矿床地球化学国家重点实验室知识库

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 situ SIMS) 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 delta O-18 values (mean values are 5.1 +/- 0.3% (2 sigma, n = 53), 5.2 +/- 0.3% (2 sigma, n = 122) and 5.3 +/- 0.3% (n = 25), respectively) coupled with generally low Fo contents (mean values are 88.7 +/- 1.4 mol% (2 sigma, n = 53), 89.8 +/- 1.8 mol% (2 sigma, n = 122) and 89.4 +/- 1.8 mol% (2 sigma, n = 25), respectively). In contrast, the olivine grains in the samples from Wulongba are characterized by elevated delta O-18 values (mean = 5.6 +/- 0.3% (2 sigma, n = 58)) coupled with generally higher Fo contents (mean = 91 +/- 2.8 mol% (2 sigma, 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 delta O-18 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 delta O-18 values suggest that at least locally the pyroxenitic source was characterized by O-18-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 fO(2) conditions of the partial melt generated in the subduction modified lithospheric mantle, or to a higher degree of partial melting.