International audienceSeveral methods have been developed to assess the thermal state of the mantle below oceanic ridges, islands, and plateaus, on the basis of the petrology and geochemistry of erupted lavas. One leads to the conclusion that mantle potential temperature (i.e., TP) of ambient mantle below oceanic ridges is 1430°C, the same as Hawaii. Another has ridges with a large range in ambient mantle potential temperature (i.e., TP = 1300–1570°C), comparable in some cases to hot spots (Klein and Langmuir, 1987; Langmuir et al., 1992). A third has uniformly low temperatures for ambient mantle below ridges, ∼1300°C, with localized 250°C anomalies associated with mantle plumes. All methods involve assumptions and uncertainties that we critically evaluate. A new evaluation is made of parental magma compositions that would crystallize olivines with the maximum forsterite contents observed in lava flows. These are generally in good agreement with primary magma compositions calculated using the mass balance method of Herzberg and O'Hara (2002), and differences reflect the well-known effects of fractional crystallization. Results of primary magma compositions we obtain for mid-ocean ridge basalts and various oceanic islands and plateaus generally favor the third type of model but with ambient mantle potential temperatures in the range 1280–1400°C and thermal anomalies that can be 200–300°C above this background. Our results are consistent with the plume model

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mantle; however, there is no evidence to suggest a high heat flux Diffuse Cenozoic volcanism in Mongolia forms part of a widespread mantle plume. Volcanism is likely to occur where localized extensional tectono-magmatic province that extends from NE China to Lake conditions are favourable.

Splitting or thinning of lithosphere above a mantle plume can result in voluminous melt generation, leading to the formation of large igneous provinces, or LIPs. Examples of LIPs include continental flood basalt provinces and oceanic plateaus. Basaltic samples from the Ontong Java Plateau, Nauru Basin and Manihiki Plateau, which are among the largest of the LIPs, have isotopic compositions within the range of ocean island basalts. The majority of continental basalts, however, record a trace element and isotopic contribution from the lithosphere through which they have erupted. We are thus unable to reconcile the available compositional data with models which derive the isotopic and large-ion lithophile element-enriched character of continental flood basalts solely from sub-lithospheric mantle plume sources. A combination of mantle sources is indicated, with the thermal energy being supplied by voluminous melts from a plume, and the lithospheric components in continental flood basalts being inherited by contamination of plume-derived melts by low melting point hydrous and carbonated fractions in the lithosphere. Successive injection of plume-derived melts serves to heat the lithosphere, reducing its viscosity and making it susceptible to rupture if allowed by regional plate forces. Furthermore, the lithosphere, including the mechanical boundary layer, may be thinned by thermal stripping from below, allowing the plume mantle to ascend and decompress further. Such a system has the potential for positive feedback leading to rapid melt generation. While we do not exclude recent models of LIP formation which require the sudden impact of a new mantle plume, we favour a model whereby the thermal anomaly builds gradually, incubating beneath a steady-state lithospheric cap.