Experimental study of coupled redox equilibria in a peridotite–metasediment–eclogite mélange and implications for diamond formation

地幔中金刚石的形成是由含碳物种与硅酸盐基质之间的氧化还原反应形成的。当流动的碳酸盐熔体穿过含有金属相的橄榄岩时，它最容易形成。然而，金属的存在并不是先决条件。事实上，氧化还原反应也可以在无铁体系中发生。例如，橄榄岩的碳化反应根据石墨或钻石的存在而被称为Emog或Emod缓冲区：Mg_2SiO_4(橄榄石)+C(钻石)+O2=MgSiO_3(辉石)+MgCO_3(镁砂)(1)这种氧缓冲反应的最大ƒO2低于榴辉岩或沉积物中类似平衡的氧缓冲反应，例如：CaMgSi_2O_6(辉石)+2C(钻石)+2O2=CaMG(CO_3)2(白云岩)+2SiO_2(2)因此，并列碳酸盐榴辉岩或沉积岩应仅通过与相邻橄榄岩的氧交换而减少。这种情况预计将在俯冲背景下的混杂带中普遍存在。实验将在CaO-MgO-Al_2O_3-SiO_2-C-O_2简单系统中进行。纯镁橄榄岩将是橄榄岩碳化反应的最终成员，适当的CaCO_3、MgCO_3、SiO_2和Al_2O_3混合物将被用作制备沉积和榴辉岩成分模型的起始原料。不同的起始材料将用石墨圆盘物理分离，以限制化学交换为氧转移。这些实验将在温度范围(1300-1500摄氏度)、10 Gpa的多顶锤装置中的钻石稳定场内进行。这一结果将对上地幔中金刚石形成的可能机制有应用价值。它们还可以解释一个看似矛盾的现象，即钻石中的大多数矿物包裹体属于橄榄岩套，而含钻石榴辉岩比含钻石橄榄岩更常见。含碳酸盐变质岩或变质玄武岩可以转化为含金刚石的无碳酸盐榴辉岩，而相邻的含钻石橄榄岩将在俯冲板块上方的混杂岩带中碳化。

Diamond formation in the mantle occurs by redox reactions between carbon-bearing species and the silicate matrix. It forms most readily when a mobile carbonatitic melt passes through peridotite containing a metallic phase. However, the presence of metal is not a prerequisite. In fact, redox reactions can also occur in Fe-free systems. For example, the carbonation reaction for peridotite is known as EMOG or EMOD buffer depending on the presence of graphite or diamond:Mg2SiO4 (olivine) + C (diamond) + O2 = MgSiO3 (pyroxene) + MgCO3 (magnesite) (1)The maximum ƒO2 of this oxygen buffer reaction is lower than that of analogous equilibria in eclogites or sediments such as: CaMgSi2O6 (pyroxene) + 2C (diamond) + 2O2 = CaMg(CO3)2 (dolomite) + 2SiO2 (2)Thus, juxtaposed carbonated eclogitic or sedimentary lithologies should become reduced solely by the exchange of oxygen with adjacent peridotite. Such situations are expected to be widespread in mélange zones in a subduction setting. Experiments will be conducted in the simple system CaO-MgO-Al2O3-SiO2-C-O2. Pure, synthetic forsterite will be the endmember for the carbonation reaction for peridotite and appropriate mixtures of CaCO3, MgCO3, SiO2, and Al2O3 will be used as starting materials to produce model sedimentary and eclogite compositions. The different starting materials will be physically separated with a graphite disk to limit chemical exchange to that of oxygen transfer. The experiments will be performed within the diamond stability field in a multianvil apparatus at 10 GPa over a range in temperature (1300 -1500°C). The results will have applications to possible mechanisms of diamond formation in the upper mantle. They may also explain the seeming paradox that the majority of mineral inclusions in diamond belong to the peridotite suite while diamondiferous eclogites are much more common than diamond-bearing peridotites. Carbonate-bearing metasediments or metabasalts can be transformed to diamond-bearing carbonate-free eclogites, while the adjacent diamondiferous peridotites will be carbonated in the mélange zone above a subducting slab.