Monazite and xenotime solubility in granitic melts and the origin of the lanthanide tetrad effect
Monazite and xenotime solubility in granitic melts and the origin of the lanthanide tetrad effect
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DOI：
10.1007/s00410-014-1100-9
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发表时间：
2015-01
影响因子：
3.5
通讯作者：
Quach Duc-Tin;H. Keppler
中科院分区：
文献类型：
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作者：
Quach Duc-Tin;H. Keppler
在2kbar和800 - 1，100 ° C下测量了具有独居石或磷钇矿结构的合成纯稀土磷酸盐（LaPO 4至LuPO 4）在含水单斜花岗岩熔体中的溶解度。实验运行长达2个月以达到平衡。独居石和磷钇矿的溶解度随熔体中磷浓度的增加而降低。独居石在长英质熔体中的溶解度方程，其中没有明确包括熔体中的磷浓度，因此应谨慎对待。磷的影响可以定量建模，如果一个假设独居石部分溶解的熔体中的离子和分子物种（REE3+和REEPO4）。溶解反应的平衡常数以及形态的定量数据来自溶解度数据。独居石和磷钇矿的溶解度强烈增加与熔体的过碱度。这种效果主要是由于离子物种的溶解度增加，这可能是稳定的熔体中的非桥接氧原子。独居石和磷钇矿在过铝熔体中的溶解度几乎是恒定的。氟对独居石和磷钇矿的溶解度没有重大影响;事实上，溶解度似乎随着氟含量的增加而略有下降。稀土磷酸盐的溶解度不是原子序数或离子半径的简单连续函数。相反，溶解度显示出“四分体”样的模式，在个别稀土元素的溶解度的几个局部最大值。相邻的稀土元素的溶解度有时会相差两倍以上;这些影响远远超出任何分析误差。四分体模式在某些过碱性熔体和富氟的金属铝熔体中特别明显。然而，一些功能，如在镱的溶解度最大，几乎在所有的熔体中看到。因此，在某些高度演化的花岗岩中的镧系元素四分体效应可能是独居石和磷钇矿分馏作用的结果。独居石和磷钇矿在硅酸盐熔体中的溶解度可能表现出四分体效应，因为这些磷酸盐矿物中的稀土元素的配位非常不寻常，与熔体的配位不同，从而导致与部分填充的稀土元素的晶场相互作用不同。花岗岩中的四分体效应不能作为流体/岩石或流体/熔体相互作用的指标，因为它可以在没有任何流体的情况下通过实验重现。
The solubility of synthetic, pure rare earth phosphates with monazite or xenotime structure (LaPO4to LuPO4) in hydrous haplogranitic melts was measured at 2 kbar and 800–1,100 °C. Experiments were run for up to 2 months to attain equilibrium. Monazite and xenotime solubility decreases with increasing phosphorus concentration in the melt. Published equations for monazite solubility in felsic melts, which do not explicitly include phosphorus concentration in the melt, should therefore be treated with caution. The effect of phosphorus can be quantitatively modeled if one assumes that monazite partially dissolves as ionic and molecular species in the melt (REE3+and REEPO4). Equilibrium constants for the dissolution reactions as well as quantitative data on speciation were derived from the solubility data. Monazite and xenotime solubility strongly increases with the peralkalinity of the melt. This effect is mostly due to an increase in the solubility of the ionic species, which are probably stabilized by non-bridging oxygen atoms in the melt. In peraluminous melts, the solubility of monazite and xenotime is nearly constant. Fluorine has no major effect on monazite and xenotime solubility; in fact, the solubility appears to slightly decrease with increasing fluorine content. The solubility of rare earth phosphates is not a simple continuous function of atomic number or ionic radius. Rather, the solubility shows a “tetrad”-like pattern with several local maxima of solubility at individual rare earth elements. The solubilities of neighboring rare earth elements sometimes differ by more than a factor of two; these effects are far outside any analytical error. The tetrad pattern is particularly clearly seen in some of the peralkaline melts and in the fluorine-rich metaluminous melts. Some features, however, such as a solubility maximum at ytterbium, are seen in virtually all melts. The lanthanide tetrad effect in some highly evolved granites may therefore be a result of monazite and xenotime fractionation. The solubility of monazite and xenotime in silicate melt probably shows the tetrad effect, because of the very unusual coordination of the rare earth elements in these phosphate minerals, which is different from the coordination of the melt and therefore causes different crystal field interactions with the partially filledforbitals of the rare earths. The tetrad effect in granites cannot be used as an indicator of fluid/rock or fluid/melt interaction, since it can be experimentally reproduced in the absence of any fluids.