Magmatic Response to Subduction Initiation: Part 1. Fore‐arc Basalts of the Izu‐Bonin Arc From IODP Expedition 352
Magmatic Response to Subduction Initiation: Part 1. Fore‐arc Basalts of the Izu‐Bonin Arc From IODP Expedition 352
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DOI：
10.1029/2018gc007731
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发表时间：
2019-01
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通讯作者：
J. Shervais;M. Reagan;E. Haugen;R. Almeev;J. Pearce;J. Prytulak;J. Ryan;S. Whattam;M. Godard;T. Chapman;Hongyang Li;W. Kurz;W. Nelson;D. Heaton;M. Kirchenbaur;K. Shimizu;T. Sakuyama;Yibing Li;S. Vetter
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作者：
J. Shervais;M. Reagan;E. Haugen;R. Almeev;J. Pearce;J. Prytulak;J. Ryan;S. Whattam;M. Godard;T. Chapman;Hongyang Li;W. Kurz;W. Nelson;D. Heaton;M. Kirchenbaur;K. Shimizu;T. Sakuyama;Yibing Li;S. Vetter
伊豆-小笠原-马里亚纳（IBM）弧前保留了约52 Ma俯冲开始期间形成的火成岩组合。国际海洋发现计划（IODP）第352次考察在小笠原高原附近的前弧中取芯，以记录俯冲开始的岩浆反应和新生俯冲带的物理，岩石学和化学地层学。其中两个地点（U1440和U1441）位于弧前玄武岩（FAB）之下。FAB具有洋中脊玄武岩（MORB）样的成分，然而，与MORB相比，FAB的高场强元素（TiO 2，P2 O 5，Zr）和Ni始终较低，Na 2 O位于MORB场的低端，FeO* 位于高端。几乎所有的FAB都是轻稀土元素贫化，具有低的总REE，并且相对于MORB具有低的高度不相容元素与较不相容元素（Ti/V，Zr/Y，Ce/Yb和Zr/Sm）的比率。孔U1440 B的化学地层趋势与形成偏离轴的最上部熔岩一致，而下部熔岩形成于扩张中心轴下方。U1440 B的轴向岩浆变得更加分馏;上覆的离轴岩浆恢复到更原始的成分。熔体模型需要一个两阶段的过程，早期石榴石场熔体提取之前，后来的尖晶石场熔体，高达23%的熔化，形成最贫化的组合物。在相对较低的压力（1-2 GPa）下，地幔平衡温度高于正常的MORB（1，400 °C-1，480 °C），这可能反映了Manus羽在俯冲开始期间的影响。我们的数据支持以前的模式FAB起源减压熔融，但意味着一个源比正常MORB源地幔更耗尽。
The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.