Age And Origin Of Researcher Ridge And An Explanation For The 14 Degrees N Anomaly On The Mid-Atlantic Ridge By Plume-Ridge Interaction

Researcher Ridge is a 400 km long, WNW-ESE oriented chain of volcanic structures, located on similar to 20 to 40 Ma old oceanic crust on the western flank of the Mid-Atlantic Ridge (MAR) at similar to 15 degrees N. Researcher Ridge has been little studied, and its age and origin are currently unclear. At roughly the same latitude (14-15 degrees N), the MAR axis is bathymetrically elevated and geochemically enriched (hereafter referred to as the 14 degrees N MAR anomaly). This study presents Ar-40/Ar-39 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 degrees 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 (Nd-143/Nd-144 = 0.51294-0.51316,Sr-87/Sr-86 = 0.70266-0.70405, (206)pb/(204)pb = 19.14-19.93, Pb-207/Pb-204 = 15.57-15.63, Pb-208/Pb-204 = 38.82-39.17, and Hf-176/Hf-177 = 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 degrees 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 (Nd-143/Nd-144 = 0.51298-0.51313, Sr-87/Sr-86 = 0.70250-0.70282, (206)pb/(204)pb = 18.90-19.31, (207)pb/(204)pb = 15.52-15.58, (208)pd/(204)pd = 38.45-38.95, Hf-176/Hf-177 = 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 degrees 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 degrees N MAR anomaly. (C) 2019 Elsevier B.V. All rights reserved.