Magma Differentiation within Lithospheric Mantle: a Possible Source for Mantle Heterogeneity

Variations in radiogenic isotope composition observed in oceanic basalt indicate that the mantle is heterogeneous at large and small scales. The recycling of subducted oceanic crust with the addition of sediment is commonly considered to be responsible for the formation of these mantle heterogeneities [1]. Alternatively, these heterogeneities observed in the OIB source could be related to the recycling of a metasomatised lithosphere [2-5]. Following the model suggested by [5], compositional variation in the mantle source, necessary to generate time distinct end-member compositions, could be explained by an evolution of metasomatic vein compositions present in the recycled heterogeneous lithosphere.. However, the magmatic process that generates these veins remains unconstrained. Here we present new experimental data, combined with a study of mantle veins from the Pyrenees, which provide constraints on phase equilibria, solid and liquid compositions along the liquid line of descent of primary basanitic magmas at lithospheric mantle pressure. A series of hydrous experiments have been performed in an end-loaded piston cylinder apparatus on a primitive basanite at 1.5 GPa and temperatures in the range 1250-980°C. Near-perfect fractional crystallization was attained in a stepwise manner using 30°C temperature steps and starting compositions corresponding to the liquid composition of the previous, higher-temperature glass composition. The minerals that crystallized during these experiments varied from cpx + olivine (4:1) in mafic liquid compositions to amphibole + accessory phases (spinel, ilmenite and apatite) in more differentiated ones. Peralkaline liquid compositions were generated below 1040°C and are characterized by a low MgO and FeO tot content, but an extreme enrichment in Na2O, K2O and Al2O3. These enrichments can be explained by the lack of feldspar crystallization during all experiments. The mineralogical compositions of the Pyrenean mantle veins show clear similarities to the fractionated phases produced by the fractional crystallization experiments. The same transition from clinopyroxenite to hornblendite and the same range of Mg# variation is observed in vein and experimental minerals. The correlation between the decrease of Cr content in cpx or amphibole and the decrease of their respective Mg# indicate a clear disequilibrium of differentiated veins with the surrounding mantle. This suggests that the formation of anhydrous and hydrous veins could be linked by a differentiation process and that the formation of these mantle veins could be approximated by a fractional crystallization process. The evolution of trace elements from basic to differentiated veins confirms that it is possible to fractionate trace element ratios, such as Nb/La and U/Pb, in a metasomatized heterogeneous lithosphere. With time, the different parts of this lithosphere will evolve to distinct isotopic compositions. Furthermore, considering the extremely high degree of trace element enrichment in veins, a small percentage of veins could significantly modify the trace element "signature" of the lithospheric mantle without significantly altering whole mineral composition of this heterogeneous mantle. Such a mantle would be a perfect candidate for OIB formation. [1] Hofmann, White. 1982 EPSL 57, 421. [2] Halliday et al. 1995 EPSL 133, 379. [3] McKenzie, K.R. O'Nions. 1995 J. Petrol. 36, 133. [4] Nui, O'Hara. 2003 J. Geophys. Res.,108, B4, 2209. [5] Pilet et al. 2005 EPSL 236, 148. [6] Pilet et al. 2004 Geology, 32, 2, 113.