Pressure of origin of primary mid-ocean ridge basalts

Abstract

Summary High-pressure phase equilibrium studies of mid-ocean ridge basalts (MORBs) have shown that some samples are in equilibrium with olivine + orthopyroxene (OPX) + clinopyroxene ± plagioclase at about 10 kbar, whereas others are not in equilibrium with OPX at this pressure. Samples that are OPX-saturated at about 10 kbar are characterized by higher SiO 2 (>49.7 wt%) than most (about >75%) MORB glasses with more than 9.5 wt% MgO, suggesting that only about 25% of these glasses could be derived from primary magmas generated at about 10 kbars. Orthopyroxenes in abyssal peridotites are characterized by Al 2 O 3 contents that range from 2.0 to 6.0 wt% (probably about 2.5–6.5 wt% before subsolidus equilibration), but orthopyroxenes in peridotite partial-melting experiments and liquidus orthopyroxenes for MORBs at about 10 kbar have only 3.5–4.3 wt% Al 2 O 3 . Liquidus orthopyroxenes for high-MgO basalts at 25 kbar and 20 kbar have about 6.6 wt% and 5.5 wt% Al 2 O 3 respectively. It appears that the least depleted (Al 2 O 3 -rich end) of abyssal peridotite orthopyroxenes indicate the dominance of melting and primary MORB genesis at pressures substantially greater than 10 kbar, and probably at 20–25 kbar.