New constraints from Central Chile on the origins of enriched continental compositions in thick-crusted arc magmas
New constraints from Central Chile on the origins of enriched continental compositions in thick-crusted arc magmas
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智利中部对厚壳弧岩浆中丰富大陆成分起源的新限制
DOI：
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发表时间：
2018
影响因子：
5
通讯作者：
G. Orozco
中科院分区：
文献类型：
--
作者：
P. Wieser;S. Turner;T. Mather;D. Pyle;I. Savov;G. Orozco
摘要建立在厚地壳上的大陆弧岩浆与覆盖在薄地壳上的岛弧和大陆弧岩浆相比，具有较高的不相容元素丰度和“富集”的放射性同位素比值。板片、地幔和上板块对这种变化的相对影响是有很大争议的。安第斯南部火山带（SVZ; 33-46°S）是研究富集大陆弧成分产生的理想环境，因为地壳厚度和岩浆化学成分沿着走向一致地变化。然而，在厚壳的北方SVZ原始岩浆的稀缺，阻碍了以前的区域研究。为了更好地解决丰富的大陆成分的起源，我们调查的地球化学（主要和微量元素丰度，87 Sr/86 Sr和143 Nd/144 Nd比值）的新的镁铁质样品唐Casimiro和Maipo火山内的Diamante-Maipo火山口复杂的北方SVZ。虽然演化的Diamante-Maipo样品显示出地壳同化的证据，但大多数镁铁质样品的微量元素和同位素富集并不是地壳加工的结果，因为没有已知的区域或全球基底岩性富含所有必要的不相容微量元素。俯冲侵蚀模型同样无法解释这些样品的同位素和微量元素富集特征。相反，我们认为，富集的北方SVZ岩浆是来自于丰富的环境地幔成分（类似于EM 1型洋岛玄武岩），叠加在一个向北下降的熔体范围。在所有纬度都需要俯冲沉积物和蚀变洋壳的大量但几乎均匀的熔融贡献。EM 1富集可能来自交代的次大陆岩石圈地幔（M-SCLM）的再循环，作为原始弧后单成锥的同位素轨迹的趋势，对整个南美洲的SCLM熔体取样的组成。空间分布的弧后中心的同位素数据表明，弧前样品中观察到的EM 1型富集程度的弧平行变化也存在于弧后海沟后方600公里处。弧后微量元素系统学需要显着的，但变量板熔体被输送到地幔楔在这些大的沟槽距离。总体而言，我们表明，一个统一的模型，将变量地幔富集，板片添加，熔融程度可以占沿着和跨弧的SVZ内的趋势。认识到地幔富集在SVZ富集大陆成分的产生中起着关键作用，这对我们理解地球的化学演化具有重要意义。如果不考虑环境地幔的富集，演化熔岩的成岩模型可能会高估地壳同化的作用，这反过来又可能导致大陆地壳生长模型高估大陆物质再循环回地幔的数量。
Abstract Magmas from continental arcs built on thick crust have elevated incompatible element abundances and “enriched” radiogenic isotope ratios compared to magmas erupted in island and continental arcs overlying thinner crust. The relative influence of the slab, mantle, and upper plate on this variability is heavily debated. The Andean Southern Volcanic Zone (SVZ; 33–46°S) is an ideal setting to investigate the production of enriched continental arc compositions, because both crustal thickness and magma chemistry vary coherently along strike. However, the scarcity of primitive magmas in the thick-crusted northern SVZ has hindered previous regional studies. To better address the origin of enriched continental compositions, we investigate the geochemistry (major and trace element abundances, 87Sr/86Sr and 143Nd/144Nd ratios) of new mafic samples from Don Casimiro and Maipo volcanoes within the Diamante-Maipo Caldera Complex of the northern SVZ. While evolved Diamante-Maipo samples show evidence for crustal assimilation, the trace element and isotopic enrichment of the most mafic samples cannot result from crustal processing, as no known regional or global basement lithologies are enriched in all of the necessary incompatible trace elements. Subduction erosion models similarly fail to account for the enriched isotopic and trace element signature of these samples. Instead, we suggest that the enrichment of northern SVZ magmas is derived from an enriched ambient mantle component (similar to EM1-type ocean island basalts), superimposed on a northward decline in melt extent. A substantial, but nearly uniform contribution of melts from subducting sediment and altered oceanic crust are required at all latitudes. The EM1-like enrichment may arise from recycling of metasomatized subcontinental lithospheric mantle (M-SCLM), as the isotopic trajectory of primitive rear-arc monogenetic cones trend towards the compositions of SCLM melts sampled across South America. Isotopic data from spatially distributed rear-arc centres demonstrate that the arc-parallel variations in the degree of EM1-type enrichment observed in arc-front samples are also present up to 600 km behind the trench in the rear-arc. Rear-arc trace element systematics require significant but variable quantities of slab melts to be transported to the mantle wedge at these large trench distances. Overall, we show that a unified model incorporating variable mantle enrichment, slab additions, and melt extents can account for along and across-arc trends within the SVZ. The recognition that mantle enrichment plays a key role in the production of enriched continental compositions in the SVZ has important implications for our understanding of the chemical evolution of the Earth. If ambient mantle enrichment is not taken into account, petrogenetic models of evolved lavas may overestimate the role of crustal assimilation, which, in turn, may lead models of continental crust growth to overestimate the amount of continental material that has been recycled back into the mantle.
影响因子：
5
作者：
G. Jacques;K. Hoernle;J. Gill;F. Hauff;H. Wehrmann;D. Garbe‐Schönberg;P. Bogaard;I. Bindeman
通讯作者：
G. Jacques;K. Hoernle;J. Gill;F. Hauff;H. Wehrmann;D. Garbe‐Schönberg;P. Bogaard;I. Bindeman
影响因子：
3.9
作者：
S. Skora;J. Blundy
通讯作者：
S. Skora;J. Blundy