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| Titel |
Geochemical provincialism in ocean islands: a new perspective on double chain volcanism from Iceland |
| VerfasserIn |
Oliver Shorttle, John Maclennan, Alexander Piotrowski |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078137
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| Zusammenfassung |
| Bilateral spatial structure in the chemical composition of mantle plumes has been
observed in a number of Pacific ocean islands, notably Hawaii. This has led to models
proposing a role for plume-lithosphere interaction, or deep mantle low velocity
domains, in controlling the spatial distribution of chemical components in the shallow
mantle. However, volcanism on most ocean islands provides a highly processed
sample of the mantle’s spatial structure, as melt is focused, mixed and erupted at a
limited number of dominant volcanic edifices. In contrast, Iceland, with 500 km of
rift zones and unparallelled sampling density, offers a high resolution spatially
continuous sample of the Earth’s mantle. This coverage enables us to constrain the
progressive nature of the geochemical shifts that occur in the Icelandic mantle,
providing a guide to interpreting the spatial geochemical structure in other ocean island
settings.
We present new Pb-Sr-Nd isotope, major and trace element data on a group of basalts from
central Iceland. We combined this new data with existing datasets and interrogated the data
using spatial statistical methods. We identify three types of spatial structure, all of which are
most strongly observed in the Pb isotopes. Firstly, the mean Pb-isotopic composition of
basalts shifts from south to north Iceland to become progressively more depleted, with our
central Iceland dataset falling at intermediate compositions. Secondly, there is a shift in the
binary mixing array that samples fall along in Pb isotope space, occurring as the volcanic
zones are stepped through south to north. Lastly, there is a shift in the Pb isotopic variability
across Iceland, with neovolcanic zones in the southwest exhibiting a greater variance in Pb
isotopes than those in the north and east. This offset in variance persists across the full range
of Mg# of the basalts, indicating that it originates in the mantle, rather than by crustal
processing.
The spatial trends we observe on Iceland are similar to those seen on Hawaii between the Loa
and Kea volcanic chains, which also show shifts in mean Pb isotopic composition and binary
mixing array. However, on Iceland we are able to see that rather than representing
dichotomous compositional domains on either side of the island, the change in mean
composition and mixing array occurs continuously—only showing segmentation on the scale
of a volcanic zone (~ 100 km).
Even with the spatially continuous sampling of the mantle present at Iceland, melt transport
and mixing places severe limitations on our ability to resolve the scale of mantle
heterogeneity. The spatial statistical methods we use attempt to maximise the information on
heterogeneity scale available from the geochemical data. However, our results emphasise
the need for a better understanding of melt region geometry and melt transport
processes before the sizes of mantle heterogeneities can be constrained on a < 100 km
scale. |
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