Oceanic plateaus are formed by a large volume of basaltic rocks on top of the oceanic lithosphere. Alteration of these basalt lava piles leads to significant chemical element exchanges between mantle and oceans that can strongly influence chemical budget. Here we report boron (B), chlorine (Cl), and other element concentrations in basalt samples from the Shatsky Rise to define alteration processes and to estimate the significance of oceanic plateaus in storing these elements. Sampling includes 121 basaltic lavas and 92 fresh glasses collected at various depths from Holes U1346A, U1347A, U1349A, and U1350A during Integrated Ocean Drilling Program Expedition 324. Loss on ignition (LOI) results indicate that alteration affected basalts from the summit sites (U1346 and U1349) more deeply than those from the flank sites (U1347 and U1350). The positive correlations between B, K, and LOI observed in the basalts indicate that low-temperature seawater-derived alteration was the predominant process affecting Shatsky Rise basalts. This is confirmed by the elevated B/K and modest Cl/K ratios of these altered basalts relative to the fresh glasses. In addition, B concentrations in the summit basalts (~132 ppm) are significantly higher than those in normal altered oceanic crust and are likely related to the presence of illite. This suggests that the Shatsky Rise oceanic plateau may be an important sink for B in the Pacific oceanic crust.

The eruptive history of the Shatsky Rise, a large oceanic plateau in the northwestern Pacific Ocean, is poorly understood. Although it has been concluded that the Shatsky Rise volcanic edifices erupted rapidly, there are few solid chronological data to support this conclusion. Similarly, the Shatsky Rise is thought to have formed near the equator, but paleolatitude data from the plateau are few, making it difficult to assess its plate tectonic drift with time. To understand the formation history of this oceanic plateau, paleomagnetic measurements were conducted on a total of 362 basaltic lava samples cored from the Shatsky Rise at 4 sites (U1346, U1347, U1349, and U1350) during Integrated Ocean Drilling Program Expedition 324. Examining changes in paleomagnetic inclinations, we gain a better understanding of eruptive rates by comparison of observed shifts in inclination with expected paleosecular variation. At three sites (U1346, U1347, and U1349) little change in paleomagnetic directions was observed, implying that the cored sections were mostly erupted rapidly over periods of <~100–200 yr. Only Site U1350 displayed directional changes consistent with significant paleosecular variation, implying emplacement over a period of ~1000 yr. The paleomagnetic data are consistent with the idea that the Shatsky Rise igneous sections were mostly emplaced rapidly, but there were some time gaps and some fl ank locations built up more slowly. Because paleosecular variation was inadequately sampled at all the Shatsky Rise sites, paleolatitudes have large uncertainties, and because of the equatorial location, magnetic polarity is also uncertain. All sites yield low paleolatitudes and indicate that the Shatsky Rise stayed near the equator during its formation. Given that the locus of magmatism moved northward relative to the Pacific plate while staying near the equator, the Pacific plate must have drifted southward relative to the spin axis during the emplacement of the plateau.