Wilson, D. S., Teagle, D. A. H., Alt, J. C., Banerjee, N. R., Umino,
S., Miyashita, S., Acton, G. D., Anma, R., Barr, S. R., Belghoul,
A., Carlut, J., Christie, D. M., Coggon, R. M., Cooper, K. M.,
Cordier, C., Crispini, L., Durand, S. R., Einaudi, F., Galli, L.,
Gao, Y. J., Geldmacher, J., Gilbert, L. A., Hayman, N. W.,
Herrero-Bervera, E., Hirano, N., Holter, S., Ingle, S., Jiang,
S. J., Kalberkamp, U., Kerneklian, M., Koepke, J., Laverne,
C., Vasquez, H. L. L., Maclennan, J., Morgan, S., Neo, N.,
Nichols, H. J., Park, S. H., Reichow, M. K., Sakuyama, T., Sano,
T., Sandwell, R., Scheibner, B., Smith-Duque, C. E., Swift,
S. A., Tartarotti, P., Tikku, A. A., Tominaga, M., Veloso, E. A.,
Yamasaki, T., Yamazaki, S., and Ziegler, C., 2006. Drilling to
gabbro in intact ocean crust. Science, 312, 1016–1020,
doi:10.1126/science.1126090.
Xu, M., Pablo Canales, J., Carbotte, S. M., Carton, H., Nedimović,
M. R., and Mutter, J. C., 2014. Variations in axial magma lens
properties along the East Pacific Rise (9
30
0 N-10
00
0 N) from
swath 3-D seismic imaging and 1-D waveform inversion. Journal of Geophysical Research, 119, 2721–2744, doi:10.1002/
2013JB010730.
Cross-references
Axial Summit Troughs
Axial Volcanic Ridges
Cumulates
Gabbro
Layering of Oceanic Crust
Lithosphere: Structure and Composition
Ocean Drilling
Oceanic Spreading Centers
Peridotites
Serpentinization
CUMULATES
Jürgen Koepke
Institute of Mineralogy, Leibniz University Hannover,
Hannover, Germany
Cumulate is a name for plutonic rocks which were formed
either by the process of accumulation of crystals (crystal
settling) or by direct crystallization (in situ crystallization)
in a magma chamber. Since “cumulate” is clearly a genetic
term, it should not be used as rock name (qv. “Gabbro,”
and the chapter on rock classification of gabbroic rocks
therein), but can be used as additional qualifier, in order
to express that rocks were formed by a distinct process
within a magma chamber.
For the oceanic crust, this term is directly related to the
accretion of oceanic gabbros, forming the lower part of the
oceanic crust (layer 3; qv. “seismic layers”). The
corresponding magma chambers are located directly
under the spreading centers of mid-ocean ridges or backarc systems (qv. “axial magma chamber”). Here, primitive
MORB (mid-oceanic ridge basalt; qv. “Mid-ocean Ridge
Magmatism and Volcanism”) melts differentiate by fractionation of mainly olivine, plagioclase, and
clinopyroxene, which may accumulate at the floor of the
magma chamber to form a dense crystal mush, resulting
in the oceanic gabbros after cooling. Thus, the vast majority of oceanic gabbros can be regarded as cumulate rocks,
formed by the process of crystal sedimentation in axial
magma chambers.
Traditionally, the term cumulate was used for layered
intrusions on continents (e.g., Skaergaard Layered Intrusion, Greenland; Rum Layered Suite, Scotland). Based
on the work of Wager et al. (1960), a scheme of textures
for cumulate formation was introduced which has been
a major contribution to the understanding of igneous textures. Reviews on that are given by Shelly (1993) or
Hunter (1996). “Cumulus phases” or “cumulus crystals”
are those phases which are accumulated (sedimented) in
a magma chamber, while the liquid around is called
“intercumulus” liquid. “Adcumulate” textures represent
an accumulation of crystals from which the intercumulus
liquid was pressed out, possibly driven by compaction.
The cumulate phases occupy the entire volume of the rock,
resulting in crystals with subhedral to anhedral shapes
which are defined by the jointly merged boundaries of
adjacent crystals. “Orthocumulate” textures represent
accumulated crystals around which a more evolved
intercumulus liquid crystallized often resulting in typical
poikilitic growth, meaning that relatively large, late crystallized minerals enclose earlier, smaller crystals.
However, the detailed cumulate terminology as
established by Wager et al. (1960) is generally not applied
to oceanic gabbros, due to several reasons. First, the terms
are rather specific, often with a strong genetic aspect,
which makes its application to oceanic gabbros difficult,
since the mechanisms by which the lower, gabbroic crust
forms are poorly constrained (qv. “crustal accretion”;
“gabbro”). Moreover, primary textures may be obscured
by later strain which may result in magmatic foliation
(especially in gabbros from fast-spreading systems) or in
crystal-plastic deformation (especially in gabbros from
slow-spreading systems). Thus, for the petrographic characterization of oceanic gabbros, common petrographic
description terms for mineral structures and rock textures
are commonly used. For example, the record of crystallization of a late, evolved melt in the interstices of earlier
crystallized minerals, producing minerals like amphibole,
oxide, orthopyroxene, apatite, and zircon occurring in
many
gabbros
from
slow-spreading
ridges
(qv. “gabbro”), can simply be described as “late assemblages crystallized interstitially,” instead of using a term
like “orthocumulate” or “mesocumulate” which might be
appropriate in some cases.
Bibliography
Hunter, R. H., 1996. Texture development in cumulate rocks. In
Cawthorn, R. G. (ed.), Layered Intrusions. Amsterdam: Elsevier,
pp. 77–101.
CUMULATES
137
S., Miyashita, S., Acton, G. D., Anma, R., Barr, S. R., Belghoul,
A., Carlut, J., Christie, D. M., Coggon, R. M., Cooper, K. M.,
Cordier, C., Crispini, L., Durand, S. R., Einaudi, F., Galli, L.,
Gao, Y. J., Geldmacher, J., Gilbert, L. A., Hayman, N. W.,
Herrero-Bervera, E., Hirano, N., Holter, S., Ingle, S., Jiang,
S. J., Kalberkamp, U., Kerneklian, M., Koepke, J., Laverne,
C., Vasquez, H. L. L., Maclennan, J., Morgan, S., Neo, N.,
Nichols, H. J., Park, S. H., Reichow, M. K., Sakuyama, T., Sano,
T., Sandwell, R., Scheibner, B., Smith-Duque, C. E., Swift,
S. A., Tartarotti, P., Tikku, A. A., Tominaga, M., Veloso, E. A.,
Yamasaki, T., Yamazaki, S., and Ziegler, C., 2006. Drilling to
gabbro in intact ocean crust. Science, 312, 1016–1020,
doi:10.1126/science.1126090.
Xu, M., Pablo Canales, J., Carbotte, S. M., Carton, H., Nedimović,
M. R., and Mutter, J. C., 2014. Variations in axial magma lens
properties along the East Pacific Rise (9
30
0 N-10
00
0 N) from
swath 3-D seismic imaging and 1-D waveform inversion. Journal of Geophysical Research, 119, 2721–2744, doi:10.1002/
2013JB010730.
Cross-references
Axial Summit Troughs
Axial Volcanic Ridges
Cumulates
Gabbro
Layering of Oceanic Crust
Lithosphere: Structure and Composition
Ocean Drilling
Oceanic Spreading Centers
Peridotites
Serpentinization
CUMULATES
Jürgen Koepke
Institute of Mineralogy, Leibniz University Hannover,
Hannover, Germany
Cumulate is a name for plutonic rocks which were formed
either by the process of accumulation of crystals (crystal
settling) or by direct crystallization (in situ crystallization)
in a magma chamber. Since “cumulate” is clearly a genetic
term, it should not be used as rock name (qv. “Gabbro,”
and the chapter on rock classification of gabbroic rocks
therein), but can be used as additional qualifier, in order
to express that rocks were formed by a distinct process
within a magma chamber.
For the oceanic crust, this term is directly related to the
accretion of oceanic gabbros, forming the lower part of the
oceanic crust (layer 3; qv. “seismic layers”). The
corresponding magma chambers are located directly
under the spreading centers of mid-ocean ridges or backarc systems (qv. “axial magma chamber”). Here, primitive
MORB (mid-oceanic ridge basalt; qv. “Mid-ocean Ridge
Magmatism and Volcanism”) melts differentiate by fractionation of mainly olivine, plagioclase, and
clinopyroxene, which may accumulate at the floor of the
magma chamber to form a dense crystal mush, resulting
in the oceanic gabbros after cooling. Thus, the vast majority of oceanic gabbros can be regarded as cumulate rocks,
formed by the process of crystal sedimentation in axial
magma chambers.
Traditionally, the term cumulate was used for layered
intrusions on continents (e.g., Skaergaard Layered Intrusion, Greenland; Rum Layered Suite, Scotland). Based
on the work of Wager et al. (1960), a scheme of textures
for cumulate formation was introduced which has been
a major contribution to the understanding of igneous textures. Reviews on that are given by Shelly (1993) or
Hunter (1996). “Cumulus phases” or “cumulus crystals”
are those phases which are accumulated (sedimented) in
a magma chamber, while the liquid around is called
“intercumulus” liquid. “Adcumulate” textures represent
an accumulation of crystals from which the intercumulus
liquid was pressed out, possibly driven by compaction.
The cumulate phases occupy the entire volume of the rock,
resulting in crystals with subhedral to anhedral shapes
which are defined by the jointly merged boundaries of
adjacent crystals. “Orthocumulate” textures represent
accumulated crystals around which a more evolved
intercumulus liquid crystallized often resulting in typical
poikilitic growth, meaning that relatively large, late crystallized minerals enclose earlier, smaller crystals.
However, the detailed cumulate terminology as
established by Wager et al. (1960) is generally not applied
to oceanic gabbros, due to several reasons. First, the terms
are rather specific, often with a strong genetic aspect,
which makes its application to oceanic gabbros difficult,
since the mechanisms by which the lower, gabbroic crust
forms are poorly constrained (qv. “crustal accretion”;
“gabbro”). Moreover, primary textures may be obscured
by later strain which may result in magmatic foliation
(especially in gabbros from fast-spreading systems) or in
crystal-plastic deformation (especially in gabbros from
slow-spreading systems). Thus, for the petrographic characterization of oceanic gabbros, common petrographic
description terms for mineral structures and rock textures
are commonly used. For example, the record of crystallization of a late, evolved melt in the interstices of earlier
crystallized minerals, producing minerals like amphibole,
oxide, orthopyroxene, apatite, and zircon occurring in
many
gabbros
from
slow-spreading
ridges
(qv. “gabbro”), can simply be described as “late assemblages crystallized interstitially,” instead of using a term
like “orthocumulate” or “mesocumulate” which might be
appropriate in some cases.
Bibliography
Hunter, R. H., 1996. Texture development in cumulate rocks. In
Cawthorn, R. G. (ed.), Layered Intrusions. Amsterdam: Elsevier,
pp. 77–101.
CUMULATES
137