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）位于西北太平洋，自约4800万至4900万年前弧体形成以来，产出了双峰式岩浆熔体谱系，其成分峰值分别位于玄武安山岩质（约53~54 wt% 二氧化硅（SiO₂））和流纹岩质（约70~72 wt% 二氧化硅（SiO₂））区间。取自大洋钻探计划（Ocean Drilling Program, ODP）782A站位的21份同期降落火山碎屑的斑晶（phenocrysts）与副矿物（accessory minerals）成分，证实了喷发熔体的双峰性与均一性。玄武安山岩质熔体与钙长石质斜长石（约An值70~95）、高镁数单斜辉石与斜方辉石以及低钛钛磁铁矿达到平衡；英安质与流纹岩质熔体则结晶出钠质斜长石（约An值40~60）、低镁数单斜辉石与斜方辉石、磷灰石、富钛钛磁铁矿，偶见钛铁矿与角闪石。据推断，伊豆弧的熔体结晶时的氧逸度相对于铁橄榄石-石英-磁铁矿（FMQ）缓冲剂约为0~+2.5 lg₁₀单位，温度区间约为775~1100 ℃，压力区间约为300~1100 兆帕（MPa），对应岩石圈深度约5~35千米。这些火山碎屑层的成分均一性（各层平均间隔约230±380千年（ka））表明，至少自约4200万年前以来，岩浆分异作用过程始终保持均一。火山碎屑记录未显示出周期性或渐进式地壳生长的迹象——这种生长本可能与弧形成、弧裂谷作用或弧后扩张的交替阶段相关，也不会表明随时间推移“地壳滤器”的效率不断提升。本次火山碎屑数据初步符合以下模型：地壳通过基性与演化熔体团块的侵入作用持续生长，而浮力控制着岩浆固结的深度。尽管火山碎屑成分证实了岩浆形成与分异作用过程随时间保持均一，但该数据无法限定分异作用的具体机制，这些机制可能包括分离结晶作用、地壳熔融、非橄榄岩质壳下岩石圈熔融，或上述过程的任意组合。