Geochemical Variation of Miocene Basalts within Shikoku Basin: Magma Source Compositions and Geodynamic Implications

Abstract

Shikoku Basin is unique as being located within a trench-ridge-trench triple junction. Here, we report mineral compositions, major, trace-element, and Sr-Nd-Pb isotopic compositions of bulk-rocks from Sites C0012 (>18.9 Ma) and 1173 (13–15 Ma) of the Shikoku Basin. Samples from Sites C0012 and 1173 are tholeiitic in composition and display relative depletion in light rare earth elements (REEs) and enrichment in heavy REEs, generally similar to normal mid-ocean ridge basalts (N-MORB). Specifically, Site C0012 samples display more pronounced positive anomalies in Rb, Ba, K, Pb and Sr, and negative anomalies in Th, U, Nb, and Ta, as well as negative Nb relative to La and Th. Site 1173 basalts have relatively uniform Sr-Nd-Pb isotopic compositions, close to the end member of depleted mantle, while Site C0012 samples show slightly enriched Sr-Nd-Pb isotopic signature, indicating a possible involvement of enriched mantle 1 (EM1) and EM2 sources, which could be attributed to the metasomatism of the fluids released from the dehydrated subduction slab, but with the little involvement of subducted slab-derived sedimentary component. Additionally, the Shikoku Basin record the formation of the back-arc basin was a mantle conversion process from an island arc to a typical MORB. The formation of the Shikoku Basin is different from that of the adjacent Japan Sea and Parece Vela Basin, mainly in terms of the metasomatized subduction-related components, the nature of mantle source, and partial melting processes.

Strontium Isotope Characteristics (δ88/86Sr, 87Sr/86Sr) of Arima‐Type Brines Originated From Slab‐Fluids

“…Modeling of slab‐fluid from the subducted PHS plate is shown on a “true" 87 Sr/ 86 Sr* versus δ 88/86 Sr diagram (Figure 3). The following values were applied, for modeling: AOC attributes can be represented as δ 88/86 Sr = 0.31‰ for the average MORB (Amsellem et al., 2018), 87 Sr/ 86 Sr = 0.7035 for the averages of oceanic basalts from neighboring region near along the Nankai Trough (West Philippine, Parece Vela, and Shikoku Basin basalts, Chen et al., 2021; Hickey‐Vargas, 1991), and Sr = 120 μg g −1 (Plank et al., 2007). Oceanic carbonates could be characterized as δ 88/86 Sr = 0.20‰ and 87 Sr/ 86 Sr = 0.7080 for the Cretaceous‐Eocene carbonates (Voigt et al., 2015; Vollstaedt et al., 2014), and Sr = 330 μg g −1 for carbonate sediments (Krabbenhöft et al., 2010).…”

“…One of the fluid‐mobile elements, strontium is a particularly useful tracer of mass transfer in subduction zones as the presence of both radiogenic and mass‐dependent isotope systems enables different processes to be resolved. The strontium isotopic data of the Arima saline water ( 87 Sr/ 86 Sr = 0.70853–0.70879; Kusuda et al., 2014) cannot be determined to be representative of slab‐fluids because the 87 Sr/ 86 Sr values overlap not only with the slab‐fluids estimated as a mixed component between the subducted sediments ( 87 Sr/ 86 Sr = 0.7100; Plank et al., 2007) and the altered oceanic crust (AOC) ( 87 Sr/ 86 Sr = 0.7035; Hickey‐Vargas, 1991; Chen et al., 2021) but also with those of the basement, the Rokko granite ( 87 Sr/ 86 Sr = 0.7084–0.7440; Terakado & Nohda, 1993). Because stable strontium isotope values (δ 88/86 Sr = [( 88 Sr/ 86 Sr) sample /( 88 Sr/ 86 Sr) SRM987 − 1] × 10 3 ) reflect isotopic fractionation without time‐integrated effects, they provide a different perspective of the resultant radiogenic strontium ( 87 Sr/ 86 Sr) composition.…”