FY 1999

Magmatism at mid-ocean ridges: Constraints from volcanological and geochemical investigations

Perfit, M.R., and W.W. Chadwick, Jr.

Geophysical Monograph 106, 59–115, doi: 10.1029/GM106p0059, In Faulting and Magmatism at Mid-Ocean Ridges, W.R. Buck, P. Delaney, J.A. Karson, and Y. Lagabrielle (eds.), AGU, Washington, D.C. (1998)

The morphological, structural, and volcanic characteristics of the neovolcanic zone at mid-ocean ridges (MOR) vary strongly with spreading rate. At fast-spreading ridges, the neovolcanic zone is narrow, has low-relief both across and along strike, is dominated by the products of fluid, fissure-fed eruptions, and exhibits morphologic and magmatic continuity along axis. At slow-spreading ridges, the neovolcanic zone is wider, has greater relief, is characterized by many discrete point-source constructs, and exhibits less morphologic and magmatic continuity along axis compared to fast-spreading ridges. Intermediate-spreading ridges typically have characteristics that vary in time and space between these two extremes. Lava flow morphology also varies markedly with spreading rate--sheet flows are dominant on fast-spreading ridges whereas pillow lavas are dominant at slow-spreading ridges. The morphological differences primarily reflect a difference in extrusion rates, and indicate that dikes are intruded at higher magma pressure at fast-spreading ridges. Even though volcanism appears to be concentrated within the neovolcanic zone, off-axis eruptions add significant volumes to the crust. Off-axis volcanism may be fed by the distal sections of magma lenses or, in the case of long-lived, near-axis seamounts, from magma sources that are independent of sub-axial magma bodies. The timing, locations, and volumes of volcanic events on the MOR are still largely unknown, but the documentation of recent eruptions have provided new insights and the first quantitative information regarding active volcanic processes on the ridge-crest. Documentation of historical eruptions has been realized by some good luck and detailed surveying of the neovolcanic zone along the southern Juan de Fuca Ridge (JdFR) and northern East Pacific Rise (EPR), but the most recent eruptions have been detected in real-time by listening with hydrophones for acoustic T-waves that are generated by small earthquakes during shallow crustal intrusive/extrusive events. Narrow grabens have formed adjacent to some of the new lava flows, where dikes have intruded near the surface. Similar dike-induced graben faulting has also been documented on rift zones of subaerial volcanoes.

Fine-scale mapping and sampling of a few neovolcanic zones and their adjacent crestal terrains, coupled with geochemical investigations and U-series radiometric dating, have provided critical information regarding the time and spatial scales of MOR magmatism. These more accurate and precise sampling and dating techniques have allowed us to better quantify rates and volumes of magmatic events and to evaluate if changes in mid-ocean ridge basalt (MORB) chemistry are temporally or spatially related (or both). ²¹⁰Po-²¹⁰Pb systematics have been successfully used to date and confirm young eruptions (age < 2 yr). New techniques for dating young MORB by mass spectrometric measurement of ²³⁸U-²³⁰Th, ²³⁰Th-²²⁶Ra, and ²³⁵U-²³¹Pa disequilibria have been successful, but show that samples from neovolcanic zones yield ages (on the order of a few ka) that must be considered "crustal residence ages" rather than true ages of eruption. Along the 9°-10°N segment of the EPR, U-Ra dates show a regional trend consistent with axial variations in topography, axial magma chamber depth and extent of magmatic fractionation which allow constraints to be placed on crystallization rates and construction of the oceanic crust. The identification of anomalously young lavas up to 4 km off-axis on the northern EPR using U-series disequilibria data, also indicates a significant amount of magmatic activity occurs off-axis and that this volcanism can result in the observed thickening of seismic layer 2A (the layer of the oceanic crust that is assumed to be composed of extrusive lavas based on seismic wave velocities and seismic reflection profiles). The chemical diversity and non-systematic distribution of lava types and ages observed on a small-scale (<600 m) across the axis of the EPR may reflect rapid changes in magma chemistry that occur during crystallization and replenishment in small magma lenses coupled with the effects of frequent low-volume eruptions both within and outside of the axial summit trough.

Although significant differences in sources and melting parameters have been shown to control the compositions of MORB on a regional/global basis, local chemical variations appear primarily to be controlled by fractional crystallization and magma mixing at shallow levels (0.5-15 km). In conjunction with detailed seafloor mapping, compositional data and quantitative models have been used to examine the volumes, rates of eruption and ranges of geochemical variation within several individual lava flows recently erupted on the MOR. Although all the flows show some degree of chemical variability, sheet flows on the northern EPR and JdFR exhibit greater homogeneity than pillow flows from the JdFR and northern Mid-Atlantic Ridge. The available data from recent eruptions are consistent with a model in which fast-spreading ridges have frequent, relatively homogeneous, small-volume eruptions whereas slow-spreading ridges have infrequent, more heterogeneous, larger eruptions. The fundamental influence on the variables that change with spreading rate along the MOR system seems to be whether or not a steady-state magma reservoir can be sustained at a given location. Where magma supply is continuous and robust, volcanic output dominates over tectonic process; where the supply is intermittent, tectonism may dominate. The critical spreading rate between steady-state and non-steady-state magma reservoirs is approximately 50 mm/yr.