Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres
Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres
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DOI：
10.1016/j.epsl.2006.12.030
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发表时间：
2007-03
影响因子：
5.3
通讯作者：
M. Python;G. Ceuleneer;Y. Ishida;J. Barrat;S. Arai
中科院分区：
文献类型：
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作者：
M. Python;G. Ceuleneer;Y. Ishida;J. Barrat;S. Arai
分布于蛇绿岩地幔段的镁铁-超镁铁岩墙一般是普通硅酸盐熔体的结晶产物。在对阿曼地幔方辉橄榄岩熔体迁移遗迹进行全球调查的框架内，我们发现了一种基本上由纯透辉石（0.95<MgMg+Fetotal<1）组成的特殊岩性，其特征与岩浆或地幔成因不匹配。反对这些透辉岩的岩浆或地幔起源的论点结合了联合收割机成分和结构证据。尽管它们的耐火组合物，但它们强烈地贫Cr（Cr2O30.2重量%）。同样，其他微量元素（Al、Ti、Na.）和稀土元素的丰度也特别低，远离岩浆分异趋势。在少数样品中，共生组合包括纯钙长石（An%高达0.99），少量镁橄榄石（Fo>0.95）或微量的钙铁榴石。当不变形时，透辉石是自形的，它们的结构指向由叶蛇纹石和/或碳酸盐组成的基质中的变质生长。透辉石的结构让人想起接触变质晕中发育的夕卡岩或蛇绿岩地壳蛇形体中经常存在的钠闪石。它们经常以岩墙（以前的裂缝）的形式出现，在透辉石岩相和其寄主岩石之间有几毫米到几十厘米宽的过渡带，其中含有大量的含水矿物。阿曼蛇绿岩中的透辉岩不是随机分布的，在浅部侵位的原软流圈底辟附近的岩石圈中，透辉岩更为丰富。我们将透辉石岩解释为海水和碳酸盐流体（>800 °C）的高温循环足迹，这些流体可能在穿透地幔之前已经浸出了富含斜长石的岩性（如发育良好的正Eu异常所示）。我们的数据证实了McCollom和Shock [T.M. McCollom，E.L.冲击，下大洋地壳中的流体-岩石相互作用：热液蚀变的热力学模型，地球物理学杂志。103（B1）（1998）547-575.]他提出普通的无水矿物如辉石、斜长石和橄榄石可以从硅酸盐熔体和超临界水之间的高温流体中结晶。这证实了在岩浆结晶和热液结晶领域之间没有明确的热化学边界。
Mafic–ultramafic dykes scattered in the mantle section of ophiolites are generally crystallisation products from common silicate melts. In the frame of a global survey of these melt migration relics in the Oman mantle harzburgites, we discovered a peculiar lithology made essentially of pure diopside (0.95<MgMg+Fetotal<1) whose characteristics do not match a magmatic or mantle origin. Arguments against a magmatic or mantle origin for these diopsidites combine compositional and textural evidence. In spite of their refractory composition, they are strongly depleted in Cr (Cr2O30.2 wt.%). By the same way other minor elements (Al, Ti, Na…) and rare earth elements have peculiarly low abundances and plot away from magmatic differentiation trends. In a few samples, the paragenetic association includes pure anorthite (An% up to 0.99), minor amounts of forsterite (Fo>0.95) or traces of andradite. When not deformed, the diopsides are automorphic and their texture points to metamorphic growth in a matrix composed of antigorite and/or carbonate. Diopsidites have textures reminiscent to that of skarns developing in contact metamorphic halos or that of rodingite frequently present in the serpentine bodies of the ophiolitic crust. They frequently appear as dykes (former cracks), with a few mm to few tens of cm wide transitional zones, which contain high amounts of hydrous minerals, between the diopsidite facies and its host rock. The diopsidites are not randomly distributed in the Oman ophiolite, being more abundant near former asthenospheric diapirs emplaced at shallow depth in the lithosphere. We interpret the diopsidites as the footprint of very high temperature circulation of seawater and carbonated fluids (>800 °C), which may have leached plagioclase rich lithologies before penetrating the mantle (as shown by a well developed positive Eu anomaly). Our data confirm the prediction of McCollom and Shock [T.M. McCollom, E.L. Shock, Fluid-rock interactions in the lower oceanic crust: Thermodynamic models of hydrothermal alteration, J. Geophys. Res. 103 (B1) (1998) 547–575.] who proposed that common anhydrous minerals like pyroxene, plagioclase and olivine may crystallise from high temperature fluids intermediate between silicate melts and supercritical water. This confirms that there is no clear-cut thermal and chemical boundary between the fields of magmatic and hydrothermal crystallisations.