Ca-Eskola incorporation in clinopyroxene: limitations and petrological implications for eclogites and related rocks
Ca-Eskola incorporation in clinopyroxene: limitations and petrological implications for eclogites and related rocks
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DOI：
10.1007/s00410-016-1311-3
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发表时间：
2016-11
影响因子：
3.5
通讯作者：
F. Schroeder-Frerkes;A. Woodland;L. Uenver-Thiele;K. Klimm;Nadia Knapp
中科院分区：
文献类型：
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作者：
F. Schroeder-Frerkes;A. Woodland;L. Uenver-Thiele;K. Klimm;Nadia Knapp
单斜辉石是榴辉岩中的必需矿物。它通常含有少量的含缺陷的Ca-Eskola(CaES，Ca0.5-0.5AlSi2O6)组分，较高的浓度通常被认为至少在柯石英稳定区内指示高压成因。压力和温度条件的变化可能导致该组分以游离的SiO_2相的形式出溶，这可能具有许多岩石学意义。这使得了解最大限度地将CAES掺入单斜辉石的因素变得重要。我们在高压和高温(4-10 GPa和1000-1350℃)下进行了一系列实验，以进一步研究类榴辉石类单斜辉石中CAES的体系结构和导致CAES含量最大的因素。选择了两个简单的化学体系，对结果进行了明确的解释：(1)CMAS+水和(2)NCMAS体系中的两种成分。所有实验产品都含有单斜辉石和石榴石，以及游离的二氧化硅相或硅酸盐熔体。共生石榴石富含粗粒石榴石，XGR≥一般为0.67。成分的变化可归因于熔体的存在或不存在，以及石榴石在不同压力-温度条件下的模态量的变化。即使少量的H2O降低了固相线温度，熔体的存在也会降低二氧化硅的活性，从而使单斜辉石中的CaES组分不稳定。单斜辉石中CaES和Ca-Tschermaks(CAT，CaAl2SiO6)组分随着硬玉摩尔分数的增加而减少，这也是压力和体积铝含量的函数。模拟X射线粉末衍射数据得到了VCaEs的CAES端元的摩尔体积=60.87(63)×cm3，这与从自然样品中估计的文献值相当吻合。在共存柯石英存在的情况下，CAES和CATS并不是独立变化的，受内部平衡2CaEs=1CaEs+3SiO_2(柯石英)控制。在简单体系(即CMAs±Na)中观察到的这种关系，在更复杂、更自然的模拟主体组成中也符合单斜辉石。对现有实验数据的评估表明，在对应于130-180公里深度的条件下，榴辉单斜辉石中的最大CAES含量为15-18亿摩尔%。CaES含量在高温下最大；即在柯石英存在的情况下，在固相线或其附近。因此，这项研究支持了CAES出溶作用在地幔榴辉岩体上涌过程中促进熔体产生的作用，但也提出了一些警告。在富钙、富铝的块体成分中，如绿辉石包体中，CaES的最大含量较高(~20%)。这种块状成分似乎也需要蓝晶石的共存。其他富钙和富铝的岩石类型，如橄榄岩，应该有可能含有富CaES的单斜辉石，除非它们是SiO_2不饱和的。这强调了除了高温外，在高压单斜辉石中实现最大CAES含量的主体成分的进一步作用。
Clinopyroxene is an essential mineral in eclogitic rocks. It commonly contains minor amounts of the defect-bearing Ca-Eskola (CaEs, Ca0.5□0.5AlSi2O6) component, with higher concentrations generally considered to indicate a high-pressure origin at least within the coesite stability field. Changes in pressure and temperature conditions can lead to exsolution of this component as a free SiO2phase, which may have a number of petrological implications. This makes it important to understand the factors that maximize CaEs incorporation in clinopyroxene. We have undertaken a series of experiments at high pressures and temperatures (4–10 GPa and 1000–1350 °C) to further investigate the systematics of CaEs incorporation in eclogite-like clinopyroxene and the factors responsible for maximizing CaEs contents. Two simple chemical systems were chosen that allow unambiguous interpretation of the results: (1) CMAS + H2O and (2) two compositions in the NCMAS system. All experimental products contained clinopyroxene and garnet along with either a free SiO2phase or a silicate melt. Coexisting garnet is grossular-rich, generally withXgr≥ 0.67. Compositional variations are attributable to the presence or absence of melt and changes in modal amounts of garnet at different pressure–temperature conditions. Even small amounts of H2O lower the solidus temperature and the presence of a melt reduces the SiO2activity, which destabilizes the CaEs component in clinopyroxene. The CaEs and the Ca-Tschermaks (CaTs, CaAl2SiO6) components in clinopyroxene decrease with increasing jadeite mole fraction, which is also a function of pressure and bulk Al content. Modeling X-ray powder diffraction data yields a molar volume for the CaEs endmember ofVCaEs= 60.87(63) cm3, which reasonably agrees with a literature value that was estimated from natural samples. In the presence of coexisting coesite, the CaEs and CaTs do not vary independently of each other, being controlled by the internal equilibrium 2CaEs = CaTs + 3SiO2(coesite). This relation, observed in simple systems (i.e., CMAS ± Na), is also obeyed by clinopyroxene in more complex, natural analog bulk compositions. An assessment of available experimental data reveals a maximum of 15–18 mol% CaEs in eclogitic clinopyroxene at conditions corresponding to 130–180 km depth. CaEs contents are maximized at high temperatures; i.e., at or near the solidus in the presence of coesite. Thus, this study supports the role of CaEs exsolution in contributing to melt generation during upwelling of eclogite bodies in the mantle, albeit with some caveats. Somewhat higher maximum CaEs contents (~20 mol%) are found in Ca and Al-rich bulk compositions, such as grospydite xenoliths. Such bulk compositions also seem to require the coexistence of kyanite. Other Ca and Al-rich rock types, like rodingites, should have the potential of containing CaEs-rich clinopyroxenes, except that they are SiO2-undersaturated. This emphasizes the further role of bulk composition, in addition to high temperatures, in achieving maximum CaEs contents in high-pressure clinopyroxene.