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

Abstract

[1] The ultraslow spreading Knipovich Ridge shows axial topographic highs that are associated with offaxis 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°30 0 and 77°50 0 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, lowpressure 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.

Crustal structure of the ultra-slow spreading Knipovich Ridge, North Atlantic, along a presumed ridge segment center

“…It is known that the volcanic most productive areas are typically regarded as the segment centers (see Cannat et al 1995;Batiza 1996). In the case of Knipovich Ridge these topographic highs are associated with mantle Bouguer gravity minima and have been interpreted to represent areas with enhanced volcanic activity and thicker crust (Crane et al 2001a, b;Okino et al 2002;Hellevang and Pedersen 2005). This is consistent with new bathymetry data showing that young axial volcanic ridges define these centers.…”

“…At the central and northern Knipovich Ridge, these axial topographic highs are associated with off-axis linear arrays of seamounts that are parallel to the flow line. These linear arrays suggest that the segmentation has been more or less stationary for at least 7-8 m/year (Hellevang and Pedersen 2005). The eastern flank of Knipovich Ridge is more subsided (*300 m) with respect to its western counterpart, and this is usually attributed to the sedimentary load accumulated from the Svalbard Margin and Barents Sea (Crane et al 1991;Faleide et al 1996;Fiedler Fig.…”

“…The first line was acquired 40 km north of a topographic high occurring at *76°30 0 N. Based on the bathymetry available at this time we believed that underlying crust could be amagmatic. However, a study by Hellevang and Pedersen (2005) demonstrated the magmatic nature of the entire ridge segment. The first line, acquired along an assumed amagmatic portion of of oceanic crustal formation, was presented by Kandilarov et al (2008).…”

Crustal thickness and earthquake distribution south of the Logachev Seamount, Knipovich Ridge

“…They have a distinct rift valley with scattered inner axial highs, which are related to focused volcanism. The rift valley can be divided into magmatic and amagmatic segments [Michael et al, 2003;Jokat et al, 2003;Hellevang and Pedersen, 2005]. The amagmatic segments seem to correlate with a massive exposure of mantle peridotites along the Gakkel Ridge [Michael et al, 2003;Standish et al, 2008].…”

“…to the east [Crane et al, 2001;Kandilarov et al, 2008], while other models assume constant full spreading rates of 15-17 mm yr À1 [Okino et al, 2002;Dick et al, 2003;Hellevang and Pedersen, 2005].…”