Geochemical variations in Japan Sea back‐arc basin basalts formed by high‐temperature adiabatic melting of mantle metasomatized by sediment subduction components

Abstract

The Yamato Basin in the Japan Sea is a back-arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D-type) and enriched (E-type) basalts based on their incompatible trace element and Sr-Nd-Pb-Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand-rich turbidite on the Pacific Plate in the NE Japan fore arc. The depleted nature of the D-type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E-type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D-type basalts are deeper and hotter than those for the E-type basalts, which appear to have formed under conditions hotter than those of normal mid-oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea.

Petrogenesis and structure of oceanic crust in the Lau back-arc basin

“…Similar high velocity lower crusts have been reported in other back-arc regions, including the Yamato Basin in the Japan Sea (Sato et al, 2014;Hirahara et al, 2015), the Izu-Bonin-Mariana Arc (Kodaira et al, 2007;Takahashi et al, 2008Takahashi et al, , 2009, and the Aleutian arc (e.g., Shillington et al, 2004;Behn and Kelemen, 2006), as well as rifted continental margins (Korenaga et al, 2000). Sato et al (2014) propose that the anomalous back-arc crust in the Japan Sea is the result of higher mantle potential temperature (e.g., Kelemen and Holbrook, 1995;Korenaga et al, 2002).…”

“…Unlike some back-arc extensional settings where broad crustal rifting and subsequent partial melting and modification of pre-existing arc crust has been proposed (e.g., Takahashi et al, 2008), most new crustal formation in the eastern Lau basin occurs primarily at wellorganized spreading centers. Seismic imaging along these spreading centers indicates a single narrow axial magmatic system (e.g., Jacobs et al, 2007;Dunn et al, 2013), and sonar and magnetic data indicate a narrow and persistent axis of crustal formation for >2 Myr (e.g., Sleeper, 2011;Austin, 2012;Sleeper and Martinez, 2014). The current spreading configuration initiated in the north and propagated southward along the Central Lau Spreading Center (CLSC), Eastern Lau Spreading Center (ELSC), and Valu Fa Ridge (VFR), resulting in the basin's current wedge shape (Fig.…”

“…ELSC3 is characterized by a transition in ridge morphology and axial depth, where the deep (∼2700 m) ridge axis in the north shoals abruptly to ∼1800-2200 m in the south (Fig. 3b), roughly coincident with the appearance of the seismically-imaged axial magma chamber (AMC) reflector (Harding et al, 2000;Jacobs et al, 2007).…”

“…Meanwhile, the low velocity layer that forms the upper crust (seismic layer 2) is abnormally thick (2.9-3.8 km) with unusually low velocities (3.4-4.5 km/s) that extend deeper than expected for a typical volcanic (porous) layer alone (Peirce et al, 2001;Jacobs et al, 2007;Arai and Dunn, 2014). These observations can be explained by the presence of more evolved rock compositions and higher porosities in the upper crust than typical for crust produced at mid-ocean ridges (Jacobs et al, 2007;Arai and Dunn, 2014).…”

“…Gray bar along the left edge denotes area where magma lens reflectors are observed in along-axis multichannel seismic data (Harding et al, 2000;Jacobs et al, 2007). The change in crustal type correlates with the disappearance of the axial magma lens reflector and significant increase in axial depth.…”