Coupled δ18O-δ17O and 87Sr/86Sr isotope compositions suggest a radiogenic and 18O-enriched magma source for Neapolitan volcanoes (Southern Italy)
Coupled δ18O-δ17O and 87Sr/86Sr isotope compositions suggest a radiogenic and 18O-enriched magma source for Neapolitan volcanoes (Southern Italy)
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耦合的 18O-17O 和 87Sr/86Sr 同位素组成表明那不勒斯火山（意大利南部）存在放射性和富含 18O 的岩浆源
DOI：
10.1016/j.lithos.2018.07.009
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发表时间：
2018
期刊：
影响因子：
3.5
通讯作者：
D’Antonio M
中科院分区：
文献类型：
--
作者：
Iovine RS;Mazzeo FC;Wörner G;Pelullo C;Cirillo G;Arienzo I;Pack A;D’Antonio M
那不勒斯火山（包括Somma-Vesuvius火山和Campi Flegrei火山（意大利南部））喷发的岩浆稳定同位素和放射性同位素组成的巨大变化的起源一直存在争议。事实上，沉积物俯冲与地壳同化在解释喷发岩浆的化学和同位素变化方面的作用和相对重要性仍不清楚。矿物与其寄主之间的同位素不平衡表明，块状岩石分析不能约束源的同位素组成。因此，我们利用那不勒斯火山区（Phlegrean火山区和Somma-Vesuvius杂岩）火山碎屑序列和熔岩流中分离矿物（长石、斜辉石和橄榄石斑晶）的同位素（87Sr/86Sr、18O/16O和17o /16O）数据来更好地约束岩浆氧和锶同位素比值。根据橄榄石和斜辉石斑晶的δ18O重新计算的岩浆值相对于SMOW的典型地幔值为5.2‰至近9‰。这些成分与典型的地幔源非常不同。为了评价这些矿物形成的岩浆演化程度，将斜辉石和橄榄石中的Mg#转化为寄主熔体Mg#，其范围为44 ~ 76。硅质地壳岩石的简单同化很难与这些估计的寄主岩浆组成的基性相协调。这表明一定存在一些镁铁质幔源岩浆，它们具有异常重的氧同位素（高达~9‰）和高的Sr同位素组成（0.7050 ~ 0.7085）。由于同位素与主微量元素特征之间缺乏联系，可以排除碳酸盐的地壳同化作用。然而，不能完全排除海西样地壳或蚀变火山碎屑岩的同化作用。然而，来自类似海西期地壳的部分熔体的同化作用必须达到~12%和~21%，才能分别解释Campi Flegrei和Somma-Vesuvius的重氧同位素值。这种程度的同化是不可能的，因为岩浆要么太基性，要么太碱性，与如此高比例的地壳成分不一致。另一些基性较弱的Campi Flegrei、Somma-Vesuvius和Ischia岩浆则具有更典型的地幔氧同位素，它们可能是在受远洋和碳酸盐沉积物的轻微污染以及随后的海西期地壳同化作用影响的地幔源中产生的。Sr-O混合模型表明，这些岩浆来自地幔源，受到高达10%的远洋和碳酸盐沉积物1:1混合物的污染。这些发现共同证明了坎帕尼亚火山区岩浆来源的高度复杂性和多样性。基于潜在同化剂中18o /16O和17o /16O比值测量的三氧同位素变化（Δ17O）与这些结论一致。
The origin of large variations in stable and radiogenic isotope compositions of magmas erupted from the Neapolitan volcanoes, including Somma-Vesuvius and Campi Flegrei (Southern Italy), has always been contentious. Indeed, the role and relative importance of sediment subduction versus crustal assimilation to explain the chemical and isotopic variations of the erupted magmas remain unclear. Isotopic disequilibrium between minerals and their host indicate that bulk rock analyses are incapable of constraining the isotopic composition of the source. Therefore, we use isotopic (87Sr/86Sr,18O/16O and17O/16O) data on separated minerals (feldspar, clinopyroxene and olivine phenocrysts) from pyroclastic successions and lava flows of the Neapolitan volcanic area (Phlegrean Volcanic District and Somma-Vesuvius complex) to better constrain magmatic oxygen and strontium isotope ratios. Magmatic values recalculated from δ18O of olivine and clinopyroxene phenocrysts range from typical mantle values of 5.2‰ to almost 9‰ relative to SMOW. These compositions are very different from those of typical mantle sources. In order to assess the degree of magma evolution from which these minerals formed, the Mg# of clinopyroxene and olivine were converted into host melt Mg#, resulting in the range 44 to 76. Simple assimilation of silicic crustal rocks is difficult to reconcile with the mafic nature of these estimated host magma compositions. This indicates that some mafic, mantle-derived magmas, having unusually heavy oxygen isotope (up to ~9‰) and high Sr isotope compositions (0.7050 to 0.7085), must exist. Crustal assimilation of carbonates can be excluded by the lack of a link between isotopes and major and trace element signatures. Assimilation of either Hercynian-like crust or altered pyroclastic rocks, however, cannot be ruled out completely. However, assimilation of partial melts from a Hercynian-like crust would have to be ~12% and ~21% to explain the heavy oxygen isotope values of Campi Flegrei and Somma-Vesuvius, respectively. Such degrees of assimilation are unlikely since the magmas are either too mafic or too alkaline to be consistent with such high proportion of crustal components. Other, less mafic Campi Flegrei, Somma-Vesuvius and Ischia magmas with more typical mantle oxygen isotopes, have been possibly generated in a mantle source affected by minor contamination by pelagic and carbonate sediments and subsequent assimilation of Hercynian-like crust.Sr-O mixing models indicate that such magmas were derived from a mantle source that was contaminated by up to 10% of a 1:1 mixture of pelagic and carbonate sediments. These findings together demonstrate the highly complex and varied magma sources in the Campania volcanic district. Triple oxygen isotope variations (Δ17O), based on measurements of both18O/16O and17O/16O ratios in potential assimilants are in agreement with these conclusions.
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