MORB melting processes beneath the southern Mid-Atlantic Ridge (40-55°S):: a role for mantle plume-derived pyroxenite

Major and selected trace element abundances of MORB dredged from the moderately slow spreading southern MAR (40-55degreesS), in the vicinity of the Shona and Discovery mantle plumes, are used to constrain their melting conditions. All samples plot to high Fe-8 relative to the Na-8-Fe-8 global trend, the high Fe-8 of N-MORB in particular being anomalous. Shona-influenced MORB plot at lower Na-8 and towards higher Fe-8 values along the global trend than associated N-MORB, consistent with deeper initiation of melting. Discovery-influenced MORB extend to higher Na8 and lower Fe-8 compositions than associated N-MORB. Anomalous Na-8 and Fe-8, and Si-8, Ca-8 and Al-8, of most Discovery-influenced MORB are interpreted to reflect increased modal clinopyroxene in their source regions. Calculated F-mean, range from 15-17% for N-MORB, 16-19% for Shona-influenced MORB, and similar to11-18.5% for selected, least-anomalous Discovery(+ LOMU)-influenced MORB. Calculated P-initial are fairly constant for N-MORB (similar to18 +/- 2 kbar), slightly greater for Shona-influenced MORB (similar to20 +/- 3 kbar), whereas for least anomalous Discovery(+ LOMU)-influenced MORB calculated P-initial group around similar to16 +/- 2 and similar to22 kbar. Calculated crustal thicknesses range mostly between 6-9 km, but for Shona- and a single Discovery(+ LOMU)-influenced MORB location the range is 8-12 km. Anomalous compositions of some Discovery-influenced MORB are interpreted to reflect variable melting of pyroxenite veins, initially formed as small-degree (2-3%) melts of garnet lherzolite within the upwelling off-axis Discovery mantle plume, and subsequently entrained in the ambient spinel lherzolite beneath the ridge axis. Because of lower solidus temperatures relative to ambient spinel lherzolite, initial melting preferentially consumed the low abundance (2.5-3.5%) pyroxenite veins. Variable mixing between vein- and host mantle-derived melts led to the range of Discovery(+ LOMU)-influenced MORB compositions. In the vicinity of transform offsets, melting is restricted to a narrow pressure interval, and melts are ineffectively homogenized and focused towards segment centres. Beneath slower spreading ridges, such as the southern MAR, reduced heat flux further retards melt homogenization and focusing, resulting in high proportions (less than or equal to 40%) of pyroxenite-derived melt in final magmas erupted up to similar to30-40 km from ridge segment ends, i.e. retaining a strong pyroxenitic source signature. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/ 10.1007/s00410-002-0376-3.