The Strontium Cycle of the NeoProterozoic was Driven by Seemingly Unique Paleogeography

Paleogeographic forcing of the strontium isotopic cycle in the Neoproterozoic

Authors:

Goddéris et al

Abstract:

The period spanning from 825 to 540 Ma is characterized by major changes in the surficial Earth system. This extraordinary interval starts with the breakup of the Rodinia supercontinent and eruption of a series of large igneous provinces and ends with the assembly of Gondwana, giving rise to the Pan-African orogenies. This paleogeographic reorganization is accompanied by a global climatic cooling, including the paroxysmal Cryogenian “snowball” glacial events. The 87Sr/86Sr of seawater displays a major long-term rise over this interval that is punctuated by episodic, smaller declines and inflections. We use a coupled deep time climate-carbon numerical model to explore the complex role of tectonics and climate on this distinct evolution in seawater 87Sr/86Sr. We show that the modulation of the weathering of the erupted large igneous provinces by continental drift explains the changes in seawater 87Sr/86Sr from 800 to 635 Ma. The subsequent sharp rise in seawater 87Sr/86Sr from 635 to 580 Ma is the result of erosion of radiogenic crust exposed in the Pan-African orogens. Coeval evolution of atmospheric CO2 displays a decrease from about 80 times the pre-industrial level around 800 Ma to 5 times just before the beginning of the Phanerozoic.

Sulfur Very Important for Volcanicly Driven Mass Extinctions?

Microanalyses link sulfur from large igneous provinces and Mesozoic mass extinctions

Authors:

Callegro et al

Abstract:

Volcanic gases can have devastating impacts on Earth's environment and life. However, measurement of volatile elements in prehistoric magmas is challenging. We present a new method for determining sulfur contents in magmas using its concentrations in clinopyroxenes and an experimentally determined crystal-melt partition coefficient. Using this method we compared magmatic sulfur contents in the Paraná-Etendeka large igneous province (LIP; South America) and two other similarly sized LIPs, the Central Atlantic magmatic province (CAMP) and the Deccan Traps (India). We found that the CAMP and Deccan Traps, both associated with major extinction events, contained high magmatic sulfur concentrations, up to 1900 ppm. Conversely, the Paraná-Etendeka LIP, lacking significant environmental effects, had substantially lower magmatic sulfur concentrations, less than 800 ppm.

They ought to look at the Permian Extinction/Siberian Traps.  I am a bit surprised they did not.  And, again, the Deccan Traps have dinosaur fossils below, between and above the lava flows.

Marnda Moorn LIP Generated by Plume-Lithosphere Interaction in Ectasian/Stenian NeoProterozoic Australia

Genesis of the 1.21 Ga Marnda Moorn large igneous province by plume–lithosphere interaction 

Authors:

Wang et al

Abstract:

The 1.21 Ga Marnda Moorn large igneous province (LIP) of the Yilgarn Craton is important for understanding the final breakup of the Nuna (Columbia) supercontinent. However, its petrogenesis is poorly understood owing to the lack of geochemical data. We conducted geochemical analyses of the Gnowangerup-Fraser Dyke Suite, a major part of the Marnda Moorn LIP, and report the first geochemical and Nd isotope data for this LIP. Results of a complementary paleomagnetic study of these dykes will be published elsewhere. Most of the studied dykes consist of predominately tholeiitic and OIB-like dolerite (Group 1) and one arc-like and more felsic dyke (Group 2). Group 1 samples have incompatible trace element compositions similar to those of tholeiitic Hawaiian plume-induced OIB and typical asthenospheric mantle-derived Nd isotopes with ɛNd(t) varying from +3.7 to +7.5, produced mainly within the spinel stability field (below 75 km depth). Their source region most likely contains recycled oceanic crust. Samples from the Group 2 dyke are characterized by extremely unradiogenic Nd isotopes with ɛNd(t) of about -12, strong depletion of Nb-Ta-Zr-Hf-Ti, chondritic Nb/Ta ratios (20–18), oversaturated silica, and strong deficiencies in CaO, FeOt, TiO2, and Ni. This implies that the dyke was produced by partial melting of enriched sub-continental lithospheric mantle. The coexistence of OIB- and arc-like end-members but mainly Hawaiian OIB-like tholeiitic mafic dykes, interpreted large-scale asthenosphere upwelling in a very short time, and the large volume of mafic magma, favour a plume origin for the Marnda Moorn LIP. The geochemical and emplacement characteristics are attributed to relief of the lithosphere–asthenosphere boundary across the Yilgarn craton and a complex interplay between the plume, heated lithosphere, normal asthenosphere, and recycled components. We propose a two-stage melting model to explain the geochemical composition and emplacement of the Marnda Moorn LIP. Our plume-lithosphere interaction model is consistent with the occurrence of synchronous ultrahigh-temperature events in the Musgrave Province of central Australia