Towards a petrological explanation for the X-Discontinuity: The role of eclogite
Final Report Abstract
This project has tested the hypothesis that the coesite-stishovite transition in eclogite bodies is responsible for the seismic X-discontinuity, lying at ~300km depth. We addressed two possibilities: i) that the CaEs component in clinopyroxene becomes unstable upon reaching the stability field of stishovite and rapidly exsolves to form new stishovite, which leads to a jump in bulk density, and ii) that enough free coesite is already present in an eclogitic bulk composition to cause the necessary impedance contract observed for the X-discontinuity as it transforms to stishovite. Our results indicate that there is no “sudden” decrease in CaEs component in clinopyroxene upon crossing into the stishovite stability field. Instead there is a gradual decrease in CaEs content over the pressure range from 4 to 10 GPa, which is opposite to that required to produce a “jump” in bulk density and thus bulk seismic velocities. In addition, the amounts of CaEs present in clinopyroxene at high pressures are inadequate to produce significant “additional” free SiO2 that might contribute to increasing bulk density. A second part of this study assessed the amount of free coesite to occur in different bulk compositions generated under several subduction scenarios. Two natural analog compositions representing different types and degrees of partial melting produced mineral assemblages with very little or even no free coesite. This indicates that any processing of the downgoing slab by partial melting will severely limit the potential amount of free coesite available to transform to stishovite upon reaching ~300 km depth. In fact, it is not enough to explain the impedance contrast observed for the X-discontinuity. Of three model compositions investigated, only a hydrothermally altered MORB that remained chemically unmodified during subduction would contain the necessary amount of coesite. However, a number of petrological considerations such as the effects of prograde metamorphism during subduction indicate that the preservation of such a bulk composition to a depth of 300 km is unlikely. Building on Knapp et al. (2013), we also sought to systematically investigate the influence of bulk composition on the ability of clinopyroxene to incorporate a significant CaEs component. In this way, we were also able to investigate the recently proposed model where partial melting of eclogite bodies in an upwelling heterogeneous mantle is driven by exsolution of CaEs from clinopyroxene. Our assessment indicates that maximum CaEs concentrations are promoted by high temperatures in the presence of coexisting coesite so that conditions at the solidus would be optimal for clinopyroxene containing the highest possible CaEs content for a given bulk composition (Schröder-Frerkes et al. submitted). On the other hand, the absolute amount of partial melting that can be caused by exsolution of CaEs to make coesite is limited to only a few percent partial melting. Thus, although viable, it is unclear to what extent CaEs exsolution can contribute to enhancing partial melting of upwelling eclogite bodies. This process must also be H2O-free since its presence dramatically lowers the solidus temperature to the point that all free SiO2 and CaEs component in clinopyroxene will be rapidly exhausted. Further assessment reveals that high CaEs contents are promoted by Ca-rich bulk compositions, rather than the more normal eclogitic compositions rich in Fe and Mg. However, such bulk compositions are much rarer in nature and are often SiO2 undersaturated, severely limiting the amount of possible CaEs incorporation. In fact, the internal equilibrium between CaEs and Ca-Tschermaks components at SiO2 saturation places constraints on the maximum CaEs concentration. Due to this, we estimate maximum CaEs contents of natural clinopyroxenes to be ~20 mol %, with somewhat lower concentrations in eclogitic bulk compositions. In summary, we conclude that the coesite-stishovite transition in eclogite bodies is not a viable petrological explanation for the X-discontinuity. Thus, other models involving either the orthopyroxene-high-P clinopyroxene transition or the formation of anhydrous phase B need to be refined, or another mineralogical explanation must be sought. Maximum Ca-Eskola contents of clinopyroxene are limited to ~20 mol % under optimal conditions and for Ca-rich, coesite-saturated bulk compositions, which are quite rare in nature.
Publications
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(2012) Incorporation of Ca-Eskola component in eclogitic clinopyroxene in CMAS and "natural" composition at upper mantle conditions. European Mineralogical Conference, Vol. 1, EMC2012-719
Knapp N, Woodland AB, Klimm K
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(2012) Incorporation of the Ca-Eskola component in eclogitic clinopyroxene at upper mantle conditions. 14th International Conference on Experimental Mineralogy Petrology and Geochmistry. Kiel
Knapp N, Woodland AB, Klimm K
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(2013) Experimental constraints in the CMAS system on the Ca-Eskola content of eclogitic clinopyroxene. European Journal of Mineralogy, 25,579-596
napp N, Woodland AB, Klimm K
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(2013) Is the X-discontinuity really related to the presence of eclogite bodies in the mantle? Geophysical Research Abstracts, 15, EGU2013-10299
Woodland AB, Knapp N, Klimm K
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(2014) Can subducted eclogite be the petrologic explanation for the X-discontinuity? Geophysical Research Abstracts, 15, EGU2014-10277
Woodland AB, Knapp N, Klimm K
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(2014) Crystal chemistry of Ca-eskolabearing eclogitic clinopyroxene in the system NCMAS. 21st General Meeting of International Mineralogical Association 1.-5. Sept. 2014, Johannesburg, RSA, abs 574
Woodland AB, Uenver-Thiele L, Klimm K, Knapp N
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(2015) Experimental constraints on coesite abundance in eclogite and implication for the seismic discontinuity. Journal of Geophysical Research Solid Earth, 120, 4917-4930
Knapp, N., Woodland, A.B., Klimm, K.
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Ca-Eskola incorporation in clinopyroxene: limitations and petrological implications for eclogites and related rocks. Contributions to Mineralogy and Petrology, December 2016, 171:101
Schroeder-Frerkes F, Woodland AB, Uenver-Thiele L, Klimm K, Knapp N
