Extremely light K in subducted low -T altered oceanic crust: Implications for K recycling in subduction zone
Extremely light K in subducted low -T altered oceanic crust: Implications for K recycling in subduction zone
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俯冲低温蚀变洋壳中的极轻钾：对俯冲带钾再循环的影响
DOI：
10.1016/j.gca.2020.03.025
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发表时间：
2020-05-15
影响因子：
5
通讯作者：
Tuller-Ross, Brenna
中科院分区：
文献类型：
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作者：
Liu, Haiyang;Wang, Kun;Tuller-Ross, Brenna
为了研究变质脱水过程中钾同位素的行为，并进一步探讨其对俯冲带钾循环的意义，采用新近建立的高精度分析方法，对西藏松岛榴辉岩全岩和分离矿物（多硅白云母、绿辉石、角闪石）的钾同位素组成进行了测定。我们的数据显示，全岩榴辉岩的δ 41 K（‰）（−1.64至−0.24）显示出比新鲜洋中脊玄武岩（MORB）（−0.43 ± 0.17）和蚀变MORB（−0.76至−0.11）中观察到的值低得多。榴辉岩的δ 41 K值与K2 O含量和K/Nb比值均呈正相关，表明榴辉岩的δ 41 K值偏低可能是俯冲过程中脱水所致。因此，同位素重钾可能被释放到地幔楔中，而轻组分则被俯冲到深部地幔中。Sumdo榴辉岩中矿物的K同位素组成高度不均匀，多硅白云母的K同位素组成范围为−0.98至+0.23，绿辉石的K同位素组成范围为−1.25至−1.10，角闪石的K同位素组成范围为−1.16至−0.09。角闪石与多硅白云母、角闪石与绿辉石之间的K同位素分馏值分别为-0.30 ~+0.25和-0.04 ~+0.63，表明角闪石与绿辉石/多硅白云母之间的K同位素不平衡，可能是俯冲变质过程中矿物多阶段生长的结果。但多硅白云母的钾同位素组成普遍比绿辉石重（Δ 41钾霞石-角闪石= +0.25）和角闪石（Δ 41钾霞石-角闪石= +0.03 <$+0.30，TB 193除外，为-0.25），这可能意味着多硅白云母（6）和绿辉石（7 - 8）/角闪石（8）之间的钾配位数差异控制了钾同位素分馏.
In order to investigate the behavior of potassium (K) isotopes during metamorphic dehydration and to further constrain its implications for K recycling in the subduction zone, we measured K isotopic compositions of whole-rocks and separated minerals (phengite, omphacite, amphibole) from Sumdo eclogites, Tibet, using a recently established high-precision analysis method. Our data reveal that the δ41K (‰) of the whole-rock eclogites (−1.64 to −0.24) displays dramatically lower values than observed in fresh mid-ocean ridge basalts (MORB) (−0.43 ± 0.17) and altered MORB (−0.76 to −0.11). In addition, the δ41K values of eclogites show a positive correlation with both K2O contents and K/Nb ratios, which suggests that the low δ41K values were most likely caused by dehydration during subduction. Thus, isotopically heavy K may be released into the mantle wedge, while the light component is subducted into the deep mantle. Therefore, the K isotope systems have the potential to trace subducted crustal materials and to create heterogeneity within the mantle.Mineral separates from Sumdo eclogites are highly heterogeneous in K isotopic compositions, ranging from −0.98 to +0.23 in phengite, from −1.25 to −1.10 in omphacite and from −1.16 to −0.09 in amphibole. The K isotope fractionations between amphibole and phengite and between amphibole and omphacite vary from −0.30 to +0.25 and from −0.04 to +0.63, respectively, indicating K isotopic disequilibrium between amphibole and omphacite/phengite, which might result from the multistage growth of minerals during subduction metamorphism. However, generally phengites show heavier K isotopic compositions than the coexisting omphacite (Δ41Kphengite-amphibole= +0.25) and amphibole (Δ41Kphengite-amphibole= +0.03 ∼ +0.30, except TB193 is −0.25), which may imply that the K isotopic fractionation is controlled by the difference in coordination numbers of K between phengite (6) and omphacite (7 to 8)/amphibole (8).