Abstract:
Volcanic accretion at the fast-spreading East Pacific Rise (EPR) occurs over a ~2-4 km
wide neo-volcanic zone on either side of the axial summit trough (AST). Eruption ages
are critical for understanding the distribution and timing of volcanic and magmatic
activity. Uranium series nuclides are susceptible to fractionation by magmatic processes
that occur beneath mid-ocean ridges, and the half-lives of 226Ra (1.6 kyrs) and 230Th (75
kyrs) make them ideally suited for determining eruption ages and placing constraints on
eruption frequency and temporal changes in magma chemistry. Accordingly, major and
trace element, and long-lived radiogenic and 238U-230Th-226Ra isotope compositions were
measured in basalts from 9º-10ºN EPR to determine eruption ages and to place temporal
constraints on volcanic and magmatic processes.
At 9º30’N EPR, 238U-230Th-226Ra compositions indicate that trace elementally and
isotopically enriched mid-ocean ridge basalt (MORB) collected off-axis erupted >8 ka
and that E-MORB magmatism is interspersed with normal, depleted MORB magmatism.
Lava ages are consistent with eruption from the AST and flow down the ridge flanks,
which is in contrast to previous studies that suggested E-MORB erupted from off-axis
vents. At 9º50’N EPR, discrete eruptive units are distinguished by high precision 238U, 232Th,
and 226Ra sample concentrations, but because the resolution of the 230Th-226Ra model age
dating technique is ~±1 kyrs, the surprisingly young ages of these lavas prohibit the
construction of an explicit, time-constrained lava stratigraphy. Nonetheless, seven
different flows identified within 0.8-2.0 km west of the AST imply greater frequency of
flows to these distances than previously recognized.
Model age dating of ferrobasalts, basaltic andesites, andesites, and dacites sampled from
the east limb of the overlapping spreading center at 9º03’N EPR is difficult due to
uncertainties in magma residence times. However, (226Ra/230Th) disequilibria indicate
recent basaltic volcanism (<<8 ka) up to ~4 km off-axis. The axial graben at the rise crest
sources the most recent volcanic activity and is the dominant location for eruption of
high-silica magmas. Major element, trace element, 87Sr/86Sr, and (234U/238U) isotope
compositions are consistent with the formation of dacite magmas by extensive
crystallization, and 238U-230Th-226Ra systematics imply crustal residence times of ~8 kyrs.
