ࡱ> @B?b jjbjb 0"hhj G.,uR  ֨<\F0G  D d SILICON MANTLE/CORE FRACTIONATION AND THE ORIGIN OF PALLASITES E. Hill1, K. Domanik1, G. R. Huss2 and M. J. Drake1. 1Lunar and Planetary Laboratory, University of Arizona, Tucson 85721. 2 Hawaii Institute of Geophysics and Planetology, University of Hawaii. E-mail:  HYPERLINK "mailto:eddy@lpl.arizona.edu" eddy@lpl.arizona.edu. Experimentally determined metal/olivine partition coefficients and diffusion rates will help answer questions relating to Earths elevated Mg/Si ratio and will provide better tools for the determination of the origin of pallasites. It is known that the Mg/Si ratio of the Earths primitive upper mantle (PUM) is elevated relative to all types of chondrites (possibly not CV chondrites), Mars, and the solar photosphere [1]. Theories to explain these differences fall into two distinct camps. On the one hand are those that propose that Earths Mg/Si ratio is a primordial property of the planet. On the other are those that propose Si fractionation into the core during metal/silicate equilibration in Earths magma ocean (e.g., [2]) or earlier on during asteroid and planetesimal differentiation [3]. If Malavergne [3] is correct, we should be able to measure such fractionation in pallasite meteorites. The value of these measurements is dependent on our knowledge of the formation of pallasites, however. Pallasite meteorites are intimate mixtures of olivine (forsterite) and metal. It has been proposed that they originated as samples of the core-mantle boundaries of now disrupted asteroids or planetesimals (e.g., [4]). Their olivines have been described as chemically homogeneous in the major elements [4] but have chemical gradients in the minor elements (e.g. Ca, Cr, Co, Ni). It is assumed that these chemical profiles are the result of diffusion during slow cooling, as the phases attempted to re-equilibrate at progressively lower temperatures. Measured chemical profiles in pallasitic olivines have been used with a one-dimensional, spherical, cooling model to obtain cooling rates for the pallasites parent bodies [5]. The results are four orders of magnitude faster than metallographic derived cooling rates (~10-6 C/yr) [4] and if correct hint at a complex history for the formation of the pallasites. In fitting the profiles [4] found that it is impossible to obtain a result using a fixed boundary condition and that a varying condition is necessary. Due to a lack of experimental data, [5] were not able to constrain how the boundary condition changes. Boundary conditions are dependent on the partition coefficient (D) of elements between olivine and the adjacent metal. We have obtained metal/olivine partition coefficients for Ni, Co, Cr and Si at 1550 C and IW -1. Ni and Co Ds reveal a small dependence on metal composition. Metal/olivine DSi values (1*10-61*10-7 to 5*10-53*10-6) are too low to account for Earths Mg/Si ratio. Olivine diffusion profiles for Cr are in line with those measured by [6] and provide validation for our method and for the values obtained for Ni and Co. Ongoing work will determine the effect of lowering temperatures and varying redox conditions on partitioning and diffusion of these elements. We expect, however, that Ds and diffusion rates will decrease with decreasing T but that DNi and DCo will increase as a function of lowering fO2. [1] M.J. Drake and K. Righter (2002) Nature 416, 39-44. [2] J. Wade and B. Wood (2005) EPSL 236, 78-95 ([3] Malavergne et al. (2004) Geochim et cosmochi acta 68, 4201-4211. [4] Buseck and Goldstein (1969) Geol. Soc. Amer. Bull. 80, 2141-2158. [5] Tomiyama and Huss (2006) LPSC XXXVII (2132) 40, A156. [6] Ito et al., (2004) LPSC XXXV (1324) ?@GHTUabrsuv  567KLORpq2 3 a b B C K L   3 4 ; < l m ̱h11h11CJPJh11h11CJNH hh11h11h110JCJjh11h11CJUjh11h11CJUh11h11CJH*h11h11CJh11h11CJh11h115CJ:@OQ;F l ij$a$gd11 $ & 0` P@7$8$H$a$gd11$ & 0` P@7$8$H$a$ $ 7a$gd11$ ` a$$a$ j $%de}ij'( $&?@ 8?Bjnor#.56:W`hijh11h115CJh11h116CJh11h11CJH*h11h11CJH*h11h11CJh11h11CJNHA.:p11|. A!"#$% DyK yK 8mailto:eddy@lpl.arizona.edu<`< 11Normal1$CJmH sH tH DA@D Default Paragraph FontRi@R  Table Normal4 l4a (k@(No ListD@D k*` Balloon TextCJOJQJaJ0U`0 11 Hyperlink>*B*j" +j@OQ;Fl  il0ʀ0ʀ0ʀ0ʀ0P`000ʀ00ʀF l@0 0$^ 0ؕ j jj 6KjX.8TVwyT`y/9GP~'/  N P > C  lu l:::::Qil@)}jp @UnknownGTimes New Roman5Symbol3 Arial3__ __Ca`Lucida Grande qhզ զ = ,= ,24d//H`$>SILICON MANTLE/CORE FRACTIONATION AND THE ORIGIN OF PALLASITES Eddy Hill Eddy Hill Oh+'0(  0< \ h t'@SILICON MANTLE/CORE FRACTIONATION AND THE ORIGIN OF PALLASITES Eddy HillNormal Eddy Hill1Microsoft Word 11.5.3@vA@"@.W= G'PICT'Z HHZ ZHHք׈ZZ         o{wskZwwwg9wwo{o{kZo{o{o{o{wswkZwo{w kZwo{wwwwkZkZo{o{wswo{o{VkZcg9ZZNs Rg9ZRkZwcVRZJRVcRVVkZg9ZNsZg9g9ZNsZR^o{kZVR ZJRcZcVZkZco{wo{wkZkZo{# ^Z^^VRkZkZVV#sso{ssDNsB^kZNsRJRg9cVJRRB^kZkZg9cVVkZg9RZ^kZc^cg9ZcF1ZckZg9Zcg9^cZRVVcZkZkZNsc^cRccg9wo{1c^g9kZccVcc^kZBg9Zg9NskZg9JRZc^NskZcc^g9kZZZcVZZckZRVcg9wVcZkZNsc^Bg9Zg9o{NssVg9^) k_VVg?F?VZk_ZcRk_V^  Go{wwwwwsswwssww?^kZ^^o{Nso{JRkZVVV^Ro{Ro{JRVRVkZNsR^co{JR^V^^RZNs^kZ^cVZZF1VV^kZ^^Zo{Ns^g9^kZkZVwwwo{wwwwo{wwwwwsV^RF1g9^Zo{^7ZRZg9o{^NskZRcR^Vo{Nsg9^RcVRVZg9cNso{o{V^ZVZV^kZ^o{ZcNs^V^kZ^o{RZkZg9Ro{+wwswswwK kZV^VR^Nso{ZV^ZkZkZV^Zo{cRR^VkZNsg9^^g9]wwwwwwo{swo{wwwwswww'kZkZZJRZ^wZNsVkZkZJRZg9cNso{VkZRZkZkZV^^Vo{Nsg9^o{Ro{^c^Nso{VRRkZcVZg9^g9^RcVRV_wwswwwwwo{wwwwwwwwCo{^Ns^c^RVkZV^^g9ZkZcVZZVo{Rg9kZJRg9g9NsVZZkZccJRZZVo{^^g9kZVg9Vo{g9RZVkZV^g9ZNso{WwwwkZw wwwssswwo{w ^VZRg9^V^o{RkZ6JRZo{Vg9R^kZ^NsRNs^g9JR^kZ^o{JRc^ZVkZo{ZRkZ^VVZ^ZVcVRVg9cRNs^ZZ^Wwwwwwo{wo{wwswwo{CVVZVVo{^NsZg9^kZVkZ^o{Z^Ns^kZVVkZ^V^Vg9Zg9cNso{o{V^Zo{g9V^o{NsRo{VNskZ^o{Z^^NsZkZNs^^kZVJRkZcR Awwwwo{swwwwCkZV^ZNs^o{Vo{^NsZg9^NsVkZ^R^Zg9^Vo{kZo{VVRZV^ZZRRNs^cZo{^V^o{RcRo{^NsZVcVRSwwo{wwwwo{wwwwwww^@VVo{Nso{Zo{Z^V^Vg9Ro{VJRo{VZc^F1kZ^cwkZZkZZo{^VV^o{ZZV^o{RcVg9kZVZNsVF1^NsZ^Rg9Nso{NsSwss wswwswwwwswwwwwwwCV^kZ^o{JR^V^ZZkZ^kZkZkZZg9VNsc^o{Ns^kZ^kZo{g9Nso{VckZw^Zg9VZ^NsV^VVRRRo{^Vg9^o{^g9^JR;swkZwwswwwCkZo{VNsZZNso{Zo{Z^VkZV^RRVRZo{^^g9kZJRcVR^RNs^g9^RVVg9^o{F1VZkZg9o{g9V^^JRV^ZkZZZJRo{9w wwswsswwW$^ZZ^g9^VcJRkZkZV^kZZNso{NsRZR^F1Vg9^^g9RZ^Zc^o{Eswswwswwwwww?cVkZVg9^^RVkZ^Ro{Rg9Vo{^RkZNso{VRR^kZ^o{^NskZNscR^kZRo{g9Ro{RkZVZVRVNskZkZVg9JR^ZQwwwwwswwwwswwC^o{ZJRg9^VkZVVkZNs^cZo{cRVRVVg9g9VRVRg9ZkZkZJRcZo{VRNsZkZ^VZ^ZVVo{^g9ZkZZZ^wVo{g9o{^F1V^wwwwwkZwwwwwwwwwCVg9RRo{Vg9Zo{^NsZ^Rg9Nso{Nsg9kZ^cwkZZkZkZJRo{o{ZkZZo{JRg9VVc^V^^ZVRco{V^VJRcVF1o{cVkZcFwwwwwwwwww-VZRRc^Z^JRg9RNs^RNsRo{^kZ^RVZZkZZNs^kZVVc^cV^Ro{cNsco{VZZRNs^RRZo{^kZ^RVZZdwo{o{o{wwwwwwwswwwswssCkZ^ccVwco{^ZwZ^kZkZZVg9g9F1VVNsVkZNs^g9^cV^Ro{cNs^RkZ^^g9Vo{^Ns^Vg9^kZZkZV^kZ^g9RZCwwwwwwwwwwCV^o{Rcg9R^wcZZ^ZcwVg9Ns^^VVg9^g9VkZ^RVkZJRRo{^s^^JRVo{Ns^Ns^o{ZZ^g9^NsVR^Zo{ ! 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