Mid-ocean ridge magmatic processes

Magmatic processes inferred from the erupted rocks

Escanaba Trough is a 130 km long basin in the southern part of the Gorda Ridge, bounded in the south by the Mendocino Fracture Zone. The rift-valley floor is filled with thick sequences of sediments derived from the continent, fallout from the great floods that carved the Columbia Gorge in the Pleistocene. Lava has intruded as sills into the sediments, uplifting broad hills. Some assimilation of the sediment into the intruded lava has changed its chemical composition. This contamination offers insight into the extent to which processes that occur as lava ascends through the crust, as opposed to original mantle source heterogeneity, influence normal mid-ocean ridge basalt composition.

Escanaba Trough, Gorda Ridge
Map © MBARI 2001

Our research on magmatic processes at mid-ocean ridges

The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.

Magmatic evolution on the northern Gorda Ridge

GORDA RIDGE - High-density, precisely located, dive and rock-corer basalt samples from the 65-km-long North Gorda ridge segment reveal compositional diversity as great as for the entire Gorda Ridge. Lava compositions along the ridge axis show considerable major and minor element diversity (MgO 9.2–4.4%, K₂O 0.04–0.36%) for lavas erupted in close proximity. Although they form a near-continuum in the higher MgO range, the samples can be separated into two groups; one is typical N-type mid-ocean ridge basalt (MORB) (K₂O/TiO₂ < 0.09), and the other is a more enriched T-MORB (K₂O/TiO₂ > 0.09). Incompatible elements also reflect this grouping with (Ce/Yb)_N < 1 and Zr/Nb > 20 for N-MORB and (Ce/Yb)_N > 1 and Zr/Nb < 20 for T-MORB.

Samples collected from off-axis, over a distance of 4 km up the eastern rift valley wall, are all light rare earth element (LREE)-depleted N-MORB with a narrower compositional range (MgO of glasses 7.7 ± 0.3%, Zr/Nb = 38–50; (Ce/Yb)_N < 1), although isotopic ratios are comparable to those on-axis. Lavas erupted in the past, before the present-day deep axial valley formed on this part of the ridge, were more uniform N-MORB, generated by larger degrees of melting when magma supply was greater. Basalts from the adjoining southern Juan de Fuca Ridge segment, with comparable spreading rate but distinctly different ridge morphology, are also all LREE-depleted N-MORB, but the narrow range of evolved compositions of the sheet flows covering the broad, U-shaped valley suggests shallower, more steady state magma reservoirs underlying this ridge segment. Basalts from Escanaba, the slowest spreading segment of Gorda Ridge, include N-, T-, and E-MORB that were erupted from isolated volcanic centers.

The pattern of incompatible element enrichment, especially in LREE, K, Ba, and ⁸⁷Sr/⁸⁶Sr, with decreasing spreading rate and magma supply is even more pronounced at the ultraslow spreading Arctic ridges where most lavas are E-MORB (Zr/Nb < 10, (Ce/Yb)_N >1.0–3.0). Arctic E-MORB compositions lie along a common mixing trend with those from North Gorda. As the magma budget and/or partial melting decreases, a similar enriched component, especially in K, Ba, and LREE, widely present in the oceanic mantle is apparently incorporated to a greater degree. At North Gorda, morphology and chemical characteristics appear to evolve with time toward that of ultraslow spreading ridges.

Reference: Davis, A. S., D. A. Clague, B. L. Cousens, R. Keaten, J. B. Paduan (2008) Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply, Geochemistry Geophysics Geosystems, 9, Q04004, doi:10.1029/2007GC001775. [Abstract] [Article]

Sediment contamination of magma

ESCANABA TROUGH – For continental volcanoes, contamination of magmas by components in the crust is easily identified, but the extent of contamination of mid-ocean ridge basalts (MORBs) by oceanic crust is a topic of intense debate. The Escanaba Trough, the southernmost segment of the Gorda Ridge, contains thick terrigenous sediments derived from floods through the Columbia River basin following the melting of Pleistocene glaciers. MORBs erupted in Escanaba Trough have greater isotopic and geochemical variability than those from the remainder of the ridge, providing a large signal-to-noise ratio with which to model the extent to which crustal processes, such as ascent, storage in magma chambers and emplacement, as opposed to heterogeneous mantle sources, may cause variability in the relatively uniform composition of mid-ocean ridge basalts.

Samples from sediment-free northern sites in the trough are depleted normal-MORB similar to those from the northern Gorda Ridge. However, samples from the southern, sediment-covered portion of the trough are significantly enriched in elements incompatible with crystals typically found in basalt. Sr and Nd isotopic ratios are shifted toward the composition of local sediments, and Pb isotopic ratios, in particular, lie on a mixing line with local sediments. Assimilation-fractional crystallization (AFC) calculations suggest that 25-40% fractional crystallization coupled with assimilation of 1-2% of local sediments can produce the enrichment observed for some Escanaba basalts. The extent of assimilation was variable, and the assimilants were compositionally heterogeneous. Assimilation of sediment by MORB magmas probably resulted when laccolithic intrusions ponded at the interface between the basement and the thick sediment cover in the southern part of the trough. The strong compositional contrast between the depleted parental magma and the continentally-derived sediment assimilant allows geochemical and isotopic detection of contamination in these lavas and provides a sensitive indicator for subtle changes to oceanic magma by crustal processes.

Reference: A.S. Davis, D.A. Clague, and W.M. White (1998) Geochemistry of basalt from Escanaba Trough: evidence for sediment contamination, Journal of Petrology, 39(5): 841-858. [Abstract]

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Questions? Comments? Please contact Jenny Paduan
Last updated: May. 08, 2008