Spreading Dynamics of an Intermediate Ridge: Endeavour Segment of the Juan de Fuca Ridge
Abstract
U/Th disequilibria analyses of 36 on- and off-axis MORB samples from the Endeavour segment of the Juan de Fuca Ridge, an intermediate spreading ridge off the Pacific Northwest coast of the USA, reflect recent spreading and time-integrated geochemical variability. Previous major and trace element and isotopic data from Endeavour samples exhibit a wide range of geochemical characteristics for samples within close spatial proximity, including EMORB, TMORB and NMORB. Morphology of Endeavour constrains lavas erupted on-axis to flow within the current axial valley, preventing axial lavas from flowing off-axis. This relationship can be used to date any MORB found outside of the axial valley, assuming all types lie on a single zero-age line from the most depleted to most enriched lavas. U-Th data indicate that all EMORB and the majority of TMORB are zero-age (<10 ka) within error and lie on a single zero-age line. EMORB with equivalent (within error) young ages on both sides of the axial valley and that span across the entire eastern flank of the ridge erupted prior to the formation of the current axial valley and within a short time interval (<10 ky). If the axial valley formed within the last 10 ky, the minimum full spreading rate falls around 8 cm/yr. If the majority of EMORB are <8 ka, the axial valley may have formed at a full spreading rate of 10 cm/yr or greater. These U-Th age constraints on EMORB yield a spreading rate faster than the time-integrated spreading rate (5 cm/yr). An EMORB sample located furthest east of the axis is at least 70 ky younger than expected based on the time-integrated spreading rate (5 cm/yr). One on-axis TMORB is not within error of the E-T zero-age line, and may be as old as 17 ka. U-Th ages of TMORB from the western flank fall between 25 and ~100 ka, older than all EMORB. NMORB generally have higher Th isotope ratios than E- or TMORB for a given U/Th ratio and lie on a second zero-age line. One NMORB, on the far western flank, is ~40 ky older than expected based on the time-integrated spreading rate, while another NMORB located farther out on the western bathymetric flat is also older than expected (in secular equilibrium, >300 ka). Two on-axis NMORBs are not within error of the NMORB zero-age line, and may be as old as 27 ka. Anomalously old on-axis NMORB and TMORB may reflect a structural horst block within the axial valley, where zero-age lavas are not able to flow. Presence of anomalously old lavas west of the axis and anomalously young lavas east of the axis indicates asymmetry in recent volcanic construction. U-Th-Ra disequilibria will confirm the timing of EMORB volcanism and formation of the current axial valley, as well as confirm zero-age relationships for different lava types. Ultimately, U-series data offer insight into intermediate spreading dynamics and spatial and temporal relationships between variable lavas types.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V11E2555S
- Keywords:
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- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 8416 VOLCANOLOGY / Mid-oceanic ridge processes