption. By integrating the microgeochronology and geochemistry of zircons from lavas with those from components crystallized within the magma chamber and incorporated during eruption, we show that the Soufrière (Saint Lucia) volcanic reservoir was instead eruptible over long (>100 ka) timescales. Together with data from other volcanic complexes, we show that arc magmas may generally be stored warm (are able to erupt for >100 ka). Thus geophysical detection of melt beneath volcanoes represents the normal state of magma storage and holds little potential as an indicator of volcanic hazard.
One of the most conserved traits in the evolution of biomineralizing organisms is the taxon-specific selection of skeletal minerals. All modern scleractinian corals are thought to produce skeletons exclusively of the calcium-carbonate polymorph aragonite. Despite strong fluctuations in ocean chemistry (notably the Mg/Ca ratio), this feature is believed to be conserved throughout the coral fossil record, spanning more than 240 million years. Only one example, the Cretaceous scleractinian coral Coelosmilia (ca. 70 to 65 Ma), is thought to have produced a calcitic skeleton. Here, we report that the modern asymbiotic scleractinian coral Paraconotrochus antarcticus living in the Southern Ocean forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. P. antarcticus and Cretaceous Coelosmilia skeletons share a unique microstructure indicating a close phylogenetic relationship, consistent with the early divergence of P. antarcticus within the Vacatina (i.e., Robusta) clade, estimated to have occurred in the Mesozoic (ca. 116 Mya). Scleractinian corals thus join the group of marine organisms capable of forming bimineralic structures, which requires a highly controlled biomineralization mechanism; this capability dates back at least 100 My. Due to its relatively prolonged isolation, the Southern Ocean stands out as a repository for extant marine organisms with ancient traits.
Abstract A series of tourmaline reference materials are developed for in situ oxygen isotope analysis by secondary ion mass spectrometry (SIMS), which allow study of the tourmaline compositions found in most igneous and metamorphic rocks. The new reference material was applied to measure oxygen isotope composition of tourmaline from metagranite, meta-leucogranite, and whiteschist from the Monte Rosa nappe (Western Alps). The protolith and genesis of whiteschist are highly debated in the literature. Whiteschists occur as 10 to 50 m tube-like bodies within the Permian Monte Rosa granite. They consist of chloritoid, talc, phengite, and quartz, with local kyanite, garnet, tourmaline, and carbonates. Whiteschist tourmaline is characterized by an igneous core and a dravitic overgrowth (XMg > 0.9). The core reveals similar chemical composition and zonation as meta-leucogranitic tourmaline (XMg = 0.25, δ18O = 11.3–11.5‰), proving their common origin. Dravitic overgrowths in whiteschists have lower oxygen isotope compositions (8.9–9.5‰). Tourmaline in metagranite is an intermediate schorl-dravite with XMg of 0.50. Oxygen isotope data reveal homogeneous composition for metagranite and meta-leucogranite tourmalines of 10.4–11.3‰ and 11.0–11.9‰, respectively. Quartz inclusions in both meta-igneous rocks show the same oxygen isotopic composition as the quartz in the matrix (13.6–13.9‰). In whiteschist the oxygen isotope composition of quartz included in tourmaline cores lost their igneous signature, having the same values as quartz in the matrix (11.4–11.7‰). A network of small fractures filled with dravitic tourmaline can be observed in the igneous core and suggested to serve as a connection between included quartz and matrix, and lead to recrystallization of the inclusion. In contrast, the igneous core of the whiteschist tourmaline fully retained its magmatic oxygen isotope signature, indicating oxygen diffusion is extremely slow in tourmaline. Tourmaline included in high-pressure chloritoid shows the characteristic dravitic overgrowth, demonstrating that chloritoid grew after the metasomatism responsible for the whiteschist formation, but continued to grow during the Alpine metamorphism. Our data on tourmaline and quartz show that tourmaline-bearing white-schists originated from the related meta-leucogranites, which were locally altered by late magmatic hydrothermal fluids prior to Alpine high-pressure metamorphism.
The Espinhaço rift system encompasses taphrogenetic events from the Statherian to Tonian in the São Francisco-Congo (SFC) paleocontinent. The magmatism is represented mainly by metamorphosed anorogenic granites and rhyolites with subordinate amphibolites. Zircon U-Pb (LA-ICPMS and SHRIMP) ages from felsic (1748 ± 3 Ma and 1740 ± 8 Ma) and mafic (1725 ± 4 Ma) rock samples, coupled with previous studies suggest that the Espinhaço igneous province erupted from ca. 1.79 Ga to ca. 1.70 Ga. The felsic rocks show characteristics of A-type magmas. The negative εHf(t) data for meta-rhyolite zircons (-12.32 to -17.58), the moderate δ18 O values (7.02 to 7.98) and the REE patterns suggest crustal melting related to an extensional environment. The mafic rock shows negative values of εHf(t) in zircons (-4.05 to -8.25) and moderate δ18 O values (5.56 to 7.87). The results disclose a basaltic magmatism in continental intraplate setting whose parental magma could have been derived from the subcontinental lithospheric mantle with contamination of crustal material. These data coupled with coeval Espinhaço magmatism and mafic dyke swarms found to the south of the Espinhaço rift system reinforce the evidence of a long-lived Statherian silicic large igneous province (SLIP) on the SFC paleocontinental block.
