Backarc cross-chain volcanism and chemistryOsamu Ishizuka (GSJ, IFREE)

1. Southern Izu-BoninNorthern end of Mariana Trough, back-arc ridge and West MarianaRidge

2. Northern Izu-BoninSeamounts in the Shikoku Basin,Kinan Seamount Chain

M. Yuasa, Y. Tamura,H. Shukuno, R.J. Stern, M. Joshima, J. Naka

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 ceasedbefore 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.

•Minami Io-to Spur and West Mariana Ridge show clear “arc signature” .

Trace element composition

•Parece Vela Rift basalts only show limited enrichment in LILE if any. → 　 almost free from slab component?

Pb isotopic composition

•Minami Io-to Spur shows less radiogenic composition relative to front, implying less contribution of slab component

NHRL

•West Mariana Ridge and Quaternary front might form a single trend.

Mariana arc

Pb isotopic composition

•West Mariana Ridge-Quaternary front trend might correspondto a mixing line between slab components, such as pelagic sediments and volcaniclastics.

Pb isotopes-Ce/Pb relationship

•West Mariana Ridge seemsto have slightly larger contribution of sediment with higher 7/4.

•WMR and Quaternary frontcan be explained by contribu-tion of melt of subducted chert, pelagic sediment and volcaniclastics.

• 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)

Slab component: trace element signature

Nd isotopes-Th enrichment

•West Mariana Ridge and Quaternary front show higher Th/Ce and low 143/144 relativeto MORB (and Parece Vela Rift),which seem to be compatible with1-2% 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.

•This is distinct from northern arc,where composition of slab component is different betweenarc front and back-arc.

•Minami Io-to Spur seems to have smaller amount of slab component with similar composition to the volcanic front.

Structure of Northern tip of Mariana Trough

GH85-55frontknolls Mariana

Trough

•Normal faulting developed in and west of the trough.•basaltic knolls in extensional stress regime.

•Basaltic knolls on WMR overlap with volcanic front.

•Slightly larger variation relative to front, and distinct from Northern Mariana Trough

Sr, Nd, Pb isotopic compositions

•HIMU component seems to have been present. What is the age?

Northern Izu-Bonin back-arc

Age of volcanism: Back-arc seamount chains

(Ishizuka et al., 2003)

Myojin knoll

Sumisu-jima

Aogashima

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)

•Kinan Seamount Chain had

been active for c. 8 m.y.

since cessation of Shikoku

Basin spreading.

Age of volcanism (Kinan Seamount Chain)

•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-arcSeamount.

•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)

• Back-arc seamounts show clear “arc signature”.

• Kinan Seamount Chain is highly enriched in incompatibleelements and shows no depletion in HFSE.

Trace element chemistry

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.

•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.

Slab input in the northern Izu-Bonin back-arc

Isotopic composition

•Back-arc seamounts might be explained by addition of subducted sediment melt (or fluid)to MORB source.

(Hochstaedter et al., 2001; Ishizuka et al., 2003, unpubl. data)

•Kinan Seamount Chainmight have enriched mantle source with relatively low 206/204 and 143/144 (EMI like).

•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

Kinan Seamount Chain: Temporal variation

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.