Subduction factory in an ampoule: Experiments on sediment-peridotite interaction under temperature gradient conditions
Subduction factory in an ampoule: Experiments on sediment-peridotite interaction under temperature gradient conditions
复制标题
DOI：
10.1016/j.gca.2017.12.012
复制
发表时间：
2018-02
影响因子：
5
通讯作者：
A. Woodland;V. Bulatov;G. Brey;A. Girnis;H. Höfer;A. Gerdes
中科院分区：
文献类型：
--
作者：
A. Woodland;V. Bulatov;G. Brey;A. Girnis;H. Höfer;A. Gerdes
为了更好地了解俯冲洋板上的过程，我们在7.5和10.5 GPa的压力下，在温度梯度从~ 100到~ 500 °C的控制下，对样品长度为~ 3 mm的沉积物和橄榄岩层进行了并置实验。沉积物起始物质含有H2O（6.9 wt%）和CO2(5.9 wt%)，其主要元素组成类似于GLOSS (Plank和Langmuir， 1998)，其中掺杂了10-100 ppm水平的微量元素。橄榄岩层由天然橄榄石（66 wt%）、正辉石（27 wt%）和石榴石（7 wt%）与~ 15 wt%的石墨混合而成。研究了几种实验配置，但“基本”设置是在寒冷地区（400-1200 °C）底部的沉积层上覆盖900-1500 °C的橄榄岩。温度分布由两个热电偶和正辉石榴石测温法测定。许多实验的共同特征是：(1)在胶囊最热的部分形成不同岩性的多层和一池含水硅酸盐或碳酸盐硅酸盐熔体；(2)偏橄榄岩中的橄榄石被正辉石取代；(3)偏橄榄岩层中石榴石的保存生长和菱镁矿的局部发育；(4)与变质橄榄岩接触处沉积层内石榴石富集；(5)底部（最冷部分）形成斜辉石-石榴石组合；(6)沉积层中含钾相（云母岩或云母岩）和碳酸盐仅在温度<700 °C时存在；(7)在最冷地区伴生锆石、金红石和磷酸盐。元素重分配方面，与初始成分相比，橄榄岩sio2富集明显，MgO、FeO和Cr2O3富集；钾被完全提取到熔体中，而Na和Ca大部分被保留在斜辉石、富钙石榴石和文石中最冷的沉积层中。熔体是部分熔体或来自沉积物的流体与橄榄岩相互作用的产物。它具有硅碳酸盐组成，SiO2、MgO、FeO和CaO含量变化，Al2O3含量低。添加Cl对元素的分布几乎没有影响，而添加F则导致含有大量Ti的黄铁矿群矿物的出现。微量元素的分布受压力、温度和矿物组合的控制。沉积层低温（<700 ℃）下，Ba、Rb、Sr、Li比REE、HFSE更具流动性，形成了较高的Ba/La、Ba/Nb、Sr/Nb等（流体交代）。在沉积物层较高温度下，熔体中Ba、Rb、Sr、LREE和U相对于Ti、MREE和HREE显著富集。熔融体Nb-Ta和Zr-Hf负异常是由于固体渣中金红石、锆石和黄铁矿群矿物的残留所致。热扩散可能影响一些高度不相容元素的比例（例如，Ta/La）。从矿物组合和微量元素比值的变化来看，讨论了这些结果在自然深俯冲中的可能应用。
To better understand processes above subducted oceanic slabs, we have undertaken experiments with juxtaposed sediment and peridotite layers at pressures of 7.5 and 10.5 GPa at a controlled temperature gradient from ∼100 to ∼500 °C per a sample length of ∼3 mm. The sediment starting material contains H2O (6.9 wt%) and CO2(5.9 wt%) and has a major-element composition similar to GLOSS (Plank and Langmuir, 1998) doped with trace elements at 10–100 ppm levels. Several experiments were conducted with ∼0.5 wt% Cl or F. The peridotite layer is composed of natural olivine (66 wt%), orthopyroxene (27 wt%) and garnet (7 wt%) mixed with ∼15 wt% graphite. Several experimental configurations were investigated, but the “basic” setup has the sediment layer at the bottom in the cold zone (400–1200 °C) overlain by peridotite at 900–1500 °C. The temperature distribution was determined by two thermocouples and orthopyroxene–garnet thermometry.Features common to many experiments are (1) the development of multiple layers of various lithologies and a pool of hydrous silicate or carbonate–silicate melt in the hottest part of the capsule; (2) replacement of olivine by orthopyroxene in the metaperidotite; (3) preservation and growth of garnet and local development of magnesite in the metaperidotite layer; (4) enrichment in garnet within the metasediment layer at the contact with the metaperidotite; (5) formation of a clinopyroxene–garnet assemblage at the bottom (the coldest part); (6) presence of K-bearing phases (phlogopite or phengite) and carbonates in the metasediment layer only at temperatures <700 °C; and (7) occurrence of accessory zircon, rutile and phosphates in the coldest regions. In terms of element redistribution, the peridotite becomes strongly enriched in SiO2compared to the starting composition, and the sediment gains MgO, FeO and Cr2O3. Potassium is fully extracted into the melt, while Na and Ca are largely retained in the coldest part of the metasediment layer in clinopyroxene, Ca-rich garnet and aragonite. The melt is a product of interaction between partial melt or fluid from the sediment and peridotite. It has a silico-carbonatite composition with variable SiO2, MgO, FeO and CaO contents and low Al2O3. The addition of Cl has almost no effect on element distribution, whereas the addition of F results in the appearance of humite-group minerals containing significant amounts of Ti. Trace-element distribution is controlled by pressure, temperature and mineral assemblages. At low temperatures in the sediment layer (<700 °C) Ba, Rb, Sr and Li are much more mobile than REE and HFSE, which results in high Ba/La, Ba/Nb, Sr/Nb etc. (fluid metasomatism). At higher temperatures in the sediment layer, the melt is markedly enriched in Ba, Rb, Sr, LREE and U relative to Ti, MREE and HREE. Negative Nb–Ta and Zr–Hf anomalies in melts are caused by the retention of rutile, zircon and humite-group minerals in the solid residue. Thermodiffusion may affect the ratios of some highly incompatible elements (e.g., Ta/La). Possible applications of the results to natural deep subduction are discussed in view of variations in mineral assemblages and trace element ratios.