GEOROC News
(11/2009)
Visit the GEOROC-GeoReM
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A GEOROC update is online (version 10/1/2009) |
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to flag typographical errors that were made either by the database team or
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scientists and can include more than just the measured data (for example,
normalized data or element ratios). |
Featured Papers
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Humphreys, E. R., Niu, Y.; Lithos 112 (2009) 118-136 |
On the composition of ocean island basalts (OIB):
The effects of lithospheric thickness variation and
mantle metasomatism |
Abstract. We have examined island-averaged
geochemical data for 115 volcanic islands with known eruption ages and ages
of the underlain lithosphere from the Pacific, Atlantic and Indian Oceans.
These age data allow calculation of the lithosphere thickness at the time of
volcanism. After correcting the basalts (including alkalic
types) (< 53% SiO2) for fractionation effect to Mg# = 0.72,
we found that the island-averaged Si72 and Al72
decrease whereas Fe72, Mg72, Ti72 and P72
increase with increasing lithosphere thickness. The island-averaged [La/Sm]CN and [Sm/Yb]CN
ratios also increase with increasing lithosphere thickness. These
statistically significant trends are most consistent with the interpretation
that the mean extent of melting decreases whereas the mean pressure of
melting increases with increasing lithosphere thickness. This is physically
consistent with the active role the lithosphere plays in limiting the final
depth of intra-oceanic mantle melting. That is, beneath a thin lithosphere, a
parcel of mantle rises to a shallow level, and thus melts more by
decompression with the aggregated melt having the property of high extent and
low pressure of melting. By contrast, a parcel of mantle beneath a thick
lithosphere has restricted amount of upwelling, and thus melts less by
decompression with the aggregated melt having the property of low extent and
high pressure of melting. This demonstrates that oceanic lithosphere
thickness variation exerts the first-order control on the geochemistry of
ocean island basalts (OIB). Variation in initial depth of melting as a result
of fertile mantle compositional variation and mantle potential temperature
variation can influence OIB compositions, but these two variables must have
secondary effects because they do not overshadow the effect of lithosphere
thickness variation that is prominent on a global scale. The mantle potential
temperature variation beneath ocean islands cannot be constrained with the
existing data. Fertile mantle source heterogeneity is required to explain the
large OIB compositional variation on a given island, between islands and
between island groups. The OIB mantle source heterogeneity must have multiple
origins, but an incipient melt in the seismic low-velocity zone and its metasomatic lithologies in the
lithosphere are best candidates that contribute to the incompatible element
enriched OIB geochemistry on two different time scales: (1) melt–lithosphere
interaction during OIB magmatism, and (2) recycled metasomatized lithosphere in the OIB source regions. |
Fig. 3. Island-averaged major element
data corrected for fractionation effect to Mg# = 0.72
plotted as a function of the lithosphere thickness. Each data point
represents average composition for a given volcanic island; error bars
represent 2 standard deviations from the mean […].The geochemical data we
used are exclusively from the GEOROC database. These include mostly bulk-rock
analyses and some glass analyses for major and trace elements of over 20,000
samples ranging in composition from highly evolved andesites/basaltic
andesites (minor), to tholeiitic
basalts (abundant), to alkali rich basalts (relatively abundant) and to rocks
highly enriched in alkalis such as basanite or
rarely nephelinite (minor) from 189 ocean islands
in the Pacific (108 islands), Atlantic (56 islands) and Indian (25 islands)
ocean basins. |