EXPERIMENTAL HYDROUS PARTIAL MELTING OF HARZBURGITE AT SHALLOW PRESSURES: IMPLICATIONS ON HYDROUS LATE-STAGE MAGMATIC PROCESSES WITHIN THE MANTLE/CRUST TRANSITION OF THE OCEANIC LITHOSPHERE

The petrological record of the MOHO transition zone (MTZ) of the Oman ophiolite indicates complex multi-stage magmatic processes for the accretion of the deep oceanic crust at fast-spreading ridge systems. For the genesis of specific lithologies within the Oman ophiolite, some models include a “second” partial melting of harzburgites triggered by water-rich fluids, e.g. for the formation of a special type of chromitite/dunite associations within the MTZ (Ahmed and Arai, 2002) or for the genesis of so-called depleted gabbronorites (Benoit et al. 1999). Moreover, a recently in the Oman ophiolite observed new type of hydrothermal circulation operating at very high temperatures has the potential to reach the sub-MOHO level causing interaction between seawater and harzburgite at magmatic temperatures (Boudier et al., 2004). In order to understand the water-triggered partial melting behavior of typical harzburgites within the MTZ level of the oceanic lithosphere, we started an experimental study. Water-saturated partial melting experiments using a typical Oman harzburgite as starting material were performed under pressures of 100, 200, and 500 MPa at reducing (corresponding to the FMQ buffer) and at oxidizing conditions (FMQ+4) in specially designed internally heated pressure vessels enabling the control of oxygen fugacity. Temperatures varied between 980 and 1220°C, run durations were up to 82 hours. Solidus and the clinopyroxene-out curve vary strongly with pressure. For example, at 100 MPa first melts and the breakdown of clinopyroxene were detected at 1100/1160 °C, respectively, while the corresponding temperatures at 500 MPa were 1060/1100 °C. As expected, the generated melts are generally SiO2-rich varying between 54 and 65 wt% SiO2. The compositions of these melts show boninitic affinities with high Ca/Na ratios but generally with much lower MgO contents (< 10 wt% MgO) than those “high-Ca boninites” experimentally studied by Falloon and Danyushevsky (2000). According to these authors, the petrogenesis of “high-Ca boninites” requires temperatures as high as ~1480°C at depths of ~45 km (~ 1.5 GPa pressure) in the mantle wedge above an intra-oceanic subduction zone. Here we show that hydrous partial melting of harzburgites at MOHO level pressures has also the potential to produce boninitic melts with very high Ca/Na ratios, but at significant lower temperatures. Due to the residual character of the starting material, the experimental melts are highly depleted in incompatible trace elements showing extremely high Ca/Na ratios. A potential fractionation of such melts could produce cumulate rocks resulting in highly depleted gabbronorites with An contents of plagioclase generally above 90 mol%. Thus, such cumulates would correspond well to those “depleted gabbronorite” dikes typically observed in the Oman ophiolite intruding the layered gabbros and the MTZ (Benoit et al. 1999). At temperatures above the breakdown of clinopyroxene, the amount of residual olivine and chromite increases while that of orthopyroxene decreases. Thus, the residual mineral paragenesis shows characteristics of typical chromite-bearing dunites observed in many ophiolites. Interestingly, the compositional features of the experimental products fit in detail with natural dunite-harzburgite relations observed in some ophiolites where dunite bodies crosscut the harzburgite host. For example, Suhr et al. (2003) presented very accurate detailed traverses across dunites and neighboring harzburgites of the Bay of Islands ophiolite in Newfoundland. Main compositional features are: olivine in dunite shows an increase in Mg# and CaO but a decrease in NiO, and spinel in dunite shows an increase in Cr# relative to olivine and spinel of the associated harzburgite. Exactly the same features can be observed in our experiments when comparing olivines and spinels produced in our experiments with those of the harzburgitic starting material. In summary, our experimental results show that the formation of distinct lithologies of the oceanic lithosphere like high-Ca boninitic rocks, depleted gabbronorites, and chromite-bearing dunites, could, in principle, be attributed to a single process of fluid-induced partial melting of harzburgite at MOHO level or within the uppermost kilometers of the sub-oceanic mantle below spreading centers. The temperatures for the heating process ranging between 1060 and 1200°C for such a model could be provided by ascending MORB magmas. The presence of water-rich fluids at MOHO level which are necessary for such a model could be derived from different sources. One possible scenario would be that seawater has the potential to reach the sub-MOHO level, due to a special type of hydrothermal circulation operating at very high temperatures (Boudier et al., 2004), triggering the partial melting of harzburgite. Another possibility would be that seawater may penetrate the deep oceanic crust due to the special tectonic conditions at segment boundaries resulting in the partial alteration of harzburgites to serpentinites. In a later stage, these could be reheated by ascending magmas causing dehydration providing the water-rich fluids needed for a hydrous partial melting process. It should be emphasized that both scenarios mentioned above may occur in normal mid-ocean ridge settings away from any subduction zone. Another mechanism providing water-rich fluids in a shallow mantle level is conceivable in a supra-subduction zone setting (SSZ). Hirai and Arai (1987) reported that olivines in harzburgites of many SSZ ophiolites often contain characteristically high amounts of water-rich fluid inclusions. Thus, in a SSZ setting water can be transported to the sub-MOHO mantle trapped within olivines providing water-rich fluids after the dehydration of olivines due to hot ascending magmas.

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