Mg-Sr isotopes of low-Mg-26 basalts tracing recycled carbonate species: Implication for the initial melting depth of the carbonated mantle in Eastern China
Mg-Sr isotopes of low-Mg-26 basalts tracing recycled carbonate species: Implication for the initial melting depth of the carbonated mantle in Eastern China
复制标题
低 Mg-26 玄武岩的 Mg-Sr 同位素追踪再循环碳酸盐物种：对中国东部碳化地幔初始熔融深度的影响
DOI：
10.1080/00206814.2016.1157709
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发表时间：
2016
影响因子：
2.6
通讯作者：
Xiao Yilin
中科院分区：
文献类型：
--
作者：
Huang Jian;Xiao Yilin
最近的研究表明，可以利用玄武岩的镁同位素来追踪再循环到地幔中的地壳碳酸盐。然而，再生碳酸盐的种类限制很差。碳酸盐岩相对于地幔具有较低的δ26Mg值和较高的87sr /86Sr比值，但由于其Sr和Mg含量的差异，不同种类的碳酸盐岩在Mg-Sr同位素图中与地幔的混合曲线不同。因此，对镁锶同位素的联合研究可以限制深层再循环碳酸盐的种类。本文对中国东部<110 Ma玄武岩的87sr /86Sr比值进行了最新测定，并结合已发表的Mg同位素资料对地幔中再生碳酸盐的种类进行了评价，并讨论了其意义。<110 Ma玄武岩δ26Mg值较低，为-0.60 ~ -0.30‰，初始87sr /86Sr比值较低，为0.70328 ~ 0.70537，表明其地幔源是由轻Mg同位素组成的再循环碳酸盐杂化而成，其对Mg的影响大于对Sr同位素比值的影响。镁锶同位素数据表明，回收碳酸盐主要由菱镁矿和文石组成，但不能排除方解石和白云石的可能性。根据碳酸化橄榄岩固相、白云岩与菱镁石+文石的平衡线以及地幔绝热岩，确定了玄武岩源区碳酸化地幔的初始熔融深度为~300 ~ 360 km。因此，西太平洋板块的俯冲深度应该大于~300 ~ 360 km，这与西太平洋板块目前停滞在地幔过渡带的地震层析成像结果一致。
Recent studies show that crustal carbonates recycled into the mantle can be traced using Mg isotopes of basalts. However, the species of recycled carbonates are poorly constrained. Carbonates have lower δ26Mg values and higher87Sr/86Sr ratios relative to the mantle, but different carbonate species display different mixing curves with the mantle in the Mg-Sr isotopic diagram because of differences in their Sr and Mg contents. Thus a combined study of Mg-Sr isotopes can constrain the species of deeply recycled carbonates. Here, we present newly determined87Sr/86Sr ratios of the <110 Ma basalts from Eastern China, and together with published Mg isotopic data we evaluate the species of recycled carbonates in the mantle and discuss their implication. The <110 Ma basalts display low δ26Mg values of ‒0.60 to ‒0.30‰ and relatively low initial87Sr/86Sr ratios of 0.70328 to 0.70537, suggesting that their mantle source was hybridized by recycled carbonates with a light Mg isotopic composition which had more significant effects on Mg than Sr isotope ratios. Mg-Sr isotopic data indicate that the recycled carbonates consist of magnesite and aragonite, but the possibility of calcite and dolomite cannot be eliminated. Based on the carbonated peridotite solidus, the equilibrium line between dolomite and magnesite + aragonite, as well as the mantle adiabat, the initial melting depth of the carbonated mantle, the source region of the studied basalts, was constrained at ~300–360 km. Thus, the subducted depth of the west Pacific slab underlying the carbonated mantle and supplying recycled carbonates should be greater than ~300–360 km, consistent with the seismic tomography result that the west Pacific slab now stagnates in the mantle transition zone.