Current Research
Submarine arc-backarc magmatism in the Papua New Guinea (PNG) - Solomons -Vanuatu region
There are two main reasons for resorting
to ship-based sampling of the products of arc and backarc magmatism.
The first of these is the fact that glass is more commonly preserved in
lavas that are quenched in water, and this material is essential for
tracking the processes of magmatic evolution. The second is that
detailed analysis and quantification of the fluxes of gases in the
arc-backarc environment is only really practicable in the submarine
realm, where the gases are trapped in the water column. Accordingly, I
have been collaborating with colleagues from the Division of
Exploration & Mining of CSIRO (North Ryde, Sydney) in the use of
the RV Franklin in cruises to the SW Pacific. We have successfully
recovered spectacular suites of young, glassy volcanic rocks in the
past two years, and discovered newly active backarc spreading centers
in the Coriolis Troughs of Vanuatu. In 2002, we have two cruises
scheduled for the Bismarck (PNG) and San Cristobal (between the
Solomons and Vanuatu) arcs. |
Asama volcano in central Honshu, Japan. |
Arc magmatism in a region of ridge subduction
It is the fate of all ridges to be
subducted, but there are a limited number of examples of this
phenomenon occurring at any one time. One of the active examples is in
the Solomon Islands, where the Woodlark Ridge is being subducted
eastwards beneath the New Georgia Group of volcanoes. A remarkable
array of lava types has been erupted in the Pliocene to Recent from
these volcanoes, including highly magnesian picrites, andesites, and
dacites. We are obtaining comprehensive geochemical analyses of these
rocks, determining their ages, and attempting to model the particular
processes of magma generation and evolution associated with ridge
subduction. |
RV Franklin |
The deep petrologic structure of island arcs
It is a fact that deep structures of
island arcs are rarely exposed (exceptions include the Horoman Complex
in Hokkaido, and the Jijal Complex in the Himalayas). Despite the
consensus that island arc magmatism is critical in formation of the
continental crust, we are ignorant of much of the plutonic processes
accompanying the more accessible volcanic products. We can predict that
considerable quantities of ultramafic and mafic cumulate complements to
the erupted basalt-andesite-dacite arc suites must exist. While there
are several explanations for the apparent lack of these complements,
there are some fortuitously exposed examples in the South Island of New
Zealand where the magmatic processes in the roots of arc volcanoes can
be studied. Over the past few years, we have been examining a number of
layered, ultramafic-mafic-intermediate plutonic complexes of
Permo-Triassic age in the so-called "Median Tectonic Zone" (or
Batholith) stretching from Bluff in the south to Nelson in the north of
the South Island. Of particular interest has been the discovery of
in-situ platinum group minerals in ordinary arc tholeiite magmas, and
the recognition of complex cumulate – percolating magma
interactions in the plutonic sequences. |
The deep petrologic structure of oceanic plateaus
Mantle plumes are ephemeral, and are
accompanied on first ascent into the outer regions of the Earth by
extensive partial melting – the magmatic products form
over-thickened portions of crust known as oceanic plateaus, and the
largest current example is the Ontong Java Plateau (OJP) in the SW
Pacific. The OJP collided with the Solomons – Vanuatu arc system
about 10 million years ago, triggering a reversal in subduction
polarity. Following the collision, the margin of the Plateau was
locally uplifted in the NE Solomon Islands of Malaita and Santa Isabel.
Associated with extensive outcrops of basaltic pillow lavas of the OJP
are gabbros and peridotites (also exposed on the island of Choiseul)
that seem to be the plutonic equivalents and residual mantle fraction
respectively of the erupted magmas. We are studying the genesis of
these rocks. |
Asama volcano in central Honshu, Japan.
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Isolating the variables – the multicomponent sources of arc-backarc magmas
The processes of magma formation in
arc-backarc systems are complex involving components from the subducted
lithosphere (i.e., metamorphosed sedimentary, igneous and residual
mantle lithologies), advecting mantle wedge, and the overriding arc
lithosphere. Because there can be considerable ambiguity in the
identification of these components and the individual mass fluxes
involved, it is important to find natural locations where the component
inputs vary. For example, we identified a region of northern Honshu
where the active volcanic chain associated with subduction of the
Pacific Plate transgresses a major terrane boundary in the arc
lithosphere, and showed that a distinct change in the Pb isotopic
character of the arc magmas is coincident with this boundary. The
importance of this observation is that the budget of Pb in arc magmas
is commonly asserted to be controlled by a subducted lithosphere
component. But clearly, our models have to become more sophisticated in
terms of the mass balances of the different components involved. We
have recently extended this type of research effort to a transect along
the volcanic front from Hokkaido (Japan) to the southern Kurile Islands
(Russia), once again crossing a major lithospheric terrane boundary
while the nature of the subducted Pacific Plate is constant. |
The geochemical and petrologic changes accompanying high pressure/temperature subduction zone metamorphism
There is clear geochemical and isotopic
evidence that some fraction of subducted lithosphere is returned to the
Earth's surface, both in arc-backarc magmas and also as exhumed
terranes in former plate collision zones. Our current knowledge of the
geochemical changes that occur over a range of pressures and
temperatures in subducted lithologies is not however, particularly
sophisticated. We have been studying the details of these changes in
suites of blueschist-eclogite rock types, particularly from Ecuador and
New Caledonia. Our aims are to map with laser ablation inductively
coupled plasma source mass spectrometry (LA-ICP-MS) the detailed trace
element distributions in the various mineral phases comprising these
rock types. We are trying to assess what fraction of the highly mobile
trace elements such as U, Pb, and Sr (characteristically
"over-enriched" in arc magmas) are lost at the various stages of
prograde metamorphic evolution of subducting lithosphere. |
The petrologic and geochemical evolution of the Rabaul Volcano (PNG)
In 1994, twin volcanic cones either side
of the Rabaul caldera (i.e., harbour) erupted in spectacular fashion,
causing considerable dislocation to the human population. Eruptions
continue from the eastern vent (Tavurvur) while the western vent
(Vulcan) is now dormant. This is merely the latest eruption episode of
a complex basalt-andesite-dacite volcano. Because of the potential
hazard, there has been a long history of monitoring at Rabaul, and more
recently, a major AUSAID project to understand the detailed seismic and
crustal environment of the volcano. We have undertaken in collaboration
with Dr. R. W. Johnson (AGSO-Geoscience Australia) and the Rabaul
Volcano Observatory, a detailed petrologic and geochemical study of the
products of the latest eruption, and a more extended study of the
history of the volcano for the past few thousand years. Given that
Rabaul is now one of the best understood active volcanoes in terms of
current seismicity, our hope is that significant integration of the
petrologic evolution with crustal structures and magma chamber
configuration can be achieved. |
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