Generation of basalt and the effect of carbon-bearing fluids on partial melting of peridotite and eclogite: from trace elements in olivine to petrological experiments

Primitive basalts are the main rock samples to probe mantle heterogeneity and understand the process of partial melting of the mantle. In comparison to the well-known Mid-Ocean Ridge Basalts and Ocean Island Basalts, the generation of Intra-plate continent basalts is still controversial. In this study, I followed two approaches to understand the genesis of these basalts: (i) a study of intra-plate basalts from southeastern Australia, through trace element analyses of olivine in combination with whole-rock compositions, (ii) an experimental study focused on the effect of fluid compositions under different oxygen fugacity conditions on the partial melting of Iherzolite and eclogite. For the study of natural samples, I collected Cenozoic primitive basalts from various locations in New South Wales, Australia. Two locations (Dubbo and Bingara/Inverell (Central) are close to a major step in thickness of the lithosphere, while two other localities lie close to small areas of anomalously thicker lithosphere (Ebor and Barrington). In addition, I used samples from Buckland in Queensland as a complement which lies in another province but also close to this lithosphere edge. Based on a correlation between Mn/Fe and Ni/Mg ratios and concentrations of Ni, Ca, TI, Zn and Li in olivine, and whole rock compositions, tholelite from Dubbo was found to originate from a plume-type pyroxenite source which had experienced a low degree of carbonatitic metasomatism. Similarly, the alkali basalt from Dubbo and Ebor also comes from a pyroxenite-associated source. Basanite from Bingara/Inverell (Central) has experienced the highest degree of carbonatitic metasomatism in the source and the basanite from Barrington comes from a source formed by metasomatism of peridotite reacted with carbonate melt. Alkali basalt and basanite from Buckland have a similar source to Barrington, Partial melting experiments were conducted on eclogite and herzolite at conditions of 2 GPa and 6 GPa, 900-1500 °C. Starting compositions contained 5 wt.% H2O for oxidizing conditions and 5wt.% H2O and CH4 for reducing conditions. Based on the mineral and melt phases and solidus temperature obtained from the experiments, it turns out that under reducing conditions, both the solidi of eclogite and herzolite with C-O-H fluids are subparallel to the anhydrous solidi and higher than those under oxidizing conditions. The mineralogy of eclogite and lherzolite residues depends on fog. At 2 GPa partial melting of therzolite with 5% HO forms a basaltic melt while partial melting of therzolite with 5% CO-H fluids forms a basaltic andesite melt. In contrast, the melt from partial melting of eclogite with 5% H20 varies from basalt, through basaltic andesite to andesite with increasing temperature. The generation depth of Cenozoic basalts from southeastern Australia is close to the conditions of my experiments. The experiments showed that at this pressure and temperature range, hydrous metasomatism of eclogite does not lead to the formation of intra-plate basalts, whereas it is possible to form intra-plate basalts by partial melting of hydrated (> 1,000 ppm H,0) peridotite. The Cenozoic basalts from Southeastern Australia were not formed through this latter mechanism, but were generated by partial melting of a pyroxenitic or plume-associated pyroxenitic source metasomatized by carbonatitic melts.