On the 40Ar/39Ar Dating of Low-Potassium Ocean Crust Basalt from IODP Expedition 349, South China Sea

Abstract:

Accurate age dates for the basement rocks in the South China Sea (SCS) basins were lacking before the execution of International Ocean Discovery Program (IODP) Expedition 349 in early 2014. This left a large margin of error in estimated opening ages for the SCS and rendered various hypotheses regarding its opening mechanism and history untested, hampering our understanding of East Asian tectonic and paleoenvironmental evolution. Therefore, high-precision ⁴⁰Ar/³⁹Ar age dating lies at the heart of Expedition 349, which in particular aimed to determine the timing of the start and cessation of seafloor spreading in the SCS. In addition, the recovery of a complete seamount apron section at Site U1431 allows ⁴⁰Ar/³⁹Ar dating of abundantly present plagioclase and biotite crystals to help establish a detailed chronology of the sedimentary and volcaniclastic sequences cored.

Here we present the first ⁴⁰Ar/³⁹Ar incremental heating ages on the low-potassium (~0.1-0.2 wt% K₂O) and the least altered (loss on ignition < 1.5%) mid-ocean ridge basalt (MORB) from the SCS. Plagioclase and groundmass samples were prepared using conventional mineral separation techniques, acid-leaching and hand-picking. Analyses were carried out using a new ARGUS-VI multi-collector noble gas mass spectrometer. Ages are expected to have precisions ranging between 0.1-0.3 Ma (2σ), which will allow us to precisely and accurately date the final emplacement of basalts at Sites U1431, U1433 and U1434 in the SCS basin, just prior to the cessation of spreading as all sites were slightly offset from the relict spreading center.