Researches on Os isotopic compositions of mantle peridotites from New Caledonia (southwest Pacific Ocean) Ophiolites (NCO) have been conducted by a research team led by Chuanzhou LIU, a research scientist from the Lab for Marine Geology in QNLM, and the Institute of Geology and Geophysics, Chinese Academy of Sciences. The NCO have been classified as the stereotype of forearc ophiolites (fig.1). The harzburgites from the NCO have experienced high degrees of fluid-fluxed melting (e.g., spinels with high Cr# values), which have produced the Tertiary boninitic lavas. Os isotopes indicate that some New Caledonia peridotites are characterized by Os concentrations of <1 ppb, yet have 187Os/188Os ratios comparable to those of the ambient oceanic mantle (i.e., 187Os/188Os< 0.13). This confirms that little crustal Os was transported to the forearc mantle via slab dehydration.
Figure 1. Geological map of New Caledonia (southwestern Pacific Ocean).
Figure 2. 187Os/188Os versus Os contents
In comparison, some mantle wedge xenoliths containing < 1 ppb have187Os/188Os ratios much more elevated than the ambient oceanic mantle (Fig. 2). This supports that little radiogenic Os was transferred from the subducting slab to the forearc mantle, suggesting Os is immobile in the oxidizing and Cl-rich fluids generated by slab dehydration. According to researcher Chuanzhou LIU, the contrasting Os isotopes between forearc peridotites and mantle wedge xenoliths may reflect the changing behavior of Os in diverse agents released from the descending slab as a function of depth through slab dehydration or melting, which is mainly controlled by the stability of sulfides in the slabs. During dehydration at shallow depths, sulfides keep stable and thus little Os is transported to the overlying mantle. In comparison, sulfides become unstable and tend to break down at deeper depths where slab melting or supercritical fluid generation occurs, and thus Os behaves like a mobile element. Such a change is well manifested in mantle wedge xenoliths from different volcanoes in the Kamchatka arc. Peridotite xenoliths from shallow depths display 187Os/188Os ratios comparable to that of ambient oceanic mantle. In comparison, peridotite xenoliths from both the Bakening and Valovayam volcanoes were derived from greater mantle depths and have been subjected to metasomatism by slab-derived melts; they display187Os/188Os ratios remarkably elevated above ambient oceanic mantle.
This paper has been published on the world- renowned journal Geology on March, 2018.（Liu et al. Limited Recycling of Crustal Osmium in Forearc Mantle during Slab Dehydration, Geology, 46(3), 239-242）。