Stable isotopic constraints on formation of continental lithospheric mantle: a case study from the Colorado Plateau

Abstract:

Mantle melt depletion is fundamental to stabilization of thick lithospheric mantle and to craton formation and survival. Three potential mechanisms to grow lithospheric mantle are: partial melting of upwelling mantle plumes; imbrication and accretion of oceanic lithosphere at subduction zones; and flux melting of the mantle wedge at volcanic arcs. Stable isotopes can distinguish these models because stable isotopes fractionate substantially at low temperature, making the mantle sensitive to the incorporation of subducted material. Correlations between stable isotopes and trace elements can then distinguish seafloor weathering processes from flux melting processes, allowing subducted oceanic lithosphere to be discerned from flux-melted lithosphere. We find that indices of melt depletion (e.g. cpx Cr#, Yb concentrations and Mg#) in xenoliths from the Colorado Plateau correlate with δ¹⁸O values of olivine. Xenoliths from The Thumb volcanic neck have δ¹⁸O_ol values ranging from +5.05 to +5.50‰ (n=12) that correlate positively with indices of melt depletion, and xenoliths from the Green Knobs diatreme have δ¹⁸O_ol values that range from +4.85 to +5.36‰ (n=9) and correlate negatively with indices of melt depletion. These trends may reflect coupled fluid input and melt depletion, typical of flux melting. The different trends at each locality may be due to fluxing of different fluids- either high δ¹⁸O fluids derived from altered oceanic crust or low δ¹⁸O fluids derived from recycled serpentinite. Canil and Lee (2009) interpreted correlations between whole rock δ¹⁸O values and MgO/SiO₂ in xenoliths from The Thumb to be due to seafloor weathering and Mg loss of abyssal peridotites followed by later accretion of the rocks to the Colorado Plateau lithosphere. However, chrome and alumina are immobile during seafloor weathering, so correlations between δ¹⁸O values and Cr# also present in The Thumb xenoliths are inconsistent with seafloor weathering. Evidence for flux melting is consistent with the results of Selverstone et al. (1999), who found that the constituent terranes of the Colorado Plateau are accreted arcs. Stable isotope analyses of older SCLM peridotites may help distinguish lithospheric mantle formation mechanisms in other cratons and constrain the timing of onset of modern plate tectonics.