Global isotopic signatures of oceanic island basalts
Citable URI
http://hdl.handle.net/1912/5473DOI
10.1575/1912/5473Keyword
Basalt; Ocean bottomAbstract
Sr, Nd and Pb isotopic analyses of 477 samples representing 30 islands or
island groups, 3 seamounts or seamount chains, 2 oceanic ridges and 1 oceanic
plateau [for a total of 36 geographic features] are compiled to form a
comprehensive oceanic island basalt [OIB] data set. These samples are
supplemented by 90 selected midocean ridge basalt [MORB] samples to give
adequate representation to MORB as an oceanic basaJt endmember. This
comprehensive data set is used to infer information about the Earth's mantle.
Principal component analysis of the OIB+MORB data set shows that the first
three principal components account for 97.5% of the variance of the data. Thus.
only four mantle endmember components [EMI, EMII, HIMU and DMM] are
required to completely encompass the range of known isotopic values. Each
sample is expressed in terms of percentages of the four mantle components,
assuming linear mixing. There is significant correlation between location and
isotopic signature within geographic features, but not between them, so
discrimination analysis of the viability of separating the oceanic islands into those
lying inside and outside Hart's (1984, 1988) DUPAL belt is performed on the
feature level and yields positive results.
A "continuous layer model" is applied to the mantle component percentage
data to solve for the spherical harmonic coefficients using approximation
methods. Only the degrees 05 coefficients can be solved for since there are only
36 features. The EMI and HIMU percentage data sets must be filtered to avoid
aliasing. Due to the nature of the data, the coefficients must be solved for using
singular value decomposition [SVD], versus the least squares method. The Ftest
provides an objective way to estimate the number of singular values to retain
when solving with SVD. With respect to the behavior of geophysics control data sets, only the degree 2 spherical harmonic coefficients for the mantle components
can be estimated with a reasonable level of confidence with this method.
Applying a "deltafunction model" removes the problem of aliasing and
simplifies the spherical harmonic coefficient solutions from integration on the
globe to summation over the geographic features due to the properties of deltafunctions.
With respect to the behavior of geophysics control data sets, at least
the degree 2 spherical harmonic coefficients for the mantle components can be
estimated with confidence, if not the degrees 3 and 4 as well. Deltafunction
model solutions are, to some extent, controlled by the nonuniform feature
distribution, while the continuous layer model solutions are not.
The mantle component amplitude spectra, for both models, show power at
all degrees, with no one degree dominating. The DUPAL components [EMI,
EMil and HIMU], for both models, correlate well with the degree 2 geoid,
indicating a deep origin for the components since the degrees 23 geoid is
in ferred to result from topography at the coremantle boundary. The DUPAL
and DMM components, for both models, correlate well [and negati vely! at degree
3 with the velocity anomalies of the ClaytonComer seismic tomography model
in the 25002900 km depth range [immediately above the coremantle boundary].
The EMil component correlates well [and positively] at degree 5 with the
velocity anomalies of the ClaytonComer model in the 7001200 km depth range,
indicating a subduction related origin. Similar positive correlations for the geoid
in the upper lower mantle indicate that subducted slabs extend beyond the 670
km seismic discontinuity and support a wholemantle convection model.
Description
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1991
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