Uniform processes of melt differentiation in the central Izu Bonin volcanic arc (NW Pacific)

The intra-oceanic Izu Bonin arc (NW Pacific) has produced a bimodal spectrum of melts with maxima in the basaltic andesitic (<em>c</em>. 53–54 wt% SiO₂) and rhyolitic range (<em>c</em>. 70–72 wt% SiO₂) since arc inception <em>c</em>. 48–49 million years ago. Composition of phenocrysts and accessory minerals from 21 contemporaneous fallout tephras from ODP Site 782A confirm the bimodality and uniformity of the erupted melts. The basaltic andesite melts equilibrated with calcic plagioclase (<em>c</em>. An_70–95), high-Mg# clino- and orthopyroxene and low-Ti titanomagnetite. Dacitic and rhyolitic melts crystallized sodic plagioclase (<em>c</em>. An_40–60), low-Mg# clino- and orthopyroxene, apatite, Ti-rich titanomagnetite in addition to occasional ilmenite and amphibole. The Izu melts are inferred to crystallize at oxygen fugacities between <em>c</em>. 0 to +2.5 log₁₀ units relative to FMQ, at temperatures between <em>c</em>. 775° and 1100 °C and at pressures between <em>c</em>. 300 and <em>c</em>. 1100 MPa, corresponding to <em>c</em>. 5–35 km lithospheric depth. The compositional uniformity of the tephra layers, which are spaced on average 230 ± 380 ka apart, suggest uniform processes of differentiation since at least <em>c</em>. 42 Ma ago. The tephra record shows no indication of periodic or progressive crustal growth that might correlate with the alternate periods of arc formation, arc rifting or backarc spreading, or would suggest an increasingly efficient ‘crustal filter’ with time. The tephra data tentatively conform to a model where crust grows steadily through intrusions of mafic and evolved melt body batches whereby buoyancy controls the level of solidification. While the tephra compositions demonstrate the uniformity of the processes of melt formation and differentiation through time, the data do not permit the differentiation processes themselves to be constrained. These may comprise fractional crystallization, crustal fusion, fusion of non-peridotitic sub-crustal lithologies, or any combination of these processes.

洋内伊豆-小笠原弧（intra-oceanic Izu Bonin arc）地处西北太平洋（NW Pacific），自约4800万至4900万年前弧起始阶段以来，便持续产出以玄武安山岩质（SiO₂含量约53~54 质量百分比（weight percent，wt%））和流纹岩质（SiO₂含量约70~72 wt%）为峰值的双峰式熔体谱系。对大洋钻探计划（Ocean Drilling Program，ODP）782A站位的21层同期降落型火山碎屑岩（fallout tephras）中的斑晶与副矿物开展成分分析，结果证实了喷发熔体的双峰特征与均一性。玄武安山岩质熔体与钙斜长石（An₇₀–₉₅）、高镁指数（Mg#）单斜辉石、斜方辉石以及低钛磁铁矿达到平衡；英安岩质与流纹岩质熔体则结晶出钠质斜长石（An₄₀–₆₀）、低镁指数单斜辉石、斜方辉石、磷灰石、富钛磁铁矿，偶见钛铁矿与角闪石。据推测，伊豆弧熔体的结晶条件为：相对FMQ氧缓冲剂（FMQ）约0至+2.5 log₁₀单位的氧逸度、约775℃至1100℃的温度，以及约300~1100 MPa的压力，对应约5~35 km的岩石圈深度。这些平均间隔为230±380 ka（千年，thousand years ago）的火山碎屑岩层，其成分均一性表明，至少自约42 Ma（百万年前，million years ago）以来，岩浆分异过程始终保持均一。该火山碎屑记录未显现周期性或渐进式地壳生长的迹象——这类生长本可与弧形成、弧裂谷作用或弧后扩张的交替阶段相关，亦无法佐证"地壳过滤器效率随时间推移逐步提升"的推论。本次火山碎屑数据初步契合一种地壳生长模型：通过基性熔体与演化熔体体的幕式侵入，由浮力控制结晶深度，进而实现地壳的持续生长。尽管火山碎屑的成分特征证实了岩浆形成与分异过程随时间保持均一，但现有数据无法限定分异过程的具体类型，其可能包括分离结晶（fractional crystallization）、地壳熔融、非橄榄岩质地壳下岩性的熔融，或是上述过程的任意组合。