CIDER 2012 Outcomes

2012 AGU Abstracts

Impacts Group

Title: "Origin and mixing timescale of Earth's late veneer"

Submitted to V012 Early Earth Differentiation

Authors: C. Prescher, A. Cernok, D. Allu Peddinti, E.A. Bell, M.M. Wielicki, L. Bello, N. Ghosh, J. Tucker, K.J. Zahnle

Abstract: Experimental studies on the partitioning behavior of highly siderophile elements (HSE) between silicate and metallic melts imply that the Earth’s mantle should have been highly depleted in these elements by core formation in an early magma ocean. However, present HSE contents of the Earth’s mantle are ~3 orders of magnitude higher than that expected by experiments. The apparent over-abundance of HSE has commonly been explained by the addition of meteoritic material in the “late veneer” which describes the exogenous mass addition following the moon forming impact and concluding with the late heavy bombardment at ~3.8-3.9 Ga. The strongest evidence for this theory is that the platinum group element (PGE) contents in today’s mantle are present in chondritic relative abundances, as opposed to a fractionated pattern expected with metal-silicate partitioning.

Archean komatiites indicate that the PGE content of the Earth’s mantle increased from about half their present abundances at 3.5 Ga to their present abundances at 2.9 Ga. This secular increase in PGE content suggests a progressive mixing of the late veneer material into the Earth’s mantle. However, this time scale also implies that the whole mantle was relatively well mixed by 2.9 Ga.

We use a compilation of existing isotopic and trace element data in order to constrain the origin and composition of the late veneer. We use PGE abundances, W abundances and W isotopic compositions in chondritic meteorites and the primitive upper mantle to compute the amount of mass delivered during the late veneer and find the late veneer mass to be ~0.6 % the mass of the bulk silicate Earth (consistent with earlier estimates). We also use the 187Re-187Os and 190Pt-186Os systems to constrain the composition and timing of delivery of the impacting population.

We model the efficiency of mantle mixing in this time frame by using 3-dimensional numerical geodynamical simulations and geochemical constraints. Initial parameters include the amount of mass delivered in the late veneer and the Archean internal heating which is at least 4 times higher than the present values, due to the higher abundance of radioactive elements. Another important parameter is the mechanism of mass addition to the Earth. We test three end-member scenarios: (1) a single very large impactor accounting for the entire mass addition, (2) sprinkling of a large number of small impactors over the whole Earth which then mix into the mantle, or (3) by using a size/frequency distribution estimated from the lunar cratering record and corrected for the difference in gravitational cross section of the Earth and the Moon.

This project results from collaborations begun at the CIDER II workshop held at KITP, UCSB, 2012.

Core Mantle Boundary

Title: On the Possibility of a Thin, Molten Silicate Basal Magma Layer at the Base of Earth’s Mantle

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

People involved: Stefanie Hempel, Xi Liu, Emma Rainey, Martina Ulvrova, Yang Wang, Lingling Ye, Yingxia Shi, Ondrej Smarek, Razvan Caracas, Ed Garnero, Dave Rubie, and Quentin Williams

The temperature and compositional changes across the core-mantle boundary (CMB) region are the largest within Earth. On either side of the CMB various compositional and thermal effects, such as horizontal layering, lateral heterogeneities, mineral phase changes, and CMB topography have both been predicted and observed. Above the CMB, heterogeneous structures exhibiting variable thicknesses and properties are present, such as large low shear velocity provinces (LLSVPs, 100’s of km thick, O~1000’s of km laterally), as well as thin ultra-low velocity zones (ULVZs, 10’s of km thick, O~100’s of km laterally). Two nearly antipodal LLSVPs are, to first order, situated beneath hotspots and away from beneath subduction. ULVZs appear to be preferentially located near LLSVPs, possibly concentrated near LLSVP margins. Seismic studies have noted seismically sharp edges to LLSVPs. That, combined with the preference of hotspots to overly LLSVP edges infers a compositionally distinct origin to LLSVPs. Below the CMB, seismic studies have modeled a 50-200 km thick zone of reduced P-wave (Vp) velocities, compared to radial reference earth models. Geodynamic considerations, on the other hand, have implied a thin (up to 10’s of km) layer of anomalously high Vp at the very top of the outer core, due to lighter alloying components accumulating near this depth. Recent electrical conductivity measurements and calculations suggest large conductivities of the outer core that would necessarily lead to stratification. In considering the core-mantle system, models including a basal magma ocean juxtapose liquid silicate magma against the fluid iron outer core for extended periods of Earth history. How do these disparate pictures fit together? One such scenario might involve long-term chemical interactions between a thin magmatic mantle basal layer and the outer core: a possible fully molten cousin of partially molten ULVZs. In this study, we investigate the seismic detectability of such a layer by producing synthetic seismograms using a reflectivity method as well as axisymmetric seismic wave propagation method for a host of seismic phases that sample the layer. Synthetic data waveforms will be carefully analyzed and processed with array methods to establish resolution levels. Either at present or within an entirely molten basal silicate boundary layer in Earth’s past, a range of processes could be operative that would affect the thermal and chemical evolution of a magma-rich boundary. These include diffusive transport and chemical reactions between the layer and the underlying core, including iron diffusion into the layer, and lighter alloying diffusion into the outer core. Indeed, characteristic length-scales of diffusion within such materials are several 10’s of km over the age of the Earth. Thus, any primordial basal magma layer that persisted to the present day, would be highly chemically altered, and would have compositionally evolved to become substantially iron-enriched. Additionally, possible renewal of this layer through descent of negatively-buoyant melts and/or melt-drainage from ULVZs may have occurred, depending on the degree of melting of the overlying mantle, abundance of negatively buoyant melts, and the rate of melt segregation.

Cratonic Lithosphere

Title: "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.

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, Zack 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.