In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton
In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton
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DOI：
10.1016/j.gca.2015.11.028
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发表时间：
2016-02
影响因子：
5
通讯作者：
A. Riches;R. Ickert;D. Pearson;R. Stern;S. Jackson;A. Ishikawa;B. Kjarsgaard;J. Gurney
中科院分区：
文献类型：
--
作者：
A. Riches;R. Ickert;D. Pearson;R. Stern;S. Jackson;A. Ishikawa;B. Kjarsgaard;J. Gurney
金伯利岩中榴辉岩捕虏体的俯冲洋壳成因一直是克拉通构造模式的基石。然而，榴辉岩捕虏体通常具有漫长而复杂的历史，涉及多个交代事件，这些事件可能会叠加一些通常被视为俯冲起源证据的关键地球化学指标（例如，石榴石δ ~（18）O值和矿物~（87）Sr/~（86）Sr组成。为了评估地幔榴辉岩中氧同位素组成通过金刚石形成流体和其他可能的交代流体而受到干扰的可能性，我们对罗伯茨维克托金伯利岩S.非洲利用西姆斯首次对榴辉岩中的δ ~（18）O值进行了结构控制的原位测量。榴辉岩中石榴石和单斜辉石的模式比例在捕虏体中是不均匀的，石榴石的成分变化范围为Mg# = 75.8-79.2，钙铝榴石的比例为8.05- 10.14mol%，绿辉石的Jd为13 - 24，Mg# = 86.6 ~ 90.0。捕虏体中矿物的稀土元素分布模式为典型的贫轻稀土石榴石和明显富集轻稀土的辉石。这些硅酸盐矿物也记录可检测的内部和间的粮食轻稀土丰度的变化。单斜辉石的HFSE和Sr丰度变化与LREE含量和主量元素的变化不相关，矿物学约束和岩石体重构表明，在小体积（<<0.03wt.%）过程中，样品可能经历了不相容元素的选择性富集交代流体的渗入可能与古代金刚石演化有关。单斜辉石粒内常量元素、LREE和HFSE变化是后期交代作用的结果。石榴石中的氧同位素组成与所有主要元素和微量元素的变化无关，石榴石δ 18 O值在各种质地环境中的捕虏体中是均匀的。石榴子石δ ~（18）O值为+6.5 ± 0.2 ‰，高于平均值（+5.19 ± 0.26 ‰）的地幔石榴石范围（+4.8-5.5 ‰）.地幔橄榄岩对富CO_2和H_2O ~-的缓冲作用模拟在金刚石稳定性范围内的温度下的丰富交代流体表明，具有外来氧同位素组成的交代流体达到地幔榴辉岩体在大深度以橄榄岩质为主的地幔中发生交代流动后，其同位素组成未发生改变，含量很低。由于我们在所研究的含金刚石榴辉岩捕虏体中没有发现交代作用引起的石榴石氧同位素变化的证据，我们得出结论，石榴石δ 18 O值升高的最可能起源是通过在相对较低温度下改变的地壳原岩的继承。这些结果具有更广泛的相关性，并支持地幔榴辉岩捕虏体的低压原岩的假设，证明了石榴石氧同位素组成的稳健性-即使在含金刚石榴辉岩。
A subducted oceanic crustal origin for most eclogite xenoliths in kimberlites has long been a cornerstone of tectonic models for craton development. However, eclogite xenoliths often have protracted and complex histories involving multiple metasomatic events that could overprint some of the key geochemical indicators typically taken as evidence of a subducted origin (e.g., garnet δ18O-values and mineral87Sr/86Sr compositions). To assess the potential for disturbance of oxygen isotopic compositions in mantle eclogites via diamond-forming and other possible metasomatic fluids, we have conducted a multi-techniquein situstudy of a diamondiferous eclogite xenolith from the Roberts Victor kimberlite, S. Africa. Using SIMS we provide the first texturally-controlledin situmeasurements of δ18O-values in eclogitic garnet in close proximity to diamond.Garnet and clinopyroxene modal proportions are heterogeneous in the xenolith and garnet compositions vary from Mg# = 75.8–79.2; grossular proportions = 8.05–10.14 mol.%, and omphacitic pyroxene has Jd13–24and Mg# = 86.6–90.0. Rare earth element patterns of minerals across the xenolith, including grains close to diamond, are typical LREE-depleted garnets and markedly LREE-enriched pyroxenes. These silicate minerals also record detectable intra- and inter-grain LREE abundance variations. Clinopyroxenes of the studied xenoliths show HFSE and Sr abundance variations that are decoupled from LREE contents and major-element variations.Mineralogical constraints and bulk-rock reconstructions indicate that the studied sample likely experienced selective incompatible element enrichment during small-volume (<<0.03 wt.%) infiltration of metasomatic fluid(s) potentially linked to ancient diamond evolution. Intra-grain major-element, LREE and HFSE variations in clinopyroxene resulted from late-stage metasomatism. Oxygen isotope compositions in garnet are decoupled from all major- and trace-element variations, with garnet δ18O-values being uniform across the xenolith in a wide variety of textural settings. Garnet δ18O-values of +6.5 ± 0.2 ‰ are higher than the mean (+5.19 ± 0.26 ‰) of the mantle garnet range (+4.8–5.5 ‰).Modelling of the buffering effect of mantle peridotite on CO2-rich and H2O-rich metasomatic fluids at temperatures within the diamond stability field indicates that the likelihood of a metasomatic fluid with exotic oxygen isotopic composition arriving at a mantle eclogite body with its isotopic composition unmodified, after percolative flow through dominantly peridotitic mantle at great depth, is very low. As we find no evidence of metasomatically induced garnet oxygen isotope variations in the studied diamondiferous eclogite xenolith we conclude that the most likely origin for the elevated garnet δ18O-values is via inheritance from a crustal protolith altered at relatively low temperatures. These results have broader relevance and support the hypothesis of a low-pressure protolith for mantle eclogite xenoliths, demonstrating the robust nature of garnet oxygen isotope compositions – even in diamond-bearing eclogites.