Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back
Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back
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DOI：
10.1016/j.epsl.2015.08.039
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发表时间：
2015-12
影响因子：
5.3
通讯作者：
S. Aulbach;K. Viljoen
中科院分区：
文献类型：
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作者：
S. Aulbach;K. Viljoen
年龄≥ 2.5Ga榴辉岩和辉石岩捕虏体的主量和微量元素组成（单斜辉石的原位Pb-Pb数据），以确定：（1）它们原岩的性质和演化，（2）它们在多大程度上保留了形成它们低压原岩的软流圈地幔源区的信息，（3）它们的低压原岩是由地幔源区的物质组成的。（3）它们的深部再循环对对流地幔放射成因同位素演化的影响。它们的元素关系与洋壳形成过程中橄榄石±斜长石和单斜辉石的低压分馏相一致，从而使残余熔体富集稀土元素、高场强元素和Y，产生了稀土元素与Eu和Sr异常大小的负相关关系。轻稀土元素贫化可能表明，在洋壳俯冲和变质（榴辉岩化）时，平均损失20%的部分熔融，但这并不有助于大陆地壳的幼年生长。榴辉岩的Sm/Nd（0.40）和Lu/Hf（0.27）与亏损地幔相似，U/Pb（0.02）和Th/Pb（0.02）较低。如果深俯冲，这些岩石不能解释非放射成因的Nd和Hf，和放射成因的铅同位素组成的一些现代洋岛玄武岩的来源。与苦橄岩相似的低不相容微量元素含量和与现代MORB相似的给定TiO 2含量下的Yb浓度表明，原岩的平均熔体分数为0.20-0.25，在≤ 2.5 - 3.0 GPa的压力下留下尖晶石橄榄岩残留物。橄榄岩固相线的这种浅相交表明，中等的西太平洋环境地幔潜在温度≤ 1420至1470° C。经单斜辉石分馏和交代作用过滤的样品的V/Sc（4.7±1.2; n= 11）表明f O 2（相对于铁橄榄石-磁铁矿-石英缓冲层= Δ FMQ为-1.9）低于现代MORB。这部分是由于在其地壳原岩形成过程中较高的平均熔体萃取压力（1.5GPa）。外推到1 GPa，类似于现今MORB生成的平均压力，得到ΔFMQ为− 1.7±1.1，对应于Fe 3+#为0.07±0.04。如果这些结果是正确的，他们建议一个更大的还原比现在的环境地幔，挥发物的物种，影响地幔固相线和火山气体的组成的影响。这对东亚大气氧化还原演化和碳及其他挥发物的再循环具有影响。
Major-and trace-element compositions of eclogite and pyroxenite xenoliths of≥ 2.5 Ga age (in situ Pb–Pb data on clinopyroxene) from the Lace kimberlite on the Kaapvaal craton were investigated in order to constrain:(1) the nature and evolution of their protoliths;(2) the extent to which they preserve information on the state of the asthenospheric mantle source that gave rise to their low-pressure protoliths; and (3) the effect of their deep recycling on the radiogenic isotope evolution of the convecting mantle. Their elemental relationships are consistent with low-pressure fractionation of olivine±plagioclase and clinopyroxene during oceanic crust formation, whereby the residual melt was enriched in rare-earth elements (REE), high field-strength elements and Y, producing inverse correlations of ΣREE with the size of Eu-and Sr-anomalies. LREE-depletion may indicate loss of on average 20% of a partial melt upon subduction and metamorphism (eclogitisation) of oceanic crust, which did not, however, contribute to juvenile growth of continental crust. The eclogites have median Sm/Nd (0.40) and Lu/Hf (0.27) similar to Depleted Mantle, and lower U/Pb (0.02) and Th/Pb (0.02). If deeply subducted, these rocks cannot explain unradiogenic Nd and Hf, and radiogenic Pb isotope compositions in the sources of some modern ocean island basalts. Low incompatible trace-element contents similar to picrites, and Yb concentrations at a given TiO 2 content similar to modern MORB, indicate derivation of the protoliths by average melt fractions of∼ 0.20–0.25 that left a spinel peridotite residue at pressures≤ 2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of≤ 1420 to 1470° C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7±1.2; n= 11) indicating lower f O 2 (− 1.9 relative to the fayalite–magnetite–quartz buffer= Δ FMQ) than modern MORB. This is in part due to the higher average melt extraction pressure (∼ 1.5 GPa) during formation of their crustal protoliths. Extrapolation to 1 GPa, similar to the average pressure of present-day MORB generation, yields ΔFMQ of− 1.7±1.1, corresponding to Fe 3+# of 0.07±0.04. If these results are correct, they suggest an Archaean ambient mantle more reducing than at present, with implications for the speciation of volatiles, which affects the mantle solidus and the composition of volcanic gases. This has implications for the Archaean atmospheric redox evolution and the recycling of carbon and other volatiles.