Traces of Recycled Fe-Ti Gabbros in the Sources of Ferropicrites

Ferropicrites are sub-alkaline or mildly alkaline primitive magmatic rocks (MgO = 12-18 wt. %) characterized by exceptionally high FeOtot contents (> 13 wt. %) compared to other picrites. They are found in continental flood basalt (CFB) provinces as relatively thin basal lava flows (e.g., Paraná-Etendeka) or, in some cases, as dikes with ambiguous age relationships (e.g., Karoo). Ferropicrites typically are nearly uncontaminated and thus provide geochemical information on the asthenospheric mantle. The combination of unusually high Fe-contents and primitive olivines (up to Fo88) in near-primary ferropicrites is indicative of high mantle potential temperatures (~1600 °C) relative to mid-ocean ridges thus bearing evidence for mantle plume sources in LIPs. High (Sm/Yb)CN ratios (3-7) and low Al2O3 contents (~10 wt. %) indicate residual garnet in the mantle source of ferropicrites and high Ni contents and melting experiments suggest that the source was dominated by olivine-free pyroxenite. Such a garnet pyroxenite source has been recognized in many hotspots and has been ascribed to melt-olivine reactions in ascending eclogite-bearing garnet peridotite. Most of the purported pyroxenite-derived picrites have lower FeOtot contents (10-12 wt. %) compared to ferropicrites, however, which suggests that the generation of ferropicritic liquids requires specific melting conditions or exceptionally Fe-rich pyroxenite composition, or both. The characteristic positive V anomaly of ferropicrites is extremely rare in continental and oceanic basalts but common in cumulate Fe-Ti gabbros. In order to explain the geochemical differences between ferropicrites and common picrites we have modeled the behaviour of REE and V during melting of mantle pyroxenite with eclogite component representing distinct parts of subducted oceanic crust. Firstly, we modeled partial melting of 'basaltic' and 'Fe-Ti gabbroic' eclogites assuming 50% modal batch melting with clinopyroxene/garnet ratio of 9:1. Partial melting of secondary pyroxenite (eclogite/peridotite = 1:2) was modeled assuming modal batch melting with source modes corresponding to 2.5 GPa and 5.0 GPa conditions. The model parameters were adopted directly from experimental studies and the compositional data for the different components represent averages or typical values. Our results suggest that ferropicrites with high (V/Lu)PMN ratios (~1.5-2.5) contain traces of subducted Fe-Ti gabbros, whereas the geochemistry of many common picrites [(V/Lu)PMN ≤ 1.2] can be explained by partial melting of mantle pyroxenite including a MORB-type eclogite component. Some ferropicrites with relatively lower (V/Lu)PMN ratios may have been generated from ferrobasalt-bearing mantle sources, as low-degree melts from common pyroxenite sources, or, in fact, may not represent primitive ferropicritic melts at all. Interestingly, some mildly alkaline Fe-rich picrites from the rejuvenated parts of the Hawaii volcanic chain have anomalous high (V/Lu)PMN and may indicate the presence of subducted Fe-Ti gabbros in the Hawaiian plume.