Subduction-zone magmas are characterized by high fluid content and Fe3+/ΣFe ratios are closely related to fluids derived from the slab during subduction. However, instead of heavy iron isotope compositions in the oxidized state, arc magmas have lighter iron isotope compositions than seafloor basalts, including mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB).
It is not clear whether the light iron isotope composition of subduction-related magmas is inherited from a depleted mantle source or is due to the addition of slab-derived oxidized fluids.
Recently, Prof. from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS). A research team led by Xiao Yuanyuan reported new Fe isotope data for low-Ca bonite from the Bonin Islands and Troodos ophiolite complex in Cyprus, thought to be the result of melting of a highly depleted refractory mantle with different effects of slab-derived fluids. Their study was published Lithos.
The researchers found that δ56The Fe values of bonin boninite were highly variable and affected by mineral cumulates, whereas δ56The Fe values of troodos boninite were relatively uniform. Average δ with literature data on bonite from New Caledonia56The Fe value of the Troodos high-Ca boninite is higher than that of the low-Ca boninite from Bonin and New Caledonia, the latter originating from a more refractory mantle source. Thus, the iron isotope variation of boninite between different sample suites reflects heterogeneous mantle composition caused by different past depletion histories.
Furthermore, the researchers found an inverse correlation of decreasing δ56Fe with increasing Ba/La in troodos boninite reflecting the addition of aqueous fluids to the mantle source for metasomatism during the long evolutionary history prior to melting. Bonin bonite magma, on the other hand, is best understood as the result of highly refractory mantle melting mixed with slab melt.
This study shows that the influence of the subduction component on the iron isotope composition of bonite may be limited (eg, Troodos bonite) or hidden (eg, Bonin bonnite), while the influence of highly refractory mantle sources is more significant. This may be responsible for the relatively lighter iron isotope composition of subduction-related magmas than that of seafloor basalts.
“Given the limited data on the iron isotope composition of boninite so far, a systematic study of the iron isotope composition of boninite is needed to further understand the petrogenesis of boninite and the iron isotope variation of subduction-related magmas,” said. Xiao Prof.
Yuanyuan Xiao et al, Diverse mantle source compositions for bonite from Bonin and Troodos, evidence from iron isotope variations, Lithos (2023). DOI: 10.1016/j.lithos.2023.107214