hypothesis is that partial melting of hydrated (altered) oceanic crust produced low
temperature (\1000 °C) mineral assemblages.
The bimodal basalt plus silica-enriched lava association during volcanic eruption
could either be due to a common origin and/or to separate melting source components. Marsh et al. (1991) suggested that while small portions of Hawaiian silica-rich
lava found in association with the basalt could be the result of fractional differentiation, in comparison, considerable amounts of rhyolites have been extruded in
association with other Icelandic flows. The origin of these silica-rich flows might be
tied to that of the associated, less evolved lavas of basaltic composition. The heat
required to produce silica-enriched lava is lower than what is necessary for basalt.
Hence another alternative hypothesis is that the silica-rich lavas are derived from an
independent source with a lower degree of partial melting.
Support for both of these interpretations is given by the isotopic composition of
basalt plus silica-rich lava suites. Indeed, the neodymium (Nd) and strontium ratios
of some basalt-silica-enriched lava associations (such the suites found on the SEPR
Pacific-Antarctic Ridge and in some intraplate regions) agree with the hypothesis of
a common parent for the basalts and the silica-rich lavas. Other cases, such as the
Icelandic suite, might not be due to a common origin but rather each one of the
associated pairs could have been derived from its own parental melt.
The criteria for recognizing lithospheric melting are: (1) the presence of disequilibrium minerals derived from previous periods of solidification and remelting
(anatexis) and (2) the inclusion of solids from the melt source. The solidus temperature of silica-rich lava is lower than the liquidus for basalt, which could supply
heat during crystallization. During partial melting and melt segregation, small
amounts of solidification during magmatic ascent could provide heat to the shallower lithosphere and thereby lower the temperature of the solidus. This will be
more significant where the oceanic crust has thickened during intense episodes of
volcanism. Also, it is important to keep in mind that the most evolved silica-rich
lavas of dacitic and rhyolitic composition are too low in their compatible element
concentrations to have been originated directly from mantle-derived melts.
Experimental work on Fo (the Forsterite content of peridotite)-Di (Diopside
content of clinopyroxene)-Qz (Quartz) and H 2 O at pressure = 20 kilobars (Kushiro 1969) has shown that if the liquid is saturated with an excess of H 2 O, it will
give rise to a rhyolitic melt at 950 °C. As soon as a liquid of rhyolitic composition
is removed and if melting continues, eventually the remaining melt will move
towards the Enstatite-Diopside field on a Kushiro (1969) diagram and give rise to
basaltic liquid. This is an elegant way of explaining the formation of both silicarich and basaltic liquid from the same parental melt source. This source could be a
mantle lherzolite, which has undergone several degrees of partial melting. If we
use this hypothesis, it is likely that the strontium isotopic ratios of both extruded
basalt and silica-rich lavas should be comparable.
Based on the positive correlation of the ratios of radioactive isotopes of
87 Sr/
86 Sr and
206 Pb/
204 Pb in an area of the Foundation Seamount Chain, Haase
et al. (2005) have speculated that mixing between a plume that is enriched in
radioactive components and some non-radioactive upper mantle material could
86
4 Sea Floor Rocks
temperature (\1000 °C) mineral assemblages.
The bimodal basalt plus silica-enriched lava association during volcanic eruption
could either be due to a common origin and/or to separate melting source components. Marsh et al. (1991) suggested that while small portions of Hawaiian silica-rich
lava found in association with the basalt could be the result of fractional differentiation, in comparison, considerable amounts of rhyolites have been extruded in
association with other Icelandic flows. The origin of these silica-rich flows might be
tied to that of the associated, less evolved lavas of basaltic composition. The heat
required to produce silica-enriched lava is lower than what is necessary for basalt.
Hence another alternative hypothesis is that the silica-rich lavas are derived from an
independent source with a lower degree of partial melting.
Support for both of these interpretations is given by the isotopic composition of
basalt plus silica-rich lava suites. Indeed, the neodymium (Nd) and strontium ratios
of some basalt-silica-enriched lava associations (such the suites found on the SEPR
Pacific-Antarctic Ridge and in some intraplate regions) agree with the hypothesis of
a common parent for the basalts and the silica-rich lavas. Other cases, such as the
Icelandic suite, might not be due to a common origin but rather each one of the
associated pairs could have been derived from its own parental melt.
The criteria for recognizing lithospheric melting are: (1) the presence of disequilibrium minerals derived from previous periods of solidification and remelting
(anatexis) and (2) the inclusion of solids from the melt source. The solidus temperature of silica-rich lava is lower than the liquidus for basalt, which could supply
heat during crystallization. During partial melting and melt segregation, small
amounts of solidification during magmatic ascent could provide heat to the shallower lithosphere and thereby lower the temperature of the solidus. This will be
more significant where the oceanic crust has thickened during intense episodes of
volcanism. Also, it is important to keep in mind that the most evolved silica-rich
lavas of dacitic and rhyolitic composition are too low in their compatible element
concentrations to have been originated directly from mantle-derived melts.
Experimental work on Fo (the Forsterite content of peridotite)-Di (Diopside
content of clinopyroxene)-Qz (Quartz) and H 2 O at pressure = 20 kilobars (Kushiro 1969) has shown that if the liquid is saturated with an excess of H 2 O, it will
give rise to a rhyolitic melt at 950 °C. As soon as a liquid of rhyolitic composition
is removed and if melting continues, eventually the remaining melt will move
towards the Enstatite-Diopside field on a Kushiro (1969) diagram and give rise to
basaltic liquid. This is an elegant way of explaining the formation of both silicarich and basaltic liquid from the same parental melt source. This source could be a
mantle lherzolite, which has undergone several degrees of partial melting. If we
use this hypothesis, it is likely that the strontium isotopic ratios of both extruded
basalt and silica-rich lavas should be comparable.
Based on the positive correlation of the ratios of radioactive isotopes of
87 Sr/
86 Sr and
206 Pb/
204 Pb in an area of the Foundation Seamount Chain, Haase
et al. (2005) have speculated that mixing between a plume that is enriched in
radioactive components and some non-radioactive upper mantle material could
86
4 Sea Floor Rocks