Magmatic segmentation of the northern Knipovich Ridge: Evidence for high-pressure fractionation at an ultraslow spreading ridge

The ultraslow spreading Knipovich Ridge shows axial topographic highs that are associated with off-axis linear arrays of seamounts that parallel the spreading direction. These linear arrays suggest that the segmentation has been stable for at least 7-8 m.y. Axial topographic highs are marked with distinct gravity anomalies, suggesting that they represent thicker crust and hence volcanic segment centers. Dredging on the northern Knipovich Ridge between 76 degrees 30' and 77 degrees 50'N shows that the axial and off-axial seamounts, and the deeper axial rift valley between the segment centers, are composed of volcanic rocks. The basalts are subdivided into high-, intermediate-, and low-FeO groups. Low-FeO basalts, with generally more enriched radiogenic isotope ratios and REE patterns, were only dredged midway between segment centers, whereas dredges close to the segment centers comprised high- and intermediate-FeO basalts. The variations in isotopes and REE patterns suggest that the mantle below the northern Knipovich Ridge is heterogeneous, and the along-axis variations can be explained by higher degrees of melting of this heterogeneous source close to segment centers. The high- and intermediate-FeO basalts define different MgO-FeO liquid lines of descents that may reflect differences in the crystallization pressures or the water contents of the magmas. Similar water contents of the high- and intermediate-FeO basalts suggest that the differences in FeO primarily are a pressure effect and that the intermediate- FeO group fractionated at higher pressure than the high- FeO group. The high- FeO basalts show both low- and high-pressure phenocryst assemblages, suggesting crystallization at high pressures (6-7 kbar) followed by shallow, low-pressure fractionation. On the basis of the new data we propose that magmas at this ultraslow spreading ridge fractionate at mantle depth, except at segment centers where magma productivity occasionally is high enough to sustain shallow level magma reservoirs.