HIGH PRECISION 40AR/39AR AGES OF BASALTIC BASEMENT FROM IODP HOLE U1365E NEAR OSBOURN TROUGH: IMPLICATIONS FOR THE BREAKUP OF ONTONG JAVA NUI

The separation of the Manihiki and Hikurangi plateaus from the larger Ontong Java Nui superplateau via Osbourn Trough is well established, yet the creation of oceanic crust separating these two plateaus during the Cretaceous Normal Superchron (CNS) prevents traditional seafloor magnetic anomaly dating and thus requires radiometric methods. We dated nine samples from IODP Hole U1365E by ⁴⁰Ar/³⁹Ar incremental step heating at Oregon State University Argon Geochronology Laboratory using ARGUS-VI multi-collector mass spectrometry. Groundmass separates for all nine samples and plagioclase separates for four samples throughout 41 m of core were analyzed and yielded preliminary plateau ages ranging from 102.16 ± 0.62 to 108.44 ± 0.29 Ma (2σ errors) at the base of the hole. Two trends are apparent in the age data: five samples in the lowest 12 m of dated core increase in age downwards, while three samples from the upper 22 m of core are roughly coeval at 103 Ma. IODP shipboard geochemical data also show a variation at roughly the same boundary between the upper and lower sections, indicated by lower MgO and higher Zr contents in the latter. The upper dated samples could be from off-axis flows (up to ~255 km) that are superimposed over the lower, older crust associated with oceanic crustal emplacement. The lowest sample with the oldest age is likely a closer approximation to the true emplacement age of oceanic crust at the location of this drill site. This results in a faster spreading rate than previously suggested (~210 mm/yr vs 190 mm/yr full rate) for the basin separating Manihiki and Hikurangi based on a Re-Os date of 103.7 ± 2.3 Ma (Zhang and Li, 2016). According to this older age and assuming separation occurred at a constant rate, cessation of spreading via Osbourn Trough occurred at ~106 Ma. However, this is older than the youngest of these dated samples, and so requires a reduction in spreading rate to <94 mm/yr at some point after 108 Ma in order for the younger flows to have erupted and flowed over the older crust at this drill site. More ages from samples within the lowest portions of this core are needed to confirm this hypothesis.

References

Zhang, G.L. and Li, C., 2016. Interactions of the Greater Ontong Java mantle plume component with the Osbourn Trough. Scientific reports, 6, p.37561.