Bottom up: Diffuse porous flow OK, prefer Wark et al. (for now) field evidence

1. Top down: MORB composition MORB focusing MORB ascent rate Arc composition Arc focusing Hotspot flux, comp, focusing Bottom up: Diffuse porous flow OK, prefer Wark et al. (for now) field evidence Melting & diapirs Magma fracture Focused porous flow Sills & lenses at “top”

2. J. Geol. 79 Marsh, J Geol 1979, suggested that buoyant, partially molten diapirs rising through denser, less melted peridotite, could explain some aspects of arc volcanism.

3. The buoyant, partially molten diapir idea was applied to mid-ocean ridges by Nicolas & Violette, Tectonophysics 1982. With this diagram, they suggested that such diapirs occur in the middle of ridge segments, with radial flow of the mantle along axis as well as along the plate spreading direction.

4. R = gd2/(U0), U0 spreading rate,  shear viscosity, d “length scale” here, Spiegelman implicitly varied  but changing permeability controls (z) and has a similar effect 0.3 1.0 With this figure from Spiegelman, EPSL 1996, I emphasize that the difference between “passive, plate drive, 2D mantle flow” and “diapiric, buoyant, 3D upwelling” is determined via the competing effects of melt buoyancy and mantle viscosity. First, note that the color scales in the two diagrams are dramatically different, with a maximum value ≥ 0.6% on the left, and a maximum value three times larger, ≥ 1.8%, on the right. Where the porosity is high or the mantle viscosity is low, buoyant diapirs may form, whereas at low porosity and/or high mantle viscosity, passive flow is dominant. Spiegelman probably varied the viscosity to produce these two different realizations, but he could as easily have varied porosity at constant viscosity. Note that the porosities near the base of the melting column beneath the ridge (left side of both diagrams) are higher in the buoyancy driven case, because solid upwelling velocities (and melting rates) are higher. This kind of feedback between viscosity, upwelling rate, and porosity can make the transition from passive to buoyancy driven upwelling very sharp.

5. The first numerical model of the ridge axis diapir concept was developed by Michel Rabinowicz in collaboration with Nicolas and co-workers. These diagrams from Nicolas & Rabinowicz 1983, and Rabinowicz et al., EPSL 1984 show the model setup and results. They assumed that melt buoyancy produced a 1% density contrast in a triangular region at shallow depth near the ridge axis. For melt with a density of 3000 kg/m^3 in peridotite with a density of 3300 kg/m^3, a 1% density contrast corresponds to a melt fraction of 10% in this entire region. They also assumed that a semi-circular region beneath the ridge axis, with a radius of about 30 km had a low viscosity, of 10^18 Pa s (10^19 Poises). While the low viscosity in this region was justified as a consequence of high melt porosity in upwelling peridotite, the low viscosity region extends beyond the melt-rich, buoyant region; frankly, I am not sure why. A half spreading rate of 0.05 m/yr was used; such rates are typical for the fast spreading East Pacific Rise. As a result of the chosen parameters, a relatively narrow zone of buoyancy driven upwelling forms beneath the spreading ridge.

6. At about the same time as the Rabinowicz et al EPSL 1984 paper, Whitehead et al. Nature 1984 proposed that buoyant diapirs fed ridge magmatism, and controlled the spacing of mid-ocean ridge segmentation.

7. MAR fracture zones SWIR & AAR axis Dick J Geol Soc 1989 Whitehead et al Nature 1984 SWIR & AAR fracture zones SWIR axial valley walls dredge hauls on fracture zones richer in peridotite; dredge hauls in axial valley richer in basalt basalt SWIR fracture zones One of the main lines of evidence for magmatic segmentation along ridges, advanced by Whitehead et al., Nature 1994, was the paucity of peridotite dredges at the center of ridge axis segments, and the abundance of peridotite together with relatively rare lavas and gabbros, along fracture zones at segment ends. This observation was more fully quantified by Dick, J Geol Soc London 1989. peridotite

8. Synthesis diagram of Dick, J Geol Soc London 1989, summarizing dredging observations and the diapiric mantle upwelling hypothesis, forming magmatic segments along slow spreading mid-ocean ridges. Here, Henry Dick implicitly proposes that the diapiric upwelling of partially molten peridotite, or melt alone (the figure is intentionally or unintentionally ambiguous), rises in discontinuous “blobs” rather than in a continuous bouyant column of the type modeled by Rabinowicz et al.

9. gravity & seismic support for hypothesis that crustal accretion is focused at ridge segment centers Tolstoy et al. Science 1993 Lin et al. Nature 1990 There is abundant evidence for focused melt accretion near the centers of ridge segments. Lin et al., Nature 1990 provided the clearest picture to date of “gravity bull’s eyes”, where gravity corrected for topography and crustal thickness shows “Residual Mantle Bouger” anomalies indicative of thick crust and/or hot mantle near spreading segment centers and comparatively thin crust and/or cold mantle near segment ends at transform faults or “non-transform offsets”. Tolstoy et al., Science 1993 provided the first quantitative seismic data showing an increase in crustal thickness from a segment end to a segment center. A much more complete and recent seismic data set showing the same type of observation is available in the paper by Hooft et al., JGR 2000.

10. Mid-Atlantic Ridge East Pacific Rise The dramatic difference in bathymetry between slow spreading and fast spreading ridges is exemplified by these two maps of the seafloor at the slow spreading Mid-Atlantic Ridge and the fast spreading East Pacific Rise. The “lumpy” morphology of slow spreading crust is often taken to indicate long term focusing of crustal accretion at ridge segment centers, and perhaps substantial time dependence in magma supply, whereas the far more regular bathymetry along the East Pacific Rise is taken to indicate less focused supply of melt from the mantle, spread more evenly along (longer) ridge segments, and/or more efficient along axis transport of crust forming melt by dikes and melt lenses within the crust.

11. These figures, from Lin & Phipps Morgan, GRL 92, were used to emphasize that evidence for focused crustal accretion in the middles of ridge segments, and associated gravity variations, were primarly observed at slow spreading ridges. This is in accord with the idea that the rate of buoyancy driven upwelling in the mantle is a bit faster than, e.g., 1 cm per year but substantially slower than 10 cm per year, so that the effects of buoyancy driven upwelling should be most evident at ridges with a half spreading rate less than or equal to ~ 1 cm/year such as the Mid-Atlantic Ridge.

12. Lin & Phipps Morgan GRL 1992

13. Lin & Phipps Morgan GRL 1992

14. Lin & Phipps Morgan, GRL 1992 proposed (or reinforced) the idea that bouyant diapirs are important features at slow spreading ridges, but “passive upwelling” in response to plate driven flow is dominant at fast spreading ridges.

15. To some extent, this influential paper by Dunn & Toomey, Nature 97, seemed to contradict the idea that focused crustal accretion toward the center of ridge segments would be confined mainly to slow-spreading ridges. They found low seismic velocities in elliptical regions within the shallow mantle beneath the fast-spreading East Pacific Rise, apparently indicating the presence of systematic variation in the proportion of melt within the shallow mantle, which in turn was taken to indicate that some 3D focusing mechanism controlled melt input to the base of oceanic crust. In this view, the lack of along strike variation in crustal thickness, and resulting lack of gravity variation, results from efficient along strike transport of melt in the crust from segment centers to segment ends, perhaps in a continuous, seismically imaged shallow melt lens.

16. ? ? ? NIcolas et al, Marine Geophys. Res., 1988, and Ceuleneer & Rabinowicz, AGU Monograph 1992, emphasized evidence from the (fast-spreading?*) Oman ophiolite for diapiric mantle upwelling focused in 3D toward ridge segment centers as interpreted from regional geological mapping. This view has become very influential. These papers, however, made some predictions which have been difficult to confirm in later work. First, melt transport features from within the mantle upwelling column (red) should be transposed and emplaced off axis by diapiric corner flow. In general, this has not been confirmed in extensive mapping of melt transport features in the Oman upper mantle section by Ceuleneer and co-workers (next few slides). Further, the diapiric flow model predicts gradual variation in the dip of foliation and the plunge of lineation, from vertical, through dips toward the spreading ridge and horizontal dips, to dips away from the ridge. Again, this has not been observed in the Oman “diapirs”. *: The spreading rate of the Oman ophiolite was inferred from a variety of different geological evidence; in my view, some of this evidence is more persuasive than other parts. For example, the observation that Oman igneous crust is extensive, and has roughly constant thickness for long distances along strike, is well documented. Slow spreading ridges such as the Mid-Atlantic, SW Indian Ocean, and Gakkel (Arctic) Ridges rarely if ever have continuously thick, gabbroic lower crust over distances of a few 100 km, and instead commonly have regions with exposed, residual mantle peridotite on the seafloor. From this perspective, the spreading rate during formation of the Oman ophiolite was probably intermediate (like the Juan de Fuca Ridge) or fast (East Pacific Rise). Tilton et al., JGR 1981, presented U/Pb zircon ages for tonalite and trondjhemite (“plagiogranite”) plutons intruding the crust, showing an approximate west to east age progression that could be interpreted in terms of a fast spreading rate. However, the uncertainties of the individual ages were so large that the spreading rate based on these data could have almost any value. Also, since the tonalite plutons intrude gabbroic crust, their ages could be substantially younger than the gabbroic oceanic lower crust, and the progression of ages need not correspond to a spreading rate. The inference of spreading rate based on the extent of depletion in residual mantle peridotite is probably not very persuasive.

17. Detailed structural map of the Oman shallow mantle and lower crust in the well known “Maqsad diapir” region of the Samail massif, from Jousellin et al. JGR 98

18. Synoptic cross section of the Maqsad diapir, from Jousselin (pers comm 2007) based on extensive work by the Montpellier group.