The earliest stage of Izu rear‐arc volcanism revealed by drilling at Site U1437, International Ocean Discovery Program Expedition 350

Abstract

The International Ocean Discovery Program Expedition 350 drilled between two Izu rear-arc seamount chains at Site U1437 and recovered the first complete succession of rear-arc rocks. The drilling reached 1806.5 m below seafloor. In situ hyaloclastites, which had erupted before the rear-arc seamounts came into existence at this site, were recovered in the deepest part of the hole (15-16 Ma). Here it is found that the composition of the oldest rocks recovered does not have rear-arc seamount chain geochemical signatures, but instead shows affinities with volcanic front or some of the extensional zone basalts between the present volcanic front and the rear-arc seamount chains. It is suggested that following the opening of the Shikoku back-arc Basin, Site U1437 was a volcanic front or a rifting zone just behind the volcanic front, and was followed at~9 Ma by the start of rear-arc seamount chains volcanism. This geochemical change records variations in the subduction components with time, which might have followed eastward moving of hot fingers in the mantle wedge and deepening of the subducting slab below Site U1437 after the cessation of Shikoku back-arc Basin opening.

Tuffaceous Mud is a Volumetrically Important Volcaniclastic Facies of Submarine Arc Volcanism and Record of Climate Change

“…The chemical evolution of the Izu volcanic front is well known from studies of bulk discrete ash and tuff intervals (sometimes called turbidites), and their individual vitriclasts, in ODP cores from the forearc (Bryant et al, ; Gill et al, ; Straub et al, ). One rationale for IODP Expedition 350 was to do the same for the rear arc, and some studies of discrete ash and tuff intervals from all Lithologic Units except III are underway (Heywood et al, ; Miyazaki et al, ; Sato et al, ; Schindlbeck et al, ). The tuffaceous mud(stone)s are compromised for this purpose because they contain variable amounts of foreign silicate material, bioclasts, and volcanic material from the volcanic front.…”

“…Analyses of both bulk rocks and individual vitriclasts suggest that most of the local component was felsic since 2 Ma. At least two different types of magmatism co‐existed in the rear arc from ∼9 Ma to at least ∼2 Ma: the geochemically enriched Rear Arc Seamount Chain (RASC) type; and a more depleted type like that of the Back Arc Knolls (BAK), or the underlying Unit VII “basement” (Sato et al, ). Both types seem to be recurring sources of the tuffaceous mud(stone) vitriclasts.…”

“…The Th and K contents and Th/K ratio all decrease below 1,280 mbsf in Unit V and become similar to those of the basaltic volcaniclastics of Unit VII that are broadly BAK-like . Following Sato et al (2017), we attribute the shallower, more enriched sediments to the onset of RASC-type magmatism in the vicinity of Site U1437 at 9 Ma which is the age of basalt dredged from the nearby Kanbun Seamount (Ishizuka et al, 2002a).…”

“…Gill et al, 1994;Straub et al, 2004). One rationale for IODP Expedition 350 was to do the same for the rear arc, and some studies of discrete ash and tuff intervals from all Lithologic Units except III are underway (Heywood et al, 2016;Miyazaki et al, 2017;Sato et al, 2017;Schindlbeck et al, 2018). The tuffaceous mud(stone)s are compromised for this purpose because they contain variable amounts of foreign silicate material, bioclasts, and volcanic material from the volcanic front.…”

“…As usual, more attention has been paid to the coarser, more obviously volcaniclastic intervals (e.g., Heywood et al, ; Miyazaki et al, ; Sato et al, ; Schindlbeck et al, ). Discrete layers are mostly ash or tuff (i.e., average grain size <2 mm); coarser material (lapilli‐ash, lapilli‐tuff, lapillistone, or breccia) is minor (<5%).…”