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| Titel |
Across-arc patterns in mafic-magma chemistry controlled by thermal and chemical gradients at the slab interface |
| VerfasserIn |
Tamsin Mather, Sebastian Watt, David Pyle, Jose Naranjo |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250092864
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| Publikation (Nr.) |
 EGU/EGU2014-7228.pdf |
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| Zusammenfassung |
| A range of recent studies demonstrate systematic across-arc variations in the volatile
and trace element contents of primary arc magmas. Most of these studies used
olivine-hosted melt inclusions to bypass upper crustal modifications, and thereby
estimate the chemical composition of parental magmas in equilibrium with the mantle.
The patterns preserved in these melts can be used to investigate variation in the
volatile-rich flux that enters the core of the mantle wedge, and which is sourced
from the subducting plate. Similarly, the implied variability in the composition of
this flux provides information about fluid and melt transport through the mantle
wedge, and of the mineral breakdown processes occurring within the downgoing
slab.
Here, we report on results from a detailed study of olivine-hosted melt inclusions sampled
from a set of scoria cones in southern Chile. These samples include some highly
primitive rocks from Apagado, with picritic composition and containing unzoned
highly-forsteritic olivine (Fo88). Such rocks are extremely rare in continental arcs. The
Chilean rocks display a variation in their water, CO2, and trace element content that
suggests that the primary-melt chemistry reflects the pattern of element release at
the subducting slab interface. This down-slab chemical gradient is consistent with
predictions from modelling, geothermometry and experiments. The flux feeding the
arc magmas becomes progressively less water-rich and increasingly dominated by
hydrous melts over a distance of a few kilometres. We suggest that this change
marks the onset of significant water-fluxed melting of sediment at the downgoing
slab-surface.
The short length scale of the across-arc chemical patterns in southern Chile is perhaps
surprising. The fact that such changes are preserved within our sampled rocks suggests that
there is limited across-arc mixing and focussing of fluids or melts as they ascend through the
mantle wedge. Our results suggest that slab-surface inputs exert a first-order control on
arc-magma chemistry. The chemical patterns that we observe are replicated in other arcs,
such as the Kamchatka and Izu-Bonin arcs, in spite of the plate-scale thermal differences
between these subduction zones (i.e. downgoing plate age and descent rate). The common
patterns between these arcs implies that sub-arc slab-surface temperature ranges may be
similar in all three settings. This unexpected result hints at a thermal control on the precise
position of volcanic arcs within subduction zones. |
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