Paleointensity results from the Jurassic: New constraints from submarine basaltic glasses of ODP Site 801C

Abstract

Tholeiite of the oldest oceanic crust was drilled during ODP Legs 129 and 185 at Hole 801C in the western Pacific. Fresh appearing submarine basaltic glass (SBG) was recovered from the tholeiites (∼167 Ma) which has been shown to be nearly ideal for determining absolute paleointensity. Paleointensities of the younger, off‐axis, alkalic basalts (∼160 Ma), overlying the tholeiites, had been studied earlier. Here we report results from the older tholeiitic (on‐axis) sequence. We subjected a total of 73 specimens from 17 cooling units to absolute paleointensity experiments. Of these, 30 specimens and six cooling unit averages met our strictest reliability criteria, yielding an average of 11.9 ± 3.9 μT. The bulk of evidence suggests a paleolatitude of the site of 14°S (with an uncertainty of 10°). This translates the intensity to a value for the virtual axial dipole moment of 28 ZAm2, slightly lower than values determined from the plagioclase crystals in the three cooling units of the younger alkalic basalts overlying the tholeiites (Tarduno & Cottrell, 2005). Our value is low when compared to the long‐term median value of the field of 42 ZAm2. Our results and those of the published literature therefore support the contention of a low magnetic field strength in the Jurassic (average of 28 ± 14 ZAm2;N = 138 individual estimates), as initially suggested by Prévot et al. (1990). Our interpretation of the body of available data argue for low field strengths for the entire Jurassic extending into the early Cretaceous.