Experimental determination of the effect of Cr on Mg isotope fractionation between spinel and forsterite
Experimental determination of the effect of Cr on Mg isotope fractionation between spinel and forsterite
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铬对尖晶石和镁橄榄岩中镁同位素分馏影响的实验测定
DOI：
10.1016/j.gca.2020.12.028
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发表时间：
2021-03
影响因子：
5
通讯作者：
Haolan Tang;I. Szumila;D. Trail;E. Young
中科院分区：
文献类型：
--
作者：
Haolan Tang;I. Szumila;D. Trail;E. Young
本文报道了在600℃、700℃、800℃和1 GPa时进行的尖晶石-镁矿镁同位素交换实验结果，建立了尖晶石相中铬替代铝的镁同位素在镁铁矿和尖晶石之间的平衡分配关系。用三同位素方法求出了镁铝铬酸镁与菱镁矿、镁铬酸镁与菱镁矿之间的平衡分馏系数。实验测得的镁同位素分馏系数分别为：Δ2 6 mg MgAlCrO4-MgS=0.96±0.2 2×10 6/T2和Δ2 6 mg MgCr2 O4-MgS=0.5 5±0.0 8×10 6/T2(2Se)。结合前人实验测定的镁镁尖晶石与镁橄榄石之间的分馏关系(Macris等人，2013年)，这两种含铬尖晶石与镁橄榄石之间的镁同位素分馏系数分别为Δ2 6 mg镁铝铬矿-Fo=0.84±0.2 3×10 6/T2和Δ2 6 mg镁铬矿-Fo=0.43±0.10×10 6/T2。实验测定的镁铬矿与镁铁矿之间的同位素分馏关系与基于镁在这些矿物中的晶体化学环境的理论预测一致。将这些新结果与已有的尖晶石和镁橄榄岩之间平衡镁同位素交换的实验定标相结合，我们得到了尖晶石和镁橄榄岩之间随温度变化的镁同位素分馏与尖晶石中铬含量的函数：Δ26Mg镁铝1-xCrx2O4-Fo=-0.67±0.26x+1.10±0.23106/T2当应用于自然样品时，尖晶石和镁橄榄岩之间测量的镁同位素分馏，尖晶石的铬浓度，以及来自独立地温计的温度估计可以用来识别镁同位素交换和阳离子交换之间闭合温度的差异。或者这些系统中不平衡的存在。
We report the results of spinel-magnesite Mg isotope exchange experiments at 600, 700, and 800° C and 1 GPa to establish the equilibrium Mg isotope partitioning between magnesite (MgCO 3) and spinel as a function of Cr substitution for Al in the spinel phase. We used the three-isotope method to obtain equilibrium fractionation factors between MgAlCrO 4 and magnesite and MgCr 2 O 4 and magnesite. The experimentally-determined temperature-dependent Mg isotope fractionations are Δ 26 Mg MgAlCrO 4-Mgs= 0.96±0.22× 10 6/T 2 and Δ 26 Mg MgCr 2 O 4-Mgs= 0.55±0.08× 10 6/T 2 (2 se). When combined with the previous experimentally determined fractionation between forsterite and magnesite (Macris et al., 2013), the corresponding Mg isotope fractionations between these two Cr-bearing spinel compositions and forsterite are Δ 26 Mg MgAlCrO 4-Fo= 0.84±0.23× 10 6/T 2 and Δ 26 Mg MgCr 2 O 4-Fo= 0.43±0.10× 10 6/T 2. The experimentally determined isotopic fractionation relationship between magnesiochromite and magnesite agrees with theoretical predictions based on the crystal chemical environment of Mg in these minerals. By combining these new results with the existing experimental calibration of equilibrium Mg isotope exchange between pure spinel and forsterite, we arrive at the temperature-dependent Mg isotope fractionation between spinel and forsterite as a function of Cr concentration in the spinel: Δ 26 M g Mg Al 1-x Cr x 2 O 4-Fo=-0.67±0.26 x+ 1.10±0.23 10 6/T 2 When applied to natural samples, the combination of measured Mg isotope fractionation between spinel and forsterite, the Cr concentrations of the spinels, and estimates of temperature from independent geothermometers can be used to identify differences in closure temperatures between Mg isotope exchange and cation exchange, or the presence of disequilibrium in these systems.