Geochemistry and radiogenic isotopes of ODP Hole 157-953C lavas

The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6-90.0 mol.% Fo, 1,722-3,915 ppm Ni, 1,085-1,552 ppm Mn, 1,222-3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr-Nd-Pb isotope compositions (87Sr/86Sr = 0.70315-0.70331, 143Nd/144Nd = 0.51288-0.51292, 206Pb/204Pb = 19.55-19.93, 207Pb/204Pb = 15.60-15.63, 208Pb/204Pb = 39.31-39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al., 2007, Science Vol 316) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with 87Sr/86Sr = 0.70337, 143Nd/144Nd = 0.51291, 206Pb/204Pb = 19.36, 207Pb/204Pb = 15.61 and 208Pb/204Pb = 39.07 (EM-type end member), and pyroxenite with 87Sr/86Sr = 0.70309, 143Nd/144Nd = 0.51289, 206Pb/204Pb = 20.03, 207Pb/204Pb = 15.62 and 208Pb/204Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al., 2009, doi:10.1016/j.epsl.2008.11.013), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.

学界普遍认为，加那利群岛的原始玄武质岩浆起源于HIMU型（HIMU-type）上涌地幔，该地幔含有同位素亏损型（正常洋中脊玄武岩（NMORB））组分，并与富集型（EM）组分发生了相互作用，而该富集组分的成因至今仍存在争议。本研究针对加那利群岛最原始的陆上熔岩及大洋钻探计划（ODP, Ocean Drilling Program）第157航次的高硅熔岩（从苦橄岩至橄榄玄武岩）中的橄榄石斑晶（Fo值为85.6~90.0 mol.%，镍含量1722~3915 ppm，锰含量1085~1552 ppm，钙含量1222~3002 ppm）的镍、锰、钙浓度，与其全岩Sr-Nd-Pb同位素组成（⁸⁷Sr/⁸⁶Sr = 0.70315~0.70331，¹⁴³Nd/¹⁴⁴Nd = 0.51288~0.51292，²⁰⁶Pb/²⁰⁴Pb = 19.55~19.93，²⁰⁷Pb/²⁰⁴Pb = 15.60~15.63，²⁰⁸Pb/²⁰⁴Pb = 39.31~39.69）之间的关联展开了研究。本研究数据表明，岩浆源区同时存在橄榄岩组分与辉石岩组分。基于Sobolev等人2007年发表于《Science》第316卷的模型——该模型提出，俯冲洋壳在高压下形成的榴辉岩发生部分熔融产生的富硅熔体，与周围橄榄岩地幔发生反应，形成不含橄榄石的反应成因辉石岩——我们得到了两个端元组分的同位素组成：橄榄岩端元的⁸⁷Sr/⁸⁶Sr = 0.70337，¹⁴³Nd/¹⁴⁴Nd = 0.51291，²⁰⁶Pb/²⁰⁴Pb = 19.36，²⁰⁷Pb/²⁰⁴Pb = 15.61，²⁰⁸Pb/²⁰⁴Pb = 39.07（即EM型端元）；辉石岩端元的⁸⁷Sr/⁸⁶Sr = 0.70309，¹⁴³Nd/¹⁴⁴Nd = 0.51289，²⁰⁶Pb/²⁰⁴Pb = 20.03，²⁰⁷Pb/²⁰⁴Pb = 15.62，²⁰⁸Pb/²⁰⁴Pb = 39.84（即HIMU型端元）。将上述两个端元的熔体以70%橄榄岩熔体+30%辉石岩熔体至28%橄榄岩熔体+72%辉石岩熔体的比例混合，即可复现加那利群岛最原始的盾状火山阶段熔岩的地球化学组成。结合本研究数据与加那利群岛西部低硅熔岩的相关研究结果（Gurenko等人2009年，doi:10.1016/j.epsl.2008.11.013），至少需要四种不同的端元组分才能解释观测到的地球化学特征。我们提出这四种端元分别为：（1）源自地幔柱核的HIMU型辉石岩组分（代表中等年龄的再循环洋壳）；（2）源自地幔柱缘的HIMU型橄榄岩组分（古老再循环洋壳被搅拌至周围地幔中）；（3）被地幔柱携带至上地幔的亏损型MORB型辉石岩组分（年轻的再循环洋壳）；（4）源自地幔柱上方软流圈或岩石圈的EM型橄榄岩组分。