Source of the Quaternary alkalic basalts, picrites and basanites of the Potrillo Volcanic Field, New Mexico, USA: lithosphere or convecting mantle?
The <80 ka basalts–basanites of the Potrillo Volcanic Field (PVF) form scattered scoria cones, lava flows and maars adjacent to the New Mexico–Mexico border. MgO ranges up to 12·5%; lavas with MgO < 10·7% have fractionated both olivine and clinopyroxene. Cumulate fragments are common in the lavas, as are subhedral megacrysts of aluminous clinopyroxene (with pleonaste inclusions) and kaersutitic amphibole. REE modelling indicates that these megacrysts could be in equilibrium with the PVF melts at ~1·6–1·7 GPa pressure. The lavas fall into two geochemical groups: the Main Series (85% of lavas) have major- and trace-element abundances and ratios closely resembling those of worldwide ocean-island alkali basalts and basanites (OIB); the Low-K Series (15%) differ principally by having relatively low K2O and Rb contents. Otherwise, they are chemically indistinguishable from the Main Series lavas. Sr- and Nd-isotopic ratios in the two series are identical and vary by scarcely more than analytical error, averaging 87Sr/86Sr = 0·70308 (SD = 0·00004) and 143Nd/144Nd = 0·512952 (SD=0·000025). Such compositions would be expected if both series originated from the same mantle source, with Low-K melts generated when amphibole remained in the residuum. Three PVF lavas have very low Os contents (<14 ppt) and appear to have become contaminated by crustal Os. One Main Series picrite has 209 ppt Os and has a {gamma}Os value of +13·6, typical for OIB. This contrasts with published 187Os/188Os ratios for Kilbourne Hole peridotite mantle xenoliths, which give mostly negative {gamma}Os values and show that Proterozoic lithospheric mantle forms a thick Mechanical Boundary Layer (MBL) that extends to ~70 km depth beneath the PVF area. The calculated mean primary magma, in equilibrium with Fo89, has Na2O and FeO contents that give a lherzolite decompression melting trajectory from 2·8 GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modelling of REE abundances in Main Series Mg-rich lavas is successful for a model invoking decompression melting of convecting sub-lithospheric lherzolite mantle ({epsilon}Nd = 6·4; Tp ~ 1400°C) between 90 and 70 km. Nevertheless, such a one-stage model cannot account for the genesis of the Low-K Series because amphibole would not be stable within convecting mantle at Tf ~ 1400°C. These magmas can only be accommodated by a three-stage model that envisages a Thermal Boundary Layer (TBL) freezing conductively onto the ~70 km base of the Proterozoic MBL during the ~20 Myr tectonomagmatic quiescence before PVF eruptions. As it grew, this was veined by hydrous small-fraction melts from below. The geologically recent arrival of hotter-than-ambient (Tp ~ 1400°C) convecting mantle beneath the Potrillo area re-melted the TBL and caused the magmatism.