structure to the oceanic crust (e.g., see discussion of the
“gabbro-glacier” and “sheeted-sill” models in Coogan,
2007 and Keleman and Aharonov, 1998). An important
characteristic of many mid-ocean ridges is the axial
magma chamber (AMC), discovered in the 1980s by seismic reflection studies. The AMC represents the shallowest
level of magma accumulation in the crust (Sinton and
Detrick, 1992) and can be segmented at a variety of spatial
and temporal scales (e.g., Carbotte et al., 2013).
Far fewer AMC reflections have been detected at
the ridges with very slow to slow-spreading rates
(i.e., < 1–2 cm/year), despite most magmas erupted there
showing the chemical and mineralogical signatures of
differentiation at crustal pressures, which implies that the
AMC bodies there are mostly too small and discontinuous
to be seismically imaged. Where they can be detected, the
size, continuity, and depth of the AMC vary substantially
through the ridge system, but overall these parameters
scale with spreading rate: the AMC generally becomes
more continuous and shallower (as little as 1.5 or 2 km
below the seafloor) at the fastest-spreading rates
(Figure 2). This depth variation has been successfully
modeled as an interplay between the rate of magma supply
from below and the intensity of hydrothermal cooling of
the shallowest crust from above (Phipps Morgan and
Chen, 1994).
Mid-ocean Ridge Magmatism and Volcanism, Figure 2 Three diagrams depicting compositional attributes of mid-ocean ridge
basalts that can be used to infer conditions of magma differentiation, as discussed in the text. Panel a, which is modified from Figure 1
of Rubin and Sinton (2007) (see that paper for details of data analysis and spreading rate calculations), shows Mg variations and
inferred eruption temperature in ridge section means from different geographic regions of the global mid-ocean ridge system using
a >11,000 basalt glass database. Note the strong linear relationships with spreading rate (the solid black line is a linear regression to
the data). Gray bars are 1 s deviations for each mean. Overall, erupted compositions are cooler and more differentiated at fasterspreading rate ridges and also more variable in differentiation-related parameters (see Rubin and Sinton, 2007 for a detailed
discussion). The central Galapagos Spreading Center, which is an example of a hotspot-influenced ridge having anomalously high
magma supply due to active upwelling, falls off the trend (diamond symbol). Panel b, axial magma chamber (AMC) depth versus Mg
content of erupted MORB showing variations in both means of ridge sections (from panel a) also having AMC depth measurements
(red symbols) and segments of the Juan de Fuca ridge having AMC depth (blue symbols). See the caption in Figure 8 in Rubin and
Sinton (2007) for details of data sources and handing. This panel shows that magmas erupted from deeper magma chambers are
generally less differentiated. Magma chambers are generally shallower at higher spreading rates and deeper at slower-spreading
rates, where overall melt supply is high and low, respectively. The linear regression includes the red symbols. Panel c, A generalized
depiction of the greater depth, smaller size, and lower connectivity of AMCs at low spreading rates, as discussed in Rubin et al., (2009).
A wide range of geophysical and geochemical parameters were used to infer these variations, as discussed briefly in this article and in
detail in Rubin et al. (2009).
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MID-OCEAN RIDGE MAGMATISM AND VOLCANISM
“gabbro-glacier” and “sheeted-sill” models in Coogan,
2007 and Keleman and Aharonov, 1998). An important
characteristic of many mid-ocean ridges is the axial
magma chamber (AMC), discovered in the 1980s by seismic reflection studies. The AMC represents the shallowest
level of magma accumulation in the crust (Sinton and
Detrick, 1992) and can be segmented at a variety of spatial
and temporal scales (e.g., Carbotte et al., 2013).
Far fewer AMC reflections have been detected at
the ridges with very slow to slow-spreading rates
(i.e., < 1–2 cm/year), despite most magmas erupted there
showing the chemical and mineralogical signatures of
differentiation at crustal pressures, which implies that the
AMC bodies there are mostly too small and discontinuous
to be seismically imaged. Where they can be detected, the
size, continuity, and depth of the AMC vary substantially
through the ridge system, but overall these parameters
scale with spreading rate: the AMC generally becomes
more continuous and shallower (as little as 1.5 or 2 km
below the seafloor) at the fastest-spreading rates
(Figure 2). This depth variation has been successfully
modeled as an interplay between the rate of magma supply
from below and the intensity of hydrothermal cooling of
the shallowest crust from above (Phipps Morgan and
Chen, 1994).
Mid-ocean Ridge Magmatism and Volcanism, Figure 2 Three diagrams depicting compositional attributes of mid-ocean ridge
basalts that can be used to infer conditions of magma differentiation, as discussed in the text. Panel a, which is modified from Figure 1
of Rubin and Sinton (2007) (see that paper for details of data analysis and spreading rate calculations), shows Mg variations and
inferred eruption temperature in ridge section means from different geographic regions of the global mid-ocean ridge system using
a >11,000 basalt glass database. Note the strong linear relationships with spreading rate (the solid black line is a linear regression to
the data). Gray bars are 1 s deviations for each mean. Overall, erupted compositions are cooler and more differentiated at fasterspreading rate ridges and also more variable in differentiation-related parameters (see Rubin and Sinton, 2007 for a detailed
discussion). The central Galapagos Spreading Center, which is an example of a hotspot-influenced ridge having anomalously high
magma supply due to active upwelling, falls off the trend (diamond symbol). Panel b, axial magma chamber (AMC) depth versus Mg
content of erupted MORB showing variations in both means of ridge sections (from panel a) also having AMC depth measurements
(red symbols) and segments of the Juan de Fuca ridge having AMC depth (blue symbols). See the caption in Figure 8 in Rubin and
Sinton (2007) for details of data sources and handing. This panel shows that magmas erupted from deeper magma chambers are
generally less differentiated. Magma chambers are generally shallower at higher spreading rates and deeper at slower-spreading
rates, where overall melt supply is high and low, respectively. The linear regression includes the red symbols. Panel c, A generalized
depiction of the greater depth, smaller size, and lower connectivity of AMCs at low spreading rates, as discussed in Rubin et al., (2009).
A wide range of geophysical and geochemical parameters were used to infer these variations, as discussed briefly in this article and in
detail in Rubin et al. (2009).
504
MID-OCEAN RIDGE MAGMATISM AND VOLCANISM