海洋地质与环境重点实验室知识库

马努斯海盆位于俾斯麦海东部，先后受到了太平洋−卡洛琳板块和所罗门海板块俯冲作用的影响，是世界上扩张速率最快的弧后盆地之一。其中，东马努斯海盆是马努斯海盆中扩张最快的部分，是近期火山活动和热液活动发生的场所，出露了从玄武岩到流纹岩的完整岩石序列，是研究快速扩张弧后盆地岩浆系统的理想场所。本文以东马努斯海盆的玄武安山岩、安山岩和英安岩为研究对象，进行了全岩主微量元素、Sr−Nd−Pb−Hf同位素以及矿物结构、原位主微量元素的测试分析工作，探讨了俯冲组分对研究区地幔源区的改造作用以及各类火山岩的成因，剖析了该区岩浆通道系统中各岩浆储层的物理化学条件及包括岩浆混合在内的岩浆过程，主要取得了以下认识：
（1）Sr−Nd−Pb同位素分析表明，马努斯海盆的火山岩源自印度洋型MORB地幔，而东马努斯海盆岩浆源区由于受到了俯冲板片熔体和板片衍生流体的影响，其火山岩表现出较高的Ba/La、Th/Yb、Ba/Th和Cl/F比值，以及较低的Hf/Nd比值。¹⁷⁶Hf/¹⁷⁷Hf−¹⁴³Nd/¹⁴⁴Nd图解显示，东马努斯海盆岩浆源区的俯冲板片熔体成分来自太平洋板块，由蚀变洋壳熔体和俯冲沉积物熔体以约72:28的比例混合而成，其在岩浆源区中占比5%左右；源区中的板片衍生流体成分来自所罗门海板块，主要为沉积物衍生流体，还有极少量蚀变洋壳脱水流体，二者比例约为95:5，且在岩浆源区中的输入量小于1%。受到俯冲输入的影响，东马努斯海盆地幔源区中至少有96−98%的Ba、75−90%的Th和14−21%的Cl来自俯冲组分的贡献，并导致其Sr−Nd−Pb同位素组成偏离了印度洋MORB地幔域的组成范围。
（2）东马努斯海盆的玄武安山岩经历了高过冷和强脱气过程，形成了大量的骨架状斜长石以及扇形分区和树枝状的单斜辉石；而安山岩及英安岩在形成过程中经历了岩浆的补给和混合，均发育有三种矿物组合。其中，安山岩中的矿物组合分别来自玄武质熔体、英安质熔体和混合熔体（安山质），经计算，混合熔体是由前两者以2:8的比例完全混合形成；英安岩中的矿物组合分别来自安山质熔体、流纹质熔体和混合熔体（英安质），斑晶的结构和成分分带表明，其经受了安山质熔体的多次侵入，此外，还经历了对围岩的捕获。对英安岩中岩浆磷灰石的主量元素及主要挥发分（F、Cl、SO₃）丰度的测定表明，其结晶自挥发分不饱和的英安质岩浆，因此可以作为寄主熔体成分的指示剂。根据计算，此时英安质岩浆的最低S浓度范围为2−65ppm或8−11ppm。由于磁铁矿分离结晶的影响，其具有较低的氧逸度（ΔFMQ = −0.2±0.9）。计算得到的英安岩中的高Cl含量表明其可能还受到了轻微的富Cl海水组分的浅层同化作用。
（3）通过构建适用于本研究的实验数据集，对基于矿物的地质温压计进行了评估和选择，表现最好的方程均来自Putirka（2008）。对于单斜辉石和斜方辉石，分别选择了Enq. 32b与Enq. 33迭代以及Enq 28b与Enq. 29a 迭代计算结晶温度和压力，对于斜长石，使用Enq. 24a计算其结晶温度。测得玄武安山质岩浆、安山质岩浆和英安质岩浆的喷发前存储温度分别为1090±13℃，1032±9℃和938±10℃，存储压力没有受到较好的约束，分别为4.3±1.4kbar，2.8±1.3kbar和2.5±1.3kbar。东马努斯海盆复杂的岩浆通道系统及其岩浆过程为该地区活跃的火山活动和热液活动提供了物源和可能的热源。

The Manus Basin, located in the eastern part of the Bismarck Sea, has been successively affected by the subduction of the Pacific−Caroline Plate and the Solomon Sea Plate and is one of the fastest spreading back−arc basins in the world. The Eastern Manus Basin is the fastest expanding part of the Manus Basin, which is the site of recent volcanism and hydrothermal activity, developing a complete sequence of rocks ranging from basalt to rhyolite, making it an ideal region to analyze the volcanic magma system of the fast−spreading back−arc basin. In this dissertation, through analyzing the petrography and geochemical characteristics of the whole rocks (major and trace elements, Sr−Nd−Pb−Hf isotopes) and minerals (in situ geochemistry) of basaltic andesite, andesite, and dacite from the Eastern Manus Basin, we discussed the modification of its mantle source region by subduction components and the genesis of various volcanic rocks, as well as analyzed the physicochemical characteristics of the magma reservoirs in the magma plumbing system of this region as well as the magma processes including magma mixing. The major findings include:
(1) Analysis Sr−Nd−Pb isotopes indicates that the Manus Basin volcanic rocks originated from the Indian MORB−type mantle, whereas the Eastern Manus Basin volcanic rocks exhibit high Ba/La, Th/Yb, Ba/Th, and Cl/F ratios, and low Hf/Nd ratios due to the influence of slab melts and slab−derived fluids on its source region.¹⁷⁶Hf/ ¹⁷⁷Hf−¹⁴³Nd/¹⁴⁴Nd illustration shows that the slab melt component of the Eastern Manus Basin magma source region is derived from the Pacific Plate and consists of a mixture of altered oceanic crust melt and subducted sediment melt in a ratio of ~72:28, which accounts for ~5% of the magma source region, and that the slab−derived fluid component of its source region is derived from the Solomon Sea Plate, which consists of mostly sediment−derived fluids, with a very small amount of altered oceanic crustal derived fluids, the ratio of the two is about 95:5, and their inputs to the magma source region is less than 1%. As a result of subduction inputs, at least 96−98% of Ba, 75−90% of Th, and 14−21% of Cl in the mantle source region of the Eastern Manus Basin are contributed by subduction components, as well as result in a Sr−Nd−Pb isotopic composition that deviates from the compositional range of the Indian MORB mantle.
(2) The basaltic andesite of the Eastern Manus Basin underwent high undercooling and intense degassing processes, which formed a large number of skeletal plagioclase as well as sector−zoned and dendritic clinopyroxene; whereas the andesite and the dacite underwent the magma recharging and mixing during their formation, and both developed three mineral assemblages. Among them, the mineral assemblages in andesite are from basaltic melt, rhyolitic melt, and mixed melt (andesitic), respectively, and the mixed melt is calculated to be formed by the mixing of the first two in the ratio of 2:8; the mineral assemblages in dacite are from andesitic melt, rhyolitic melt and mixed melt (dacitic), respectively, and the textural and chemical zoning of phenocrysts indicate that the dacitic melt experienced several injections of andesitic melt, and that it also experienced the capture of the wall rocks. Experimental data (abundances of major elements and F, Cl, SO₃ components) indicate that all the magmatic apatites in the dacite crystallized from volatile−undersaturated melts, so their chemical compositions can be used as indicators of dacitic magma compositions. The calculated minimum S concentration of the dacitic melt at that moment ranges from 2−65 ppm or 8−11 ppm. Due to the effect of magnetite fractional crystallization, the dacitic melt has a low oxygen fugacity (ΔFMQ = −0.2 ± 0.9). The calculated high Cl content in the dacite indicates that it may also have been slightly influenced by shallow assimilation of Cl−rich seawater−derived components.
(3) The performance of different mineral−based thermobarometers has been assessed by constructing the experimental datasets applicable to this study, and the best−performing thermobarometers are all from Putirka（2008）. For clinopyroxene and orthopyroxene, the iteration of Enq. 32b with Enq. 33 and Enq. 28b with Enq. 29a are chosen to calculate the crystallization temperatures and pressures, respectively. For plagioclase, Enq. 24a is chosen for calculating the crystallization temperatures. Pre−eruption storage temperatures for basaltic andesitic, andesitic, and dacitic magmas are 1090±13 °C, 1032±9 °C, and 938±10 °C, respectively, with storage pressures not well constrained at 4.3±1.4 kbar, 2.8±1.3 kbar, and 2.5±1.3 kbar, respectively. The complex magma plumbing system beneath the Eastern Manus Basin provides the material as well as heat sources for the volcanism and hydrothermal activity in this region.