Mantle source heterogeneity and magmatic evolution at Carlsberg Ridge (3.7A degrees N): constrains from elemental and isotopic (Sr, Nd, Pb) data
Mantle source heterogeneity and magmatic evolution at Carlsberg Ridge (3.7A degrees N): constrains from elemental and isotopic (Sr, Nd, Pb) data
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Carlsberg Ridge（北纬 3.7°）地幔源异质性和岩浆演化：来自元素和同位素（Sr、Nd、Pb）数据的约束
DOI：
10.1007/s11001-016-9292-1
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
Dong Yanhui
中科院分区：
文献类型：
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作者：
Chen Ling;Tang Limei;Yu Xing;Dong Yanhui
我们提出了新的主元素，ICP-MS微量元素和Sr-Nd-Pb同位素数据的玄武岩从四个地点沿着嘉士伯海岭（CR），北方印度洋。玄武岩是低钾拉斑玄武岩，具有7.52-9.51重量% MgO、49.40-50.60重量% SiO2、0.09-0.27重量% K2 O、2.55-2.90重量% Na 2 O和0.60-0.68重量% Mg#+[2]。玄武岩的微量元素含量与MORB的平均值相似，表现为LREE亏损型，（La/Sm）N比值为0.55-0.69，但部分样品富集K、Rb等大离子亲石元素，可能是地幔源区改造的结果。相容元素[如Ni、Cr和Sr（分别与橄榄石、单斜辉石和斜长石有关）]与不相容元素（如Zr和Y）之间的相关性较差，而Zr与Zr/Y和Nb与Nb/Y曲线的正相关性表明岩浆演化主要受地幔熔融而非分离结晶控制。我们的结果将CR玄武岩范围扩展到更高放射成因Pb同位素和更低的143 Nd/144 Nd。这些玄武岩和来自北方印度洋海岭的玄武岩的143 Nd/144 Nd和87 Sr/86 Sr值比亏损地幔（DM）的低，表明它们具有远洋沉积成分的趋势。CR 3-4°N玄武岩的Pb同位素比值沿着于DM与上陆壳之间的成分混合线。而CR 9-10°N玄武岩的低放射成因Pb位于DM与下陆壳的混合线上。如果源地幔被大陆岩石圈地幔污染，MORB的Pb同位素比值会降低，我们认为CR中含有大陆岩石圈物质，导致不同脊段下地幔的不均匀性。大陆岩石圈物质是在冈瓦纳大陆裂解之前或裂解过程中进入软流圈的。这些结果支持了大陆物质在大洋地幔中的长期保存，这将对印度洋MORB的同位素异常产生重要影响。
We present new major element, ICP-MS trace element, and Sr–Nd–Pb isotope data of basalts from four locations along the Carlsberg Ridge (CR), northern Indian Ocean. The basalts are low-K tholeiites with 7.52–9.51 wt% MgO, 49.40–50.60 wt% SiO2, 0.09–0.27 wt% K2O, 2.55–2.90 wt% Na2O, and 0.60–0.68 Mg#. Trace element contents of the basalts show characteristics similar to those of average normal MORB, such as LREE depleted patterns with (La/Sm)Nratio of 0.55–0.69; however, some samples are enriched in large-ion lithophile elements such as K and Rb, suggesting probable modification of the mantle source. Poor correlations between the compatible elements [e.g. Ni, Cr, and Sr (related to olivine, clinopyroxene and plagioclase, respectively)] and the incompatible elements (e.g. Zr and Y), and positive correlations in the Zr versus Zr/Y and Nb versus Nb/Y plots suggest a magmatic evolution controlled mainly by mantle melting rather than fractional crystallization. Our results extend the CR basalt range to higher radiogenic Pb isotopes and lower143Nd/144Nd. These basalts and basalts from the northern Indian Ocean Ridge show lower143Nd/144Nd and higher87Sr/86Sr values than those of the depleted mantle (DM), defining a trend towards pelagic sediment composition. The Pb isotopic ratios of basalts from CR 3–4°N lie along the compositional mixing lines between the DM and the upper continental crust. However, the low radiogenic Pb of basalts from CR 9–10°N lie on the mixing line between the DM and lower continental crust. Since the Pb isotopic ratio of MORB would decrease if the source mantle was contaminated by continental lithospheric mantle, we suggest that CR contains continental lithospheric material, resulting in heterogeneous mantle beneath different ridge segments. The continental lithospheric material was introduced into the asthenosphere before or during the breakup of the Gondwana. These results support the long-term preservation of continental material in the oceanic mantle which would significantly influence the isotopic anomaly of the Indian Ocean MORB.