David W. Muenow

Abundance and distribution of water, carbon and sulfur in the glassy rims of submarine pillow basalts

Mass spectrometric analyses of phenocrysts, containing glass-vapor inclusions quenched in glassy rims of tholeiitic submarine pillow basalts from spreading centers and Hawaii, indicate that water released from the inclusions upon thermal decrepitation is much less than is released from equivalent volumes of the matrix glass enclosing the same phenocrysts. If water released represents water quenched in the two types of glass (inclusion and matrix) at the time of eruption, then the inclusions contained far less water than the magma surrounding the phenocrysts. Such a condition is consistent with a water depleted source mass for the basalts and an influx of water into the melt after inclusion formation but before quenching at the ocean floor. Glass-vapor inclusions within phenocrysts in tholeiitic pillow basalts from the Marianas Interarc Basin released abundant water even though the adjacent matrix glasses contain only 50% more water than Hawaiian matrix glasses. Water contents of matrix glasses from the Marianas pillow rims are very close to experimental saturation values. In contrast, matrix glasses from MORs and Hawaii are distinctly below water saturation. Vesicles in these glasses are most reasonably accounted for by a CO2-dominated vapor phase, because CO2 contents of those basalt glasses closely approximate experimental solubility limits. Glass-vapor inclusions in phenocrysts from the same pillows appear to contain more CO2 than adjacent matrix glasses, implying that a CO2-dominated vapor developed and outgassed prior to eruption and after inclusion formation. Water influx and CO2 degassing combine to make the CO2(CO2 + H2O) ratio of seafloor basalt glasses a minimum value for the magma source mass. The sulfur content of the inclusion glasses is normally very similar to that of the enclosing matrix glass.

Volatiles in submarine volcanic rocks from the Mariana Island arc and trough

High temperature mass spectrometric analyses of glasses from quenched pillow rims of andesites dredged from 1170 m water depth in the northern portion of the Mariana Island arc indicate substantially less H2O (~ 1 wt.%) and more CO2 (~ 0.24 wt.%) than previously reported for volcanic arc rocks. Glass-vapor inclusions within plagioclase phenocrysts from quenched rims have CO2H2O ratios of 1:1. These results are similar to analyses of basaltic samples from the Mariana Trough (a back-arc basin). Generally, F and Cl contents are higher and S lower in the arc rocks compared to the samples from the back-arc basin. These results favor models for the production of island arc magmas which involve melting of the subducted slab, rather than just melting of the overlying mantle wedge because of the high volatile content needed to produce island arc magmas from peridotite (10–15 wt.%). The trough samples, although similar in non-volatile composition to mid-ocean ridge rocks, have much higher H2O. somewhat higher CO2 and lower S contents. Either near surface addition of voiatiles has enriched the magmas or H2O must be a more important component in the generation and evolution of back-arc basin lavas than in the genesis of mid-ocean ridge basalts.

Volatiles in basaltic glasses from the East Pacific Rise at 21°N: implications for MORB sources and submarine lava flow morphology

Pillow-rim glasses from a suite of moderately evolved lavas erupted along the axis of the East Pacific Rise (EPR) at 21°N were analyzed by high-temperature mass spectrometry for volatile content. Concentrations of H2O, F and Cl in the 21°N glasses are low and correlate inversely with Mg# yielding well-defined trends. These and other geochemical data indicate derivation of the 21°N glasses from similar parental magmas produced from a highly depleted, nearly homogeneous source [1]. Total volatile, H2O, F and S contents are lower and CO2 content is somewhat higher in these samples than in previously analyzed Mid-Atlantic Ridge (MAR) glasses. 21°N glasses are similar in volatile content to glasses from the Galapagos Spreading Center (GSC) at 95°W except in Cl (lower) and CO2 (higher). Unlike most MAR and GSC glasses, CO2 is the dominant volatile in all of the 21°N glasses with Mg# > 62. All of the EPR and GSC glasses contain reduced carbon species (CO and CH4), unlike most previously analyzed MAR samples. These data indicate that sources for mid-ocean ridge basalts are extremely volatile-poor (< 0.10 wt.%) and are probably dominantly reduced (below quartz-fayalite-magnetite buffer). Sheet flow and pillow basalts contain identical volatile contents. Thus, volatile abundance is not a factor controlling flow morphology. Extrusion rate and/or surface topography are probably the most important factors influencing flow morphology in submarine basalts. H2O-release patterns are however related to flow morphology. Mass pyrograms for sheet flow and pillow basalt glasses both show bimodal H2O-release behavior at the same temperatures (800°C and1000°C ± 50°C). However, the dominant H2O-release peak for sheet flow glasses is at the lower temperature; for pillow-rim glasses it is at the higher temperature. Infrared spectroscopic studies indicate that H2O in these glasses is present only as hydroxyl groups. Thus, the cause of differences in the bimodal H2O-release and how it is related to flow morphology is unclear.

Volatiles in pillow rim glasses from Loihi and Kilauea volcanoes, Hawaii

Abstract Volatiles and major elements in submarine glasses from Loihi seamount and Kilauea volcano. Hawaii were analyzed by high temperature mass spectrometry and the electron microprobe. Loihi glasses are subdivided into three groups: tholeiitic, transitional and alkali basalts. The glasses are evolved: Mg numbers range from 48–58. The alkalic lavas are the most evolved. Total volatiles range from 0.73 to 1.40 wt.%. H 2 O shows a positive linear correlation with K 2 O content [ H 2 O = 0.83 (± .09) K 2 O + 0.08 (± .06)]. Concentrations of H 2 O are higher in the alkalic lavas, but Cl and F abundances are highly variable. Variations in ratios of incompatible elements (K 2 O, P 2 O 5 , H 2 O) indicate that each group was derived from a distinct source. CO 2 contents range from 0.05 to 0.19 wt.% but show no systematic correlation with rock type or Mg #. A well-defined decrease in glass CO 2 content with increasing vesicularity is shown by the alkalic lavas. CO 2 may have been outgassed from the tholeiitic and transitional magmas prior to eruption during storage in a shallow magma chamber. Reduced carbon species (CO and CH 4 ) were found in small amounts in most of the alkalic samples. Although the redox histories of Hawaiian lavas are poorly known, these new data indicate the presence of a reduced source for Loihi magmas. The Kilauea tholeiitic glasses are evolved (Mg # 48.3 to 55) and have higher H 2 O contents (av. 0.54 wt.%) than Loihi tholeiites (av. 0.42 wt.%) at the same Mg # (~55). Cl is distinctly lower in Kilauea glasses (0.01 wt.%) compared to Loihi glasses (0.09 wt.%). The data indicate significant source differences for the two volcanoes, consistent with results of other geochemical studies. Loihi tholeiites have distinctly higher 3 He/ 4 He ratios than Kilauea tholeiites and are the highest measured in submarine basalts (KURZ et al. , 1983). These high ratios have been used to invoke a primitive source for Loihi basalts. The high Cl content of these basalts, the highest we have ever measured in submarine basalts, may be a fingerprint of this primitive source, as previously noted for Icelandic basalts ( Schilling et al. 1980).

Volatiles in basalts and andesites from the Galapagos Spreading Center, 85° to 86°W

Abstract Glasses from submarine lavas recovered by the ALVIN submersible from the Galapagos Spreading Center (GSC) near 86°W have been analyzed by electron microprobe for major elements and by high-temperature mass spectrometry for volatiles. The samples studied range in composition from basalt to andesite and are more evolved than typical MORBs. Previous studies indicate that they are related to normal MORB by extensive crystal fractionation in small, isolated magma chambers. The H 2 O, Cl and F contents of these lavas are substantially higher than any previously reported for MORBs. H 2 O, Cl and F abundances increase linearly with P 2 O 5 content, which is used as an indicator of the extent of crystal fractionation. The Fe 2 O 3 (FeO + Fe 2 O 3 ) ratios measured in the andesite glasses progressively decrease with increasing P 2 O 5 content and are probably related to fractionation of Fe-Tioxides. Reduced carbon gas species, principally CH 4 and CO, were discovered in these glasses. The presence of reduced carbon species in GSC glasses may be indicative of a more reduced oxidation state of the upper mantle than is commonly assumed.

VOLATILES IN SUBMARINE VOLCANIC-ROCKS FROM THE SPREADING AXIS OF THE EAST SCOTIA SEA BACK-ARC BASIN

Abstract The volatile content of glassy pillow rims from East Scotia Sea back-arc basin (BAB) lavas are unlike those of mid-ocean ridge (MOR) pillow-rim glasses, although non-volatile compositions of the two rock groups overlap. The East Scotia Sea samples have three to ten times greater water contents and nearly twice the average CO 2 and Cl contents of MOR samples; F contents are similar. S contents are only one-third those from MOR samples. H 2 O and CO 2 contents of glassy pillow rims from Mariana island arc andesites are similar to those in the BAB lavas studied. Nevertheless, volatiles in the East Scotia Sea BAB magmas are probably not directly derived from the subducted slab, because there is no seismic evidence that the slab extends within 200 km of the spreading axis of the East Scotia Sea. Available data do not preclude the possibility that the magmas were contaminated by seawater prior to eruption or that the mantle under the East Scotia Sea spreading center is volatile-rich. The volatiles may have been added to the mantle during an earlier period of subduction, perhaps during the initial formation of the East Scotia Sea basin.

Volatile contents and ferric-ferrous ratios of basalt, ferrobasalt, andesite and rhyodacite glasses from the Galapagos 95.5°W propagating rift

Volatiles and major elements in abyssal glasses ranging in composition from basalt, ferrobasalt, andesite to rhyodacite from the Galapagos Spreading Center (GSC) near 95°W were analyzed using electron microprobe and high temperature mass spectrometry. Total volatile content ranged from 0.32 wt.% to 2.74 wt.%. Volatile abundances of MORB glasses from the 95.5°W propagating rift are similar to those from the adjacent normal rift (avg. 0.34 wt.%) and lower than those of N-type MORB from the Mid-Atlantic Ridge (avg. 0.49 wt.%). Although both propagating and non-propagating rift glasses contain trace amounts of methane (<0.01 wt.%) and carbon monoxide (0.04 wt.%), significantly higher 100 Fe2O3FeO + Fe2O3 ratios are observed for the primitive propagating rift glasses. Water contents of the most primitive GSC glasses are ~0.09 wt.% suggesting a water content for the mantle source of ~0.02 wt.% which indicates that source masses with very low water content can be involved in the generation of MORB. In fractionated ferrobasalt, andesite and rhyodacite glasses from the 95.5°W propagating rift, increasing abundances of H2O, Cl and F indicate highly incompatible behavior, whereas CO2 and reduced carbon species appear to decrease in abundance with increasing differentiation. Ferric-ferrous ratios increase from basalt to andesite and reduce to near zero in the rhyodacite. These values are not distinguishable from those previously reported for similar fractionated glasses from the Galapagos 85°W propagating rift, despite the apparent suppression of oxide precipitation in the 85°W suite.

Volatiles from Hawaiian submarine basalts determined by dynamic high temperature mass spectrometry

Abstract Quantitative measurements of volatiles from Hawaiian submarine basalts have been made using a Knudsen cell dynamic-mass spectrometer system. The principal advantage of the technique is the ability to determine simultaneously the absolute amounts of more than one volatile released from the same sample. From mass pyrograms it was observed that the release of water was bimodal, with the major release occurring above 600°C. Water released below this temperature is believed not to have been present in the magma at the time of extrusion. Sulfur dioxide was evolved only after the bulk of the water was released and coincided with the general expansion and melting of the sample. Sulfur and carbon- containing gases which were released in surges (above 1000°C) correspond to the bursting of bubbles from the softened basalt. The molar amounts of vesicle gases were plotted as a function of extrusion depth. A change in the slope of the resulting linear curve indicates saturation of the basalt with respect to water.

Micro-Raman studies of gypsum in the temperature range between 9 K and 373 K

Abstract Raman spectra were collected for synthetic gypsum (CaSO4⋅2H2O) powder between 9 and 373 K under atmospheric pressure with special emphasis on the temperature dependence of the OH-stretching modes. The stretching bands of the water molecules in gypsum were found to shift in opposite directions as a result of the different degree of intermolecular hydrogen-bonding between nonequivalent water H atoms and the O atoms of nearby SO4 ions. The anharmonic parameters of the OH-stretching modes are calculated using the temperature derivatives measured from the present investigation and existing pressure derivatives. These parameters are -4.7 × 10-6 K-1 and -0.6 × 10-6 K-1 for the 3407 and 3494 cm-1 bands, respectively. The dehydration of gypsum into γ-CaSO4 and the subsequent rehydration of γ-CaSO4 into hemihydrate are clearly identified in the Raman spectra by the observed variation in Raman shifts of the OH and ν1(SO4) bands. The latter increases as the mineral becomes increasingly anhydrous (1007 cm-1 in gypsum; 1014 cm-1 in hemihydrate; 1026 cm-1 in γ-CaSO4), which can be used as a fingerprint for the remote detection of these minerals on planetary surfaces.

The abundance of volatiles in Hawaiian tholeiitic submarine basalts

High-temperature mass spectrometric studies have been made to determine the distribution of volatiles within glassy rims of submarine pillow basalts dredged from the east rift zone of Kilauea volcano, Hawaii. The CO2/H2O mole ratio for glass-vapor inclusions within olivine phenocrysts in the glassy rims is greater than 30 : 1 compared to 0.06 for matrix glasses. Enclosing matrix glasses contain 0.53–0.74 wt.% H2O, 0.02–0.04 wt.% carbon, 0.08–0.12 wt.% sulfur, 0.012–0.028 wt.% chlorine and 0.012–0.077 wt.% fluorine.