PETROLOGICAL-SEISMIC CONSTRAINTS ON POTENTIAL TEMPERATURES ALONG OCEANIC RIDGES

On the basis of volatile-free phase relations, the major-element compositional systematics of primitive mid-oceanic ridge basalts (MORBs) have been explained by large potential temperature (Tp) variations (1160-1510°C), and the higher temperatures have been associated with "hot spots" on or close to ridges.  For a volatile-free lherzolite, melting at the highest Tp of 1510°C would begin at a depth of about 125 km.  However,  both H₂O and CO₂ are known to be present throughout the oceanic mantle.  The small amounts of H₂O (~100-300 ppm) generally understood to exist in the oceanic mantle beneath ridges do not significantly lower the volatile-free peridotite solidus.  In contrast, even the smallest amount of CO₂ decreases the solidus abruptly by ³ 300°C at P > 1.9 GPa, and greatly increases the depth of initial melting.  The lowest Tp capable of producing basaltic magmas cannot avoid small amounts of melting at depths of ~65-300 km, a nearly perfect match for the depth range of the seismic low-velocity zone (LVZ), which has repeatedly been shown to track the oceanic ridge system closely.  Therefore, the LVZ appears clearly to be caused by small amounts of melting of a CO₂-bearing mantle.  Melts at and near the solidus over this entire depth range are carbonatitic.  Basaltic melts would be produced at larger melt fractions and shallower depths.  Extrapolation of the carbonated lherzolite solidus to extreme depths shows that only the lowest Tp range capable of producing basalts (~1240-1260°C) is consistent with the global (except for Iceland and possibly Afar) disappearance of the LVZ beneath ridges at depths >250 km.  Higher potential temperatures ranging up to 1400-1500°C would produce melting at mantle transition zone depths and possibly into the lower mantle, which is not supported by seismic data.  This result is inconsistent with the existence of hot diapirs beneath most "hot spots" associated with ridges and models that explain the major-element systematics of MORBs by a large Tp range.  Instead, the MORB systematics are best explained by source heterogeneity and shallow, passive upwelling from an oceanic mantle most of which has a relatively constant and low Tp of ~1240-1260°C.