Applying a new olivine-melt Ni thermometer to K-rich primitive lavas from Colima cones, western Mexico: new constraints on melt segregation conditions and geodynamic implications

A new olivine-melt thermometer based on the partitioning of Ni (Pu et al., 2017) is applied here to a suite of mantle-derived K-rich minettes and absarokites from the Colima rift in W. Mexico in order to determine the temperature and minimum H₂O concentrations at the onset of olivine growth (1060-1223°C and 6.7% H₂O, respectively). Encouragingly, the H₂O estimates match H₂O measurements from melt inclusions in these samples (Vigouroux et al., 2008; Maria and Luhr, 2008). The depth of melt segregation from the mantle for these rocks is determined by comparison to experimental partial melts of phlogopite-bearing lherzolite (Condamine and Medard, 2014; Condamine et al., 2016), and is found to be around 80km (2.5GPa). This estimated depth, and the relative pressure independence of the Ni thermometer, permit a temperature correction for adiabatic ascent to the calculated Ni temperature, thereby constraining temperatures at the time of melt segregation to be 1120-1285 °C.

Comparison of the conditions of melt segregation for the K-rich Colima melts (2.5GPa, up to 6.7% H₂O) to those for calc-alkaline basalts (1.5GPa, up to 5.7%H₂O) erupted to the east side of the Colima Rift demonstrates that the K-rich Colima melts segregated at the base of a much thicker lithosphere beneath the Jalisco block of western Mexico. The suture between the older (Cretaceous) Jalisco block to the west, and the younger (Oligocene) lithosphere to the east, has a NE-SW orientation that is located beneath the N-S Colima rift, and the different thickness of these two lithosphere blocks may be the cause for the different dips in the subduction of Rivera and Cocos plates. When the P,T conditions for melt segregation are mapped onto arc mantle wedge, the results are consistent with available models of the thermal structure of typical subduction zones (Kelemen et al., 2003; Johnson et al., 2009). The initial temperature estimates from the olivine-melt Ni-thermometer provide critical constraints on the contrasting thermal conditions of the two sets of samples erupted from the Colima Rift and the subduction zone volcanic front, and demonstrates the use of the Ni-thermometer to constrain the temperature in arc basalts without prior knowledge of melt H₂O content.