Mid-ocean ridge magmatic processes
The axial graben of the Endeavour Segment of the Juan de Fuca Ridge is heavily faulted due to a period of low magmatic activity while the sea floor continued to spread. Summit Seamount, mapped by the MBARI Mapping AUV (right), erupted just prior to the formation of the modern axial valley and remnant blocks are strewn across the entire width of the axial valley from its dissection by extensional faults. Age dating of sediments on the seamount gives a minimum age for the seamount, and therefore for the beginning of the most recent tectonic phase of Endeavour’s cyclic geologic history.
The ridge segment is currently in a hydrothermal phase while magmatic activity is beginning to increase. The faults farther south on the ridge provide pathways for hydrothermal fluids that vent as black smokers and have built large sulfide chimneys for which the ridge segment is most well known.
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.
Our research on magmatic processes at mid-ocean ridges
Spatial and temporal scales of mantle enrichment
ENDEAVOUR SEGMENT – Major +/- trace element and Sr–Nd–Pb–Hf–He isotope data are presented for more than 300 geo- chemically diverse basalt samples collected by submersible from the Inflated Central Endeavour Segment of the Juan de Fuca Ridge. Seven chemically distinct basalt types are present, from depleted (D-) to enriched mid-ocean ridge basalt (E-MORB). By combining the geochemical data with high-resolution bathymetry and age determinations, the detailed spatial and temporal scale of on-axis mantle-derived basalt heterogeneity is determined. The basalts define binary mixing arrays in all isotope plots that are usual in their correlations, but unusual in the limited range of Sr–Nd–Hf isotope compositions for D- to E-MORB, and greater range in Pb isotopes. The basalts also define two different styles of enrichment of moderately incompatible elements. Geochemical enrichment began when the currently inflated axial ridge formed <105 years ago. One enrichment style (the Inflated Ridge Trend) characterizes basalts erupted across the ~5 km wide ridge from >10,000 to ~4000 years ago, whereas the other enrichment style (the Graben Trend) characterizes most basalt types erupted within the axial graben after it formed ~2300 years ago. We attribute the Inflated Ridge Trend to a relatively high proportion of pyroxenite (or melt derived therefrom) to enriched peridotite in the mantle during a phase of ridge inflation that lasted at least 6000 years. The Graben Trend reflects the reduced effect of pyroxenite after the axial graben formed. Because at least 14 different samplings of mantle components occurred within <1 km of ridge length and width during a time when <1 km of upwelling occurred, we infer that the scale of mantle heterogeneity far from a plume is < 1 km. The enriched mantle component at Endeavour is young with 206Pb/204Pb ~19.0; Hf and He isotope ratios trend toward HIMU characteristics. These traits are regionally widespread and are shared with the next two ridge segments to the north (West Valley and Explorer).
Reference: Gill, J. P. Michael, J. Woodcock, B. Dreyer, F. Ramos, D. Clague, J. Kela, S. Scott, K. Konrad, and D. Stakes (2016) Spatial and Temporal Scale of Mantle Enrichment at the Endeavour Segment, Juan de Fuca Ridge. J. Petrol. 1-33. doi: 10.1093/petrology/egw024.
Xenoliths in Hawaii offer clues to processes at mid-ocean ridges
Hualalai Volcano – The patterns of axial hydrothermal circulation at mid-ocean ridges both affect and are influenced by the styles of magma plumbing. Therefore, the intensity and distribution of hydrothermal alteration in the lower oceanic crust (LOC) can provide constraints on LOC accretion models (e.g., “gabbro glacier” vs. “multiple sills”). Gabbroic xenoliths from Hualalai Volcano, Hawaii include rare fragments of in situ Pacific lower oceanic crust. Oxygen and strontium isotope compositions of 16 LOC-derived Hualalai gabbros are primarily within the range of fresh MORB, indicating minimal hydrothermal alteration of the in situ Pacific LOC, in contrast to pervasive alteration recorded in LOC xenoliths from the Canary Islands. This difference may reflect less hydrothermal alteration of LOC formed at fast ridges than at slow ridges. Mid-ocean ridge magmas from slow ridges also pond on average at greater and more variable depths and undergo less homogenization than those from fast ridges. These features are consistent with LOC accretion resembling the “multiple sills” model at slow ridges. In contrast, shallow magma ponding and limited hydrothermal alteration in LOC at fast ridges are consistent with the presence of a long-lived shallow magma lens, which limits the penetration of hydrothermal circulation into the LOC.
Most Hualalai gabbros have geochemical and petrologic characteristics indicating derivation from Hualalai shield-stage and post-shield-stage cumulates. These xenoliths provide information on the evolution of Hawaiian magmas and magma storage systems. MELTS modeling and equilibration temperatures constrain the crystallization pressures of 7 Hualalai shield-stage-related gabbros to be ∼2.5–5 kbar, generally consistent with inferred local LOC depth. Therefore a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt–crust interaction between Hawaiian melts and in situ Pacific crust during magma storage partially overprinted clinopyroxene Sr and Nd isotope compositions of LOC-derived gabbros. Although minor assimilation of Pacific crust by Hawaiian melts cannot be excluded, the range of oxygen isotope compositions recorded in Hawaiian lavas and cumulates cannot be generated by assimilation of the in situ LOC gabbros, which have relatively uniform and MORB-like δ18O values. To first order, the isotopic heterogeneity observed in Hawaiian melts appears to derive from the heterogeneous plume source(s), rather than assimilation of local oceanic crust.
Reference: Gao, R., J.C. Lassiter, J.D. Barnes, D.A. Clague, W.A. Bohrson (2016) Geochemical investigation of Gabbroic Xenoliths from Hualalai Volcano: Implications for lower oceanic crust accretion and Hualalai Volcano magma storage system. Earth and Plan. Sci. Lett. 442:162-172. doi: 10.1016/j.epsl.2016.02.043.
Geologic history of the Endeavour ridge segment
ENDEAVOUR SEGMENT – High-resolution bathymetric surveys from autonomous underwater vehicles ABE and D. Allan B. were merged to create a co-registered map of 71.7 km2 of the Endeavour Segment of the Juan de Fuca Ridge. Radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle Doc Ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history. The ages range from Modern to 10,700 marine-calibrated years before present (yr BP). During a robust magmatic phase from >10,700 yr BP to ~4300 yr BP, flows erupted from an axial high and many flowed >5 km down the flanks; some partly buried adjacent valleys. Axial magma chambers (AMCs) may have been wider than today to supply dike intrusions over a 2-km-wide axial zone. Summit Seamount formed by ~4770 yr BP and was subsequently dismembered during a period of extension with little volcanism starting ~4300 yr BP. This tectonic phase with only rare volcanic eruptions lasted until ~2300 yr BP and may have resulted in near-solidification of the AMCs. The axial graben formed by crustal extension during this period of low magmatic activity. Infrequent eruptions occurred on the flanks between 2620-1760 yr BP and within the axial graben since ~1750 yr BP. This most recent phase of limited volcanic and intense hydrothermal activity that began ~2300 yr BP defines a hydrothermal phase of ridge development that coincides with the present-day 1-km wide AMCs and overlying hydrothermal vent fields.
Reference: Clague, D.A., B.M. Dreyer, J.B. Paduan, J.F. Martin, D.W. Caress, J.B. Gill, D.S. Kelley, H. Thomas, R.A. Portner, J.R. Delaney, T.P. Guilderson, M.L. McGann (2014) Eruptive and tectonic history of the Endeavour segment, Juan de Fuca Ridge, based on AUV mapping data and lava flow ages. Geochem., Geophys., Geosyst., 15(8): 3364-3391. doi: 10.1002/2014GC005415.
Petrological variability at Axial Seamount
AXIAL SEAMOUNT – A combined study of mapping, observational, age constraint, and geochemical data at the summit of Axial Seamount, Juan de Fuca Ridge, has revealed its recent petrological history. Multiple basalt types erupted at the summit in a time sequence. At least three different magma batches have been present beneath the Axial Summit caldera during the last millennium, each with a range in differentiation. The first, prior to 1100 CE, was compositionally diverse, dominantly aphyric T-MORB. The second, from ∼1220 to 1300 CE, was dominantly plagioclase-phyric, more mafic N-MORB erupted mostly in the central portion of the caldera. Since ∼1400 CE, lavas have been more differentiated, and nearly aphyric T-MORB mostly erupted in the caldera’s rift zones. Parental magmas vary subtly due to small coupled differences in the degree of melting and sources, but all share a uniform differentiation trend indicating pooling at similar depths. Thus, melts percolate through melt-rich lenses that remain partially isolated in space and/or time. Centennial magmatic timescales at Axial Seamount are similar to those for fast spreading ridge segments. The fluctuation between aphyric and plagioclase-phyric lava likely reflects different pathways or velocities of melt migration.
Reference: Dreyer, B.M, D.A. Clague, J.B. Gill (2013) Petrological variability of recent magmatism at Axial Seamount summit, Juan de Fuca Ridge. Geochem., Geophys., Geosyst., 14(10): 4306-4333. doi:10.1002/ggge.20239.
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.