Dataset Information

Core formation and core composition from coupled geochemical and geophysical constraints.

ABSTRACT: The formation of Earth's core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal-silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth's magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7-5% oxygen along with 2-3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.

SUBMITTER: Badro J

PROVIDER: S-EPMC4603515 | biostudies-literature | 2015 Oct

REPOSITORIES: biostudies-literature

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Core formation and core composition from coupled geochemical and geophysical constraints.

Badro James J Brodholt John P JP Piet Hélène H Siebert Julien J Ryerson Frederick J FJ

Proceedings of the National Academy of Sciences of the United States of America 20150921 40

The formation of Earth's core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal-silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevail ...[more]

PMID: 26392555

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Project description:The black shale in the upper Permian Dalong Formation is considered as an excellent source rock in the Lower Yangtze region. However, mechanisms of organic matter (OM) accumulation in such a setting are scarcely understood. Here, the characteristics of total organic carbon (TOC) and elemental geochemistry of 33 rock samples from GD1 well are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that the lower and middle parts of Dalong Formation (section A) display high TOC contents ranging from 1.19 to 6.45% (average 3.19%), whereas the upper part (section B) exhibits medium TOC contents varying from 1.18 to 4.90% (average 2.09%). These data also elucidate that the target shales were deposited in a complex paleoenvironment with moderate to strong water-mass restriction that was characterized by warm and semiarid-semihumid paleoclimate, high biotic productivity, fluctuating plaeoredox conditions, and a relatively high sedimentary rate. Compared to the organic-rich shales from section A mainly developed under an anoxic condition, shales from section B formed in an oxic-to-dysoxic water environment exhibited a comparatively higher sedimentary rate. Moreover, among all these factors that might affect OM accumulation, the paleoredox conditions appear to be the dominant controlling factors for section A, whereas the biotic productivity, paleoredox conditions, and sedimentary rate are the main controlling factors for section B. Finally, two formation models for OM accumulation in Dalong Formation shale in the Lower Yangtze region are proposed. The "preservation model" for OM accumulation in section A emphasizes that the reducing deep-water environment, which was mainly caused by the regional sea level rise, is favorable for OM preservation. The "integrated model" for OM accumulation in section B stresses that greater biotic productivity intensifies respiratory oxygen consumption in a water column and a higher sedimentary rate can greatly shorten OM exposure time for respiration by oxygen, both of which cause OM accumulation under an oxidizing water environment. These findings also add to our knowledge that despite the oxygenated water environment during shale deposition, TOC contents are not necessarily lower.

Dataset Information

Core formation and core composition from coupled geochemical and geophysical constraints.

Publications

Core formation and core composition from coupled geochemical and geophysical constraints.

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