Gabbro Lenses: Evidence for Magma Fracture and Plutonic Emplacement in the Crust-Mantle Transition Zone

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 “jury is out” Magma fracture: dikes in TBL, often depleted Focused porous flow abundant, power law in Oman = “MORB” Sills & lenses at “top”

2. In the shallowest mantle sections of the Oman ophiolite, lenses of layered gabbro are found within massive dunites and, less commonly, residual mantle peridotites. This photo, from Korenaga & Kelemen, JGR 1997, shows Greg Hirth standing on such an outcrop, pointing to interdigitated contacts between light colored gabbro lenses and darker dunites in the Tuf area, NW of the Maqsad diapir structure in the Samail massif of the Oman ophiolite.

3. Although this picture is taken from lower crustal gabbros, we also commonly find such modally graded mineral layering in the gabbro lenses in dunite and peridotite in the crust-mantle transition zone. These textures would not have been preserved if the rocks had undergone large shear strains, and so we think that the gabbro lenses in the crust-mantle transition zone are plutonic rocks that were emplaced into their ultramafic host rocks, well below the depth of “neutral buoyancy” for basaltic melt. Photo from Pallister & Hopson, JGR 1981.

4. Chemical layering within gabbro lenses in the crust-mantle transition zone. Korenaga & Kelemen, JGR 1997.

5. Continuous compositional variation in minerals from gabbro lenses in the crust-mantle transition zone (MTZ) together with lower crustal gabbros, indicates that the MTZ lenses are genetically related to the crustal gabbros, and were among the first crystal products from the primitive melts that ascended to form the oceanic crust in the Oman ophiolite. Data from MTZ samples in the Maqsad region of the Samail massif (Korenaga & Kelemen) and the nearby Nakhl-Rustaq massif (Browning); crustal gabbro data from the Wadi Al Abyad - Wadi Bani Karous section in the Nakhl-Rustaq massif, from the PhD thesis of Browning, Open Unversity, 1982.

6. REE in whole rocks (right) and minerals (left) for lower crustal and MTZ gabbros from the Oman ophiolite. Right hand panels show compiled data for the entire ophiolite, with a vertical progression from cumulate gabbros with little or no “trapped melt” in the lower crust, through “upper gabbro” layer of variable thickness with transitional cumulate-liquid compositions, to sheeted dike and lava compositions that represent ”frozen” liquids. Right hand panels show cpx and plagioclase (plag) REE contents from MTZ and lower crustal gabbros in the Samail and Wadi Tayin massifs. Wide grey lines, INAA analyses of mineral separates from Wadi Tayin lower crustal gabbros (Pallister & Knight, JGR 1981). Open symbols, 6 lower crustal gabbros from the Samail and Wadi Tayin massifs. Thin lines without symbols, 12 gabbro lenses from the Samail massif MTZ. The similarity in MTZ and lower crustal mineral compositions further supports the hypothesis that the MTZ gabbro lenses formed from the same magmas as the lower crustal gabbros. Figure from Kelemen et al., EPSL 1997.

7. Cpx in gabbros from lower crust and MTZ is in REE exchange equilibrium with lavas and sheeted dikes. Thus, MTZ lenses formed on axis, together with lower crustal gabbros.

8. Based on observations of MTZ gabbro lenses, and chemical and textural similarity between MTZ lenses and lower crustal gabbros, Kelemen et al., EPSL 1997, suggested that both lenses and lower crustal gabbros crystallized in a series of “sheeted sills” spanning most of the lower crust, with rocks crystallized at the level where they are now found in the crustal section. Korenaga & Kelemen, JGR 1997, EPSL 1998; and Kelemen & Aharonov, AGU Monograph 1998, expanded upon this hypothesis. We proposed that the gabbro lenses in the MTZ formed beneath a permeability barrier created by crystallization of cooling magma in pore space in the shallowest mantle peridotites. Rising melt pressure in gabbro melt lenses trapped below this permeability barrier led to increasing magma pressure, and ultimately to magma fracture, carrying melts higher in the crustal section.

9. If our hypothesis for formation of gabbro lenses beneath a permeability barrier at the base of the conductively cooled shallow mantle is correct, then one would infer that such lenses should form at much greater depths beneath some slow-spreading ridges as intrusions into mantle peridotite over a broad depth interval.

10. There is evidence that gabbroic crystallization does begin at depth beneath some slow spreading ridges, as in this example from Grove et al., AGU Monograph 2002, showing that lavas from the MARK area, just south of the Kane Fracture Zone along the Mid-Atlantic Ridge, preserve a chemical trend characteristic of crystal fractionation at 4 to 6 kb. Similar interpretations of compositional trends in basaltic lavas have been proposed by others, as listed in red.

11. The hypothesis that gabbroic plutons are commonly emplaced into residual mantle peridotites beneath slow spreading ridges is supported by results from ODP Leg 209 (Kelemen, Kikawa, Miller et al., ODP Initial Reports, 2004; Kelemen,Kikawa, Miller et al., ODP Scientific Results, 2007), where gabbroic lenses within residual mantle peridotite form about 30% of recovered core from 10 holes. ODP Leg 209 was designed to sample residual mantle peridotites, exposed on the seafloor from 14 to 16°N along the Mid-Atlantic Ridge.

12. Unusual igneous textured plagioclase-spinel lherzolites from ODP Site 1275 can be used to estimate the pressure and temperature at which the minerals last equilibrated.

13. Here, PT estimates for metamorphic equilibration of plagioclase-spinel lherzolites from ODP Site 1275 (purple diamonds with 1 sigma uncertainties shown as ellipses) are compared to PT estimates for melt-peridotite equilibration for primitive (Mg# > 50%) lavas sampled along the Mid-Atlantic Ridge between 14 to 16°N (blue circles, grey 1 sigma uncertainty estimates, red average and standard error of the mean), assuming that these melts equilibrated with a plagioclase lherzolite assemblage. Estimated pressures of 4 to 8 kb confirm the hypothesis that gabbroic lenses are emplaced into peridotite over a large depth range beneath this part of the slow spreading Mid-Atlantic Ridge. Kelemen, Kikawa, Miller et al., ODP Scientific Results, 2007.

14. Inferred geotherm for the 14 to 16°N region along the Mid-Atlantic Ridge, based on data in the previous slide. The conductive boundary layer in this region extends to 15 to 25 km depth below the seafloor, and gabbroic lenses are probably emplaced into mantle peridotite over this entire depth interval.

15. Strikingly, residual mantle peridotites from ODP Leg 209 preserve delicate, igneous textures, showing that they have not undergone penetrative, ductile deformation since they cooled below 1250 to 1300°C.

16. Ductile shear zones were sampled in drill core at all but one of the six sites where we drilled mantle peridotite and gabbroic rocks. In several cases, more than one ductile shear zone was sampled in an individual hole. the orientaion of these shear zones, and faults exposed on the seafloor, indicate that some of the shear zones and faults intersect at high angles. This suggests that rift valley normal faults, with dips toward the ridge axis, were rotated into an orientation in which some dip away from the ridge axis. Kelemen, Kikawa, Miller et al., ODP Initial Reports, 2004; ODP Scientific Results, 2007.

17. Our observations call to mind the hypothesis of Proffett, GSA Bull 1977, who proposed that low angle normal faults, and apparent reverse faults, in the Basin and Range region of the western US, formed as high angle normal faults. As they rotated during extension, they reached low dips where normal stress on the fault surface inhibited frictional failure. Then, new faults formed, cutting and rotating older fault surfaces. We proposed that some of the low angle normal faults and apparent reverse faults observed in drill core from ODP Leg 209 formed as high angle normal faults, by analogy with the better studied faults observed by Proffett. This may also provide an explanation for abyssal hill structure … ??? Kelemen, Kikawa, Miller et al., ODP Initial Reports, 2004; ODP Scientific Results, 2007.

18. To summarize results from ODP Leg 209, the thick thermal boundary layer beneath the slow spreading Mid-Atlantic Ridge at 14 to 16°N caused the formation of gabbro lenses within residual peridotite beneath the ridge axis over a vertical interval of 15 to 25 km beneath the seafloor. Corner flow and exhumation of undeformed blocks of residual peridotite took place along a network of abundant high temperature shear zones and lower temperature normal faults. Progressive rotation of blocks within the fault network resulted in exposure of low angle fault surfaces at and near the seafloor. Kelemen, Kikawa, Miller et al., ODP Initial Reports, 2004; ODP Scientific Results, 2007.

20. Note the great similarity between the results of ODP Leg 209 and the hypotheses outlined by Mathilde Cannat in her review paper in JGR in 1996!