An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part II: Applications to Degassing |
| |
| Authors: | DIXON JACQUELINE EABY; STOLPER EDWARD M |
| |
| Institution: | DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CA 91125, USA |
| |
| Abstract: | Degassing processes in basaltic magmas rich in both water andcarbon dioxide can be modeled using the solubilities of theendmember systems and the assumption of Henry's law. Suitesof vapor-saturated basaltic melts having a range of initialCO2/H2O ratios and erupted over a narrow depth interval willdefine negatively sloped arrays on an H2O vs CO2 plot. It isimportant that all of the major volatile species be consideredsimultaneously when interpreting trends in dissolved volatilespecies concentrations in magmas. Based on measured concentrations of water and carbon dioxidein basaltic glasses, the composition of the vapor phase at 1200°Cthat could coexist with a basaltic melt and the pressure atwhich it would be vapor saturated can be calculated. The rangein vapor compositions in equilibrium with submarine basaltsreflects the range in water contents in the melts characteristicof each environment. The ranges in the molar proportion of CO2in vapor phases (X CO2) calculated to be in equilibrium withsubmarine tholeiitic glasses are 0•93–1•00 formid-ocean ridge basalts (MORB), 0•60–0•99 forglasses from Kilauea representative of ocean island basalts(OIB)] and 0–0•94 for glasses from back-arc basins(BABB). MORB glasses from spreading centers ranging from slow(e.g. the Mid-Atlantic Ridge) to fast (e.g. East Pacific Rise,9–13°N) are commonly supersaturated with respect toCO2-rich vapor, resulting from magma ascent rates so rapid thatmagmas erupt on the sea-floor without having been fully degassedby bubble nucleation and growth during ascent. In contrast tothe MORB glasses, volatile contents in submarine glasses fromKilauea are consistent with having been in equilibrium witha vapor phase containing 60–100 mol% CO2 at the pressureof eruption, reflecting differences in average magma transportrates during eruptions at mid-ocean ridges and hotspot volcanoes. Degassing during decompression of tholeiitic basaltic magmais characterized by strong partitioning of CO2 into the vaporphase. During open system degassing, CO2 is rapidly removedfrom the melt with negligible loss of water, until a pressureis reached at which the melt is in equilibrium with nearly purewater vapor. From this pressure downward, the water contentof the melt follows the water solubility curve. During closedsystem degassing, water and CO2 contents in vapor-saturatedbasaltic magmas will depend strongly on the vapor compositionas determined by the initial volatile concentrations. Deviationfrom open system behavior, toward lower dissolved H2O and CO2saturation concentrations at a given pressure, will be greatestin melts having high total volatile concentrations and highCO2:H2O ratios. Closed system degassing of basaltic melts havingthe low initial H2O and CO2 contents typical of MORB and OIB,however, are similar to the open system case. KEY WORDS: mid-ocean ridge basalts; water and carbon dioxide solubility; degassing |
| |
| Keywords: | |
| 本文献已被 Oxford 等数据库收录！ |
|
