Trace Element Constraints on the Differentiation and Crystal Mush Solidification in the Skaergaard Intrusion, Greenland
Trace Element Constraints on the Differentiation and Crystal Mush Solidification in the Skaergaard Intrusion, Greenland
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DOI：
10.1093/petrology/egy032
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发表时间：
2018-03
影响因子：
3.9
通讯作者：
O. Namur;M. Humphreys
中科院分区：
文献类型：
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作者：
O. Namur;M. Humphreys
&NA;通过对Skaergaard岩体辉长岩中斜长石和单斜辉石的主量元素和微量元素的分析，研究了主岩浆体和结晶体的结晶机制。结晶岩心显示出层状系列中连续的化学地层演化，相容元素（如单斜辉石中的Cr和Ni）自下而上逐渐贫化，而不相容元素（如斜长石和单斜辉石中的Sr、Ba、REE）逐渐富集。我们进行了数值模型，并表明，这些趋势可以解释一个简单的过程中的分离结晶，除了上20%的侵入斜长石和单斜辉石微量元素组合物离开分离结晶的趋势。在岩浆房凝固的这一阶段，分离结晶变得不那么有效，并被大部分原位平衡结晶所取代。微量元素也显示显着的变化，在斜长石和单斜辉石堆晶的间隙过度生长。它们是由液晶+晶体糊状物中的间隙液体结晶产生的。然而，不相容的元素，特别是稀土元素，在斜长石边缘（> 20 ppm Ce），强烈超过了斜长石核心观察到的最高浓度（高达4 ppm Ce）在层状系列的顶部显示出一定程度的富集。这种强烈的富集很难与晶体糊状物中间隙液体的分离结晶的简单过程相协调，但可能与晶体糊状物中硅酸盐液体不相容性的发展或磷灰石的延迟成核有关。在斜长石和单斜辉石过度生长结晶之后，在0.1 - 0.25百万年期间的扩散再平衡显著改变了原始分带剖面。Ce通量从单斜辉石到斜长石晶格的相邻晶体也可能有助于观察到的强烈的Ce富集在斜长石轮辋。
&NA; New major and trace element analyses of plagioclase and clinopyroxene in gabbros from the Skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush. Crystal cores show a continuous chemostratigraphic evolution in the Layered Series, with compatible elements (e.g. Cr and Ni in clinopyroxene) being progressively depleted from the bottom up, whereas incompatible elements (e.g. Sr, Ba, REE in plagioclase and clinopyroxene) become progressively enriched. We performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization, except for the upper 20% of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends. At this stage of magma chamber solidification, fractional crystallization becomes less efficient and is replaced by a major proportion of in situ equilibrium crystallization. Trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals. They result from crystallization of the interstitial liquid in the liquid + crystal mush. However, incompatible elements, and especially REE, show a degree of enrichment in plagioclase rims (> 20 ppm Ce) that strongly exceeds the highest concentrations observed in plagioclase cores (up to 4 ppm Ce) at the top of the Layered Series. Such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush, but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite. After the crystallization of plagioclase and clinopyroxene overgrowths, diffusive re‐equilibration during a period of 0·1–0·25 Myr significantly changed the original zoning profiles. Ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong Ce enrichment observed in plagioclase rims.