CIDER 2012 Outcomes

Revision as of 03:03, 11 August 2012

2012 AGU Abstracts

Impacts Group

Core Mantle Boundary

Reconciling geophysical and geochemical observations to understand craton lithosphere architecture

Submitted to T014 Evolution of the Continental Lithosphere

Cratonic Lithosphere Group: Andrew Lloyd, P. Boehnke, P. Bouilhol, C. Doherty, E. Emry, M. Li, E. Paulson, H. Yue, R. Carlson, D. Wiens, H. Yuan

The composition of the thick lithospheric mantle beneath cratons is only accessible through geophysical investigation and mantle xenolith records (i.e. lithospheric mantle fragments sampled by volcanoes). How this composition relates to craton formation, evolution and persistence through time remains poorly understood. Geophysical and petrologic observations reveal heterogeneous “layering” in some cratons, while others are less stratified, and apparently do not exhibit internal boundaries. When present, the geophysical characteristics of these different internal structures (e.g. Vs) are partly inherited from the composition of the rocks that form the lithosphere, but also from the dynamic processes by which they form. The xenolith record allows us to tests of whether certain compositions are responsible for the geophysical characteristics. One of the main problems is that it is difficult to know at what time in geological history the layering is acquired, and how it relates in time with craton formation and evolution. Discrepancies arise when there are inconsistencies between the crust and mantle formation ages, as well as between the xenolith age record (i.e. Re-depletion age) within the lithospheric mantle itself. These differences lead to the ultimate question of how and when the lithospheric craton architecture developed, and how it evolved to form stable cratonic roots. Nevertheless, the geophysical and geochemical data cannot always be reconciled, and thus there is not a well established model for the presence or absence of a stratified lithospheric mantle, and how it relates to the formation and evolution of continents through time. Here we take an interdisciplinary approach by bringing together available geophysical and geochemical data aiming to understand the formation and evolution of craton lithospheric lithosphere architecture, and use this these data to test dynamical models of craton formation and destruction. We selected threeThree cratons with available geochemical and seismological data were selected, which differ in structure and chemical composition and may represent cratonic structural end-members. These include the Kaapvaal craton, Slave craton, and North Atlantic craton. By identifying the nature of stratification, we can then reconstruct the viable geodynamic scenarios for its formation and evolution can then be reconstructed. Understanding the nature of the stratified craton and its dynamics of formation is a prerequisite for understanding the survival of such structures and the formation of the earliest continental lithosphere.

Mg/Si

Magma Oceans

Inner Core

Title: Investigating the translation of Earth's inner core

Submitted to DI013. The Earth's Complex Core: Bridging our Understanding from Seismology to Mineral Physics and Beyond

Authors: Elizabeth Day, Vernon Cormier, ZAch Geballe, Marine Lasbleis, Mohammad Youssof, Han Yue

Abstract:The Earth's inner core provides unique insights into processes that are occurring deep within our Earth today, as well as processes that occurred in the past. The seismic structure of the inner core is complex, and is dominated by anisotropic and isotropic differences between the Eastern and Western ‘hemispheres’ of the inner core. Recent geodynamical models suggest that this hemispherical dichotomy can be explained by a fast translation of the inner core. In these models one side of the inner core is freezing, while the other side is melting, leading to the development of different seismic properties on either side of the inner core. A simple translating model of the inner core, however, does not seem to easily explain all of the seismically observed features, including the innermost inner core; the observed sharp lateral gradient in seismic properties between the two hemispheres; and a complex hemispherical and radial dependence of anisotropy, attenuation, and scattering in the uppermost inner core.

We explore the compatibility of geodynamic models of a translating inner core with seismic observations. Using a relatively simple set of translation models we map the age of material in the inner core and apply mineral physics models for the evolution of grain size to estimate likely changes in seismic properties throughout the inner core. We then compare these predictions to the observations of seismic studies that target two regions that are highly sensitive to the translation of the inner core: the boundary between the two hemispheres and the regions of freezing and melting at the inner core boundary.

To constrain the sharpness of the boundary between the two hemispheres of the inner core we collate a data set of PKiKP-PKIKP, PKP-PKIKP and P′P′bc-P′P′df differential travel times consisting of paths that sample the core near to the proposed hemisphere boundaries. This combination of body wave data samples a range of depths (and consequently ages) in the inner core, and provides an insight into the nature of hemispheres and their compatibility with our predictions for models of a translating inner core. Additionally, we investigate the structure at the base of the outer core and the inner core boundary by analyzing PKP-Cdiff waves. The search for observable PKP-Cdiff is particularly concentrated in regions that are predicted to be actively freezing and melting, and spans a range of frequencies, allowing us to fully investigate any regional differences around the inner core boundary that may result from the translation of the inner core.

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