Interpretation of Mantle Upwelling and Diapirism in Ophiolite Structures

1. Jousselin et al. JGR 98 interpreted their structural data in terms of a highly focused cylindrical column of ascending, partially molten mantle, turning over and flowing radially outward in a narrow region - just a few km thick - immediately beneath the igneous crust.

2. diapir model of Jousselin et al., JGR 1998 It is worthwhile to show this concept in true scale compared to the synoptic cross-section on which the Jousselin et al. interpretation is based. Clearly, the field evidence does not provide a strong constraint on the vertical extent or shape of focused upwelling of residual mantle peridotite in the melting region. One cal also see in the synoptic cross-section on the lower right that no dips toward the ridge axis are observed in the center of the “diapir, and instead foliations fan from vertical through dips away from the ridge axis that become increasingly shallow “off axis”. Jousselin et al. section in same scale as diapir model synoptic cross section of Jousselin et al., JGR 1998

3. The lack of evidence for corner flow within the top of the diapir led Jousselin & Nicolas, Marine Geophys Res, 2000a, to propose that corner flow must have occurred within the dunite of the crust-mantle transition zone. Some of the dunites within the Maqsad diapir region have disrupted olivine fabrics, with individual grains preserving evidence for viscous deformation at high temperature, but a random lattice preferred orientation on the hand specimen scale interpreted as resulting from suspension of deformed grains in a high porosity region just beneath the base of the crust (Boudier & Nicolas, J Petrol 1995). Thus, if corner flow occurred in this region, the associated structures might not be preserved, obliterated in the high melt fraction suspension region. Jousselin & Nicolas, 2000a, went further, to propose that dunites now found within the mantle section of the Oman ophiolite formed in the crust-mantle transition zone and were transported downward in the hanging walls of ductile normal faults or shear zones. I show this figure out of fairness and to ensure that you get a complete picture of debates about the Maqsad diapir and related, less well studied features elsewhere in the Oman ophiolite. In my view, there is no evidence for corner flow within residual mantle peridotites anywhere in the Oman ophiolite, whereas the theory of Jousselin et al., JGR 1998 and Jousselin & Nicolas MGR 2000 absolutely requires that corner flow generally occurs within residual mantle peridotites, not in dunites, since the diapir is supposed to supply the entire upper mantle section from a narrow upwelling column, and more than 90% of the shallow mantle section is composed of residual peridotite, not dunite. Note that the interpretive “cartoon” from Jousselin & Nicolas (bottom six panels) does not illustrate the mechanism of corner flow, with inward dipping foliation, anywhere, even in the top left and lower right panels where dunites, and melt rich regions beneath the crust, are missing. Furthermore, there is no evidence for normal faults or shear zones carrying dunite into the residual mantle section. Instead, it seems likely to me that the Maqsad diapir should be interpreted in terms of the waning phase of ridge spreading and magmatism in the Oman ophiolite, just before it was obducted. A new rift opened within older, colder mantle lithosphere and crust, and a ductile “horst” of mantle rose to fill the gap. This horst did not experience corner flow, and mantle peridotites within the horst did not spread outward. Instead, the peridotites surrounding the horst formed via spreading and magmatism along another, earlier spreading ridge.

4. We return to this figure from Spiegelman, EPSL 1996, in order to make a second point. Spiegelman made these diagrams mainly in order to empasize the very different mantle (white) and melt (black) flow trajectories predicted in two end-member models of mantle upwelling beneath spreading ridges. On the left is a passive upwelling scenario, where flow of the mantle is driven entirely by plate spreading. In this scenario, melt flows inward toward the ridge, while residual peridotites flow upward and then turn the corner to become part of the spreading plate at a range of depths from the bottom to the top of the model domain. On the right is a focused, diapiric upwelling scenario where flow of the mantle is driven primariy by the relative buoyancy of partially molten peridotite compared to melt-free peridotite. In this latter scenario, melt flows vertically beneath the ridge, and residual mantle peridotites mainly flow from near the bottom of the model doman to near the top. From a chemical perspective, the main difference between these two end-members is that the average degree of melting is much lower in the passive spreading scenario compared to the buoyancy driven scenario, because the average extent of decompression of peridotite is much less on the left than on the right. This point was made earlier, though far less elegantly, by Langmuir et al., AGU Monograph 1992.

5. Spiegelman, EPSL 1996, used these diagrams to illustrate the difference in the resulting melt compositions for the passive versus diapiric (here, “active”) scenarios. The top panel shows melt compositions in terms of the apparent fraction of melting (F) recorded by each element (with different distribution coefficients, on the horizontal axis), while the bottom panel shows the concentration of elements in the melt normalized to their initial composition in the solid mantle source (Cf = concentration in “fluid” or melt; Cso = concentration in the initial solid). With the shaded boxes in both panels, I have indicated the approximate composition of typical mid-ocean ridge basalts. One can see that mid-ocean ridge basalts (MORB) correspond much better to the melts produced in Spiegelman’s passive upwelling scenario than to the melts produced in the diapiric scenario, as was already pointed out by Langmuir et al., AGU Monograph 1992. In this context, lavas in the Oman ophiolite correspond very closely to mid-ocean ridge basalts, casting additional doubt on the hypothesis of Jousselin et al., JGR 1998, in which all the mantle peridotite that melted to form the crust above the Maqsaid diapir rose in a narrow, 50 km high diapiric column. MORB MORB

6. The idea that the upper mantle in the Maqsad diapir is a horst that did not flow outward to form the surrounding shallow mantle section is supported by the geochemical study of Godard et al. EPSL 2000, who found significant geochemical differences between the peridotites within the steeply lineated and foliated diapir and the surrounding, transposed peridotites with foliation approximately parallel to the base of the crust. These differences suggest that the diapir peridotites had a significantly different initial composition, prior to melting, and/or experienced a very different extent and type of melting, compared to the peridotites that are peripheral to the diapir. This, the diapir may have intruded, rather than fed, the surrounding shallow mantle section.

7. Map of gabbroic sills and dikes in the Maqsad region from Ceuleneer et al, Nature 1996, showing mainly high temperature features in the diapir, and progressively lower temperature features in the surrounding shallow mantle section. If all of the shallow mantle peridotites were fed by radial flow out of the diapir, then one would expect to see both older, high temperature and younger, low temperature features in the periphery of the diapir, but high temperature features are not found in this region, especially in the “really cold” area to the north. Again, these data support a model in which the hot diapir intruded older, colder, pre-existing shallow mantle peridotites that melted and were transposed beneath another spreading ridge. The presence of a few high temperature dikes and sills can be interpreted in terms of outward intrusion of magma from the hot horst into its older, colder wall rocks. REALLY COLD COLD HOT

8. Regional map of gabbroic dikes and sills in the mantle section of the entire Oman ophiolite, from Python & Ceuleneer, G-cubed 2003, showing a pattern very similar to that documented for the Maqsad region by Ceuleneer et al., Nature 1996.

9. old, cold, unrelated “graben”? new, hot “horst”? Synoptic cross section from Rabinowicz & Ceuleneer, EPSL 2006, intepreted (by me) in terms of intrusion of a hot horst into older, colder, genetically unrelated shallow mantle peridotites.

10. Even for slow spreading ridges, it is not axiomatic that formation of thick igneous crust at ridge segment centers, and thinner crust at segment ends, is related to three dimensionally focused upwelling of mantle peridotite (left, ridge axis parallel, vertical section at top, map view just below the Moho at bottom, solid flow in black, melt flow in red). Instead, focused crustal accretion could result from focused melt transport within passively upwelling mantle peridotite (right).

11. Sparks & Parmentier (several papers, beginning with EPSL, 1991) proposed that focused melt transport to spreading ridge axes occurred by nearly horizontal flow beneath a permeability barrier, formed by crystallization of cooling melt in the overlying thermal plate. This process has been quantitatively modeled by Spiegelman, Phil Trans Roy Soc London 1993; Ghods & Arkani Hamed, GJI 2000, and Rabinowicz & Ceuleneer EPSL 2006.

12. Magde & Sparks, JGR 97 and Magde et al., EPSL 97 found that their models for focused MELT transport (lower right) provided a slightly better fit to inferred crustal thickness (red, upper curves) and observed gravity variation (red, lower curves) along the Mid-Atlantic Ridge near the Oceanographer Fracture Zone, compared to models of diapiric, focused SOLID upwelling (upper left). Thus, it is not necessary to explain crustal thickness and gravity variation along slow spreading ridges as the result of diapiric, 3D focused upwelling of buoyant, melt-rich upper mantle peridotite.

13. 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 “jury is out” Magma fracture: dikes Focused porous flow Sills & lenses at “top”

14. Schematic cross section of a typical mantle section in the Oman ophiolite, redrawn after Lippard et al., 1986, by Kelemen et al., Nature 1995. Lower crustal gabbroic rocks in black. Dunites in red, including mantle dunites roughly concordant with foliation in host residual mantle peridotites, and thicker, massive dunites in the crust-mantle transition zone. Gabbroic and pyroxenite dikes in black, cutting mantle peridotite. In the 1980’s and early 90’s, the dikes were commonly invoked as examples of melt extraction features which transported melt from the melting region to form the igneous crust at the Oman spreading ridge, and by analogy at other ophiolites and mid-ocean ridges. This interpretation is incorrect. As shown in the figure, the gabbroic dikes are almost exclusively discordant to foliation, with steep paleo-dips. They typically have strikes parallel to the sheeted dikes near the top of the Oman crustal section, as documented in several papers including Nicolas et al., in Ryan, ed., Magmatic Systems, 1994. The foliation in residual mantle peridotites is roughly parallel to the base of the crust and the paleo-seafloor, consistent with the hypothesis that the shallow mantle underwent corner flow and transposition to a horizontal orientation beneath a spreading ridge. Thus, it is clear from the structural relationships of the gabbroic dikes that they did not form in the upwelling mantle beneath the Oman spreading ridge, since the dikes are not transposed along with their host peridotites. Instead, they intruded the mantle off-axis, after corner flow was complete. Furthermore, the presence of cumulous minerals indicates that the dikes formed by partial crystallization from a cooling magma, which shows that they formed in the conductively cooled, “lithospheric” mantle along the base of the thermal plate, and not in the adiabatically upwelling, partial melting mantle beneath the spreading ridge. In addition, more than a few km from the crust-mantle transition zone, gabbroic dikes in the Oman mantle section are dominantly orthopyroxene-bearing gabbronorites and websterites, which is inconsistent with their being related to the magmas that formed lower crustal gabbros and upper crustal lavas in the Samail and Wadi Tayin massifs, where gabbros lack orthpyroxene (opx) and primitive lavas are MORB-like, far from saturation in opx.

15. There are, however, pyroxene-rich bands in the residual peridotite of the Oman ophiolite, as in the mantle sections of other ophiolites and orogenic massif peridotites. Similar features have been dredged from the mid-ocean ridges. These transposed pyroxene-rich bands are the oldest feature in exposed mantle peridotite outcrops. I show them here in order to emphasize the distinction between these bands and the sharp sided, discordant gabbroic and pyroxenite dikes. The formation of the old, transposed pyroxene-rich bands is enigmatic. They could be recycled oceanic crustal fragments, or igneous dikes formed prior to the last stage of upwelling and partial melting. In general, structural relationships and mineral compositions indicate that these bands underwent partial melting along with their peridotite host rocks.

16. Not-so-good pictures of discordant, gabbroic and pyroxenite dikes in the Oman mantle section, from Python & Ceuleneer, G-cubed 2003. Top left: Fine grained gabbronorites. Bottom left: coarse websterite. Bottom right, internal structure in coarse gabbronorite dike.

17. Diagram from Benoit et al., Chem Geol 2000, emphasizing presence of two end-member suites of gabbroic dikes and sills in shallow mantle peridotite of the Maqsad diapir area. One (filled stars) resembles lower crustal gabbros from mid-ocean ridges and from the Samail and Wadi Tayin massifs of the Oman ophiolite, and the other (open stars) is extremely depleted in incompatible elements (elements that partition preferentially into melt rather than solid minerals), orthopyroxene-rich, and - generally - high in radiogenic 87Sr/86Sr. This latter suite of dikes dominates the dikes found deeper in the Wadi Tayin mantle section, far from the crust-mantle transition zone.

18. In the diagram on the upper right, Kelemen et al., Phil Trans Roy Soc London, 1997 illustrated analyses of rare earth elements (REE) in clinopyroxene (cpx) in pyroxenite, gabbronorite, and gabbroic dikes from the Wadi Tayin massif of the Oman ophiolite (bold lies) compared to analyses of cpx in abyssal peridotites (grey field, Johnson et al., JGR 1990; Johnson & Dick, JGR 1992) and residual mantle peridotites from Wadi Tayin (Kelemen et al., Nature 1995). The diagram on the lower right includes unpublished data on cpx in dikes from Kelemen et al., Nature 1995 (red) as well as the data published by Kelemen et al, Phil Trans Roy Soc London 1997 (blue). All but three of the mantle dikes have cpx that is strongly depleted in light REE, similar to residual mantle peridotites from the mid-ocean ridges and the Oman mantle section, and very different from cpx in equilibrium with MORB and with the magmas that formed Oman lavas and lower crustal gabbros. In conclusion, most or all of the dikes in the Oman mantle section, more than a few km from the crust-mantle transition zone, were not the primary conduits for transport of melt from the melting region through the shallow mantle to form the igneous crust