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首页> 外文期刊>Journal of Volcanology and Geothermal Research >B/Nb and δ~(11)B systematics in the Phlegrean Volcanic District, Italy
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B/Nb and δ~(11)B systematics in the Phlegrean Volcanic District, Italy

机译：意大利Phlegrean火山区的B / Nb和δ〜（11）B系统

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摘要

Boron concentration and isotopic compositions were determined in representative products from the Phlegrean Volcanic District (PVD), which includes Campi Flegrei (CF), and the islands of Procida and Ischia. The most primitive products (from Procida and Ischia islands) are moderately enriched in B (4.6-12 ppm), whereas more evolved products of CF and Ischia Island are more enriched (21-118 ppm). The content of B is positively correlated with Nb, Zr, Th, La and, generally, with all incompatible elements. δ~(11)B values are generally lower and more uniform in samples from CF (-6.8 to - 10.6 per thousand) compared with those from Procida (-3.6 to - 8.5 per thousand) and Ischia (-2.8 to -8.4 per thousand) islands. Overall, B-enrichments relative to fluid-immobile elements of PVD are only slightly higher than those observed in mid-ocean ridge basalts and there is no significant correlation between δ~(11)B and B/immobile element ratios. For CF samples including the Campanian Ignimbrite (CI) and most post-CI products, δ~(11)B is negatively correlated with ~(87)Sr/~(86)Sr, with extreme compositions represented by evolved products of the CI (higher δ~(11)B, lower ~(87)Sr/~(86)Sr) and less evolved shoshonite (lower δ~(11)B, higher ~(87)Sr/~(86)Sr). In contrast, samples from Procida, Ischia, and pre-CI products and one young shoshonite from CF define a distinct correlation with primitive trachybasalt as the high δ~(11)B, low ~(87)Sr/~(86)Sr end-member, and young shoshonites as the low δ~(11)B, high ~(87)Sr/~(86)Sr end-member. The overall interpretation of the geochemical and isotopic data suggests three conclusions: (1) Geochemical and isotopic variations of post-CI products from CF can be explained by mixing/mingling between at least two distinct magmas, i.e. the CI and the young shoshonite (Minopoli 1). (2) The isotopic trend described by Procida trachybasalts, Ischia samples, pre-CI samples from CF and Pigna St. Nicola shoshonite is a mantle trend suggesting the involvement in their genesis of a distinct component depleted in B, with low δ~(11)B and high ~(87)Sr/~(86)Sr. A similar component has been invoked to explain the differences between Stromboli potassic lavas and more typical calc-alkaline rocks of the Aeolian Islands. This component is consistent with 'sediments or melts thereof introduced into the mantle by subduction, but depleted in B due to metamorphic reactions accompanying subduction. (3) The displacement from the mantle trend of CI and Minopoli 1 shoshonite magmas is probably due to crustal contamination processes.
机译：测定了来自Phlegrean火山区（PVD）（包括Campi Flegrei（CF））以及Procida和Ischia岛的代表性产品中的硼浓度和同位素组成。最原始的产品（来自Procida和Ischia岛）的B含量中等（4.6-12 ppm），而CF和Ischia岛的进化产物更丰富（21-118 ppm）。 B的含量与Nb，Zr，Th，La以及通常与所有不相容元素呈正相关。与Procida（-3.6至-8.5 /千）和Ischia（-2.8至-8.4 /千）相比，CF（-6.8至-10.6 /千）样品中的δ〜（11）B值通常更低且更均匀）岛屿。总体而言，相对于PVD流体固定元素的B富集仅略高于在洋中脊玄武岩中观测到的富集，并且δ〜（11）B与B /固定元素之比之间没有显着相关性。对于包括Campanian Ignimbrite（CI）和大多数CI后产物的CF样品，δ〜（11）B与〜（87）Sr /〜（86）Sr呈负相关，其极端成分以CI的演化产物表示（较高的δ〜（11）B，较低的〜（87）Sr /〜（86）Sr）和少析出的菱铁矿（较低的δ〜（11）B，较高的〜（87）Sr /〜（86）Sr）。相比之下，来自Procida，Ischia和CI前产品的样品以及来自CF的一种年轻的shoshonite定义了与原始鱼玄武岩的明显相关性，即高δ〜（11）B，低〜（87）Sr /〜（86）Sr δ-（11）B，高〜（87）Sr /〜（86）Sr末端成员。地球化学和同位素数据的整体解释提出了三个结论：（1）CF的后CI产物的地球化学和同位素变化可以通过在至少两个不同的岩浆即CI和年轻的shoshonite（Minopoli）之间混合/混合来解释。 1）。 （2）由Procida trachybasalts，Ischia样品，CF和Pigna St.Nicola shoshonite的CI前样品描述的同位素趋势是地幔趋势，表明参与了其贫化的B中独特组分的产生，δ〜（11 B和高〜（87）Sr /〜（86）Sr。人们援引了一个类似的成分来解释斯特龙博利钾盐熔岩与风神群岛的更典型的钙碱性岩石之间的差异。该组分与通过俯冲作用引入地幔中的沉积物或熔体相一致，但由于伴随俯冲作用的变质反应而被贫化。 （3）CI和Minopoli 1 shoshonite岩浆地幔趋势的位移可能是由于地壳污染过程造成的。

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