THE GENERATION OF URANIUM SERIES DISEQUILIBRIA BY PARTIAL MELTING OF SPINEL PERIDOTITE - CONSTRAINTS FROM PARTITIONING STUDIES
THE GENERATION OF URANIUM SERIES DISEQUILIBRIA BY PARTIAL MELTING OF SPINEL PERIDOTITE - CONSTRAINTS FROM PARTITIONING STUDIES
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DOI：
10.1016/0012-821x(93)90091-m
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发表时间：
1993-06-01
影响因子：
5.3
通讯作者：
BEATTIE, P
中科院分区：
文献类型：
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作者：
BEATTIE, P
用二次离子质谱仪测定了橄榄石-斜方辉石-单斜辉石-熔体对在实验电荷中的碱土、铅和榄系元素的分配系数。尖晶石橄榄岩与玄武岩熔体的平衡分配系数U和Th分别为1.1×10~(-4)和1.7×10~(-4)，均以单斜辉石的存在为主。这些新数据表明，即使水合相或副相在固相线上是稳定的，铀在尖晶石橄榄岩中的总体分配系数也低于对钍的分配系数。对于与碳酸盐熔体平衡的尖晶石橄榄岩来说，U与Th类似地不相容。这些结果表明，尖晶石橄榄岩在化学平衡状态下的部分熔融不能解释在MOB中观察到的(Th-230/U-238)超额。尖晶石橄榄岩不平衡部分熔融产生的(Th-230)超额需要Th的扩散系数大于U的扩散率，这是不可能的，因为Th的离子半径大于U。同样，只有当Th的体积分配系数小于U的体积分配系数时，熔体渗流才会产生(Th-230)超额，因此在尖晶石稳定领域不会产生(Th-230)超额。如果D(U*)GT/L比D(Th*)GT/L和D(Th*)cpx/L大，则石榴石稳定场中缓慢的近部分熔融可能产生大于1的(Th-230/U-238)比值。
Alkaline earth, Pb and actinide partition coefficients have been measured for olivine-, orthopyroxene- and clinopyroxene-melt pairs in experimental charges using secondary ion mass spectrometry. The bulk partition coefficients for spinel peridotites in equilibrium with basaltic melts are 1.1 X 10(-4) and 1.7 x 10(-4) for U and Th, respectively; both are dominated by the presence of clinopyroxene. These new data demonstrate that the bulk partition coefficient for uranium is lower than that for thorium in spinel peridotites, even if hydrous or accessory phases are stable at the solidus. U is similarly incompatible compared to Th for spinel peridotite in equilibrium with carbonate melts. These results demonstrate that the partial melting of a spinel peridotite at chemical equilibrium cannot account for the (Th-230/U-238) ratios in excess of unity observed in MORB.The generation of the observed (Th-230) excesses by disequilibrium partial melting of spinel peridotite requires that the diffusivity of Th is greater than that of U, which is improbable, as the ionic radius of Th is greater than that of U. Similarly, (Th-230) excesses can be produced on melt percolation only if the bulk partition coefficient of Th is smaller than that of U, and cannot therefore be generated in the spinel stability field. (Th-230/U-238) ratios greater than unity might be produced by slow, near fractional partial melting in the garnet stability field, if D(U*)gt/L is large compared to D(Th*)gt/L and D(Th*)cpx/L.