Ocean Island Lavas: Garnet Clinopyroxenite or CO2-bearing Mantle Lherzolite?

Shantanu Keshav1, Gudmundur H Gudfinnsson2, Gautam Sen1, Dean C Presnall2& Yingwei Fei2

1Dept. Earth Sciences, Florida International University, Miami, FL 33199, USA
2Geophysical Lab., Carnegie Inst. Washington, Washington DC, DC 20015, USA

We combine new results of an experimental melting study of a Hawaiian garnet clinopyroxenite (SL) at 2.0-2.5 GPa pressure with data from lherzolite (+CO2) in the CMAS and natural systems to evaluate the origin of alkalic basalts (AB) that erupt at oceanic islands. The experimental data on SL are also used to map the behavior of partial melts as a function of pressure (P), temperature (T), and degree of melting (F). The solidus of SL, a tholeiitic picrite with ~ 1 wt % Na2O, is bracketed at 1,295±15 and 1,335±15°C at 2.0 and 2.5 GPa, respectively. These brackets are slightly lower than those of anhydrous mantle lherzolite at identical pressures. Chemically, the high and low-F melts are ol-hy normative and moderately to strongly ne-normative, which can be ascribed to the effect of Na and Fe in expanding the “eclogite surface” in natural systems. There is very little overlap between primitive AB and partial melts of SL. These differences are most pronounced for MgO, Al2O3, CaO, CaO/Al2O3, and CaO/MgO. SL melts partially resemble AB only in terms of SiO2 and Na2O+K2O. Partial melts of anhydrous mantle lherzolite also partially overlap AB, but even the lowest degree melts analyzed are far removed from most of the AB. However, partial melts generated at 3-6 GPa from carbonated mantle lherzolite in CMAS-CO2 and natural systems can generate the AB clan of lavas from oceanic islands. Experimental melting studies in simple CMAS-CO2 and natural lherzolitic systems demonstrate that isobaric increases in F lead to a moderate decrease in CaO+MgO in partial melts. With increasing F, CaO/MgO and CaO/Al2O3 drop sharply in the partial melts. Based on CaO/Al2O3, CaO, CaO+MgO, AlAl2O3O3, and Mg# systematics, it is proposed that the Hawaiian, Samoan, and Polynesian lavas have tapped the shallowest (~3.0-3.5 GPa) part of the melting column within the garnet stability field in the presence of a low-to-moderate amount of COAl2O3 in their respective mantle sources. In addition, on the basis of CaO and CaO/MgO systematics, Hawaiian, Samoan, and Polynesian lavas appear to be produced by relatively high F. Within Hawaii, lavas from Oahu may have equilibrated at a slightly higher pressure than those from Koloa. On the other hand, based on higher CaO, CaO/Al2O3, and CaO+MgO, coupled with lower CaO/MgO, and Al2O3, it is inferred that lavas from the Gran Canaria have equilibrated at a slightly higher pressure (~3.5-4.5 GPa) in the presence of slightly higher CO2. Chemical systematics also suggest that lavas from Gran Canaria may have been products of relatively lower F. It is proposed here that major element systematics of AB and also nephelinites/melilitites cannot be modeled by garnet clinopyroxenite (with anhydrous mantle lherzolite) at the pressures of investigation. CO2 is required in the mantle source regions of AB on ocean islands.