et al., 2006). About 200
C below the liquidus of the system, orthopyroxene and Fe-Ti oxides crystallize (the latter
strongly controlled by the prevailing oxygen fugacity),
followed by amphibole, indicating that the water content
in the residual melt has been raised to significant amounts,
due to the crystallization of only water-free minerals in the
main stage crystallization. At this stage, most of the melts
are already crystallized and these minerals form interstitial
parageneses.
Geochemistry of oceanic gabbros
Students’ textbooks often state that gabbros are equivalent
in composition to basalts and that the difference between
the two rock families is their grain size, which is a direct
function of the cooling rate (high cooling rates produce
basalts; low cooling rates gabbros). However, oceanic
gabbros are cumulates (¼accumulated crystals; qv “cumulate”), and thus their bulk composition is strongly dependent of the presence and the modal amount of
accumulated crystals. For example, if oceanic gabbros
are strongly enriched in oxides (“oxide gabbro”; see
above), the silica content of the bulk rock can be <40 wt%
SiO 2 , due to the high concentration of the heavy element
oxide FeO. If crystallization of plagioclase dominates,
anorthosites (Figure 1) may be formed with very high
Al 2 O 3 contents (>20 wt%), and abundant crystallization
of olivine produces olivine-rich troctolites with MgO
contents >25 wt%. In contrast, MORBs are frozen melts
with more homogeneous major element composition,
which varies only within a few wt% for the major element
oxides as SiO 2 , Al 2 O 3 , and MgO. Another consequence of
the cumulate nature of oceanic gabbros is that the concentrations of incompatible trace elements are generally lower
than those of MORBs. For example, the average content
of K 2 O and P 2 O 5 in typical gabbros from the drilled core
from Hole 735B at SWIR is 0.04 and 0.01 wt%, respectively (data set of Dick et al., 2000), while the
corresponding value for typical primitive MORB is 0.07
and 0.09 (e.g., average from the data base PETD, Lehnert
et al., 2000), respectively. This depleted nature of typical
oceanic gabbros is a consequence of the accumulation of
the cotectic assemblage olivine + plagioclase +
clinopyroxene (Æorthopyroxene) resulting in rocks which
often represent nearly pure cumulates with very low proportions of residual melt (~1.4 % as average value for
the gabbros from the SWIR drill core, Natland and Dick,
2001). Hence, the bulk content of incompatible elements
of oceanic gabbros is more or less a direct function of
the amount of interstitial late crystallization where incompatible elements are enriched in phases like amphibole and
apatite, and which is in general decoupled from the main
stage of crystallization.
Summary
Gabbros are the most common rocks in the oceanic crust
and form its lower part (layer 3). They represent slowly
cooled crystal mushes composed mainly of olivine,
plagioclase, and clinopyroxene, corresponding to the
accumulated phases during differentiation of primitive
MORB in shallow magma chambers under the ridges.
Their mode of accretion is strongly controlled by the
spreading rate, with the formation of layered gabbros at
fast-spreading ridges and more isotropic gabbros at
slow-spreading ridges. The latter often show signs of plastic deformation and vast crystallization of evolved interstitial parageneses.
Bibliography
Bedard, J. H., 1991. Cumulate recycling and crustal evolution in the
Bay of islands ophiolite. Journal of Geology, 99, 225–249.
Boudier, F., Nicolas, A., and Ildefonse, B., 1996. Magma chambers
in the Oman ophiolite: fed from the top and the bottom. Earth
and Planetary Science Letters, 144, 239–250.
Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at
mid-ocean ridges. Journal of Geophysical Research-Solid Earth,
98, 4163–4172.
Coleman, R. G., and Donato, M. M., 1979. Oceanic plagiogranite
revisited. In Barker, F. (ed.), Trondhjemites, Dacites, and
Related Rocks. Amsterdam: Elsevier, pp. 149–167.
Coogan, L. A., 2014. The lower oceanic crust. In Turekian, K., and
Holland, H. D. (eds.), Treatise on Geochemistry, 2nd edn.
Amsterdam:
Elsevier,
doi:10.1016/B978-0-08-0959757.00316-8.
Coogan, L. A., Thompson, G., and MacLeod, C. J., 2002. A textural
and geochemical investigation of high level gabbros from the
Oman ophiolite: implications for the role of the axial magma
chamber at fast-spreading ridges. Lithos, 63, 67–82.
Detrick, R. S., Buhl, P., Vera, E., Mutter, J., Orcutt, J., Madsen, J.,
and Brocher, T., 1987. Multichannel seismic imaging of
a crustal magma chamber along the East Pacific Rise. Nature,
326, 35–41.
Dick, H. J. B., Natland, J. H., Alt, J. C., Bach, W., Bideau, D., Gee,
J. S., Haggas, S., Hertogen, J. G. H., Hirth, G., Holm, P. M.,
Ildefonse, B., Iturrino, G. J., John, B. E., Kelley, D. S., Kikawa,
E., Kingdon, A., LeRoux, P. J., Maeda, J., Meyer, P. S., Miller,
D. J., Naslund, H. R., Niu, Y. L., Robinson, P. T., Snow, J., Stephen, R. A., Trimby, P. W., Worm, H. U., and Yoshinobu, A.,
2000. A long in situ section of the lower ocean crust: results of
ODP Leg 176 drilling at the Southwest Indian Ridge. Earth
and Planetary Science Letters, 179, 31–51.
Dick, H. J. B., Tivey, M. A., and Tucholke, B. E., 2008. Plutonic
foundation of a slow-spreading ridge segment: oceanic core
complex at Kane megamullion, 23 degrees 30 ' N, 45 degrees
20 ' W. Geochemistry, Geophysics, Geosystems, 9.
Expedition Scientific Party, 2005. Oceanic core complex formation,
Atlantis Massif – oceanic core complex formation, Atlantis Massif, Mid-Atlantic Ridge: drilling into the footwall and hanging
wall of a tectonic exposure of deep, young oceanic lithosphere
to study deformation, alteration, and melt generation. Integrated
Ocean Drilling Program Preliminary Reports, 305, http://iodp.
tamu.edu/publications/PR/305PR/305PR.PDF.
Feig, S., Koepke, J., and Snow, J., 2006. Effect of water on tholeiitic
basalt phase equilibria: an experimental study under oxidizing
conditions. Contributions to Mineralogy and Petrology, 152,
611–638.
Gaetani, G. A., Grove, T. L., and Bryan, W. B., 1993. The influence
of water on the petrogenesis of subduction-related igneous rocks.
Nature, 365, 332–334.
Gillis, K. M., Snow, J. E., Klaus, A., Abe, N., Adriao, A. B.,
Akizawa, N., Ceuleneer, G., Cheadle, M. J., Faak, K., Falloon,
T. J., Friedman, S. A., Godard, M., Guerin, G., Harigane, Y.,
GABBRO
267
C below the liquidus of the system, orthopyroxene and Fe-Ti oxides crystallize (the latter
strongly controlled by the prevailing oxygen fugacity),
followed by amphibole, indicating that the water content
in the residual melt has been raised to significant amounts,
due to the crystallization of only water-free minerals in the
main stage crystallization. At this stage, most of the melts
are already crystallized and these minerals form interstitial
parageneses.
Geochemistry of oceanic gabbros
Students’ textbooks often state that gabbros are equivalent
in composition to basalts and that the difference between
the two rock families is their grain size, which is a direct
function of the cooling rate (high cooling rates produce
basalts; low cooling rates gabbros). However, oceanic
gabbros are cumulates (¼accumulated crystals; qv “cumulate”), and thus their bulk composition is strongly dependent of the presence and the modal amount of
accumulated crystals. For example, if oceanic gabbros
are strongly enriched in oxides (“oxide gabbro”; see
above), the silica content of the bulk rock can be <40 wt%
SiO 2 , due to the high concentration of the heavy element
oxide FeO. If crystallization of plagioclase dominates,
anorthosites (Figure 1) may be formed with very high
Al 2 O 3 contents (>20 wt%), and abundant crystallization
of olivine produces olivine-rich troctolites with MgO
contents >25 wt%. In contrast, MORBs are frozen melts
with more homogeneous major element composition,
which varies only within a few wt% for the major element
oxides as SiO 2 , Al 2 O 3 , and MgO. Another consequence of
the cumulate nature of oceanic gabbros is that the concentrations of incompatible trace elements are generally lower
than those of MORBs. For example, the average content
of K 2 O and P 2 O 5 in typical gabbros from the drilled core
from Hole 735B at SWIR is 0.04 and 0.01 wt%, respectively (data set of Dick et al., 2000), while the
corresponding value for typical primitive MORB is 0.07
and 0.09 (e.g., average from the data base PETD, Lehnert
et al., 2000), respectively. This depleted nature of typical
oceanic gabbros is a consequence of the accumulation of
the cotectic assemblage olivine + plagioclase +
clinopyroxene (Æorthopyroxene) resulting in rocks which
often represent nearly pure cumulates with very low proportions of residual melt (~1.4 % as average value for
the gabbros from the SWIR drill core, Natland and Dick,
2001). Hence, the bulk content of incompatible elements
of oceanic gabbros is more or less a direct function of
the amount of interstitial late crystallization where incompatible elements are enriched in phases like amphibole and
apatite, and which is in general decoupled from the main
stage of crystallization.
Summary
Gabbros are the most common rocks in the oceanic crust
and form its lower part (layer 3). They represent slowly
cooled crystal mushes composed mainly of olivine,
plagioclase, and clinopyroxene, corresponding to the
accumulated phases during differentiation of primitive
MORB in shallow magma chambers under the ridges.
Their mode of accretion is strongly controlled by the
spreading rate, with the formation of layered gabbros at
fast-spreading ridges and more isotropic gabbros at
slow-spreading ridges. The latter often show signs of plastic deformation and vast crystallization of evolved interstitial parageneses.
Bibliography
Bedard, J. H., 1991. Cumulate recycling and crustal evolution in the
Bay of islands ophiolite. Journal of Geology, 99, 225–249.
Boudier, F., Nicolas, A., and Ildefonse, B., 1996. Magma chambers
in the Oman ophiolite: fed from the top and the bottom. Earth
and Planetary Science Letters, 144, 239–250.
Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at
mid-ocean ridges. Journal of Geophysical Research-Solid Earth,
98, 4163–4172.
Coleman, R. G., and Donato, M. M., 1979. Oceanic plagiogranite
revisited. In Barker, F. (ed.), Trondhjemites, Dacites, and
Related Rocks. Amsterdam: Elsevier, pp. 149–167.
Coogan, L. A., 2014. The lower oceanic crust. In Turekian, K., and
Holland, H. D. (eds.), Treatise on Geochemistry, 2nd edn.
Amsterdam:
Elsevier,
doi:10.1016/B978-0-08-0959757.00316-8.
Coogan, L. A., Thompson, G., and MacLeod, C. J., 2002. A textural
and geochemical investigation of high level gabbros from the
Oman ophiolite: implications for the role of the axial magma
chamber at fast-spreading ridges. Lithos, 63, 67–82.
Detrick, R. S., Buhl, P., Vera, E., Mutter, J., Orcutt, J., Madsen, J.,
and Brocher, T., 1987. Multichannel seismic imaging of
a crustal magma chamber along the East Pacific Rise. Nature,
326, 35–41.
Dick, H. J. B., Natland, J. H., Alt, J. C., Bach, W., Bideau, D., Gee,
J. S., Haggas, S., Hertogen, J. G. H., Hirth, G., Holm, P. M.,
Ildefonse, B., Iturrino, G. J., John, B. E., Kelley, D. S., Kikawa,
E., Kingdon, A., LeRoux, P. J., Maeda, J., Meyer, P. S., Miller,
D. J., Naslund, H. R., Niu, Y. L., Robinson, P. T., Snow, J., Stephen, R. A., Trimby, P. W., Worm, H. U., and Yoshinobu, A.,
2000. A long in situ section of the lower ocean crust: results of
ODP Leg 176 drilling at the Southwest Indian Ridge. Earth
and Planetary Science Letters, 179, 31–51.
Dick, H. J. B., Tivey, M. A., and Tucholke, B. E., 2008. Plutonic
foundation of a slow-spreading ridge segment: oceanic core
complex at Kane megamullion, 23 degrees 30 ' N, 45 degrees
20 ' W. Geochemistry, Geophysics, Geosystems, 9.
Expedition Scientific Party, 2005. Oceanic core complex formation,
Atlantis Massif – oceanic core complex formation, Atlantis Massif, Mid-Atlantic Ridge: drilling into the footwall and hanging
wall of a tectonic exposure of deep, young oceanic lithosphere
to study deformation, alteration, and melt generation. Integrated
Ocean Drilling Program Preliminary Reports, 305, http://iodp.
tamu.edu/publications/PR/305PR/305PR.PDF.
Feig, S., Koepke, J., and Snow, J., 2006. Effect of water on tholeiitic
basalt phase equilibria: an experimental study under oxidizing
conditions. Contributions to Mineralogy and Petrology, 152,
611–638.
Gaetani, G. A., Grove, T. L., and Bryan, W. B., 1993. The influence
of water on the petrogenesis of subduction-related igneous rocks.
Nature, 365, 332–334.
Gillis, K. M., Snow, J. E., Klaus, A., Abe, N., Adriao, A. B.,
Akizawa, N., Ceuleneer, G., Cheadle, M. J., Faak, K., Falloon,
T. J., Friedman, S. A., Godard, M., Guerin, G., Harigane, Y.,
GABBRO
267