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

橄榄岩-变质沉积物-榴辉岩混杂岩中耦合氧化还原平衡的实验研究及其对钻石形成的影响

• 批准号: 445062436
• 负责人: Professor Dr. Alan Butler Woodland
• 金额: --
• 依托单位: Abteilung Petrologie und Geochemie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2020
• 资助国家: 德国
• 起止时间: 2019-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/445062436?language=en
• 关键词: Experimental; study; coupled; redox; equilibria

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.

地幔中的金刚石形成是通过含碳物质与硅酸盐基质之间的氧化还原反应发生的。当移动的碳酸岩熔体通过含有金属相的橄榄岩时，它最容易形成。然而，金属的存在不是先决条件。事实上，氧化还原反应也可以在无铁体系中发生。例如，橄榄岩的碳酸化反应被称为EMOG或EMOD缓冲，这取决于石墨或金刚石的存在：Mg 2SiO 4（橄榄石）+ C（金刚石）+ O2 = MgSiO 3（辉石）+MgCO 3（菱镁矿） （1）这种氧缓冲反应的最大CO_2O_2值低于榴辉岩或沉积物中类似平衡的最大CO_2O_2值：CaMgSi_2 O_6（辉石）+2C（金刚石）+2O_2 = CaMg（CO_3）_2（白云石）+2SiO_2 （2）因此，相邻的碳酸盐榴辉岩或沉积岩只会通过与相邻橄榄岩交换氧而还原。预计这种情况在俯冲背景下的混杂带中很普遍。实验将在简单的CaO-MgO-Al_2O_3-SiO_2-C-O_2体系中进行。纯的合成镁橄榄石将是橄榄岩碳酸化反应的端元，CaCO 3、MgCO 3、SiO2和Al 2 O3的适当混合物将用作产生模型沉积物和榴辉岩组合物的起始材料。不同的起始材料将用石墨盘物理分离，以将化学交换限制为氧转移。实验将在多砧装置中在10 GPa下在温度范围（1300 - 1500 ° C）内的金刚石稳定性场中进行。研究结果将对上地幔金刚石形成的可能机制有一定的应用价值。这也可以解释金刚石中的矿物包裹体大多数属于橄榄岩系列，而含金刚石榴辉岩比含金刚石橄榄岩更常见这一看似矛盾的现象。含碳酸盐的变质沉积岩或变质玄武岩可转变为不含金刚石的榴辉岩，而相邻的含金刚石的橄榄岩则在俯冲板片之上的混杂岩带中发生碳酸盐化。