Petrological systematics of mid-ocean ridge basalts: Constraints on melt generation beneath ocean ridges
Abstract
Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath
ocean ridges, and contain much information concerning melt formation, melt migration
and heterogeneity within the upper mantle. MORB major element chemical systematics
can be divided into global and local aspects, once they have been corrected for low
pressure fractionation and interlaboratory biases. Regional average compositions for
ridges unaffected by hot spots (“normal” ridges) can be used to define
the global correlations among normalized Na2O, FeO, TiO2 and SiO2 contents, CaO/Al2O3
ratios, axial depth and crustal thickness. Back-arc basins show similar correlations,
but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores
and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships
between FeO, Na2O and depth over distances of 1000 km.Local variations in basalt chemistry
from slow- and fast-spreading ridges are distinct from one another. On slow-spreading
ridges, correlations among the elements cross the global vector of variability at
a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the
elements are distinct from both global and slow-spreading compositional vectors, and
involve two components of variation. Spreading rate does not control the global correlations,
but influences the standard deviations of axial depth, crustal thickness, and MgO
contents of basalts.Global correlations are not found in very incompatible trace elements,
even for samples far from hot spots. Moderately compatible trace elements for normal
ridges, however, correlate with the major elements. Trace element systematics are
significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions
of the MAR are very depleted in REE, with little variability; hot spots cause large
long wavelength variations in REE abundances. Normal EPR basalts are significantly
more enriched than MAR basalts from normal ridges, and still more enriched basalts
can erupt sporadically along the entire length of the EPR. This leads to very different
histograms of distribution for the data sets as a whole, and a very different distribution
of chemistry along strike for the two ridges. Despite these differences, the mean
Ce/Sm ratios from the two ridges are identical.Existing methods for calculating the
major element compositions of mantle melts [Klein and Langmuir, 1987; McKenzie and
Bickle, 1988; Niu and Batiza, 1991] are critically examined. New quantitative methods
for mantle melting and high pressure fractionation are developed to evaluate the chemical
consequences of melting and fractionation processes and mantle heterogeneity. The
new methods rely on new equations for partition coefficients for the major elements
between mantle minerals and melts. The melting calculations can be used to investigate
the chemical compositions produced by small extents of melting or high pressures of
melting that cannot yet be determined experimentally. Application of the new models
to the observations described above leads to two major conclusions: (1) The global
correlations for normal ridges are caused by variations in mantle temperature, as
suggested by Klein and Langmuir [1987] and not by mantle heterogeneity. (2) Local
variations are caused by melting processes, but are not yet quantitatively accounted
for. On slower spreading ridges, local variations are controlled by the melting regime
in the mantle. On the EPR, local variations are predominantly controlled by ubiquitous,
small scale heterogeneites. Volatile content may be an important and as yet undetermined
factor in affecting the observed variations in major elements.We propose a hypothesis,
similar to one proposed by Allegre et al [1984] for isotopic data, to explain the
differences between the Atlantic and Pacific local trends, and the trace element systematics
of the two ocean basins, as consequences of spreading rate and a different distribution
of enriched components from hot spots in the two ocean basins. In the Atlantic, the
hot spot influence is in discrete areas, and produces clear depth and chemical anomalies.
Ridge segments far from hot spots do not contain enriched basalts. Melting processes
associated with slow-spreading ridges vary substantially over short distances along
strike and lead to the local trends discussed above, irrespective of hot spot influence.
In the Pacific, enriched components appear to have been more thoroughly mixed into
the mantle, leading to ubiquitous small scale heterogeneities. Melting processes do
not vary appreciably along strike, so local chemical variations are dominated by the
relative contribution of enriched component on short time and length scales. Thus
the extent of mixing and distribution of enriched components influences strongly the
contrasting local major element trends. Despite the difference in the distribution
of enriched components, the mean compositions of each data set are equivalent. This
suggests that the hot spot influence is similar in the two ocean basins, but its distribution
in the upper mantle is different. These contrasting relationships between hot spots
and ridges may result from differences in both spreading rate and tectonic history.
Unrecognized hot spots may play an important role in diverse aspects of EPR volcanism,
and in the chemical systematics of the erupted basalts.The observations and successful
models have consequences for melt formation and segregation. (1) The melting process
must be closer to fractional melting than equilibrium melting. This result is in accord
with inferences from abyssal peridotites [Johnson et al., 1990]. (2) Small melt fractions
generated over a range of pressures must be extracted rapidly and efficiently from
high pressures within the mantle without experiencing low pressure equilibration during
ascent. This requires movement in large channels, and possibly more efficient extraction
mechanisms than nonnally envisaged in porous flow models with small residual porosity.
(3) Diverse melts from the melting regime produce variations in basalts that are observable
at the surface. (4) Basalt data can be used to constrain the melting process (e.g.
active vs. passive upwelling) and its relationship to segmentation. The data cannot
be used to constrain the shape of the melting regime, however, for many shapes lead
to similar chemical results. (5) Highly incompatible elements and U-series disequilibria
results appear not yet to be explained by melting models, and may require additional
processes not yet clearly envisaged.
Type
Book sectionSubject
3035 Marine Geology and Geophysics: Midocean ridge processes3610 Mineralogy, Petrology, and Rock Chemistry: Composition of the crust
Volcanism—Congresses
Earth—Mantle—Congresses
Mid-ocean ridges—Congresses
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https://hdl.handle.net/10161/8316Published Version (Please cite this version)
10.1029/GM071p0183Collections
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Emily M. Klein
University Distinguished Service Professor
Dr. Klein's research focuses on the geochemistry of oceanic basalts, using diverse
tools of major, trace and isotopic analyses. Her research involves sea-going expeditions
to sample and map the ocean floor.

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