Mush, Melts and Metasediments: a History of Rhyolites from the Okataina Volcanic Centre, New Zealand, as Captured in Plagioclase
Mush, Melts and Metasediments: a History of Rhyolites from the Okataina Volcanic Centre, New Zealand, as Captured in Plagioclase
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糊状、融化物和变沉积物：斜长石中捕获的新西兰奥卡泰纳火山中心流纹岩的历史
DOI：
10.1093/petrology/egab038
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发表时间：
2021
影响因子：
3.9
通讯作者：
Nakagawa, Mitsuhiro
中科院分区：
文献类型：
--
作者：
Sas, May;Shane, Phil;Kuritani, Takeshi;Zellmer, Georg F;Kent, Adam J;Nakagawa, Mitsuhiro
Okataina火山中心（OVC），位于新西兰陶波火山区，是一个主要的流纹质岩浆系统在弧设置，其中喷发被认为是由镁铁质补给驱动。在这里，Sr-Pb同位素，成分和纹理的变化斜长石斑晶从10流纹岩矿床（两个破火山口，一个立即后破火山口，四个内破火山口，和三个额外的破火山口）被用来调查的OVC岩浆系统，并确定在这个不同的糊状区的来源和同化物。斜长石内部表现出正常和反向分带，通常与其伴随的玻璃，熔融包裹体和全岩成分不平衡。这表明晶体在不同于其载体岩浆的熔体中成核。与此相反，晶体的最外层边缘表现出正常的分区，这是在喷发前冷却和分馏熔体的增长组成一致。在晶体内尺度上，87 Sr/86 Sr比率的总套件是高度可变的（0.7042 - 0.7065 ± 0.0004平均2SE）;然而，大多数（95%）晶体在误差范围内是内部均匀的。在全晶体尺度下（获得更好的精度），87 Sr/86 Sr比值更均匀（平均2SE为0·70512-0·70543 ± 0·00001），并与其寄主全岩Sr同位素比值重叠。全晶体铅同位素比值也在很大程度上与全岩铅比值重叠。斜长石和全岩同位素组成表明，亏损洋中脊地幔岩浆源对托勒斯状变质沉积物（局部基底岩石）的地壳同化作用（≥ 20%），Pb同位素需要可变的流体主导俯冲通量。新的数据支持以前的OVC岩浆的岩石成因模型，需要在一个复杂的断开熔体和糊状水库的成分和热不同的岩浆晶体生长。这些水库是由底侵玄武岩岩浆作为喷发触发器恢复活力。然而，斜长石的最外边缘意味着，在很大程度上仅限于热量和挥发分的转移，并导致快速动员和同喷发混合的流纹岩熔体之间的相互作用熔体和喷发触发镁铁质流入。最后，斜长石相对均匀的同位素组成表明，在OVC岩浆系统的寿命来自地壳和地幔的平衡的贡献。
The Okataina Volcanic Centre (OVC), located in the Taupo Volcanic Zone, New Zealand, is a dominantly rhyolitic magmatic system in an arc setting, where eruptions are thought to be driven by mafic recharge. Here, Sr–Pb isotopes, and compositional and textural variations in plagioclase phenocrysts from 10 rhyolitic deposits (two caldera, one immediately post-caldera, four intra-caldera, and three extra-caldera) are used to investigate the OVC magmatic system and identify the sources and assimilants within this diverse mush zone. Plagioclase interiors exhibit normal and reverse zoning, and are commonly in disequilibrium with their accompanying glass, melt inclusions, and whole-rock compositions. This indicates that the crystals nucleated in melts that differed from their carrier magma. In contrast, the outermost rims of crystals exhibit normal zoning that is compositionally consistent with growth in cooling and fractionating melts just prior to eruption. At the intra-crystal scale, the total suite of87Sr/86Sr ratios are highly variable (0·7042–0·7065 ± 0·0004 average 2SE); however, the majority (95 %) of the crystals are internally homogeneous within error. At whole-crystal scale (where better precision is obtained),87Sr/86Sr ratios are much more homogeneous (0·70512–0·70543 ± 0·00001 average 2SE) and overlap with their host whole-rock Sr isotopic ratios. Whole-crystal Pb isotopic ratios also largely overlap with whole-rock Pb ratios. The plagioclase and whole-rock isotopic compositions indicate significant crustal assimilation (≥20 %) of Torlesse-like metasediments (local basement rock) by a depleted mid-ocean ridge mantle magma source, and Pb isotopes require variable fluid-dominant subduction flux. The new data support previous petrogenetic models for OVC magmas that require crystal growth in compositionally and thermally distinct magmas within a complex of disconnected melt-and-mush reservoirs. These reservoirs were rejuvenated by underplating basaltic magmas that serve as an eruption trigger. However, the outermost rims of the plagioclase imply that interaction between silicic melts and eruption-triggering mafic influx is largely limited to heat and volatile transfer, and results in rapid mobilization and syn-eruption mixing of rhyolitic melts. Finally, relatively uniform isotopic compositions of plagioclase indicate balanced contributions from the crust and mantle over the lifespan of the OVC magmatic system.