Geochemical Evolution of Arc and Slab Following Subduction Initiation: a Record from the Bonin Islands, Japan

Abstract

Volcanism following the initiation of subduction is vital to our understanding of this specific magma-generation environment. This setting is represented by the first development of the Izu-Bonin-Mariana arc system as subduction commenced along the Western Pacific margin in the Eocene. A new collection of volcanics recovered from the islands and exposed crustal sections of the Bonin Ridge span the first 10 Myr of arc evolution. An elemental and radiogenic isotope dataset from this material is presented in conjuction with new 40Ar/39Ar ages and a stratigraphic framework developed by a detailed mapping campaign through the volcanic sections of the Bonin Islands. The dating results reveal that both the locus and type of magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at around 7-8 Myr. Following initial MORB-like spreading-related basalt magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite/high-Mg andesite, then finally tholeiitic/calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominate the slab input to the shallow source of high-Si boninites at 49 Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45 Ma, volcanism switched to low-Si boninite and the pelagic sediment signature was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMU ocean island source. These low-Si boninites are isotopically consistent with a slab component comprising variable proportions of HIMU volcaniclastics and Pacific MORB. In turn, this signature was replaced by a Pacific MORB-dominated flux in the post 45 Ma tholeiite and calcalkaline volcanics. Notably, each change in slab-derived flux coincided with a change in the magma type. Fluctuations in the slab-derived geochemical signature were superimposed on a change in the mantle wedge source from highly-depleted harzburgite to a depleted MORB-type mantle-type source. In turn, this may correspond to the increasing depth of the leading edge of the slab through this 5 Myr period.

Crustal Accretion in a Slow Spreading Back‐Arc Basin: Insights From the Mado Megamullion Oceanic Core Complex in the Shikoku Basin

“…Whole rock chemistry of Mado Megamullion basalts (Table S1 in the supporting information) was determined at the Geological Survey of Japan/AIST following the method described in Ishizuka et al (2020). Major elements were analyzed on glass beads, prepared by fusing 1:10 mixtures of 0.5 g subsamples and lithium tetraborate.…”

“…Instrument calibration was performed using five to six calibration solutions made from international rock standard materials (including BIR‐1, BCR‐1, AGV‐1, JB1a, and BEN). Reproducibility is generally better than ±4% (RSD) for the REE and better than ±6% (RSD) for other elements, except those with very low concentration and Ni (see BHVO2 analysis in Ishizuka et al, 2020). Loss on ignition (LOI) was determined for both samples and yield rather low values (3.20–3.42 wt%; Table S1).…”

Slabitization: Mechanisms controlling subduction development and viscous coupling

“…A classic example is the Western Pacific Izu-Bonin/Marianas (IBM) forearc formed following early Eocene subduction initiation (e.g., Stern and Bloomer, 1992;Reagan et al, 2019 and references therein). Short-lived mantle upwelling, following subduction initiation, is documented at ~ 52-51 Ma by the eruption of forearc basalts (FAB; representing melting of undepleted but dry mantle), then boninites ~1 Ma later, and finally classical calc-alkaline volcanics at ~ 45-44 Ma (Reagan et al, 2010(Reagan et al, , 2019Ishizuka et al, 2011Ishizuka et al, , 2020. But no oceanic lithosphere was obducted nor metamorphic soles exhumed, so that the start of subduction cannot be firmly established.…”

“…(e) From stage (d) onwards, subduction is mature/steady, with stable viscous coupling, formation of (rarely exhumed) LT eclogites, and melting of the hydrated mantle wedge at sub-arc depths marked by calc-alkaline magmatism. The Izu-Bonin example suggests that this may have happened ~7 Ma after ophiolitic lithosphere formation (Ishizuka et al, 2020).…”

Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

“…Younger (~45-44 Ma), boninite and 2-pyroxene andesites outcrop in the northern parts of the Chichijima Island group 19 . Tholeiitic basalts and andesitic ("calc-alkaline") rocks 44-37 Ma age occur in the Hahajima islands, south of Chichijima 20 .…”