Backarc cross-chain volcanism and chemistry Osamu Ishizuka (GSJ, IFREE) M. Yuasa, Y. Tamura, H. Shukuno, R.J. Stern, M. Joshima, J. Naka 1. Southern Izu-Bonin Northern end of Mariana Trough, back-arc ridge and West Mariana Ridge 2. Northern Izu-Bonin Seamounts in the Shikoku Basin, Kinan Seamount Chain

Minami Io-to Spur & West Mariana Ridge

Age of volcanism •Volcanism on Minami Io-to Spur seems to have been active after cessation of spreading of Parece Vela Basin. •West Mariana Ridge had been active at least until c. 4 Ma and ceased before initiation of rifting in Mariana Trough.

Major element composition •Basalts from Minami Io-to Spur is less alkaline relative to West Mariana Ridge. •West Mariana Ridge overlaps with Quaternary front in K2O content.

Trace element composition •Minami Io-to Spur and West Mariana Ridge show clear “arc signature” . •Parece Vela Rift basalts only show limited enrichment in LILE if any. →　almost free from slab component?

Pb isotopic composition •West Mariana Ridge and Quaternary front might form a single trend. •Minami Io-to Spur shows less radiogenic composition relative to front, implying less contribution of slab component Mariana arc NHRL

Pb isotopic composition •West Mariana Ridge-Quaternary front trend might correspond to a mixing line between slab components, such as pelagic sediments and volcaniclastics.

Pb isotopes-Ce/Pb relationship •West Mariana Ridge seems to have slightly larger contribution of sediment with higher 7/4. •WMR and Quaternary front can be explained by contribu- tion of melt of subducted chert, pelagic sediment and volcaniclastics.

Slab component: trace element signature • Enrichment of Ba for West Mariana Ridge and Minami Io-to Spur seem to be compatible with contribution of melt of sediment. • Parece Vela Rift basalts fit to global MORB-OIB trend. (no enrichment of Ba)

Nd isotopes-Th enrichment •West Mariana Ridge and Quaternary front show higher Th/Ce and low 143/144 relative to MORB (and Parece Vela Rift), which seem to be compatible with 1-2% contribution of melt of sediment and volcaniclastics. •Minami Io-to Spur has less contribution of slab-derived melt.

Endmembers •West Mariana Ridge and Quaternary front might have contribution of 1-2% of mixture of melt of non-volcanic sediment and HIMU volcaniclastics + mafic oceanic crust. •Minami Io-to Spur seems to have smaller amount of slab component with similar composition to the volcanic front. •This is distinct from northern arc, where composition of slab component is different between arc front and back-arc.

Structure of Northern tip of Mariana Trough •Normal faulting developed in and west of the trough. •basaltic knolls in extensional stress regime. GH85-55 front knolls Mariana Trough

Sr, Nd, Pb isotopic compositions •Basaltic knolls on WMR overlap with volcanic front. •Slightly larger variation relative to front, and distinct from Northern Mariana Trough •HIMU component seems to have been present. What is the age?

Northern Izu-Bonin back-arc

Age of volcanism: Back-arc seamount chains Aogashima Myojin knoll Sumisu-jima (Ishizuka et al., 2003)

Age of volcanism (Eastern Shikoku Basin) •Isolated seamounts show similar age range to back-arc seamount chains (17-3 Ma), but correspond to the oldest age range (14.4-8.9 Ma). (Ishizuka et al., 1998, 2002, 2003, unpubl. data)

Age of volcanism (Kinan Seamount Chain) •Kinan Seamount Chain had been active for c. 8 m.y. since cessation of Shikoku Basin spreading. •No systematic temporal variation of locus of volcanism

Temporal variation of locus of volcanism •Age of Kinan Seamount Chain volcanism overlap with that of back-arc Seamount. •However Kinan Seamount Chain does not fit to the correlation curve between age of volcanism and longitude of volcano.

Major element composition • More alkaline compared to volcanic front • Kinan Seamount Chain is more alkaline than back-arc seamounts. (Taylor & Nesbitt, 1998; Hochstaedter et al., 2000; Ishizuka et al., 2003, Tamura et al., 2004)

Trace element chemistry • Back-arc seamounts show clear “arc signature”. • Kinan Seamount Chain is highly enriched in incompatible elements and shows no depletion in HFSE.

Across-arc variation •More enriched in incompatible element toward back-arc side (west). •Kinan Seamount Chain is rich in incompatible elements, but shows no clear spatial variation.

Enrichment of Th in the back-arc •Kinan Seamount Chain has high Th, but no enrichment relative to other incompatible element.→ No subduction input? •Back-arc seamounts seem to have enrichment of Th caused by slab input.

Slab input in the northern Izu-Bonin back-arc •Back-arc seamounts have weak Ba enrichment, which becomes weaker toward west. •Kinan Seamount Chain appears to have no Ba enrichment, again compatible with no subduction input.

Isotopic composition •Back-arc seamounts might be explained by addition of subducted sediment melt (or fluid) to MORB source. •Kinan Seamount Chain might have enriched mantle source with relatively low 206/204 and 143/144 (EMI like). (Hochstaedter et al., 2001; Ishizuka et al., 2003, unpubl. data)

Kinan Seamount Chain: Temporal variation •More enriched in incompatible elements with time •Melting of more enriched mantle and/or lower degree of melting with time after back-arc spreading ceased

Summary 1. Across-arc geochemical variation can be recognized along cross-chains and back-arc edifices both in northern and southern Izu-Bonin arc. 2. Major difference between the two is: In the northern arc composition of slab-derived component seems to be significantly different between the front and back-arc (fluid vs. melt, etc.). In the southern arc slab component in the back-arc may have similar composition to the front, and amount of contribution is smaller (melt? both in front and back-arc)

Summary 3. Distinct isotopic characteristics in the southernmost part of the Izu-Bonin arc seems to have persisted since Miocene. But HIMU character might have appeared much later. 4. Two contrasting volcanism had been active in the back-arc since cessation of spreading of Shikoku Basin. Both of them are characterized by general enrichment in incompatible elements. However, a) Back-arc seamount chains and seamounts in the eastern Shikoku Basin show clear signature of slab input (e.g. sediment melt), i.e., product of arc volcanism. b)Kinan Seamount Chain has no indication of slab input. Low degree of melting of enriched mantle source seems to have been caused by residual, waning heat flux or convection after the cessation of back-arc spreading.