The normal Mid Ocean Ridge Basalt (N-MORB) is deprived in calcium
and potassium oxide (Na 2 O ? K 2 O = \ 3 %), and has various chemical
ratios such as potassium to titanium (K/Ti = \ 0.15), zircon to yttrium (Zr/
Y = \ 3), as well as chemical contents such as Zr (50–100 ppm), Y
(20–40 ppm), strontium (Sr = 50–120 ppm) and niobium (Nb = \ 4 ppm)
when compared to Transitional MORB (T-MORB). The Transitional MORB
has intermediate values of Na 2 O ? K 2 O (\3–3.5 %), K/Ti (\0.14–0.25), Zr/
Y (\2–4) in comparison to the Enriched rocks (E-MORB), which contain
higher Na 2 O ? K 2 O ([3.5 %), K/Ti ([0.25), Zr/Y (1–2), Zr (120–200 ppm),
Y (30–60 ppm) and Nb (7–10 ppm).
Alkali Basalts/Intraplate Oceanic Basalts (IOBs)
Alkali basalts are also called Intraplate Oceanic Basalts (IOBs) and these types of
basalts are found on islands and hotspot generated seamounts along volcanic chains
or in ocean basins. They differ from MORBs since the IOB (also known as
‘‘hotspot basalts’’) have a different chemical composition for their clinopyroxene
and plagioclase minerals.
The normal Mid-Ocean Ridge Basalt (MORB) contains calcium enriched and
sodium–potassium depleted plagioclase and clinopyroxene and is less enriched in
incompatible elements than an IOB. In comparison, the alkali basalts from intraplate (IOB) regions and hotspots are enriched in incompatible elements and have
higher contents of LILE (Large Ion Lithophile Elements) such as Na 2 O ? K 2 O
(3.5–5 %), K/Ti ([0.45), Zr/Y (5–16) Zr (300–400 ppm), Y (25–30) ppm, Sr
(600–1000 ppm) and Nb (15–140 ppm) than do the MORBs.
The IOB rocks, which are enriched in LILE, could be the result of the partial
melting of a different mantle source than that of the MORBs. Hoffman and White
(1982) have suggested that enriched melts are due to the mixing of sediment and
mantle material as a result of recycling the subducted oceanic lithosphere within
the mantle (See Chap. 2). Since the continental crust is made up essentially of
silica and alkali enriched material, the processes of selective partial melting and
fractionation of mantle material producing the IOB in today’s modern-ocean could
be related to the events that once gave rise to ancient continents during the geological time-span of our planet’s formation.
Different origins for the MORBs and IOBs could be a valid theory for
explaining the chemical differences observed between the erupted volcanics found
in diverse geological provinces such as spreading ridges or ocean basins. However,
this idea is more controversial for explaining small-scale geochemical variations.
For example, when MORBs and alkali basalts are extruded on the same site and/or
close to each other in the vicinity of the same volcanic structure (\100 km apart)
80
4 Sea Floor Rocks
and potassium oxide (Na 2 O ? K 2 O = \ 3 %), and has various chemical
ratios such as potassium to titanium (K/Ti = \ 0.15), zircon to yttrium (Zr/
Y = \ 3), as well as chemical contents such as Zr (50–100 ppm), Y
(20–40 ppm), strontium (Sr = 50–120 ppm) and niobium (Nb = \ 4 ppm)
when compared to Transitional MORB (T-MORB). The Transitional MORB
has intermediate values of Na 2 O ? K 2 O (\3–3.5 %), K/Ti (\0.14–0.25), Zr/
Y (\2–4) in comparison to the Enriched rocks (E-MORB), which contain
higher Na 2 O ? K 2 O ([3.5 %), K/Ti ([0.25), Zr/Y (1–2), Zr (120–200 ppm),
Y (30–60 ppm) and Nb (7–10 ppm).
Alkali Basalts/Intraplate Oceanic Basalts (IOBs)
Alkali basalts are also called Intraplate Oceanic Basalts (IOBs) and these types of
basalts are found on islands and hotspot generated seamounts along volcanic chains
or in ocean basins. They differ from MORBs since the IOB (also known as
‘‘hotspot basalts’’) have a different chemical composition for their clinopyroxene
and plagioclase minerals.
The normal Mid-Ocean Ridge Basalt (MORB) contains calcium enriched and
sodium–potassium depleted plagioclase and clinopyroxene and is less enriched in
incompatible elements than an IOB. In comparison, the alkali basalts from intraplate (IOB) regions and hotspots are enriched in incompatible elements and have
higher contents of LILE (Large Ion Lithophile Elements) such as Na 2 O ? K 2 O
(3.5–5 %), K/Ti ([0.45), Zr/Y (5–16) Zr (300–400 ppm), Y (25–30) ppm, Sr
(600–1000 ppm) and Nb (15–140 ppm) than do the MORBs.
The IOB rocks, which are enriched in LILE, could be the result of the partial
melting of a different mantle source than that of the MORBs. Hoffman and White
(1982) have suggested that enriched melts are due to the mixing of sediment and
mantle material as a result of recycling the subducted oceanic lithosphere within
the mantle (See Chap. 2). Since the continental crust is made up essentially of
silica and alkali enriched material, the processes of selective partial melting and
fractionation of mantle material producing the IOB in today’s modern-ocean could
be related to the events that once gave rise to ancient continents during the geological time-span of our planet’s formation.
Different origins for the MORBs and IOBs could be a valid theory for
explaining the chemical differences observed between the erupted volcanics found
in diverse geological provinces such as spreading ridges or ocean basins. However,
this idea is more controversial for explaining small-scale geochemical variations.
For example, when MORBs and alkali basalts are extruded on the same site and/or
close to each other in the vicinity of the same volcanic structure (\100 km apart)
80
4 Sea Floor Rocks