HR: 0800h
AN: V51C-1512    [Abstracts]
TI: Geochemistry of Basalt Lava and Hyaloclastite From Young (President Jackson) and old (Taney) Near-ridge Seamount Chains
AU: * Davis, A S
EM: davisa@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 9644 United States
AU: Clague, D A
EM: clague@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 9644 United States
AU: Paduan, J B
V51C-1512 AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 9644 United States
AB: Short linear chains of seamounts are common near Pacific spreading centers. They are most abundant near fast spreading centers but also occur at moderate and slow spreading ridge segments. We explored two such near-ridge chains of vastly different ages. The young President Jackson chain, located at 42°20' N west of the northern Gorda Ridge, resides on ocean crust that is 2.2 to 4.2 million years old. An Ar-Ar age of ~26 Ma (G.B. Dalrymple as cited by Davis et al., 1998) for the Taney seamounts indicates they formed during the Miocene on the later subducted Farallon-East Pacific Rise spreading center. Three dives with MBARI's ROV Tiburon explored the eastern end of the President Jackson seamounts. Two dives explored two of the eastern seamounts of the Taney chain. Volcanoes in both chains are similar steep-sided, flat-topped structures with multiple, nested, overlapping calderas. The Taney seamounts, however, are considerably larger with volumes roughly three times that of the typical seamount in the President Jackson chain. Samples of glassy rimmed pillow lava and hyaloclastite were recovered from both chains, although those from the Taney chain are covered with thick manganese oxide crusts. The hyaloclastite samples from both sites consist of dense angular glass shards compositionally identical to pillow rims, suggesting they formed by quench granulation. One of the President Jackson Seamount samples is loosely consolidated ash, containing fluidal and bubble wall fragments, indicating some mildly explosive eruptions occurred. Glass data for the eastern President Jackson seamounts are all low-K₂O N-MORB (0.04-0.13% K₂O). Two distinct groups can be identified. One more depleted composition with exceedingly low K₂O and MgO >8% and the second, overlapping group includes somewhat more evolved (7.3 -8.3% MgO) and slightly more enriched (>0.1-0.15% K₂O) compositions. Both compositions erupted on a single volcano. Glass compositions of dive samples from the Taney Seamounts are also all N-MORB although whole rock samples dredged from the westernmost seamount include E-MORB and an alkalic basalt. The N-MORB glasses from the Taney's also include high MgO (>8%) and low K₂O (0.05%) compositions, but many are more evolved (MgO 6.4-7.6%) with slightly higher K₂O (0.1-0.15%). They are higher in Na₂O at comparable MgO than the President Jackson glasses, possibly suggesting melting at greater depth. Like the President Jackson seamounts, both highly depleted and more enriched compositions exist on the same volcano. Similar to other near-ridge seamount chains, the President Jackson glass compositions are generally high-MgO, relative to basalts from the adjacent Gorda Ridge (Davis and Clague, 1987, 1990). Because of the primitive compositions found on most near ridge seamounts, the prevailing view has been that magma bypasses magma chambers under the spreading center and erupts without storage in shallow crustal reservoirs. The large calderas require magma storage at shallow depths within the volcanoes. As suggested by Clague et al. (2002), these collapse features probably formed by a series of voluminous eruptions of magma that rapidly passed through the crustal reservoirs. The >8% MgO lavas may represent those erupting rapidly whereas the slightly lower MgO lava resided briefly in the crustal reservoir where they cooled and fractionationated to some extent.
DE: 1021 Composition of the oceanic crust
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1036 Magma chamber processes (3618)
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005