2-D modeling, simplistic

1. 2-D modeling, simplistic Boundary layer thicknesses in dunite and host peridotite. Illustration from Morgan et al., in prep. 2007

2. Case 1. No flow across interface Early Time Steady State For flow parallel to dunite-harzburgite contacts, boundary layers are expected on both sides of the contact, with a wider boundary layer within the peridotite where advective flow is slower.

3. Case 2. Flow from dunite Early Time Steady State For flow out of dunites, a thick chemical boundary layer is predicted in the host peridotite, with essentially no boundary layer in the dunite. Illustration from Morgan et al., in prep. 2007

4. Case 3. Flow into dunite Early Time Steady State For flow into dunites, a chemical boundary layer is predicted on both sides of the contaact, but - unlike the case of flow parallel to the contact - in some cases the chemical boundary layer is predicted to be wider in the dunite than in the host peridotite

5. Josephine ophiolite mineral chemistry returning to the example from the Josephine peridotite, the presence of a chemical boundary layer that is wider in the dunite than in the host rock suggests that melt was flowing from the host rock into the dunite.

6. Chemical transects across dunites of varying sizes from the Samail massif of the Oman ophiolite. Braun & Kelemen, in prep. 2007. Here, spinel TiO2 content is illustrated.

7. Chemical transects across dunites of varying sizes from the Samail massif of the Oman ophiolite. Braun & Kelemen, in prep. 2007. Here, molar Cr/(Cr+Al), or Cr#, in spinel is illustrated.

8. Correlated variation of Ni in olivine and Ti in spinel in dunites and host peridotites from the Samail massif of the Oman ophiolite. Wadi Tayin data are for more regional samples, mostly depleted, residual mantle peridotites far from large dunites.

9. Chemical composition of mantle dunites in the Oman ophiolite varies with dunite width.

10. Chemical boundary layers in dunites from the Samail massif of the Oman ophiolite occur mainly within the dunites, again perhaps indicative of melt flow dominantly from the host peridotite into the dunite.

11. In many ophiolites with highly depleted residual mantle peridotites (harzburgites) dunites apparently record melt flow from host peridotites into dunite. This is true for the Josephine peridotite example developed here, for the Bay of Islands dunite data of Suhr et al. (G-cubed, 2003), and in the Samail massif dunite data of Braun & Kelemen, in prep. 2007.

12. Trinity ophiolite Abundant dunites in the China Mountain area of the Trinity ophiolite were mapped in detail by Quick, CMP 1981. Kelemen et al., Nature 1992, made detailed transects across dunite-peridotite contacts and reported on REE contents in cpx from samples in these transects. Morgain et al., in prep. 2007, resampled some transects in more detail, and analyzed major element variation in minerals.

13. spinel clinopyroxene orthopyroxene Chemical boundary layer around the Eunice Bluff dunite in the Trinity peridotite show chemical boundary layers mainly in the peridotite host rocks, not in the dunites. Interestingly, chemical boundary layers are in different positions for different elements, with a strong gradient on Ti in cpx, opx and spinel apprent 8 to 20 m from the dunite contact, and a gradient in Ni content in olivine evident from 2.5 to 5 m from the same contact. olivine

14. Publshed and unpublished data on trace element variation in cpx as a function of distance from dunite contacts for three other transects. Variation of concentrations of highly incompatible elements Nd and Zr differs markedly from one transect to another, whereas variation of moderately incompatible elements Ti and Yb is essentially the same in all transects.

15. Detailed view of Ni in olivine variation across the Eunice Bluff sample transect. Open symbols show calculated original olivine compositions, before possible re-equilibration of olivine, opx and cpx from igneous to metamorphic temperatures. See Kelemen et al., EPSL 1998, for effect of temperature on Ni partitioning between olivine, opx and cpx. Morgan et al., in prep. 2007

16. Case 2. Flow from dunite Early Time Steady State Thus, unlike the other examples from the Josephine, Bay of Islands, Horoman and Oman peridotites, it seems that melt flowed outward from Trinity dunites into the surrounding peridotite. This is likely related to impingment of dunite conduits on the base of the thermal plate, with crystallization downstream driving a drop in permeability, which in turn would cause divergent, increasingly diffuse melt flow near the top of conduits. It could be that such processes produced the Trinity plagioclase lherzolites by refertilization or “impregnation” of previously highly depleted, residual mantle peridotite. Note that - not very well documented - compositions of peridotites far from dunites seem to support this idea, because they are more depleted (less refertilized?) than peridotites within the chemical boundary layers around dunites. The idea that lherzolites commonly represent refertilized, previously depleted mantle peridotites was suggested for plagioclase lherzolites in the Horoman peridotite massif by Saal et al., J Petrol 2001, and more recently for plagioclase lherzolite in the Lanzo peridotite massif (Piccardo et al EPSL 2007) and lherzolites in the Lherz massif (LeRoux et al EPSL 2007) . Moregan et al., in prep. 2007.

17. Lherzolite subtype ophiolite In summary, dunites in lherzolite-rich peridotite massifs show evidence for melt flow both inward and outward. Outward flow may be related to refertilization of previously depleted peridotites.

18. lungs Finally, we will look at size-frequency statistics for dunites in mantle peridotite. Kelemen et al., Nature 1995 and JGR 1995, together with Aharonov et al., JGR 1995, proposed that dunites formed a coalescing network of conduits for focused porous flow of melt, with many small conduits feeding a few large ones. Kelemen et al., G-cubed, 2000 elaborated on this idea, suggesting that coalescing channel networks generally show power law relationships between frequency and flux, and - where flux is related to channel width - a power law relationship between frequency and width. In this illustration, they demonstrated this relationship for a proposed model of the arterial network in mammalian lungs.

19. Using a blimp to supend a camera about 7 meters above outcrops, Kelemen et al., G-cubed 2000, mapped dunite networks in the mantle section of the Ingalls ophiolite in the Washington Cascades

20. Resulting photo-mosaic map of dunites (black) in residual mantle peridotites from the Ingalls ophiolite. The full resolution image has mm scale resolution over 100’s of square meters.

21. Power law relationship of frequency to width for dunites in the Ingalls peridotite (left) and extrapolated to predict the frequency of ten to 100 m wide dunites in the outcrop area of the Wadi Tayin massif mantle section in the Oman ophiolite. Armed with this result (from Kelemen et al., G-cubed, 2003) we set out to make similar observations over a broader range of scales in the Oman ophiolite.

22. Photomosaic from the Muscat massif in the Oman ophiolite, showing tan dunites in brown residual peridotites.

23. Peter Kelemen Greg Hirth Photomosaic from the Muscat massif in the Oman ophiolite, showing tan dunites in brown residual peridotites.

24. 100 m thick dunite Air photo of part of the Muscat massif in the Oman ophiolite, showing tan dunites in brown residual peridotites. Dashed square shows approximate area of photomosaic in previous slide. Black areas to N and E are the Gulf of Oman beyond the coast line. Figure from Braun & Kelemen, G-cubed 2002.

25. Size-frequency statistics for dunites within residual mantle peridotite in the Muscat, Samail and Wadi Tayin massifs of the Oman ophiolite. Upper right panel shows results of photo interpretation, which together yield a power law relationship over five orders of magnitude. Large, lower left panel shows a single transect measured with tape and compass in the Samail massif, which also shows a power law size-frequency relationship over more than four orders of magnitude. Braun & Kelemen, G-cubed 2002.

26. lungs By analogy with such networks as the mammalian arterial network, the dunite size-frequency results are consistent with the hypothesis that the mantle dunites form a coalescing network with many small channels feeding a few large ones.

27. Braun & Kelemen, G-cubed 2002, also used the dunite size-frequency data to constrain the proportion of melt flowing through large dunites, in order to determine whether the dunite network was sufficient to carry the flux of magma that formed the oceanic crust in the ophiolite. We assumed that the frequency of dunite channels of a given flux had a power law relationship with flux, as is the case for coalescing fluid flow networks. We then combined this assumption with the observed power law relationship between frequency and dunite width, to yield a power law relationship between flux and width.

28. Calculating the total magma flux ~100 m Braun & Kelemen, G-cubed, 2002 Using our derived relationship between dunite width and flux, and constraints from diffusivity of, e.g., SiO2 in melt, we investigated whether the observed dunite network in the Oman ophiolite could carrry sufficient flux, via porous flow of melt in dunites wide enough to preserve disequilibrium between migrating melt and surrounding, residual mantle peridotite. We found that flux through a porous channel network with the observed size-frequency relationship of Oman dunites would be more than sufficient to form oceanic crust, provided that the largest dunites had a melt porosity exceeding about 2%.

29. Based on our results, we proposed that observed mantle dunites in the Oman ophiolite are dismembered, transposed relicts of a coalescing network of high porosity conduits formed by dissolution of pyroxene from peridotite during ascent of olivine-saturated melt through the melting region beneath a mid-ocean ridge. Figure from Braun & Kelemen, G-cubed 2002.

30. As I will discuss in more detail in Part 3, it has been proposed that high porosity channels for focused flow of melt can form at the base of the conductive boundary layer in oceanic plates, with diagonal migration of melt toward the ridge axis beneath a permeability barrier formed by crystallization of cooling melt in pore space. If so, perhaps our interpretation of mantle dunites as having formed deep within the melting region beneath ridges is partly or completely incorrect. Instead, could some or all dunites form in a shallow dipping orientation coincident with the proposed focused flow of melt along the base of the thermal plate? Left hand figures from Spiegelman, Phil Trans Roy Soc London, 1993. Right hand figures from Rabinowicz & Ceuleneer, EPSL 2006.

31. My tentative answer is that few if any all mantle dunites in the Oman ophiolite formed in a sub-horizontal orientation. If they had, we would still expect to see vertical conduits carrying olivine-saturated melt from depth, through shallow residual mantle peridotites (more on this in Part 3). Thus, if some dunites formed in a sub-horizontal orientation, others would have formed in steep orientations far from the ridge axis. One would not expect these steep, off-axis dunites to be transposed by corner flow, and so they should preserve high angle orientations with respect to the crust-mantle boundary, the paleo-seafloor, and the hypothetical sub-horizontal dunites. Instead, in Oman, dunites are almost all transposed parallel to the crust-mantle transition and the paleo-seafloor, and dunites rarely if ever show high angle intersections. Illustration of a possible melt transport network from Sohn & Sims, Geology 2005.