Calcium diffusion in enstatite, with application to closure temperature of the Ca-in-opx thermometer
Calcium diffusion in enstatite, with application to closure temperature of the Ca-in-opx thermometer
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顽辉石中的钙扩散，适用于 Ca-in-opx 温度计的闭合温度
DOI：
10.1016/j.gca.2022.06.018
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发表时间：
2022
影响因子：
5
通讯作者：
Liang, Y.
中科院分区：
文献类型：
--
作者：
Cherniak, D.J.;Liang, Y.
钙的化学扩散的特点是在天然顽火辉石缓冲条件下（IW，NNO）和在空气中。使用合成粉末源进行实验，用卢瑟福背散射光谱法测量Ca扩散分布。扩散剂的各种来源被用来调查的二氧化硅活性和Ca浓度对扩散的影响。钙扩散出现相对不敏感的结晶取向和氧逸度的范围内的调查条件下，也出现在扩散源中的二氧化硅活性和钙浓度的差异影响不大。对于Ca在顽火辉石中的扩散，我们得到了以下的Arkenius扩散关系，在750-1150 °C的温度范围内，D = 1.17 × 10− 10 exp（−240 ± 10 kJ mol−1/RT）m2 sec − 1。在顽火辉石中，Ca的扩散比Mg、Cr和Fe慢，但比较大的二价阳离子Pb和Eu快，Pb和Eu也可能优先占据M2位。顽火辉石中钙的扩散速度也快于三价稀土和四价钛。顽火辉石中的钙扩散系数介于单斜辉石和橄榄石中的钙扩散系数之间，顽火辉石中的钙扩散比透辉石中的钙自扩散快2个数量级，比橄榄石中的钙扩散慢2个数量级。通过耦合封闭温度和钙在opx温度计，这是可能的约束橄榄岩和辉石的冷却速率。兰佐地块和阿曼蛇绿岩橄榄岩的应用。
Chemical diffusion of Ca has been characterized in natural enstatite under buffered conditions (IW, NNO) and in air. Experiments were conducted using synthesized powder sources, with Ca diffusion profiles measured with Rutherford Backscattering Spectrometry. A variety of sources of diffusant were used to investigate the effects of silica activity and Ca concentration on diffusion. Calcium diffusion appears relatively insensitive to crystallographic orientation and oxygen fugacity under the range of investigated conditions, and also appears little influenced by differences in silica activity and Ca concentration in the diffusant source. For Ca diffusion in a natural enstatite, we obtain the following Arrhenius relation for diffusion over the temperature range 750–1150 °C,D = 1.17 × 10−10exp(−240 ± 10 kJ mol−1/RT) m2sec−1.Calcium diffusion is slower than Mg, Cr and Fe in enstatite, but faster than larger divalent cations Pb and Eu that also are likely to preferentially occupy the M2 site. Calcium diffusion is also faster than that for trivalent REE and tetravalent Ti in enstatite. Calcium diffusivities in enstatite are intermediate between Ca diffusivities in clinopyroxene and olivine, with Ca diffusion in enstatite about 2 orders of magnitude faster than Ca self-diffusion in diopside and 2 orders of magnitude slower than Ca diffusion in olivine.Diffusion parameters obtained from this study were used to develop a simple model for closure temperature of the Ca-in-opx thermometer of Brey and Köhler (1990). By coupling closure temperature and the Ca-in-opx thermometer, it is possible to constrain cooling rates of peridotites and pyroxenites. Applications to peridotites from the Lanzo Massif and the Oman ophiolite are presented.