To investigate the role of carbonate recycling in generating ultrapotassic magmatic rocks, we carried out detailed MgZn isotope analyses of silica-saturated to silica-unsaturated ultrapotassic volcanic rocks from the Central Mediterranean basin. These rocks have lower Mg isotopic compositions (δMg = −0.40‰ to −0.21‰, = 20) than the mantle and exhibit MORB-like Zn isotopic compositions (δZn = 0.23‰ to 0.30‰, = 16). Postmagmatic alteration, crustal assimilation, magmatic differentiation, and diffusion processes were found to have insignificant effects on MgZn isotope variations of the ultrapotassic rocks. Based on the relationships between δMg and major and trace element contents and Nd isotopic compositions, the Mg isotope variations cannot be induced by adding carbonated eclogite to the mantle source. Instead, the low Fe/Mn, Hf/Hf, Ti/Ti and high Ca/Al ratios and good correlations of Hf/Sm vs. Ca/Al and La/Yb vs. Ti/Eu suggest that the light Mg isotopic compositions of the ultrapotassic rocks were caused by carbonate melt metasomatism. In addition, the combined MgZn isotopic compositions suggest that the metasomatic agent is recycled carbonate-bearing silicate sediments. Silica-unsaturated ultrapotassic rocks have slightly lower δMg values than silica-saturated rocks, which is consistent with the greater amounts of carbonates in the recycled carbonate-bearing silicate sediments. The highly radiogenic Sr and unradiogenic Nd isotopic compositions of the studied rocks and two end-member mixing models further document that the mantle sources contained silicate-rich components, possibly Italian basements. The transition from silica-saturated to silica-unsaturated ultrapotassic rocks corresponds to the transformation of sediments from metapelites to carbonated metapelites. Therefore, by integrating Mg–Zn–Sr–Nd isotopic compositions, this study constrains the composition of subducted sediments and documents the important role of carbonate melt metasomatism in shifting the compositions of ultrapotassic rocks from silica-saturated to silica-unsaturated.

中文翻译：

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镁锌同位素追踪了循环沉积物成分在地中海中部硅饱和超钾岩向硅不饱和超钾岩转变过程中的作用


为了研究碳酸盐循环在生成超钾岩浆岩中的作用，我们对地中海盆地中部硅饱和至硅不饱和超钾火山岩进行了详细的 MgZn 同位素分析。这些岩石的镁同位素组成（δMg = -0.40‰至-0.21‰，= 20）低于地幔，并表现出类似MORB的锌同位素组成（δZn = 0.23‰至0.30‰，= 16）。研究发现岩浆后蚀变、地壳同化、岩浆分异和扩散过程对超钾岩的 MgZn 同位素变化影响不大。从δMg与主微量元素含量及Nd同位素组成的关系来看，地幔源区中添加碳酸盐榴辉岩并不能引起Mg同位素变化。相反，低 Fe/Mn、Hf/Hf、Ti/Ti 和高 Ca/Al 比率以及 Hf/Sm 与 Ca/Al 和 La/Yb 与 Ti/Eu 的良好相关性表明，超钾岩是由碳酸盐熔融交代作用形成的。此外，结合的 MgZn 同位素组成表明，交代剂是回收的含碳酸盐硅酸盐沉积物。二氧化硅不饱和超钾岩的 δMg 值略低于二氧化硅饱和岩石，这与回收的含碳酸盐硅酸盐沉积物中碳酸盐含量较高是一致的。研究岩石的高放射性 Sr 和非放射性 Nd 同位素组成以及两个端元混合模型进一步证明地幔源含有富含硅酸盐的成分，可能是意大利基底。从二氧化硅饱和到二氧化硅不饱和超钾岩的转变对应于沉积物从变泥岩到碳化变泥岩的转变。 因此，通过整合 Mg-Zn-Sr-Nd 同位素组成，本研究限制了俯冲沉积物的组成，并记录了碳酸盐熔体交代作用在超钾岩组成从二氧化硅饱和转变为二氧化硅不饱和的过程中的重要作用。