Meter Scale Heterogeneities in the Oceanic Mantle Revealed in Ophiolite Peridotites
Haller, Mitchell Bruce
Walker, Richard J
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The upper oceanic mantle is the largest accessible terrestrial geochemical reservoir. Numerous aspects of the upper oceanic mantle’s current state, as well as its chemical evolution through time, remain obscure. Although studies of Mid Ocean Ridge Basalts (MORB), and other oceanic mantle derived melts have provided important insights into the nature of their sources, previous studies have shown that they fail to capture the full range of end-member compositions present in oceanic peridotites. Ophiolites are especially useful in interrogating this issue as field-based observations can be paired with geochemical investigations over a wide range of geologic time. Grid sampling methods (3m x 3m) at the 497 Ma Leka Ophiolite Complex (LOC), Norway, and the 1.95 Ga Jormua Ophiolite Complex (JOC), Finland, offer an opportunity to study mantle domains at the meter and kilometer scale, and over a one billion year timespan. The lithology of each locality predominately comprises harzburgite, hosting layers and lenses of dunite and pyroxenite. Here, we combine highly siderophile elements (HSE) and Re-Os isotopic analysis of these rocks with major and trace element measurements. Two grids sites are studied within the LOC harzburgite mantle section. Harzburgites at individual LOC grid sites show variations in initial γOs(497 Ma) (-2.1 to +2.2) at the meter scale. Analyses of dunites within the same LOC grid, reveal that dunites may either have similar γOs to their host harzburgite, or different, implying interactions between spatially associated rock types may differ at the meter scale. A harzburgite sample is characterized by low initial 187Os/188Os (<0.121), reflecting Proterozoic melt depletion. Preservation of Os isotopic compositions consistent with ancient melt depletion is a common characteristic in oceanic peridotites. Grid sampling of adjacent harzburgites and dunites reveal that the geometry of these refractory domains can be constrained to be < 1 m3. TMA model ages of an LOC websterite reveals at least one other stage of partial melting in the LOC, which broadly corresponds to the opening of the Iapetus Ocean (~620 – 550 Myr). Averaged γOs values between the mantle sections of two LOC grid sites (+1.3 and -0.4) separated by ~5 km, indicate km-scale heterogeneity in the convecting upper mantle. Major and trace element compositions suggest that the km-scale heterogeneity in the LOC, is a result of variable melt-extraction at different depths, and local scale processes. Analyses of two, 1 cm thick orthopyroxenite veins, hosted by harzburgite near Kvaløya-moen, are more radiogenic than host harzburgites, and suggest vein formation had minimal impact on the host harzburgite. Whole rock major and trace element data, and thin sections of relict olivine grains, are also examined to shed light on the causes of the isotopic heterogeneities in the LOC. Two grids sites are studied within the JOC serpentinite mantle section. Serpentinites at JOC grid JU15-16, display modest heterogeneities at the meter scale in γOs(1.95 Ga) (-0.5 to -3.0). TRD model ages show evidence of melt depletion at least 400 Ma prior to the accepted age of the ophiolite. Re-Os systematics of a separate JOC grid site, JU15-18 (~3 km away), show evidence of Re addition/loss at the age of the ophiolite (~1.95 Ga). LREE-enriched REE patterns suggest that this grid location was subsequently affected by metasomatic processes possibly associated with gabbroic dykes, affecting the geochemical and Os isotopic compositions of these JOC serpentinites. Enrichments of fluid mobile elements including Re, Ba, and Sr, may implicate recent Re mobilization caused by weathering and ground-water interactions. Trends in major elements show signs of variable MgO and SiO2 loss by serpentinization.