HR: 1340h
AN: T33B-1379    [Abstracts]
TI: U-Series Disequilibria From the East Pacific Rise (9N, 10N and 11N): evidence for off-axis magmatism along fast spreading ridges?
AU: * Beier, C
EM: cbeier@els.mq.edu.au
AF: GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
AU: Turner, S
EM: sturner@els.mq.edu.au
AF: GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
AU: Niu, Y
EM: yaoling.niu@durham.ac.uk
AF: Department of Earth Sciences, Durham University, Durham, DH1 3LE, United Kingdom
AB: Lavas erupted along Mid-Ocean Ridges provide important information on melt formation and movement beneath the oceanic lithosphere. Despite the fact that the majority of lavas are erupted along the spreading axis itself, it has been proposed that a small quantity of lavas are erupted off-axis at distances >5 km [1]. The East Pacific Rise has a fast (5.5 cm/yr) half spreading rate and so the age of lavas sampled off-axis are well constrained assuming an on-axis origin. Zou et al. [2] tested this assumption by comparing U-series disequilibria in lavas sampled away from the ridge with that observed at the present day axis. They found larger U-Th disequilibria than predicted by decay of the ridge signal in a number of lavas and interpreted this to indicate that these lavas were erupted off-axis. These are important results and so we have analysed U-Th disequilibria in lavas from three traverses along the East Pacific Rise (9°3 N, 10°4N and 11°5N) extending to a maximum distance of 31 km East and West from the ridge axis (corresponding to an age ≤550 ka), including reanalysing the critical samples analysed by Zou et al. [2]. Our analyses of four samples analysed by Zou et al. [2] confirm greater U-Th disequilibria than predicted by decay of the axis signal. However, we could not reproduce the U-excesses in two lavas. Relative to a calculated decay curve assuming an initial (²³⁰Th/²³⁸U) of 1.23 at the ridge, all of our analyses from 9°3N have greater Th excesses (2-6%) than predicted. Similarly, the lavas from the traverse at 10°4N have (²³⁰Th/²³⁸U) ratios up to 1.067 (6% Th excess) some 13 km from the ridge axis which is comparable to ratios of 1.042 (4% Th excess) 23 km distant from the ridge at 9°3N. Thus, lavas in a zone 13-30 km off- axis to both sides of the axis have greater Th excess than predicted by decay from the ridge. In contrast, we have found U excesses in lavas from the traverse at 11°5N. ²²⁶Ra data will be presented for those samples with disequilibria in excess of that predicted by decay of the axis signature. If the observed Th excess result from melting at the ridge axis, anomalously high initial (²³⁰Th/²³⁸U) ratios are required at the ridge (≥2). This would require unrealistically small degrees of partial melting and very slow melting and inferred upwelling rates. Dynamic melting models show that even <2% partial melting at an upwelling rate of 0.5cm/a produces initial (²³⁰Th/²³⁸U) ratios that are too low (~1.6) to explain the off-axis disequilibria. Moreover, the inferred upwelling rate is too low to be consistent with a half spreading rate of 5.5 cm/a. Small degree melts may be consistent with the off-axis eruption and be largely independent from the melting regime beneath the ridge axis. The frequent occurrence of off-axis ²³⁰Th excess at two different traverses at similar distance from the axis (5-20 km) may provide evidence that bending stresses open up tensile cracks to provide ascent paths for melts [1]. [1]R.A. Sohn, K.W.W. Sims, Bending as a mechanism for triggering off-axis volcanism on the East Pacific Rise, Geology (Boulder) 33(2005) 93-96. [2]H. Zou, A. Zindler, Y. Niu, Constraints on melt movement beneath the East Pacific Rise from 230Th-238U disequilibrium, Science 295(2002) 107-110.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 9355 Pacific Ocean
SC: Tectonophysics [T]
MN: 2007 Fall Meeting