A Partial Record of Mixing of Mantle Melts Preserved in Icelandic Phenocrysts
A Partial Record of Mixing of Mantle Melts Preserved in Icelandic Phenocrysts
复制标题
DOI：
10.1093/petrology/egr031
复制
发表时间：
2011-09
影响因子：
3.9
通讯作者：
B. Winpenny;J. Maclennan
中科院分区：
文献类型：
--
作者：
B. Winpenny;J. Maclennan
地幔熔体在岩浆房中的混合的记录，以前曾观察到的组成中的橄榄石为主体的熔融包裹体Borgarhraun，一个原始的玄武岩流从Theistareykir火山系统，北方冰岛。Borgarhraun还含有高Mg值（85^92）单斜辉石，存在于多晶结核和斑晶中。在这些单斜辉石的成分区中进行了一致的主量和微量元素分析，并使用仔细选择的晶体熔体分配系数计算了与每个单斜辉石区化学平衡的熔体的Ce/Yb比。这些计算允许直接比较单斜辉石组合物与现有的熔体包裹体数据。晶体和平衡熔体中Ce/Yb比的范围不能单独通过结晶来解释，需要同时混合和结晶成分可变的地幔熔体。然而，在Ce/Yb的范围内，这些高镁数单斜辉石的平衡熔体是小于熔融包裹体托管的橄榄石具有相等的Fo含量。此外，单斜辉石生长的熔体的平均组合物具有显着较低的Ce/Yb比橄榄石托管的熔体包裹体。因此，单斜辉石的地幔熔体的变化记录偏向于更亏损（低Ce/Yb）的熔体组合物。这种偏见可以理解，如果微量元素的变化在Borgarhraun父母熔体耦合到主要元素的变化，预期从岩石学参数化地幔熔融。主量元素的变化影响了近莫霍面岩浆房分离结晶过程中的相关系，控制了液相线相的出现。在石榴石存在下形成的小度深熔体具有高Ce/Yb比。在冷却，这些熔体有一个更长的橄榄石只结晶路径比熔体来自浅地幔。当这些深源熔体最终成为单斜辉石饱和，他们有太低的镁数结晶高镁数单斜辉石，如发现在Borgarhraun。相反，浅，亏损熔体饱和单斜辉石在高镁数。单斜辉石结晶在富集熔体中的延迟发生，再加上同时混合和结晶的熔体在地幔中的深度范围内产生的，可以解释的高镁数熔体中的橄榄石和单斜辉石饱和的微量元素组成的分布的差异。因此，微量元素组成的高镁数单斜辉石在Borgarhraun地幔熔体的混合只提供了一个部分和偏见的记录。以及显示熔体混合可以保存在斑晶组合物，结果表明，微量元素之间的不平衡晶体和载体熔体可以是岩浆混合的后果，而不是必要的捕虏晶起源的晶体。此外，必须小心使用单斜辉石分离原始玄武岩检查成分的不均匀性时，因为它们提供了一个记录的化学演化的岩浆系统，偏向于亏损的组合物，因此不完整。
The record of mixing of mantle melts in magma chambers has previously been observed in the compositions of olivine-hosted melt inclusions from Borgarhraun, a primitive basalt flow from the Theistareykir volcanic system, northern Iceland. Borgarhraun also contains high Mg-number (85^92) clinopyroxenes, which exist in polycrystalline nodules and as phenocrysts. Coincident major and trace element analyses were made in compositional zones of these clinopyroxenes, and Ce/Yb ratios of the melts in chemical equilibrium with each of the clinopyroxene zones were calculated using carefully selected crystal^melt partition coefficients. These calculations allow direct comparison of clinopyroxene compositions with existing melt inclusion data. The range of Ce/Yb ratios in the crystals and in the equilibrium melts cannot be accounted for by crystallization alone, requiring simultaneous mixing and crystallization of compositionally variable mantle melts. However, the range in Ce/Yb for melts in equilibrium with these high Mg-number clinopyroxenes is smaller than that of melt inclusions hosted by olivines with equivalent Fo contents. Also, the mean composition of the melts from which clinopyroxene grew has significantly lower Ce/Yb than the olivine-hosted melt inclusions. The record of mantle melt variability in clinopyroxenes is thus biased towards more depleted (low Ce/Yb) melt compositions. This bias can be understood if the trace element variation in the Borgarhraun parental melts is coupled to major element variation, as expected from petrological parameterizations of mantle melting. The major element variation influences the phase relationships and controls the appearance of liquidus phases during fractional crystallization in near-Moho magma chambers. Small-degree, deep melts, formed in the presence of garnet, have high Ce/Yb ratios. On cooling, these melts have a longer olivine-only crystallization path than melts derived from the shallow mantle. When these deep-sourced melts eventually become clinopyroxene saturated, they have too low Mg-numbers to crystallize high Mg-number clinopyroxenes such as are found in Borgarhraun. In contrast, shallow, depleted melts saturate in clinopyroxene at high Mg-number. The delayed onset of clinopyroxene crystallization in the enriched melts, coupled with concurrent mixing and crystallization of melts generated at a range of depths in the mantle, can account for the difference in the distribution of the trace element composition of high Mg-number melts saturated in olivine and clinopyroxene. The trace element compositions of high Mg-number clinopyroxenes in Borgarhraun therefore provide only a partial and biased record of the mixing of mantle melts. As well as showing that melt mixing may be preserved in phenocryst compositions, the results illustrate that trace element disequilibrium between crystals and carrier melt can be a consequence of magma mixing, rather than necessitating a xenocrystic origin for the crystals. Furthermore, care must be taken when using clinopyroxene separates from primitive basalts to examine compositional heterogeneity, as they provide a record of the chemical evolution of the magmatic system that is biased towards depleted compositions and therefore incomplete.