A “high ⁴He/³He” mantle material detected under the East Pacific Rise (15°4′N)

Abstract

We investigate in details helium isotope data reported in Mougel et al. (2014) for 14 basaltic samples collected on the East Pacific Rise by submersible (15°4′N) where the ridge interacts with the Mathematician seamounts. Samples locations are separated by only few hundred meters across a 15 km along-axis profile. The data reveal a strong geochemical variability that has never been observed at such high spatial resolution for helium isotope compositions. Moreover, they reveal an unusually high 4 He/ 3 He mantle component also characterized by unradiogenic lead, atypical in oceanic basalts. He-Pb systematics suggests a mixture between a nonradiogenic lead and radiogenic helium pyroxenitic component, recycled from the deep continental lithosphere and the ambient peridotitic mantle. The He isotope difference between these two end-members can be interpreted as a time evolution of two distinct mantle sources after a slight (U + Th)/ 3 He fractionation, likely due to some ancient degassing during the formation of deep continental pyroxenites.

Detailed Analysis of Near Tectonic Features Along the East Pacific Rise at 16°N, Near the Mathematician Hot Spot

“…It is the broadest and shallowest segment of the entire northern EPR with an unusually prominent along-axial-ridge, up to 15 km wide at its base and up 750 m high with respect to the adjacent abyssal hills (e.g., Baker et al, 2001;Carbotte et al, 2000;Carlut et al, 2004;Le Saout et al, 2014;Scheirer & Macdonald, 1995;Weiland & Macdonald, 1996). This morphology has been attributed to the influence of a hot spot (e.g., Carbotte et al, 2000;Carlut et al, 2004;Scheirer & Macdonald, 1995;Weiland & Macdonald, 1996), named "Mathematician hot spot" (Le Mougel et al, 2014Mougel et al, , 2015. A relationship between these features is supported by (1) the presence of the Mudskipper volcanic seamount ridge, located less than 5 km away from the ridge axis, on the western flank of the 16°N axial rise (e.g., Baker et al, 2001;Carlut et al, 2004;Macdonald et al, 1992;Weiland & Macdonald, 1996); (2) the location of the shallowest part of the dome, at 2,200 m depth, near the projected intersection of the Mudskipper volcanic chain with the ridge axis (15°42 0 N); (3) a geochemical anomaly identified in the axis-lavas that indicates a mixing between a MORB and a more enriched and heterogeneous source associated with the hot spot magmatism (Mougel et al, 2014(Mougel et al, , 2015; and (4) the presence of extinct spreading axes east of the ridge axis, indicating at least two westward jumps of 7 and 9 km in the direction of the hot spot at 150 and 300 kyr (e.g., Carbotte et al, 2000;Shah & Buck, 2006;Weiland & Macdonald, 1996).…”

High‐Resolution Isotopic Variability Across EPR Segment 16°N: A Chronological Interpretation of Source Composition and Ridge‐Seamount Interaction

“…In line with the work of Donnelly (2002), and Mougel et al. (2014, 2015), Mallick et al. (2019) confirmed that at least three components are necessary to account for the chemical and isotopic variability of the northern EPR: a depleted mantle, an enriched mantle, and an ancient gabbroic source (converted into pyroxenites).…”

“…To the west, the volcanic structures are attributed to the presence of a small heterogeneous hotspot (i.e., “Mathematician Hotspot”) responsible for the formation of the “Western plateau,” the “Northern Seamounts” and the “Mudskipper Seamounts” (Donnelly, 2002; Langmuir et al., 1998), while the origin of the “Eastern Seamounts” still remains uncertain. Finally, the tectonic and magmatic complexity of this region is also recorded in the geochemical composition of basalts from the 16°N segment, which displays a unique and remarkable isotopic variability (Donnelly, 2002; Mallick et al., 2019; Mougel et al., 2014, 2015).…”

“…For example, Mougel et al. (2014, 2015) demonstrated that the isotopic variability recorded over a single 10 km portion of an EPR segment, sampled every 200 m, can be equivalent to that of the entire EPR database (∼9,000 km), and yet reveal the existence of peculiar mantle source signatures.…”

Geochemical Variability Along the Northern East Pacific Rise: Coincident Source Composition and Ridge Segmentation

“…The high abundance of plagioclase is consistent with the low Ce/Pb and U/Pb the latter resulting in low 206 Pb/ 204 Pb isotopic compositions. As noticed by Donnelly (2002) and Mougel et al (2014Mougel et al ( , 2015 this component is different than any of the named mantle endmembers; they argue for a metagabbroic pyroxenite.…”