EXPERIMENTAL STUDY ON CRUSTAL WEHRLITES OF THE OMAN OPHIOLITE: EXPERIMENTAL OUTLINE AND MICROANALYTICAL RESULTS

Abstract

The “ophiolite model”, which is mainly based on studies on the Oman ophiolite, imply that typical lower oceanic crust formed at fast-spreading ridges consists of a relatively simple “pan cake” sequence of (from bottom to top) the Moho-transition zone (MTZ), layered gabbro developing to isotropic gabbros, and sheeted dikes. However, the “real” lower Oman ophiolite crust is much more complicated, characterized by numerous complex intrusions of different rock types. Among these, the most enigmatic rocks are wehrlites, which are consisting mainly of olivine and clinopyroxene and can make up in some areas of the Oman ophiolite more than 20% of the crust (Juteau et al., 1988). The genesis of these rocks is up to now not clear. Current models for their formation are based on an origin from melt-impregnated mantle peridotites or on a magmatic accumulation of melts with very special composition (e.g., very depleted melts related to a subduction zone environment, or picritic melts). Koga et al. (2001) showed that the clinopyroxenes of typical wehrlites were in equilibrium with a “normal” MORB melts. This result, however, conflicts with the typical crystallization order observed in typical MORB systems, where plagioclase crystallizes before clinopyroxene. This implies that the parental melts were wet, since it is well-known from experiments in water-rich systems that the crystallization of plagioclase is suppressed. This is supported by new experimental work of Feig et al. (2005, submitted) who studied the influence of water on the phase equilibria in a gabbroic system. Their results clearly show that at pressures > 0.1 MPa under high water activities the order of crystallization is olivine, clinopyroxene, plagioclase which clearly demonstrates that wet MORB-tpye magmas have the potential to form wehrlite by the accumulation of early crystallized olivine and clinopyroxene. In order to evaluate the genesis of the Oman crustal wehrlites we started a study where we combine experimental work using natural wehrlites as starting material with investigations on natural rocks, especially by using microanalytical tools like SIMS and electron microprobe. For this, we investigated a section through the lower oceanic crust of the Oman ophiolite exposed in the Wadi Haymiliyah (Halyn block). Here, distinct discordant wehrlite bodies ranging in size from decameters to hundreds of meters intrude the layered gabbro series at different crustal levels, starting from the MTZ up to the sheeted dikes. Common feature of all wehrlite bodies is the dominance of olivine and clinopyroxene with high Mg# (MgO/(MgO+FeO)*100, molar) varying between 85 and 95. In addition, each body shows characteristic features, e.g., in terms of amount and presence of poikilitic or interstitial minerals like orthopyroxene, pargasite, and plagioclase or in terms of textural details (e.g., “classical” cumulate structures versus recrystallized textures; amount and size of poikilitc clinopyroxene grains, if present). There is a clear relation between petrographical evolution and structural height of the bodies. The wehrlites deep in the section from the MTZ show only olivine and clinopyroxene, strongly recrystallized and often forming monomineralic patches or layers, while the wehrlites from the top of the section show typical cumulate textures, with the crystallization order olivine, clinopyroxene, orthopyroxene, pargasite, plagioclase. Often, pyroxene and pargasite characteristically form mm-sized poikilitic crystals. In addition to the phase equilibria experiments of Feig et al. (2005, submitted), the presence of mm-sized poikilitic pargasite as a primary phase indicates that the associated melts showed a high water activity. SIMS analyses of clinopyroxenes from different wehrlite bodies reveal REE and trace element patterns suggesting that the clinopyroxenes were crystallized from tholeiitic, MORB-type melts which were highly depleted, not comparable to normal primitive MORB. In a sample from the top of the section the clinopyroxene exhibit trace element patterns with depleted character while the coexisting pargasites show strongly enriched pattern suggesting complex multi-stage differentiation processes or magma mixing. These pargasites show high concentration of chlorine suggesting that at least the fluids involved in the latemagmatic stage of the wehrlite formation may influenced by seawater. With the help of an experimental study under controlled fO2 and aH2O at pressures between 100 and 200 MPa we want to evaluate the conditions of the wehrlitic magmas within the lower crust. Especially, we want to clarify whether the crustal wehrlites from the Oman ophiolite are derived from wet tholeiitic magmas or not. Starting material are mixtures of natural wehrlites and a synthesized glass representing the “lost” equilibrium melt. The composition of this glass was calculated using the major and trace element compositions of the corresponding olivines and clinopyroxenes and the well-known relations on element partitioning between minerals and melt in tholeiitic systems from the literature or from own experimental series. First results are presented.