Abstract
Researcher Ridge (RR) is a 400km long, WNW-ESE oriented chain of
volcanic seamounts, located on ~20 to 40 Ma old oceanic
crust on the western flank of the Mid-Atlantic Ridge (MAR) at
~15°N. RR remained nearly unstudied, and thus its age
and origin are currently unclear. At roughly the same latitude, the MAR
axis is bathymetrically elevated and produces geochemically enriched
lavas (the well-known 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 RR
and along the MAR between 13-14°N. The results reveal that RR lavas have
geochemically enriched ocean island basalt (OIB) compositions
([La/Sm]N=1.7-5.0, [Ce/Yb]N=1.58-11.3) with isotopic signatures
(143Nd/144Nd = 0.51294-0.51316, 206Pb/204Pb = 19.14-19.93, 176Hf/177Hf =
0.28307-0.28312) trending to or overlapping the ubiquitous FOZO (Focal
Zone, e.g., Hart et al., 1992, Science 256) mantle composition. Major
and trace element characteristics denote that RR lavas formed by small
degrees of melting from a deep source in the garnet stability field and
experienced high pressure fractionation beneath a lithospheric lid.
Although the sparseness of samples suitable for 40Ar/39Ar dating
prevents establishing a clear age progression for the seamount chain,
one well constrained basalt groundmass age of 28.75 ± 0.14 Ma (2σ) for
one seamount near the western end of RR indicates that this volcano
formed ~11 Ma later than the underlying lithosphere.
Taken together, RR is interpreted as a hotspot track, albeit formed by a
relatively weak melting anomaly. Compared to RR, the lavas from the 14°
N MAR anomaly have slightly less enriched compositions, exhibiting
enriched (E)-MORB compositions ([La/Sm]N=1.81-2.29). Their isotopic
ratios largely overlap with the RR compositions, thus suggesting a
genetic relationship. We therefore propose that the enigmatic 14°N MAR
anomaly is caused by deflection of upwelling RR plume material towards
the approaching (westward migrating) MAR, causing the production of
E-MORBs with nearly similar isotopic compositions to the RR lavas. Once
the plume was captured by the spreading ridge, off-axis hotspot track
volcanism ceased, resulting in a 300 km wide gap of seamount formation
between the eastern end of RR and the MAR.