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
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 contribution 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.
knolls
Mariana
Trough
GH85-55
front
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 backarc 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 backarc 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.