**2. Mantle plumes, and the significance of xenoliths, xenocrysts, and magma mixing**

Convection of hot rock within the Earth's mantle has been proposed as the mechanism for the formation of a magmatic plume or mantle plume (acting like a diapir) that results in volcanic hotspots such as those located at Hawaii or Iceland, and for large igneous provinces such as the Deccan Traps [27–29]. Mantle plumes are thought essentially to be areas of hot, upwelling magma, with a hotspot that develops above the plume (this is the Wilson-Morgan hypothesis of hotspots, typified by high heat flow, positive gravity anomaly and alkalic volcanism, resulting in surface expressions of mantle plumes rising by thermal convection [30]). Magma generated by hotspots rises through the more rigid overlying lithosphere and produces active volcanoes and lava flows at the Earth's surface [31–34]. Accompanying, and critical to this process is the fact that, on its ascent, a plume will entrain rock fragments from deep in the Earth's crust (evident as mafic and ultramafic xenoliths) or, when traversing the lithosphere such as sedimentary basins higher in the Earth's crust, entrain Phanerozoic and Proterozoic xenoliths such as sandstone, shale, coal, low-grade metamorphic rock, and granite [21, 30, 32–36]. Where there is melting or partial melting, xenocrysts can be released [30, 37–40]. Also, where there is melting or partial melting of the lithosphere there is geochemical contamination of magma [37, 38].

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**Figure 3.**

*Map of eastern Australia showing outcrops of Cainozoic volcanism and their ages, and the general trackway of* 

*the Cosgrove volcanic chain (modified from Davies* et al*. [21]).*

To account for geochemical heterogeneity in hotspot and flood basalt lavas, Farnetani & Richards [37] suggest either inherent plume-source heterogeneity or contamination from the lithosphere through which the primary magma ascended. We accord with the latter, *i.e.*, contamination from a heterogeneous lithosphere, particularly when the magma rises through a thick lithosphere [13] or through a complex sedimentary basin.
