Margins A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (1989) / Chapter Skim
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9 Active Margins: Group 3
9 Active Margins: Group 3
Pages 95-108
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From page 95... ...
- ~ reasons, active margins are the melting pots of the For these earth. The last decade has seen implementation of new geophysical and geochemical techniques, yielding a rapid improvement in the amount and quality of observational data on subduction systems.
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From page 96... ...
between slab-derived and wedge-derived components, with the relative effect of the slab contribution apparently decreasing away from the plate boundary. The mantle wedge is thus the melting pot, into which is fed new mantle plus material transferred from subducted Oceanic crust, and from which magmas and fluids are extracted into the crust of the active margin (#4~.
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From page 97... ...
Hign temperature alteration, driven by the forced hydrothermal circulation of seawater near the ridge axis ~ sty ips The basaltic crust of elements, such as K, Rb, and B However, nydrothermal circulation wanes due to cooling in the newly formed crust as it moves away from the ridge, and all crust older than 20 Ma may have similar hydrothermal circulation signatures.
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From page 98... ...
will provide a better understanding of the mechanisms of sediment incorporation in arc magmas, a distinction between contributions from sediment and the altered basaltic crust, and a better constraint on the balance of sediment accretion and subduction at active margins. Fundamental advances in understanding of the subduction process await resolution of two paradoxes that have emerged from mass tracer studies.
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From page 99... ...
Examples of these reactions and segregation processes are the serpentinite diapirs in the Mariana forearc, and the slab-derived fluid which is inferred to be a component in volcanic arc and backarc lavas. Al though the composition of the components segregated from the slab must change as a function of temperature and pressure r the important reactions controlling compositions of segregated phases are not known.
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From page 100... ...
The integrated field' geochemical, and geophysical database for selected arc systems can then be used to model the thermal structure, magma generation, and fluid flow within the mantle wedge. Magma generation and segregation processes have been the subject of detailed modelling along active spreading centers, and geochemica~ tests are being developed to evaluate them in areas of continental extension and rifting (see Passive Margins Group 1, page 27~.
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From page 101... ...
In this motley, the mantle wedge varies significantly from place to place, being highly depleted beneath convergent margins ~ hat yield island arc tholeiite suites, and relatively enriched beneath continental arcs . This concept needs to be properly eval utter by detailed transverses across magmatic arcs in oceanic and continental areas, characterized by different age provinces in the overriding p7 ate.
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From page 102... ...
In continental arcs, the preexisting crust is older continental crust that has a complex previous history. In extensional regimes, arc magmas may pass with little fractionation to shallow level magma chambers which periodically feed voicanos.
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From page 103... ...
In addition to being important as hazards and sources of economic mineralization , they also sample the current output along and across arc systems e By a detailed sampling program combined with a judicious choice of geochemical tracers, the red ative role of the various input parameters can be constrained. Older volcanic rocks and ve in mineral i z at ions make it pass ibI e to study pa st outputs as well, so that time sequences can be developed.
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From page 104... ...
Episodicity of Volcanism: Reflection of a Nonlinear System Explosive volcanism poses significant threats to inhabitants of active margins, as the 1980 eruption of Mount Saint Helens demonstrated powerfully. This explosibi~ity is a direct consequence of the melting pot.
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From page 105... ...
For example, if heterogeneities exist in the downgoing plate, such as chemical singularities clue to subduction of an oceanic ridge, then the duration of the same chemical s ingularities in the voicanos may give the-best available information about the convective pattern and the efficiency of mass transfer through the mantle wedge. Because the fundamental control of volcano location appears to be in the source region, the migration of volcanic loci along and across the strike of any arc should be used to monitor changes in the physical state of the mantle wedge.
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From page 106... ...
The analytical program must provide coordinated and thorough multi-element, multi-isotope studies of both the inputs and outputs at the three types of arcs. Special emphasis should be placed on tracers, which may distinguish input components or mass transfer processes' and which constrain time scales of mass transfer through the subduction system, Voicanic rocks, fluids, and veins must be studied in sufficient detail to identify chemical features acquired within the crust,and dated with sufficient accuracy to relate them to the local geodynamic history e The experimental program should also include as geochemical objectives the partitioning of elements between a range of residual minerals and H~-CO2 fluids plus alkali-rich silicate melts, under the P-T-fO2 conditions of the subducting plate and mantle wedge.
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From page 107... ...
of study of the processes relevant to arc subsystems have prepared us for this larger objective; that is, to define the relationships between the various subsystems and thereby to achieve an integrated view of the whole arc system.
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