The magnesium isotopic composition of the mantle
The magnesium isotopic composition of the mantle
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DOI：
10.1016/j.gca.2023.08.011
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发表时间：
2023-08
影响因子：
5
通讯作者：
Xiao-Ning Liu;R. Hin;C. Coath;M. Bizimis;Li Su;D. Ionov;E. Takazawa;R. Brooker;Tim Elliott-
中科院分区：
文献类型：
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作者：
Xiao-Ning Liu;R. Hin;C. Coath;M. Bizimis;Li Su;D. Ionov;E. Takazawa;R. Brooker;Tim Elliott-
为了更好地约束地幔的Mg同位素组成，我们分析了28个样品的大洋和大陆橄榄岩使用高精度，临界混合物双spiking方法。未改变的样品在外部分析不确定度中没有变化δ 26 Mg，并产生-0.236 ± 0.006‰（2 s.e.）为可接近的地幔，证实其非南极组成。我们还确定了矿物间镁同位素分馏的一个子集的样品。我们记录了橄榄石和辉石之间δ 26 Mg的微小但显著的差异，Δ26/24 Mgol/cpx= −0.118 ± 0.018‰和Δ26/24 Mgol/opx= −0.056 ± 0.018‰，与橄榄岩矿物测温记录的温度为1000 °C的从头计算结果非常一致。橄榄石与尖晶石δ ~（26）Mg的差异（Δ26/24 Mgol/sp）的变化较大，一般高于相应温度下的理论计算值，这可能是由于包体在喷发后冷却过程中Fe-Mg扩散交换不完全所致。部分熔融对残余橄榄岩δ 26 Mg的影响可以忽略不计。这有助于解释新鲜地幔橄榄岩中δ 26 Mg变化极小的原因。然而，在较宽的熔融条件范围内，原始地幔熔体的δ 26 Mg值都明显高于其源区（橄榄岩和辉石岩源区的代表性部分熔体的Δ 26 Mg值分别为0.06‰和0.123‰）。这种δ 26 Mg值的升高在目前的幔源熔体数据集中并没有普遍观察到。我们建议，这种不一致性可能是低Mg/Fe熔体迁移通过高Mg/Fe地幔途中的表面之间的部分再平衡过程中的扩散分馏的后果。
In order to better constrain the Mg isotopic composition of the mantle, we have analysed twenty-eight samples of both oceanic and continental peridotite using a high-precision, critical mixture double spiking approach. The unaltered samples show no variability δ26Mg in outside analytical uncertainty and yield a value of −0.236 ± 0.006‰ (2 s.e.) for the accessible mantle, substantiating its non-chondritic composition. We have also determined inter-mineral Mg isotopic fractionations for a sub-set of samples. We document small but significant differences in δ26Mg between olivine and pyroxenes, Δ26/24Mgol/cpx= −0.118 ± 0.018‰ and Δ26/24Mgol/opx= −0.056 ± 0.018‰, in excellent agreement withab initiocalculations for temperatures ∼1000 °C, as recorded by mineral thermometry in the peridotites. The differences in δ26Mg between olivine and spinel (Δ26/24Mgol/sp) are more variable and generally higher than theoretical calculations at corresponding temperatures, likely due to incomplete Fe-Mg diffusive exchange during post-eruptive cooling of the xenoliths.Using these data, together with a recently determined olivine-melt fractionation factor for Mg isotopes, we show that partial melting has a negligible influence on the δ26Mg of residual peridotites. This helps account for the minimal variability of δ26Mg in fresh, mantle peridotites. However, the δ26Mg of primary mantle melts are predicted to be discernibly higher than their sources (Δ26Mg ∼ 0.06‰ and ∼0.123‰ for representative partial melts of peridotitic and pyroxenitic sources respectively) across a wide range of melting conditions. Such elevated δ26Mg values are not generally observed in the current dataset of mantle derived melts. We propose that this inconsistency is likely a consequence of diffusive fractionation during partial re-equilibration between low Mg/Fe melts migrating through high Mg/Fe mantle en route to the surface.