lithospheric environment prior to the lava’s eruption. The volatile contents in
oceanic rocks usually form minor constituents when compared to the other elements. For example, volatiles such as HCl, CH 4 , CO 2 , CO, SO 2 H 2 S and H 2 O,
constitute less than 1 % of the bulk rock and usually only 0.50 % of sea floor
basalts.
The amount of dissolved water and carbon measured by Pineau et al. (2004) on
Mid-Ocean Ridge Basalts (MORBs) collected from the Mid-Atlantic Ridge segment at 34°50
0 N during the Oceanaut cruise ranged from 1125 to 5253 ppm (part
per million or 0.11–0.52 % of the bulk rock) up to 20–119 ppm (0.002–0.011 %).
The basalts most enriched in their incompatible elements (E-MORB = enriched
MORB) have the highest contents of water and carbon. Even with such a low
concentration of volatiles, repetitive magmatic intrusions trapping the volatiles
within the magmatic column are likely to increase their concentration and pressure
with respect the surrounding formation.
It was shown (Hekinian et al. 2000) that there is a correlation between an
erupted rock’s vesicularity and its composition (such as P and K) as well as a
correlation between the amount of total water and the CO 2 content. This implies
that both gaseous carbon and water contents will depend on the initial concentration of these elements in the mantle and this will also be related to the rate of
partial melting in the mantle. Pineau et al. (2004) studied the CO 2 content of lava
erupted on the MAR and have shown that carbon saturation is reached at
increasing depths and pressure. They found, on the basis of laboratory measurements, that carbon is saturated in magma at pressures from 2.6 to 12.8 kbar, which
corresponds to depths of 9–44 km in the mantle. Thus the degree of vesicularity is
mainly conditioned by the magma’s original carbon enrichment and its carbon
exsolution during magma ascent (Bottinga and Javoy 1989). When volatiles are
trapped within a melt, eventually vesicle expansion will lead to explosive activity
and the formation of pyroclastites, vitric tuffs and hyaloclastites (Fisher and
Schmincke 1984).
Pinneau et al. (1998) and Hekinian et al. (2000) have shown that the partition
between water and CO 2 is such that during magmatic production water tends to
dissolve in the melt and will remain in the magma, while CO 2 will rise more
rapidly and accumulate at the top of the magmatic column. In the magma column,
CO 2 will form bubbles and the rapid ascent of volatile enriched melts in a magmatic column will produce an increase in vesicularity (Fig. 5.13). Thus, CO 2 is the
main gaseous phase existing in the vesicles just before their quenching (rapid
solidification). An increase in vesicularity will accelerate the decoupling or separation of the magma-bubbles system. Consequently, the gas pressure exerted on
the magma column (CO 2 in the bubbles) will rise until it overcomes the load
pressure overlying a volcanic construction and thereby enhance an explosive
event.
One of the best documented studies on the distribution of volatile contents in
sea floor lava was made on a suite of samples from an eruption on the East Pacific
Rise at 9°50
0 N which occurred in 2005–2006. The samples were collected by
submersible at about 200 m intervals along the pathway of a single eruptive event
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5 Earth’s Mantle Melting and Volcanism
oceanic rocks usually form minor constituents when compared to the other elements. For example, volatiles such as HCl, CH 4 , CO 2 , CO, SO 2 H 2 S and H 2 O,
constitute less than 1 % of the bulk rock and usually only 0.50 % of sea floor
basalts.
The amount of dissolved water and carbon measured by Pineau et al. (2004) on
Mid-Ocean Ridge Basalts (MORBs) collected from the Mid-Atlantic Ridge segment at 34°50
0 N during the Oceanaut cruise ranged from 1125 to 5253 ppm (part
per million or 0.11–0.52 % of the bulk rock) up to 20–119 ppm (0.002–0.011 %).
The basalts most enriched in their incompatible elements (E-MORB = enriched
MORB) have the highest contents of water and carbon. Even with such a low
concentration of volatiles, repetitive magmatic intrusions trapping the volatiles
within the magmatic column are likely to increase their concentration and pressure
with respect the surrounding formation.
It was shown (Hekinian et al. 2000) that there is a correlation between an
erupted rock’s vesicularity and its composition (such as P and K) as well as a
correlation between the amount of total water and the CO 2 content. This implies
that both gaseous carbon and water contents will depend on the initial concentration of these elements in the mantle and this will also be related to the rate of
partial melting in the mantle. Pineau et al. (2004) studied the CO 2 content of lava
erupted on the MAR and have shown that carbon saturation is reached at
increasing depths and pressure. They found, on the basis of laboratory measurements, that carbon is saturated in magma at pressures from 2.6 to 12.8 kbar, which
corresponds to depths of 9–44 km in the mantle. Thus the degree of vesicularity is
mainly conditioned by the magma’s original carbon enrichment and its carbon
exsolution during magma ascent (Bottinga and Javoy 1989). When volatiles are
trapped within a melt, eventually vesicle expansion will lead to explosive activity
and the formation of pyroclastites, vitric tuffs and hyaloclastites (Fisher and
Schmincke 1984).
Pinneau et al. (1998) and Hekinian et al. (2000) have shown that the partition
between water and CO 2 is such that during magmatic production water tends to
dissolve in the melt and will remain in the magma, while CO 2 will rise more
rapidly and accumulate at the top of the magmatic column. In the magma column,
CO 2 will form bubbles and the rapid ascent of volatile enriched melts in a magmatic column will produce an increase in vesicularity (Fig. 5.13). Thus, CO 2 is the
main gaseous phase existing in the vesicles just before their quenching (rapid
solidification). An increase in vesicularity will accelerate the decoupling or separation of the magma-bubbles system. Consequently, the gas pressure exerted on
the magma column (CO 2 in the bubbles) will rise until it overcomes the load
pressure overlying a volcanic construction and thereby enhance an explosive
event.
One of the best documented studies on the distribution of volatile contents in
sea floor lava was made on a suite of samples from an eruption on the East Pacific
Rise at 9°50
0 N which occurred in 2005–2006. The samples were collected by
submersible at about 200 m intervals along the pathway of a single eruptive event
126
5 Earth’s Mantle Melting and Volcanism