Iron isotope fractionation during crystallization and sub-solidus re-equilibration: Constraints from the Baima mafic layered intrusion, SW China
Iron isotope fractionation during crystallization and sub-solidus re-equilibration: Constraints from the Baima mafic layered intrusion, SW China
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结晶过程中的铁同位素分馏和亚固相线再平衡：来自中国西南白马镁铁质层状岩体的约束
DOI：
10.1016/j.chemgeo.2014.04.020
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发表时间：
2014-07
期刊：
影响因子：
3.9
通讯作者：
Yi Junnian
中科院分区：
文献类型：
--
作者：
Chen Liemeng;Song Xieyan;Zhu Xiangkun;Zhang Xiaoqi;Yu Songyue;Yi Junnian
为了更好地了解镁铁质岩浆结晶和亚固相线再平衡过程中镁铁质矿物和氧化物之间的Fe同位素分馏，对中国西南白马镁铁质层状岩体全岩和分离矿物（橄榄石、单斜辉石、磁铁矿和钛铁矿）的Fe同位素进行了研究。分离出的矿物的δ 57 Fe值显示出系统性的降低，从磁铁矿（0.15至0.51‰）到橄榄石（− 0.11至0.15‰）和单斜辉石（− 0.35至0.05‰），然后到钛铁矿（− 0.82至− 0.10‰），表明这些矿物之间有规律的分馏。除少数样品外，大部分橄榄石和单斜辉石的Fe同位素特征与地幔捕虏体相似，表明岩浆结晶过程中达到的Fe同位素平衡得到了较好的保存。磁铁矿和钛铁矿的Fe同位素则通过氧化物之间的Fe ~（3+）与Ti ~（4+）和Fe ~（2+）交换而发生亚固相线再平衡。此外，铁同位素的亚固相线再平衡强烈地受控于岩石中磁铁矿和钛铁矿的比例。因此，磁铁矿/钛铁矿比值高达6-10的下带岩石，虽然δ 57 FeIm值因亚固相线再平衡作用而明显降低，但磁铁矿仍保持其原有的Fe同位素组成。相比之下，由于中等的磁铁矿/钛铁矿比例（4-7），中间带的大多数磁铁矿和钛铁矿的铁同位素都因亚固相线再平衡而发生了显著变化。低带旋回单元中δ 57 FeMt和δ 57 FeOlup的降低表明，磁铁矿早期广泛的分离结晶作用导致岩浆中较重的Fe同位素亏损。另一方面，橄榄石和单斜辉石的早期结晶作用使中、上带旋回单元δ 57 FeO 1值向上略有升高。δ 57 FeMt和δ 57 FeOl值的地层反转表明岩浆多次充注。下部带底部的δ 57 FeO 1值（0.10 ~ 0.15‰）表明，由于深部硅酸盐矿物的广泛分馏，母岩浆的Fe同位素较重。研究结果表明，在岩浆结晶和亚固相线再平衡过程中，铁同位素在硅酸盐和氧化物之间发生了分馏。
To better understand Fe isotope fractionation between mafic minerals and oxides during crystallization of mafic magma and sub-solidus re-equilibration, the Fe isotopes of whole-rocks and separated minerals (olivine, clinopyroxene, magnetite and ilmenite) of the Baima mafic layered intrusion, SW China, have been investigated. The separated minerals show a systematical decrease in δ57Fe values, from magnetite (0.15 to 0.51‰) to olivine (− 0.11 to 0.15‰) and clinopyroxene (− 0.35 to 0.05‰) and then to ilmenite (− 0.82 to − 0.10‰), demonstrating regular fractionation between these minerals. Except for a few of samples, most of the olivine and clinopyroxene are similar to those of mantle xenoliths in Fe isotopes, indicating that Fe isotope equilibrium reached during magma crystallization was well preserved. By contrast, the Fe isotopes of the magnetite and ilmenite may be evidently modified by sub-solidus re-equilibration via the Fe3 +versus Ti4 +and Fe2 +exchange between the oxides. Furthermore, the sub-solidus re-equilibration in Fe isotope is strongly controlled by the proportions of magnetite and ilmenite in rocks. Therefore, although the δ57FeIlmof the Lower Zone rocks with magnetite/ilmenite ratios as high as 6–10 was reduced evidently by the sub-solidus re-equilibration, the magnetite preserved their original Fe isotope compositions. By contrast, the Fe isotopes of both most magnetite and ilmenite in the Middle Zone had been markedly modified by sub-solidus re-equilibration owing to the moderate magnetite/ilmenite ratios (4–7). The decreases of both δ57FeMtand δ57FeOlupwards in the cyclic units of the Lower Zone reveal that extensive early fractional crystallization of the magnetite resulted in depletion of heavier Fe isotopes in the magma. On the other hand, early crystallization of olivine and clinopyroxene gave rise to the slight elevation of δ57FeOlvalues upwards in the cyclic units of the Middle and Upper zones. The stratigraphic reversals in the δ57FeMtand δ57FeOlvalues suggest multiple magma recharges. Additionally, the δ57FeOlvalues in the base of the Lower Zone (0.10 to 0.15‰) indicate that the parental magma were heavy in Fe isotope due to extensive silicate mineral fractionation at depth. This study indicates fractionation in Fe isotope between silicates and oxides during magma crystallization and sub-solidus re-equilibration.
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影响因子：
5.8
作者：
Yigang Xu;B. He;Sun‐Lin Chung;M. Menzies;F. Frey
通讯作者：
Yigang Xu;B. He;Sun‐Lin Chung;M. Menzies;F. Frey
影响因子：
6.1
作者：
J. Shellnutt;S. Denyszyn;R. Mundil
通讯作者：
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影响因子：
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作者：
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通讯作者：
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影响因子：
3.9
作者：
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通讯作者：
F. Richter;N. Dauphas;F. Teng
影响因子：
3.1
作者：
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通讯作者：
M. McCanta;M. Dyar;M. Rutherford;J. Delaney