An integrated chemical and oxygen isotopic study of primitive olivine grains in picrites from the Emeishan Large Igneous Province, SW China: Evidence for oxygen isotope heterogeneity in mantle sources
An integrated chemical and oxygen isotopic study of primitive olivine grains in picrites from the Emeishan Large Igneous Province, SW China: Evidence for oxygen isotope heterogeneity in mantle sources
复制标题
中国西南峨眉山大火成岩省苦铁矿中原始橄榄石颗粒的化学和氧同位素综合研究：地幔源中氧同位素异质性的证据
DOI：
10.1016/j.gca.2017.08.007
复制
发表时间：
2017
影响因子：
5
通讯作者：
Chusi Li
中科院分区：
文献类型：
--
作者：
Yu Song-Yue;Shen Neng-Ping;Song Xie-Yan;Edward Ripley;Chusi Li
由于岩浆迁移到地表时可能与地壳污染有关的叠印，因此，对大陆溢流玄武岩潜在地幔来源的性质的认识变得复杂（Arndt和Christensen，1992年）。然而，在苦橄质熔岩流中，形成于结晶序列早期的原始橄榄石斑晶可能提供其地幔来源的未扰动信息。对峨眉山五龙坝、乌龟井、塘浪河和牦牛坪等地苦橄岩中的原始橄榄石（Fo为88 ~ 92.6mol%）进行了化学和氧同位素综合研究。我们使用这些数据来评估地幔来源的岩浆的原始橄榄石结晶的地球化学性质。牦牛坪、乌龟井和塘浪河样品中的原始橄榄石颗粒具有类似地幔的δ 18 O值特征平均值分别为5.1 ± 0.3‰（2σ，n= 53）、5.2 ± 0.3‰（2σ，n= 53）（2σ，n= 122）和5.3 ± 0.3‰（n= 25），且Fo含量普遍较低（平均值分别为88.7 ± 1.4mol%（2σ，n= 53），89.8 ± 1.8mol%（2σ，n= 122）和89.4 ± 1.8mol%（2σ，n= 25））。与之相反，乌龙坝样品中的橄榄石颗粒具有较高的δ 18 O值（平均值= 5.6 ± 0.3‰（2σ，n= 58））和较高的Fo含量（平均值= 91 ± 2.8mol%（2σ，n= 58））。基于橄榄石成分，牦牛坪、塘浪河和乌龟井苦橄岩中的原始橄榄石与橄榄岩和辉石岩/榴辉岩成分比例相似的混合地幔源的成因一致。这些原始橄榄石的δ 18 O值与地幔柱源区物质的熔融相一致。乌龙坝原始橄榄岩的化学成分也与橄榄岩/辉石岩混合源相一致，但高δ 18 O值表明辉石岩源至少局部具有18 O富集的特征，这与海水与洋壳原岩的高度相互作用有关。另一种解释是，五龙坝苦橄岩中的橄榄石反映了新元古代岩石圈地幔被俯冲相关过程所改造的产物。乌龙坝苦橄岩的高Fo含量可能是由于俯冲改造岩石圈地幔产生的部分熔融的fO 2条件较高，或部分熔融程度较高所致。
Recognition of the nature of potential mantle sources of continental flood basalts is complicated by possible overprinting related to crustal contamination as magmas migrate to the surface (Arndt and Christensen, 1992). However, in picritic lava flows primitive olivine phenocrysts that formed early in the crystallization sequence can potentially provide unperturbed information of their mantle source. We have carried out an integrated chemical and oxygen isotopic (in situSIMS) study of primitive olivine grains (Fo ranging from 88 to 92.6 mol%) in the Emeishan picrites at different locations (Wulongba, Wuguijing, Tanglanghe and Maoniuping). We use these data to evaluate the geochemical nature of mantle sources for magmas from which the primitive olivine crystallized. The primitive olivine grains in the samples from Maoniuping, Wuguijing and Tanglanghe are characterized by mantle-like δ18O values (mean values are 5.1 ± 0.3‰ (2σ,n= 53), 5.2 ± 0.3‰ (2σ,n= 122) and 5.3 ± 0.3‰ (n= 25), respectively) coupled with generally low Fo contents (mean values are 88.7 ± 1.4 mol% (2σ,n= 53), 89.8 ± 1.8 mol% (2σ,n= 122) and 89.4 ± 1.8 mol% (2σ,n= 25), respectively). In contrast, the olivine grains in the samples from Wulongba are characterized by elevated δ18O values (mean = 5.6 ± 0.3‰ (2σ,n= 58)) coupled with generally higher Fo contents (mean = 91 ± 2.8 mol% (2σ,n= 58)) than primitive olivine in the samples from the other locations. Based on olivine compositions, primitive olivine in picrites from Maoniuping, Tanglanghe and Wuguijing are consistent with derivation from hybrid mantle sources containing similar proportions of peridotite and pyroxenite/eclogite components. The δ18O values of these primitive olivine grains are consistent with melting of plume source materials. The chemical composition of the primitive olivine from Wulongba are also consistent with derivation from a hybrid peridotite/pyroxenite source, but the high δ18O values suggest that at least locally the pyroxenitic source was characterized by18O-enrichment related to a higher degree of seawater interaction with the oceanic crust protolith. An alternative explanation is that olivine in the Wulongba picrite reflects derivation from lithospheric mantle that was modified by subduction-related processes in the Neoproterozoic. The high-Fo content of the Wulongba picrites may be due to the higher fO2conditions of the partial melt generated in the subduction modified lithospheric mantle, or to a higher degree of partial melting.