Abstract
ABSTRACTUltraslow spreading ridges are poorly understood plate boundaries consisting of magmatic and amagmatic segments that expose mostly mantle peridotite and only traces of basalt and gabbro. The slowest part of the global spreading system is represented by the eastern Gakkel Ridge in the Central Arctic Ocean, where crustal accretion is characterized by extreme focusing of melt to discrete magmatic centers. Close to its eastern tip lies the unusual 5,310‐m‐deep Gakkel Ridge Deep (GRD) with limited sediment infill, which is in strong contrast to the broader sediment‐filled rift valleys to the east and west. Here, we report an 40Ar/39Ar age of 3.65 ± 0.01 Ma for a pillow basalt from a seamount located on the rim the GRD, confirming ultraslow spreading rates of ~7 mm/year close to the Laptev Sea as suggested from aeromagnetic data. Its geochemistry points to an alkaline lava, attributed to partial melting of a source that underwent prior geochemical enrichment. We note that the GRD extracts compositionally similar melts as the sparsely magmatic zone further west but at much slower spreading velocities of only ~6−7 mm/year, indicating the widespread occurrence of similarly fertile mantle in the High Arctic. This enriched source differs from subcontinental lithospheric mantle that influences magmatism along the Western Volcanic Zone (Goldstein et al., 2008, https://doi.org/10.1038/nature06919) and is similar to metasomatized mantle, shown to influence melt genesis along the Eastern Volcanic Zone.
Highlights
The Gakkel Ridge extending for 1,800 km under the Arctic ice is the slowest spreading ridge on Earth with estimated full spreading rates varying from ~13 mm/year in the west to only ~6 mm/year in the east (Brozena et al, 2003; Karasik, 1968; Vogt et al, 1979)
Based on the first systematic geophysical/petrological study along most of the Gakkel Ridge between 7°W and 85°E (Jokat et al, 2003; Michael et al, 2003) the Gakkel Ridge was divided into three tectonic segments, composed of robust Western (WVZ) and Eastern (EVZ) Volcanic Zones separated by a “Sparsely Magmatic Zone” (SMZ) (Figure 1)
Lavas with low Mg# are rare along the Gakkel Ridge but occur subordinate in the EVZ and E‐SMZ (Mg# ~39 to 49 at MgO = 4.5 to 5.9 wt%)
Summary
The Gakkel Ridge extending for 1,800 km under the Arctic ice is the slowest spreading ridge on Earth with estimated full spreading rates varying from ~13 mm/year in the west to only ~6 mm/year in the east (Brozena et al, 2003; Karasik, 1968; Vogt et al, 1979). Because it contains no significant transform offsets that can act as boundaries to mantle flow and composition, it is an ideal place to study mantle source heterogeneity and melting dynamics at ultraslow spreading rates and the processes contributing to the composition of the Arctic mantle (Dick et al, 2003; Goldstein et al, 2008; Michael et al, 2003; Schlindwein & Schmidt, 2016). Based on the first systematic geophysical/petrological study along most of the Gakkel Ridge between 7°W and 85°E (Jokat et al, 2003; Michael et al, 2003) the Gakkel Ridge was divided into three tectonic segments, composed of robust Western (WVZ) and Eastern (EVZ) Volcanic Zones separated by a “Sparsely Magmatic Zone” (SMZ) (Figure 1). Basalts were recovered by dredging almost exclusively in this segment (Figure 1d), carrying a pronounced magnetic anomaly of up to 500 nT
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