during cyclic volcanic eruptions, it is unlikely that separate melt sources could
account for the differences observed. In this case, the differences between alkali
basalts and MORBS erupted in the same area are probably due to the partial
melting of different source components (minerals).
Model for Explaining Small-Scale Magma Heterogeneities
in Sea Floor Basalt
Most models suggested that the compositional variations in basalt, such as the
difference between N, T and E- MORBs, are due to the partial melting of a
heterogeneous mantle source and the passive upwelling of melt in the lithosphere
(Yoder 1976; McKenzie and Bickle 1988; Plank and Langmuir 1992). It was also
suggested that compositional differences could result from fractional crystallization and a subsequent mixing in shallow level magma chambers (e.g. Christie and
Sinton 1981). Although these models are valid for explaining some large-scale
(spatial and temporal) compositional variability, they do not provide a satisfactory
answer for the production of the different types of basalt extruded in a limited area
(only a few tens of meters distance between sampling sites) nor can these models
explain the differences noted in basalt extruded in small batches underneath the
ridge axis.
From 1981 to 1984, several French cruises (SeaRise, Clipperton, Cyatherm,
Geocyarise) carried out on the EPR near 13 and 11°N revealed the diversities in
composition of volcanic products erupted at relatively short distances from each
other (See Chap. 7). Basalts collected on the ridge axis as well as on off axis
provinces located at less than 20 km from each other were found to differ in their
chemical composition. The extrusion of the diverse types of MORBs as well as
alkali basalts, all of which were found on the same edifice as well as at short
distances (\2 km) between sampling sites, was very puzzling. It was difficult to
explain the reasons for these differences simply by means of the classical views
referring to partial melting of a heterogeneous mantle or on the basis of the
fractional crystallization processes taking place inside magma chambers. Indeed,
based on the chemistry of their incompatible elements, each individual type of
basalt seemed to indicate a different parental lineage.
Therefore, in order to explain the diversity of MORBs and alkali basalts found
in the same area, Hekinian et al. (1989) and Bideau and Hekinian (1995), proposed
an alternative hypothesis to the one elaborated by Hoffman and White (1982) and
others. The hypothesis of Bideau and Hekinian (1995) is based on a multi-stage
melt extraction model, which involves several steps of magma extraction and
accumulation above the melting region of a heterogeneous mantle source. The
model assumes that magma supply is discontinuous, or in other words, that there
are periods with a lack of magma supply, which agrees with the observed cyclic
nature of undersea volcanism. The multistage melting and extraction theory
Alkali Basalts/Intraplate Oceanic Basalts (IOBs)
81
account for the differences observed. In this case, the differences between alkali
basalts and MORBS erupted in the same area are probably due to the partial
melting of different source components (minerals).
Model for Explaining Small-Scale Magma Heterogeneities
in Sea Floor Basalt
Most models suggested that the compositional variations in basalt, such as the
difference between N, T and E- MORBs, are due to the partial melting of a
heterogeneous mantle source and the passive upwelling of melt in the lithosphere
(Yoder 1976; McKenzie and Bickle 1988; Plank and Langmuir 1992). It was also
suggested that compositional differences could result from fractional crystallization and a subsequent mixing in shallow level magma chambers (e.g. Christie and
Sinton 1981). Although these models are valid for explaining some large-scale
(spatial and temporal) compositional variability, they do not provide a satisfactory
answer for the production of the different types of basalt extruded in a limited area
(only a few tens of meters distance between sampling sites) nor can these models
explain the differences noted in basalt extruded in small batches underneath the
ridge axis.
From 1981 to 1984, several French cruises (SeaRise, Clipperton, Cyatherm,
Geocyarise) carried out on the EPR near 13 and 11°N revealed the diversities in
composition of volcanic products erupted at relatively short distances from each
other (See Chap. 7). Basalts collected on the ridge axis as well as on off axis
provinces located at less than 20 km from each other were found to differ in their
chemical composition. The extrusion of the diverse types of MORBs as well as
alkali basalts, all of which were found on the same edifice as well as at short
distances (\2 km) between sampling sites, was very puzzling. It was difficult to
explain the reasons for these differences simply by means of the classical views
referring to partial melting of a heterogeneous mantle or on the basis of the
fractional crystallization processes taking place inside magma chambers. Indeed,
based on the chemistry of their incompatible elements, each individual type of
basalt seemed to indicate a different parental lineage.
Therefore, in order to explain the diversity of MORBs and alkali basalts found
in the same area, Hekinian et al. (1989) and Bideau and Hekinian (1995), proposed
an alternative hypothesis to the one elaborated by Hoffman and White (1982) and
others. The hypothesis of Bideau and Hekinian (1995) is based on a multi-stage
melt extraction model, which involves several steps of magma extraction and
accumulation above the melting region of a heterogeneous mantle source. The
model assumes that magma supply is discontinuous, or in other words, that there
are periods with a lack of magma supply, which agrees with the observed cyclic
nature of undersea volcanism. The multistage melting and extraction theory
Alkali Basalts/Intraplate Oceanic Basalts (IOBs)
81