ALBERT

Description: Highlights: • Researcher Ridge, a chain of volcanic seamounts in the central Atlantic is identified as a classical hotspot track; • The underlying small mantle plume is believed to get captured by the westward migrating Mid-Atlantic Ridge; • The bathymetric/geochemical anomaly of the Mid-Atlantic-Ridge at 14° N can therefore be explained by plume-ridge interaction. Abstract: Researcher Ridge is a 400 km long, WNW-ESE oriented chain of volcanic structures, located on ~20 to 40 Ma old oceanic crust on the western flank of the Mid-Atlantic Ridge (MAR) at ~15° N. Researcher Ridge has been little studied, and its age and origin are currently unclear. At roughly the same latitude (14–15° N), the MAR axis is bathymetrically elevated and geochemically enriched (hereafter referred to as the 14° N MAR anomaly). This study presents 40Ar/39Ar age data, major and trace elements, and Sr-Nd-Pb-Hf isotopic compositions of volcanic rocks dredged from several seamounts of the Researcher Ridge. In addition, new geochemical data of MORBs from two 13–14° N dredge sites on the MAR are also presented. The results reveal that Researcher Ridge lavas have geochemically enriched ocean island basalt compositions (chondrite-normalized [La/Sm]N = 1.7–5.0, and [Ce/Yb]N = 1.58–11.3) with isotopic signatures (143Nd/144Nd = 0.51294–0.51316, 87Sr/86Sr = 0.70266–0.70405, 206Pb/204Pb = 19.14–19.93, 207Pb/204Pb = 15.57–15.63, 208Pb/204Pb = 38.82–39.17, and 176Hf/177Hf = 0.28307–0.28312) trending towards the FOZO or HIMU mantle end member composition. Based on the new age and geochemical data, Researcher Ridge is interpreted as a classic hotspot track, albeit formed by a relatively weak melting anomaly. The lavas from the 14° N MAR anomaly have an enriched E-MORB type composition ([La/Sm]N = 1.81–2.29, [Ce/Yb]N = 1.6–3.9). Their isotopic compositions (143Nd/144Nd = 0.51298–0.51313, 87Sr/86Sr = 0.70250–0.70282, 206Pb/204Pb = 18.90–19.31, 207Pb/204Pb = 15.52–15.58, 208Pb/204Pb = 38.45–38.95, 176Hf/177Hf = 0.28315–0.28320) plot at the enriched end of the local MORB array and partly overlap the Researcher Ridge lava compositions, suggesting a genetic relationship. We propose that the 14° N MAR anomaly is caused by deflection of upwelling Researcher Ridge plume material towards the westward migrating MAR, causing the production of E-MORBs with similar isotopic compositions to the Researcher Ridge lavas. Once the plume was captured by the spreading ridge, off-axis hotspot track volcanism ceased, resulting in a seamount gap between the eastern end of the Researcher Ridge and the 14° N MAR anomaly.