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首页> 外文期刊>Contributions to Mineralogy and Petrology >Small-scale transport of trace elements Nb and Cr during growth of titanite: an experimental study at 600 °C, 0.4 GPa
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Small-scale transport of trace elements Nb and Cr during growth of titanite: an experimental study at 600 °C, 0.4 GPa

机译：钛酸盐生长过程中微量元素Nb和Cr的小规模迁移：在600°C，0.4 GPa的条件下进行的实验研究

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摘要

Trace element distribution in titanite overgrowths on rutile has been investigated experimentally at 600 °C, 400 MPa and fO2 near NiNiO buffer. Compositionally homogenous Cr- or Nb-doped synthetic rutile single crystals or Nb-containing natural rutile crystals were the source of Cr, Nb and Ti to synthesize titanite using the double-capsule technique. All element exchange with the source of Si, Ca and Al occurred via a NaCl–H2O fluid. Titanite forms quickly and exclusively around the rutile crystals. The titanite overgrowth separates rutile from the bulk fluid, and all elements from rutile dissolution have to pass through the titanite rim. Trace element concentrations in titanite show a considerable scatter in experiments with and without Al, although the average concentrations of Cr or Nb of titanite around compositionally homogeneous synthetic rutile approach the expected values for closed system conditions. Variability of Al with Cr or Nb in the titanite is not correlated. The Al zoning is irregular and patchy, and also the distribution of trace elements does not show systematic trends in the spatial distribution. In experiments using zoned natural rutile, the concentrations of Nb in titanite are related to the Nb zoning in rutile, but the contents also vary unsystematically. Under the controlled conditions of the experiment, the explanation for the strongly irregular spatial distribution is most likely due to variations in elemental concentrations during transport from the rutile along the titanite grain boundaries. The transport pathway is complex because grain boundary migration is important during titanite growth. Such irregular element distribution is also found in a natural sample of titanite overgrowth on rutile from an eclogite with retrograde overprint in the amphibolite facies. Transport of Ti and trace elements was focused on grain boundaries and shielded from the rutile as a source of these elements. We conclude that this type of zoning is not related to changes in P–T or composition in an open system, but solely controlled by transport in and through the titanite rim.
机译：在NiNiO缓冲液附近，在600°C，400 MPa和fO2 下，通过实验研究了金红石上钛铁矿过度生长中的痕量元素分布。组成均质的Cr或Nb掺杂的合成金红石单晶或含Nb的天然金红石晶体是Cr，Nb和Ti的来源，使用双胶囊技术合成了钛矿。所有与硅，钙和铝源的元素交换均通过NaCl–H2 O流体进行。钛铁矿快速且专门围绕金红石晶体形成。钛铁矿的过度生长将金红石与散装流体分离开来，来自金红石溶解的所有元素都必须穿过钛铁矿的边缘。在有铝和无铝的实验中，尽管在组成均匀的合成金红石周围，钛矿的Cr或Nb的平均浓度接近封闭系统条件下的预期值，但在含铝和不含铝的实验中，钛元素中的痕量元素浓度仍显示出相当大的分散性。钛中铝与铬或铌的变异性无关。 Al的分区是不规则且不规则的，而且痕量元素的分布在空间分布中没有显示出系统的趋势。在使用分区天然金红石的实验中，钛铁矿中Nb的浓度与金红石中Nb的分区有关，但其含量也没有系统地变化。在实验的控制条件下，极不规则的空间分布的解释很可能是由于从金红石沿钛矿晶界传输过程中元素浓度的变化所致。传输路径很复杂，因为在钛矿生长过程中晶界迁移很重要。这种天然元素的不规则分布也存在于天然辉石中的钛铁矿过度生长的天然样品中，而辉石来自角闪石，在角闪石相中逆行叠印。钛和痕量元素的传输集中在晶界，并被金红石屏蔽，作为这些元素的来源。我们得出的结论是，这种类型的分区与开放系统中的PT或组成的变化无关，而仅受钛酸盐边缘中和穿过钛酸盐边缘的传输控制。

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