The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study
The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study
复制标题
DOI：
10.1007/s004100100266
复制
发表时间：
2001-09
影响因子：
3.5
通讯作者：
O. Müntener;P. Kelemen;T. Grove
中科院分区：
文献类型：
--
作者：
O. Müntener;P. Kelemen;T. Grove
暴露的，俯冲相关的岩浆弧通常包括部分超镁铁质深成岩，由纯橄榄岩，韦尔岩，辉石岩。在这项实验研究中，我们研究了可变的H2O浓度上的相的比例和组成的火成岩和相关的超镁铁质深成岩的影响。岩浆结晶实验模拟自然，弧岩浆成分在1.2 GPa，对应于条件的弧下地壳。增加液体中的H2O浓度会改变结晶顺序。液体中的低H2O浓度稳定斜长石早于石榴石和角闪石，而衍生物液体保持石英规范。较高的水含量（>3%）抑制斜长石，导致角闪石和石榴子石结晶，从而产生衍生的标准安山岩液体。实验表明，只要没有主要的铝相结晶，液体中的氧化铝与辉石中的铝呈正相关。将这种相关性外推到塔尔凯特纳和科希斯坦弧段的天然辉石岩中，表明具有低Ca-Tschermaks组分的单斜辉石代表原始富Si含水岩浆的近液相线阶段。对剩余固体组合的密度计算表明，超镁铁质深成岩的密度始终高于上地幔岩石，约为0.05至0.20 g/cm 3。液体中高压和高H2O浓度的组合抑制了斜长石结晶，因此超镁铁深成岩在结晶间隔的很大比例上形成（超镁铁岩石从初始的地幔衍生液体中结晶高达50%）。这表明，在俯冲相关的岩浆弧地震莫霍面可能比岩石学壳幔过渡浅。因此，基于地震数据的计算可能高估了标准斜长石含量（例如，SiO_2、Al_2O_3）。
Exposed, subduction-related magmatic arcs commonly include sections of ultramafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the effects of variable H2O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H2O concentration in the liquid changes the crystallization sequence. Low H2O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H2O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas.Density calculations on the residual solid assemblages indicate that ultramafic plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm3. The combination of high pressure and high H2O concentration in the liquid suppresses plagioclase crystallization, so that ultramafic plutonic rocks form over a significant proportion of the crystallization interval (up to 50% crystallization of ultramafic rocks from initial, mantle-derived liquids). This suggests that in subduction-related magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO2, Al2O3) of igneous crust in arcs.