Suggesting new origin of the HIMU lavas from trace element analysis
Chemical differentiation of the silicate Earth is driven primarily by mantle melting and recycling through plate tectonics. During mantle melting, contents of each element and isotopic ratio in the rock are changed by preferential enrichment or leaching to specific phase and state. This tendency is dependent on intrinsic property of each element. Like every people have different characteristics and personalities, each element shows different pattern and degree for chemical differentiation. By using this concept, we can go near the inside of Earth, and sometimes we can get evidence at different angle from previous idea.
Here, introduce one paper (Weiss et al., 2016) which investigated chemical composition of olivine phenocrysts in HIMU (having a high U/Pb ratio) lavas to reveal its origin. HIMU lava are one of puzzling study subject in oceanic basalt in sea floor system, and its origin is not clearly understood despite of many previous studies. Generally, HIMU lavas has been considered to have origin of recycled basaltic oceanic crust via hydrothermal alteration and slab dehydration process, which makes the alkali metals and Pb leached. But, Pb isotopes in MORB do not trend towards HIMU lavas, and lithology of recycled basalt (pyroxenitic mantle) is not sufficient for explaining minor and trace element compositions in olivine phenocrysts found in HIMU basalts indicating olivine-dominated (peridotitic) mantle rocks.
Dr. Weiss and his team suggested new evidences for the origin of these HIMU lavas from analysis of their trace element composition. They analyzed HIMU lavas from Mangaia, Tubuai, and Karthala. Comparing with MORBs, MTK (Mangaia, Tubuai, and Karthala) lavas has low Mg/Fe ratios (Fo88-77 verses Fo92) which decreases as magmas undergo crystallization. Also, in the similar Fo#, Ni abundances indicate a dominant role for peridotite lithologies in the HIMU source. Additionally, Mn/Fe ratio also provides evidence for production of HIMU magmas by peridotite melting. This observation support that HIMU lavas are from peridotite rocks, not recycled basalts.
Ca/Al ratios in HIMU and Karthala olivines are anomalously high than MORBs, but temperature condition doesn’t explain the high Ca/Al ratios in HIMU olivine, because Ca/Al ratios decrease with increasing temperature. Thus, Weiss et al suggested that high Ca/Al ratios indicate an enrichment process that involves carbonatitic fluids. Trace-element patterns of HIMU lavas are similar to Group I kimberlites, and carbonatitic melts in diamond fluid inclusions. Both are subcontinental lithospheric mantle (SCLM). Furthermore, melt trace-element patterns from HIMU lavas are very similar to SCLM sources metasomatized by carbonatitic fluids such as diamond inclusions.
Isotope composition shows low Os isotope ratios in SCLM xenolith, but with the high values in HIMU lavas. But the most melt-depleted xenoliths, harzburgite, often show elevated Re/Os and Os isotope ratios higher than HIMU values. Its results is consistent with deep carbonatitic metasomatism such as diamond inclusions. Thus, Weiss and his team concluded that Archean carbonatite-metasomatized lithospheric mantle is the origin of HIMU lavas from the evidence of high Ca/Al ratio, isotope trends including high Os ratio, and mass-independent fractionate sulfur isotopes. But, the timing of isotopic evolution and fractionation mechanism remain unconstrained, so further studies may be conducted to fully understand the generation of HIMU lavas.
Figure 1 | Conceptual model for the evolution of the HIMU mantle source. (Weiss et al., 2016)
(1) Subduction during the Archean or early Proterozoic recycled incompatible elements and mass-independent fractionated sulfur from the Earth’s surface to the mantle. (2) The subducting slab released fluidsmelts with carbonatitic affinity that metasomatized the (melt-depleted) SCLM and also produced diamonds (white diamonds), and are related to kimberlite eruption (purple line); metasomatized SCLM develops high Pb and Os isotope ratios with time. (3) The SCLM delaminated and was transported to a mantle boundary layer, probably the core–mantle boundary. (4) Entrainment of recycled SCLM into upwelling plume at the core–mantle boundary. (5) The upwelling plume generated the HIMU OIB.
