Unraveling Mantle Dynamics Beneath Ophiolites: A Study of Dunites in Oman

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 Sills & lenses at “top”

Returning to this cross-section from the mantle section of the Oman ophiolite, please turn your attention to the red, mantle dunites. Structurally, they have the contact relationships of features that could have formed in the upwelling mantle beneath a spreading ridge, prior to corner flow and transposition of both dunites and host residual mantle peridotites into their current orientation, approximately parallel to the crust-mantle transition and the paleo-seafloor. (Note transitional features, gabbroic dikes with dunite selvages, which are observed in the shallower parts of the Oman mantle section, and tend to be more discordant with host peridotites than larger, gabbro-free dunites).

Josephine peridotite photo, Zach Morgan (pers comm 2006) Locally, in Oman and in other ophiolites, dunite contacts cut banding and foliation in host peridotites. The extent of this discordance depends a lot on when the dunites formed with respect to the last episode of penetrative ductile deformation in host peridotite. Thus, the high degree of concordance between dunites and host peridotites in Oman, while not perfect, suggests that the dunites formed before or during the last episode of penetrative ductile deformation in the host peridotite.

Trinity peridotite photo, Zach Morgan (pers comm 2006) In some lherzolite host rocks, dunites show well developed harzburgite (olivine + opx + spinel) reaction zones separating dunite (olivine + spinel, virtually no opx or cpx) from lherzolite (olivine + opx + cpx + spinel)

this photo, with a castle and phone poles (right hand skyline) for scale, shows 10 to 20 m thick, nearly concordant dunites (tan) within foliated, residual mantle peridotites (brown) in the Muscat block of the Oman ophiolite

near the previous photo, abundant 50 cm to 2 meter dunites (tan) in residual mantle peridotite (brown) with a mosque in the foreground for scale

residual peridotites are out of trace element as well as major element equilibrium with MORB: provides a convenient way of identifying veins and dikes that ARE in equilibrium with MORB REE in cpx in Oman residual peridotites (symbols in upper panel) in the mantle section of the Wadi Tayin massif, and most abyssal peridotites (grey field in upper panel), can be inverted to yield equilibrium melt compositions (calc liquids: harzburgites, in lower panel). These compositions are very different from the liquids that formed the crust in Oman and with primitive MORB (symbols in lower panel).

dunites are in major & trace element equilibrium with MORB REE in trace cpx in Oman dunites (symbols, upper panels in the mantle section of the Wadi Tayin massif can also be used to calculate equilibrium liquids. In contrast to the equilibrium liquids calculated for depleted residual peridotites (calc liquids: harzburgite), liquids in equilibrium with Oman dunites have the same trace element concentrations as primitive MORB and the melts that formed the Oman lavas and lower crustal gabbros.

This diagram basically changed my career. It is from a paper by Boudier & Nicolas in a conference volume (H. Dick, ed., Chapman Conference Proceedings, Bulletin of the Oregon Dept. of Mines and Mineral Industries, 1977), which in turn cites earlier papers by Boudier & Nicolas, 1972. It shows a small dunite (note 10 cm scale) cutting foliated peridotite in the Lanzo peridotite of northern Italy. The foliation in the peridotite is shown with thin lines labeled S0. The lherzolite includes a concordant pyroxene-rich band with abundant spinel shown in back. The small dunite cuts the lherzolite foliation at a high angle. Where the dunite crosses the pyroxene rich band, there is a train of relict spinel crossing the dunite, showing that the dunite formed by replacement of the host lherzolite and the pyroxene-rich band, not as a dike. If it were not for the pyroxene rich band, might be interpreted as a dike, a fracture filled with cumulate olivine precipitated from migrating magma. Also, note that the dunite formed as a result of reaction between peridotite and migrating, olivine saturated melt, not via melting and melt extraction from the host peridotite. The dunite cannot be the residue of partial melting of the peridotite, because (a) contacts with the peridotite are very sharp, not gradational, (b) the pyroxene-rich band in the host rock did not melt, and (c ) the width of the dunite is far to small to have sustained a significant temperature gradient on mantle melting time scales.

Photo from the shallow mantle section of the Samail massif in the Oman ophiolite, showing contact relationships similar to that documented by Boudier & Nicolas, 1972, 1977, in the Lanzo peridotite. Photo from David Jousselin (pers comm 2007).

Cross-cutting of spinel bearing orthopyroxene layers (s0) in the harzburgite by a dunite vein (a: cartoon from a study of Lanzo peridotites; b: photographies from the Oman peridotites). The continuity of the layer through the dunite vein is shown by a trail of spinel grains in the dunite. This demonstrates that the dunite is not an intrusive cumulate dike, but the product of pyroxene dissolution. Such dunite reaction zone is common at the rims of dikes injected when the peridotite was close to (or at) asthenospheric conditions Photos from the shallow mantle section of the Samail massif in the Oman ophiolite, showing contact relationships similar to that documented by Boudier & Nicolas, 1972, 1977, in the Lanzo peridotite. Photos from David Jousselin (pers comm 2007).

{ pyroxenes dissolve olivine precipitates, SiO2 up peridotite dissolves (even olivine), MgO up { liquid adiabat olivine saturation pyroxene saturation  Depth mantle solidus Temperature  This diagram, reproduced from similar diagrams in Kelemen, J Petrol 1990 and Kelemen et al., JGR 1995, illustrates how ascending melt can dissolve pyroxenes in surrounding mantle peridotites, and replace them with olivine. Mantle derived melt rising adiabatically on its own composition will follow a PT slope steeper than its saturation surface. If the melt is passing through peridotite, conductive cooling, compositional change, and dissolution of peridotite with concomitant cooling due to the heat of fusion will bring it to the olivine saturation surface. At this point, melt saturated in olivine will dissolve pyroxenes, and precipitate a smaller mass of olivine at constant temperature, enthalpy or entropy.

Calculating reaction stoichiometry for constant temperature or constant enthalpy reaction in which melt 1 + pyroxene => melt 2 + olivine. The dissolution of pyroxene, with concomitant precipitation of olivine, produces about a 30% increase in the melt mass. Definition and use of apparent heat of fusion, rather than heat of fusion of minerals at their melting temperatures, from Kelemen, J Petrol 1990.

As shown in previous slide, dissolution of opx in olivine-saturated melt produces about a 30% increase in melt mass and a 30% decrease in solid mass. This reaction occurs while melt is saturated in olivine but not pyroxene. Opx is more abundant than cpx in residual mantle peridotites, so reaction will tend to drive liquids through the olivine-only primary phase volume toward saturation in opx. With the simplest possible assumptions, and some example calculations using the MELTS thermodynamic model for silicate liquids, Kelemen et al., JGR 1995, showed that the solubility of opx in decompressing mantle melts increases by about 1 wt% for each kilobar of decompression. This estimate is based on calculations in which MELTS predicts opx saturation after addition of 10% opx at 2 kb to a mantle melt formed at 12 kb, and 20% opx at 2 kb to a mantle melt formed at 22 kb.

Lherzolite dissolution in basalt 1300°C, 1 GPa, 8 hours Morgan and Liang, submitted CMP Experimental similation of dunite and harzburgite formation from initial lherzolite via pyroxene dissolution in olivine saturated melt (Morgan & Liang, CMP 2005). Basaltic glass is the lightest grey color. Olivine is a medium grey. Opx is dark grey. Cpx is the second lightest shade of grey. Opx appears abruptly at the dunite/harzburgite boundary, and cpx appears abruptly at the harzburgite/lherzolite boundary.

Lherzolite dissolution in alkali basalt Mg x-ray map Ca x-ray map Dunite Harzburgite Increasing abundance Lherzolite 250 mm Mg and Ca x-ray maps of the experimental products of Morgan & Liang, CMP 2005. On left, basaltic glass is dark blue, olivine is orange, opx is green, and cpx is light blue. On right, basaltic glass is green, olivine is dark blue, opx is light blue, and cpx is orange to red. Note sharp reaction boundaries, and presence of abundant melt, as well as new olivine, in zones where opx and cpx, or cpx alone, were dissolved by reaction with melt. These experiments confirm the reaction stoichiometry predicted by Kelemen, J Petrol 1990, and Kelemen et al., JGR 1995. Note that the experiments probably also indicate that sharp reaction boundaries between dunite, harzburgite and lherzolite, can form via diffusion alone, since there was little if any melt flow across reaction boundaries during the experiments.

General illustration of how elongate dissolution channels can form where a fluid solvent migrates by porous flow through a partially soluble porous matrix. There is positive feedback between permeability, permitting increased flux of the solvent, and dissolution, increasing the local permeability and porosity, which leads to the exponential growth of high porosity dissolution channels in the primary direction of fluid flow. The earliest literature on this “reactive infiltration instability” were by Chadam et al., J. App. Math., 1986; Ortoleva et al., Am J Sci, 1987; Steefl & Lasaga, ACS Symposium Series, 1990. Daines & Kohlstedt, GRL 1994, Kelemen et al., JGR 1995, and Aharonov et al., JGR 1995 proposed that dunites in mantle peridotite arise via the reactive infiltration instability, where olivine-saturated melt dissolves pyroxene while ascending through mantle peridotite. I’ve expanded on this idea at some length in Part 3 of these lectures.

Returning to outcrop scale observations, here is a famous map of dunites (stippled pattern) in residual mantle peridotites (white) with pyroxene-rich banding, from Savel’yeva et al., Geotectonics 1980, as reproduced by Nicolas, in M. Ryan, ed., Magma Transport & Storage 1989 and then Kelemen et al., JGR 1995.

Summary diagram from Kelemen et al., Nature 1995, showing contact relationships of dunites. A. As noted for the Savel’yeva map in the previous slide, bands intersecting dunites at an angle continue across the dunite along the same vector. If the dunites were dikes, then bands would show displacement across the dunite, as in Ab. B. Relationship shown earlier, where a train of relict spinel crossing dunite indicates where a spinel bearing pyroxene rich band has been replaced by dunite. C. As noted in the Savel’yeva map, pyroxene-rich bands locally show selective replacement by dunite. Excellent examples of this relationship are also documented in Kelemen & Dick, JGR 1995. D. Dunites commonly widen without any displacement in the foliation of host peridotite. Good examples of this relationship abound in the Trinity peridotite of N California. E. Very commonly, as in the Savel’yeva map, dunites form anastamosing networks enclosing relict islands of host peridotite that are not rotated with respect to surrounding peridotites. Panels F and G illustrate common structure in which cm to 10 cm wide, tabular dunites have a medial pyroxenite or gabbroic dike. In panel G, the dikes appear to “hook” toward each other, a texture which is present in three or four outcrops at Vicki Bluff in the Trinity peridotite, which probably indicates elastic interaction of crack tips. Thus, although many dunites could have formed as dissolution channels via focused porous flow of olivine-saturated melt dissolving pyroxene in host peridotite, at least some dunites seem to have formed as reaction zones around melt-filled fractures.

This illustration, drawn by Henry Dick in 1981 though never published, illustrates Henry’s understanding of the replacive nature of anastamosing dunite networks, but also his hypothesis that the largest dunites once hosted melt-filled diapirs or dikes, which later disappear without a trace. I show this illustration to emphasize that (a) Henry knew everything I know now, already in 1981, while I was still a child, and (b) that it is difficult or impossible to use field relationships to demonstrate the absence of such melt-filled diapirs and cracks, provided that they vanished without a trace. In fact, a melt-filled crack within the adiabatically upwelling mantle beneath ridges would be unlikey to leave behind any trace, other than a reaction rim of olivine within host peridotites, so the presence of such “cryptic cracks” is plausible. Finally, I like to show this figure because I think it looks quite a bit like Henry Dick, round and fiery.

Movie of Henry Dick, slide 2

Nevertheless, in the work we have done, we are strongly influenced by the obvious evidence for replacement of peridotite in olivine-saturated melt migrating by focused porous flow, and we have chosen to assume that melt-filled cracks did not exist where their presence is not evident. More on this in Part 3 of this lecture series.

Josephine ophiolite Slightly discordant dunite cutting banding in a host peridotite in a mantle shear zone in the Josephine peridotite, described by Kelemen & Dick, JGR 1995. Photo from Zach Morgan (pers. comm. 2006).

Josephine ophiolite mineral chemistry In this outcrop. there is a chemical boundary layer marked by variation in olivine, spinel and (?) opx. In this case, the boundary layer is developed almost entirely within the dunite. Morgan, Liang & Kelemen, in prep., 2007

Additional observations of composition gradients What can we learn from these composition gradients? Bay of Islands (Suhr et al., 2003) Horoman (Takahashi, 1992) Compositional boundary layers, here illustrated using percent molar Mg/(Mg+Fe) , or Mg#, also known as Forsterite content, in olivine, are also observed in and around dunites in mantle section of the the Bay of Islands ophiolite in Newfoundland (Suhr et al., G-cubed 2003) and the Horoman peridotite massif on Hokkaido, Japan (Takahashi, Nature 1992).

Models of flow in dunite One way to interpret the chemical boundary layers in and around dunites is to consider the effects of melt flow (1) parallel to dunite contacts, in which case diffusive boundary layers may exist in both dunite and harzburgite, (2) into dunites, in which case diffusive boundary layers may be advected into the host peridotite, and (3) into dunite, in which case diffusive boundary layers may be advected into the dunite. Assuming that melt flow velocities in the dunite, seen as a high porosity conduit for focused porous flow of melt, are always faster than in host peridotite, one can make semi-quantitative estimates of the relative thickness of chemical boundary layers inside and outside dunites in each of these three scenarios. Illustration, and reasoning, from Morgan et al., in prep., 2007.

Unraveling Mantle Dynamics Beneath Ophiolites: A Study of Dunites in Oman