Abstract
The ridge crest at 9°N–10°N East Pacific Rise (EPR) is dominated by overlapping lava flows that have overflowed the axial summit trough and flowed off‐axis, forming a shingle‐patterned terrain up to ∼2–4 km on either side of the axial summit trough. In this study, we employ 230Th‐226Ra dating methods, in conjunction with geochemistry and seafloor geological observations, in an effort to discern the stratigraphic relationships between adjacent flows. We measured major and trace elements and 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, and 238U‐230Th‐226Ra for lava glass samples collected from several flow units up to ∼2 km away from the axial summit trough on the ridge crest at 9°50′N EPR. Statistical analysis of the 238U‐230Th‐226Ra data indicates that all but one measured sample from these flows cannot be resolved from the zero‐age population; thus, we cannot confidently assign model ages to samples for discerning stratigraphic relationships among flows. However, because groups of samples can be distinguished based on similarities in geochemical compositions, particularly incompatible element abundances with high precision‐normalized variability such as U and Th, and because the range of compositions is much greater than that represented by samples from the 1991–1992 and 2005–2006 eruptions, we suggest that the dive samples represent 6–10 eruptive units despite indistinguishable model ages. Geochemical variability between individual flows with similar ages requires relatively rapid changes in parental melt composition over the past ∼2 ka, and this likely reflects variations in the relative mixing proportions of depleted and enriched melts derived from a heterogeneous mantle source.
Highlights
All measured lava samples are low-K, tholeiitic, incompatible element-depleted normal mid-ocean ridge basalts (MORB) (N-MORB; K2O/TiO2 Â 100 < 11; Smith et al [2001]), but dive 3974 and 3963 samples extend to slightly higher values of K2O/TiO2 Â 100 (6.6–10.8 6 14.1% (2)) compared to previously measured samples from the 1991–1992 (5.8–9.6; Sims et al [2002]) and 2005–2006 eruptions (6.1–8.9; Goss et al [2010])
Individual spots were analyzed with an accelerating voltage of 15 kV, a beam current of 10 nA, a beam diameter defocused to 20 mm to avoid Na loss during analysis, and a count time of 20 s for all elements except for F, S, and Cl, which were counted for 60, 40, and 40 s, respectively
We used a maximum likelihood, double-error regression to fit a zero-age trend line [Sohn and Menke, 2002] to a data set including previous data obtained on samples collected within the axial summit trough from 9370–9540N East Pacific Rise (EPR) [Sims et al, 2002] and data obtained in this study for samples collected from flows associated with the 2005–2006 eruption (Figure 6b)
Summary
Tivey [1] The ridge crest at 9N–10N East Pacific Rise (EPR) is dominated by overlapping lava flows that have overflowed the axial summit trough and flowed off-axis, forming a shingle-patterned terrain up to $2–4 km on either side of the axial summit trough. We employ 230Th-226Ra dating methods, in conjunction with geochemistry and seafloor geological observations, in an effort to discern the stratigraphic relationships between adjacent flows. Because groups of samples can be distinguished based on similarities in geochemical compositions, incompatible element abundances with high precision-normalized variability such as U and Th, and because the range of compositions is much greater than that represented by samples from the 1991–1992 and 2005–2006 eruptions, we suggest that the dive samples represent 6–10 eruptive units despite indistinguishable model ages.
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