Mid-Cretaceous marine Os isotope evidence for heterogeneous cause of oceanic anoxic events

During the mid-Cretaceous, the Earth experienced several environmental perturbations, including an extremely warm climate and Oceanic Anoxic Events (OAEs). Submarine volcanic episodes associated with formation of large igneous provinces (LIPs) may have triggered these perturbations. The osmium isotopic ratio (187Os/188Os) is a suitable proxy for tracing hydrothermal activity associated with the LIPs formation, but 187Os/188Os data from the mid-Cretaceous are limited to short time intervals. Here we provide a continuous high-resolution marine 187Os/188Os record covering all mid-Cretaceous OAEs. Several OAEs (OAE1a, Wezel and Fallot events, and OAE2) correspond to unradiogenic 187Os/188Os shifts, suggesting that they were triggered by massive submarine volcanic episodes. However, minor OAEs (OAE1c and OAE1d), which do not show pronounced unradiogenic 187Os/188Os shifts, were likely caused by enhanced monsoonal activity. Because the subaerial LIPs volcanic episodes and Circum-Pacific volcanism correspond to the highest temperature and pCO2 during the mid-Cretaceous, they may have caused the hot mid-Cretaceous climate.

Ore Geology, RE–OS Isotope Geochemistry of the Au and Au-Sb Mineralizations, Kular–Nera Terrane, Northeast Asia: Implications for Time of Formation and Ore Genesis

The paper presents the first results of investigation of the Re–Os isotope system of native gold from the Malo-Tarynskoe, Khangalas, Bazovskoe, and chalcopyrite from the Dvoinoe orogenic gold deposits and stibnite from the Maltan Au-Sb depositin the Kular–Nera terrane, Northeast Asia. The deposits are spatially related to NW-trending lithospheric-scale major brittle faults or controlled by subsidiary faults and fracture zones. Such zones served as pathways for fluids rising from below the crust, and they have a long tectonic and reactivation history. The Kular–Nera terrane consists of Upper Permian, Triassic, and Lower Jurassic clastic sedimentary-rock sequences, metamorphosed to initial stages of greenschist facies. Magmatism is manifested by Kimmeridgian–Berriasian S- and I-types granitoids and mafic dikes of the Tas–Kystabyt magmatic belt. Re concentration in gold varies from 0.168 to 6.997 ppb, and that of osmium – from 0.068 to 1.443 ppb. Chalcopyrite from the Dvoinoe deposit occurrence contains 0.1522 ppb Re and 0.499 ppb Os. Stibnite from the Maltan Au-Sb depositoccurrence contains 0. 236 ppb Re and 0.903 ppb Os. The Re–Os ages of gold from the Malo-Tarynskoe (147.8 ± 3.8 Ma) and Bazovskoe (147.2 ± 1.8 Ma) and Khangalas (137.1 ± 7.6 Ma) orogenic deposits and the Maltan Au-Sb deposits (69.7±1.9 Ma) are determined. Malo-Tarynskoe and Bazovskoe represent the earliest known orogenic gold mineralization in the Kular–Nera terrane. The data obtained permit us to correlate the initiation of orogenic gold-ore systems with the completion of the formation at the end of the Late Jurassic Uyandina–Yasachnaya volcanic belt, crystallization and subsequent cooling in the Late Jurassic–early Early Cretaceous of granitoid massifs of the Tas-Kystabyt magmatic belt, and subduction–accretionary events at the northeastern active continental margin of the Siberian craton. Maltan Au-Sb deposit is related to completion of the formation of the Albian-Late Cretaceous Okhotsk–Chukotka volcano-plutonic belt. Contrasting mantle and/or crustal sources of ore-forming material are established. The osmium initial isotopic ratio in gold 187Os/188Os = 0.2210-0.4275 and antimonite (0,2543-0,2976) is typical for the ore-forming material from the fertile mantle reservoir, and for chalcopyrite (3.1904) – from the crust.

Open System Re-Os Isotope Behavior in Platinum-Group Minerals during Laterization?

In this short communication, we present preliminary data on the Re-Os isotopic systematics of platinum-group minerals (PGM) recovered from different horizons in the Falcondo Ni-laterite in the Dominican Republic. The results show differences in the Os-isotope composition in different populations of PGM: (i) pre-lateritic PGM yield 187Os/188Os varying from 0.11973 ± 0.00134 to 0.12215 ± 0.00005 (2σ uncertainty) whereas (ii) lateritic PGM are more radiogenic in terms of 187Os/188Os (from 0.12390 ± 0.00001 to 0.12645 ± 0.00005; 2σ uncertainty). We suggest that these differences reflect the opening of the Re-Os system in individual grains of PGM during lateritic weathering. The implications of these results are twofold as they will help to (1) elucidate the small-scale mobility of noble metals in the supergene setting and therefore the possible formation of PGM at these very low temperatures, (2) better refine the Os-isotopic datasets of PGM that are currently being used for defining dynamic models of core–mantle separation, crustal generation, and fundamental plate-tectonic processes such as the opening of oceans.

Supplemental Material: Identification of ca. 520 Ma mid-ocean-ridge–type ophiolite suite in the inner Cathaysia block, South China: Evidence from shearing-type oceanic plagiogranite

Table S1: Zircon SIMS U-Pb data and d18O values for the meta-felsic rocks from Shitun area, Cathaysia block, South China; Table S2: LA-ICP-MS analysis of trace elements in zircon from the meta-felsic rocks, Shitun area, Cathaysia block, South China; Table S3: Zircon Hf isotope compositions of the meta-felsic rocks from Shitun area, Cathaysia block, South China; Table S4: Major- and trace-element compositions of the serpentinites, meta-ultramafic rocks, and meta-felsic rocks from Shitun area, Cathaysia block, South China; Table S5: Whole-rock Re-Os isotope compositions of the serpentinites from Shitun area, Cathaysia block, South China; and Table S6: Sr-Nd isotope compositions of the meta-ultramafic and meta-felsic rocks from Shitun area, Cathaysia block, South China.

Supplemental Material: Identification of ca. 520 Ma mid-ocean-ridge–type ophiolite suite in the inner Cathaysia block, South China: Evidence from shearing-type oceanic plagiogranite

Table S1: Zircon SIMS U-Pb data and d18O values for the meta-felsic rocks from Shitun area, Cathaysia block, South China; Table S2: LA-ICP-MS analysis of trace elements in zircon from the meta-felsic rocks, Shitun area, Cathaysia block, South China; Table S3: Zircon Hf isotope compositions of the meta-felsic rocks from Shitun area, Cathaysia block, South China; Table S4: Major- and trace-element compositions of the serpentinites, meta-ultramafic rocks, and meta-felsic rocks from Shitun area, Cathaysia block, South China; Table S5: Whole-rock Re-Os isotope compositions of the serpentinites from Shitun area, Cathaysia block, South China; and Table S6: Sr-Nd isotope compositions of the meta-ultramafic and meta-felsic rocks from Shitun area, Cathaysia block, South China.