Mantle flow between the southern EPR and eastern South Pacific Superswell: Geochemical
constraints from the Rano Rahi seamounts and Pukapuka ridges
P.E. Janney, L.S. Hall, J.J. Mahoney and J.M. Sinton
The southern East Pacific Rise between 14 and 19°S exhibits marked physical asymmetries
between its eastern and western flanks. The western flank is shallower for a given crustal age
than the eastern, and has an extraordinary concentration of volcanic ridges and seamounts that is
unmatched on the eastern flank. Additionally, results of the MELT experiment have shown that
mantle characteristics in the southern EPR area are strongly asymmetrical. Shear-wave velocities
are significantly lower at a given distance from the ridge crest on the western flank, suggesting
the presence of small amounts of interstitial melt in the upper mantle as far as several hundred km
west of the southern EPR. A possible eastward flow of hotspot-derived mantle from the western
Pacific to the southern EPR was suggested by isotopic characteristics of axial lavas (Mahoney et
al., EPSL, 1994; Phipps Morgan et al., JGR, 1995), and Toomey et al. (EPSL, 2002) have
proposed that the physical asymmetries also can be explained by eastward flow of hot material
derived from hotspots in the South Pacific Superswell region, located 3000-5000 km to the west
of the EPR axis.
Geochemical data for seamount and volcanic ridge lavas recovered from the Rano Rahi
volcanic field and the Pukapuka ridges (hereafter termed the Rano Rahi volcanic province),
located on the western flank of the southern EPR as far as 2600 km from the ridge axis, provide
important constraints on the likelihood of such mantle flow. Compared to near-ridge seamounts
from the northern and central EPR, the Rano Rahi province lavas display, on average, higher
Na2O and Al2O3 contents at a given MgO content, but they have essentially identical mean
CaO/Al2O3 values. This suggests that the Rano Rahi province lavas have unusually fertile mantle
sources, consistent with shallow eastward flow of fertile plume-derived material from the
Superswell region toward the southern EPR. The Rano Rahi province lavas range from strongly
incompatible-element-depleted to more highly incompatible-element-enriched than yet found for
any other near-ridge seamount lavas in the Pacific. Similarly, the Rano Rahi province lavas
extend to more radiogenic Sr and Pb isotope ratios than any other Pacific near-ridge seamounts
yet analyzed. The samples with the greatest incompatible element enrichment and greatest
isotopic affinity to hotspot-type ocean island basalts formed farthest from the EPR, up to 400 km
(Hotu Matua) or 1000-1300 km (western Pukapuka ridges) west of the ridge axis.
Importantly, the isotopic compositions of the Rano Rahi province lavas are consistent
with binary mixing between a Pacific MORB-source end-member and a component having
87
Sr/86Sr ≈ 0.7035, 206Pb/204Pb ≈ 19.8 and Nd ≈ 5 (Janney et al., EPSL 2000). Isotopic
compositions very similar to this are present in the Marquesas, Society and Pitcairn volcanic
chains, and this composition is within the range of values inferred for the proposed “Focus Zone”
(Hart et al., Nature 1992) and “Common” (Hanan and Graham, Science 1996) mantle components
argued to represent lower mantle or transition-zone material entrained by mantle plumes. Thus,
the available geochemical data for the Rano Rahi volcanic province indeed support the hypothesis
that plume material is being advected eastward from the Superswell region toward the southern
EPR.