Partitioning of H and C between the mantle and core during the core formation in the Earth: Its implications for the atmospheric evolution and redox state of early mantle
Partitioning of H and C between the mantle and core during the core formation in the Earth: Its implications for the atmospheric evolution and redox state of early mantle
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地球核心形成过程中H和C在地幔和地核之间的分配：其对大气演化和早期地幔氧化还原状态的影响
DOI：
10.1029/96je00940
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发表时间：
1996
影响因子：
--
通讯作者：
T. Matsui
中科院分区：
文献类型：
--
作者：
K. Kuramoto;T. Matsui
本文根据气体在硅酸盐熔体和熔融金属铁中溶解度的最新研究成果，采用热力学模型，对温度为2000-2500 K、压力为0.2-5 GPa的生长地球中H和C在流体、硅酸盐熔体和熔融金属铁中的分配进行了估算。考虑了溶解在熔融金属铁中的H、C和S之间的排斥作用。结果表明，氢在熔融金属铁和硅酸盐熔体之间的分配系数随压力和温度的升高而增大。在富铁金属存在下，硅酸盐熔体中的CO2含量很低，因为在这种条件下，氧逸度低。假设星子的吸积是均匀的，其组成由对Ringwood [1977]和Waanke [1981]稍加修改的双组分模型给出，结果表明，C优先分配到熔融金属铁中，而很少分配到硅酸盐熔体中，而相当大比例的H以H2O的形式分配到硅酸盐熔体中，也以间隙原子的形式分配到熔融金属铁中。对于这样的H、C和S浓度，它们在熔融金属铁中的热力学相互作用的影响不足以引起石墨的过饱和。分配到熔融金属铁中的H和C可能是地核密度不足的重要原因。分配到硅酸盐熔体中的H2O的估计量可能足够大，足以解释（1）水圈和地幔中的H2O，（2）地幔中将亚铁部分氧化为三价铁的氧气，以及（3）可能无法分离到地核的金属的氧化剂，并作为现今地球地幔中高度亲铁元素的来源。较高的H/C和较低的C/36 Ar比在硅酸盐地球包括水圈相比，各类陨石可能解释，如果这些元素是来自早期地幔物质后的H和C分区熔融金属铁和核心偏析。晚期单板物质的增生，如高度氧化的Cl-辉长岩样物质，似乎很难解释这种元素丰度模式，而不引起地幔中未知的大挥发性储层。
Partitioning of H and C among fluid, silicate melt, and molten metallic iron within a growing Earth at temperatures 2000–2500 K and pressures 0.2–5 GPa is estimated by using a thermodynamic model based on the recent knowledge on gas solubility into silicate melts and molten metallic iron. The repulsive interactions among H, C, and S dissolved in molten metallic iron are taken into account. It is shown that partition coefficient of H between molten metallic iron and silicate melt increases with pressure and temperature. Under the presence of Fe-rich metal, CO2 content in silicate melt is suggested to be very low because of low oxygen fugacity under such condition. Assuming a homogeneous accretion of planetesimals with the composition given by the two-component model slightly modified from Ringwood [1977] and Waanke [1981], it is shown that C is preferentially partitioned to molten metallic iron and quite less to silicate melt, whereas a substantial proportion of H is partitioned to silicate melt as H2O and also to molten metallic iron as interstitial atoms. For such concentrations of H, C, and S, the effect of thermodynamic interaction among them in molten metallic iron is not strong enough to cause the oversaturation of graphite. H and C partitioned to molten metallic iron may account for a significant portion of the density deficit in Earth's core. The estimated amount of H2O partitioned to silicate melt is possibly large enough to explain the sources for (1) H2O in the hydrosphere and mantle, (2) oxygen which partially oxidizes ferrous iron to ferric iron in the mantle, and (3) oxidant for metals which may fail to segregate to the core and act as the source of the highly siderophile elements in the mantle of the present Earth. The higher H/C and the lower C/36Ar ratios in the silicate Earth including the hydrosphere compared to various classes of meteorites are possibly explained if these elements are derived from the early mantle material after the H and C partitioning to molten metallic iron and core segregation. Accretion of the late veneer material such as the highly oxidized, CI chondrite-like material seems difficult to explain such elemental abundance pattern without invoking unknown large volatile reservoir in the mantle.