Petrology of high-MgO bronzite andesite resembling boninite at DSDP Hole 60-458

A high-MgO andesite which is texturally similar to boninite and a variolitic basalt collected from Site 458, about 100 km west of the Mariana Trench, have been studied through microprobe analyses and melting experiments at high water pressures. The boninite-type andesite is very similar in composition and texture to a boninite from Bonin Islands, except that the former is more calcic than the latter. The variolitic basalt contains magnesian pigeonite (Ca12Mg74Fe14) in cores of augite microphenocrysts. This pigeonite crystallized at temperatures above 1200°C. In the melting experiments of the boninite-type rock, clinopyroxene crystallizes as a liquidus phase at pressures at least above 8 kbar. No olivine crystallizes near the liquidus temperatures, indicating that the magma of this rock cannot be in equilibrium with the upper mantle periodotite (lherzolite) at depths at least greater than 25 km. The boninite-type rock is probably a product of fractional crystallization of a more primitive magma (e.g., olivine-bearing boninite magma) by separation of olivine and orthopyroxene. The magma of the variolitic basalt also cannot be in equilibrium with the upper mantle peridotite, and may be a product of fractional crystallization of a more primitive basaltic magma.

采自马里亚纳海沟以西约100公里的458站位、结构与玻安岩（boninite）相似的高镁安山岩（high-MgO andesite），以及一颗球颗玄武岩（variolitic basalt），已采用电子探针分析（microprobe analyses）与高压水条件下的熔融实验（melting experiments）开展相关研究。该玻安岩型安山岩的成分与结构与博宁群岛产出的玻安岩极为相近，仅前者相较于后者更富钙。该球颗玄武岩的普通辉石（augite）微晶核心中含有镁质易变辉石（pigeonite，化学式为Ca₁₂Mg₇₄Fe₁₄），该易变辉石的结晶温度高于1200℃。在该玻安岩型岩石的熔融实验中，单斜辉石（clinopyroxene）会在至少高于8千巴的压力下作为液相线相（liquidus phase）结晶析出。在液相线温度（liquidus temperatures）附近无橄榄石（olivine）结晶析出，这表明该岩石的岩浆至少在深度大于25公里的环境中，无法与上地幔橄榄岩（peridotite，二辉橄榄岩lherzolite）达到平衡状态。该玻安岩型岩石可能是由更原始的岩浆（如含橄榄石的玻安岩岩浆）经橄榄石与斜方辉石的分离结晶作用（fractional crystallization）形成的产物。该球颗玄武岩的岩浆同样无法与上地幔橄榄岩达到平衡状态，其可能源自更原始的玄武质岩浆的分离结晶作用。