Contamination and melt aggregation processes in continental flood basalts: constraints from melt inclusions in Oligocene basalts from Yemen

Melt inclusions from Oligocene continental flood basalts (CFB) erupted in Yemen provide unique insight into the timing and nature of the processes that lead to crustal contamination and melt aggregation in CFB magmas. Large variations in trace element indices that are sensitive to the degree and composition of assimilated crustal material (e.g. K2O = 0.20-1.94 wt%, Ba = 13-543 ppm, K/Nb = 128-1603, Ba/Th = 9-303) are evident in many inclusions, even where these derive from the same host lava, and reflect the complexity of the processes that lead to contamination within individual CFB melting and melt-transport systems. The compositions of melt inclusions relate to differences in the degree of contamination, but in addition require that there is substantial heterogeneity in the composition of the contaminant material itself. Many inclusions also appear to contain more primitive melts than typical Yemen CFB lava compositions, and as such would be highly sensitive to addition of crustal materials. Overall melt inclusions provide a markedly better record of the diversity of melt compositions present within given CFB magma systems than the bulk compositions of erupted lavas. Highly contaminated melts (with K/Nb and Ba/Nb up to 957 and 22, respectively) trapped within fosterite-rich olivines (Fo(85-90)) require very high rates of assimilation relative to crystal fractionation, with M-a/M-c values (the mass ratio of assimilated to crystallized material) much greater than 1. Such rapid assimilation may reflect decoupling of heat and mass transfer at the margins of larger magma chambers, within feeder dyke complexes, or at other sites where primitive magma is juxtaposed against wall-rocks that are already heated to temperatures near, or above, their solidus. In addition, relatively little assimilation appears to have occurred after crystallization of the phases that host melt inclusions, consistent with a thermal link between assimilation and phenocryst formation. Melt inclusions also show trace element variations related to mantle source compositions and mantle melting processes. Two inclusions with unusually Sr-rich and rare earth element-poor compositions are similar to those recognized from Mauna Loa, Hawaii, and may be related to melting of recycled gabbroic material within the upwelling Afar plume. In addition, many melt inclusions with crustally contaminated compositions also show large variations in trace element ratios that are essentially insensitive to crustal contamination (e.g. Zr/Y = 2-10) but are fractionated during progressive partial melting within an upwelling mantle column. The presence of both mantle and crustal-derived trace element signatures in the same inclusions demonstrates that melt transport systems in Yemen CFB were capable of transporting compositionally distinct melt batches, without complete mixing, through the asthenospheric and lithospheric mantle until final mixing and aggregation (and contamination) within crustal magma reservoirs. Thus, regardless of contamination, the ultimate compositions of many CFB lavas may be determined by magma mixing within the crust, rather than representing primitive compositions derived directly from the mantle. (C) 2002 Elsevier Science B.V. All rights reserved.