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首页> 外文期刊>Contributions to Mineralogy and Petrology >The origin of high-Mg magmas in Mt Shasta and Medicine Lake volcanoes, Cascade Arc (California): higher and lower than mantle oxygen isotope signatures attributed to current and past subduction
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The origin of high-Mg magmas in Mt Shasta and Medicine Lake volcanoes, Cascade Arc (California): higher and lower than mantle oxygen isotope signatures attributed to current and past subduction

机译：喀斯喀特弧（加利福尼亚州）沙斯塔山和梅迪奇湖火山中高镁岩浆的起源：高于和低于地幔氧同位素特征，归因于当前和过去的俯冲

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We report the oxygen isotope composition of olivine and orthopyroxene phenocrysts in lavas from the main magma types at Mt Shasta and Medicine Lake Volcanoes: primitive high-alumina olivine tholeiite (HAOT), basaltic andesites (BA), primitive magnesian andesites (PMA), and dacites. The most primitive HAOT (MgO > 9 wt%) from Mt. Shasta has olivine δ18O (δ18OOl) values of 5.9–6.1‰, which are about 1‰ higher than those observed in olivine from normal mantle-derived magmas. In contrast, HAOT lavas from Medicine Lake have δ18OOl values ranging from 4.7 to 5.5‰, which are similar to or lower than values for olivine in equilibrium with mantle-derived magmas. Other magma types from both volcanoes show intermediate δ18OOl values. The oxygen isotope composition of the most magnesian lavas cannot be explained by crustal contamination and the trace element composition of olivine phenocrysts precludes a pyroxenitic mantle source. Therefore, the high and variable δ18OOl signature of the most magnesian samples studied (HAOT and BA) comes from the peridotitic mantle wedge itself. As HAOT magma is generated by anhydrous adiabatic partial melting of the shallow mantle, its 1.4‰ range in δ18OOl reflects a heterogeneous composition of the shallow mantle source that has been influenced by subduction fluids and/or melts sometime in the past. Magmas generated in the mantle wedge by flux melting due to modern subduction fluids, as exemplified by BA and probably PMA, display more homogeneous composition with only 0.5‰ variation. The high-δ18O values observed in magnesian lavas, and principally in the HAOT, are difficult to explain by a single-stage flux-melting process in the mantle wedge above the modern subduction zone and require a mantle source enriched in 18O. It is here explained by flow of older, pre-enriched portions of the mantle through the slab window beneath the South Cascades.
机译：我们报告了Mt Shasta和Medicine Lake火山的主要岩浆类型中熔岩中橄榄石和邻位邻苯二甲酚异位晶的氧同位素组成：原始高铝橄榄石胆石（HAOT），玄武质安山岩（BA），原始镁质安山岩（PMA）和达西斯。来自山的最原始的HAOT（MgO> 9 wt％）。 Shasta的橄榄石δ 18 O（δ 18 O Ol ）值在5.9-6.1‰，比在伊斯坦布尔观测到的值高1‰。正常的地幔衍生岩浆中的橄榄石。相比之下，Medicine Lake的HAOT熔岩的δ 18 O Ol 值范围为4.7至5.5‰，与处于地幔平衡状态的橄榄石相似或低于其值。衍生岩浆。两个火山的其他岩浆类型都显示出中等的δ 18 O Ol 值。大多数镁质熔岩的氧同位素组成不能用地壳污染来解释，而橄榄石表晶的痕量元素组成排除了热释铁质地幔源。因此，研究的大多数镁质样品（HAOT和BA）的高δδsups18 O Ol 签名都来自橄榄岩地幔楔本身。由于HAOT岩浆是由浅地幔的无水绝热部分融化产生的，它在δ 18 O Ol 中的1.4‰范围反映了浅地幔源的非均质成分。受俯冲流体和/或融化作用的影响。 BA和可能的PMA举例说明了由于现代俯冲流体引起的通量熔化而在地幔楔中产生的岩浆显示出更均匀的成分，变化仅为0.5‰。在镁质熔岩中（主要是在HAOT中）观测到的高δ 18 O值很难通过现代俯冲带上方地幔楔的单阶段通量熔融过程来解释，因此需要地幔源富含 18 O。此处解释的是，地幔中较早的，预富集的部分通过南小瀑布下方的平板窗口流动。

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