First data on composition of the NW Pacific Oceanic Lithosphere exposed along the Stalemate Fracture Zone.

The Stalemate Fracture Zone (FZ) is a 500 km long SE-NW trending transverse ridge between the northernmost late-Cretaceous Emperor Seamounts and the Aleutian Trench. Sampling at the Stalemate FZ and the fossil Kula-Pacific Rift valley was carried out during the R/V SONNE cruise SO201 Leg 1b in 2009 in frame of Russian-German project KALMAR. We present in this work results on mineralogical composition of mantle peridotite dredged at the station DR37 from the northernmost  part of the Stalemate FZ on border with Aleutian Trench. The composition of primary minerals were analyzed using an electron microprobe (major elements) and  LA-ICP-MS (trace elements). This data was used to characterize the mantle melting process and the residue lherzolite formation from the Stalemate FZ. 

The results suggest that the lherzolites dredged at SFZ are mantle residues after 17-20% of near-fractional melting of mantle peridotite. The mantle melting started in the garnet stability field at a depth more than 100 km (about 10%) and than continue in the spinel stability field (7-10%). The high degree of melting in the garnet stability field are uncommon for typical abyssal peridotite and indicate a high-grade mantle source. The close location of Emperor Ridge to the Stalemate Fracture Zone suggest the influence by Hawaiian Plume on the peridotites exposed along the Stalemate Fracture Zone and on the melting process that took place 100 Ma ago. 

The strongly altered dunites are light red rocks with rare (<3%) macroscopically visible spinel relicts. The petrographic examination of these rocks revealed a texture typical of altered dunites. However, the main mineral of the altered dunites is quartz rather than serpentine. Serpentinized lherzolites of the Stalemate Fracture Zone show evidence for moderate temperature oceanic metamorphism. It should be pointed out that some samples of serpentinized herzolites contain quartz (or amorphous silica), the content of which is much lower than that in the altered dunites. Secondary alteration of the peridotites included serpentinization and also silicification of the dunites caused strong enrichment of the rocks in fluid mobile elements (U, Li, Sb, Ba). The enrichment of amorphous silica and quartz and unusually high SiO2 (up to 88.7 wt %) and low MgO (up to 1.4 wt %) clearly distinguish these rocks from the known products of the hydrothermal alteration and low-temperature weathering of peridotites in the oceanic crust. There are two type of evolution: submarine and subaerial. In order to determine which of the two environments was responsible for the silicification of the Stalemate Ridge peridotites we used a thermodynamic model in the GEOCHEQ program package. The model data indicate also that the geochemical and mineralogical effects observed in the silicified dunites of the Stalemate FZ are most similar to the expected results of the low-temperature alteration of oceanic serpentinites under subaerial conditions. 

Data briefly presented above allowed us to reconstruct the formation conditions of the basement rocks and to interpret their tectonic evolution. Spinel lherzolite–dunite assemblage recognized in Stalemate FZ is typical for spreading centers characterized by moderate spreading rate.