1Division of Marine Geology and Geophysics, Rosenstiel School of
Marine and Atmospheric Science, University of Miami, Miami, FL
33149, USA and 2Monterey Bay Aquarium Research Institute, Moss
Landing, CA 95039-0628, USA3Ocean Drilling Program and Department
of Geology and Geophysics, Texas A&M University, College Station,
TX 77845-9547, USA4Department of Geological Science, University
of Rochester, Rochester, NY 14627, USACorresponding author Email:
jdixon@rmas.miami.edu
ABSTRACT
The North Arch volcanic field is a submarine suite of alkali
basaltic to nephelinitic lavas of the seafloor north of Oahu at
water depths of 3900-4380 m. Glasses from these lavas were
analyzed for H2O, CO2, Cl, S, Fe3+/[Sigma]Fe, and noble gases to
investigate the role of volatiles in the generation, evolution,
and degassing of these alkalic series lavas. In contrast to the
systemic negative correlation between concentrations of SiO2 and
nonvolatile incompatible elements (e.g. P2O5), the behavior of
the volatile components is much more irregular. Concentrations of
H2O in glasses vary by a factor of two ( 0.69-1.42 wt %) and show
a poor correlation with melt composition, whereas concentrations
of dissolved CO2 in glasses (260-800 p.p.m) increase with
increasing alkalinity of the glasses. The H2O2 concentrations in
the glasses are in equilibrium with an H2O-CO2 vapor at the depth
of eruption (400 bar pressure). Samples collected directly from
vent structures are highly vesicular, suggesting that these
samples were gas rich upon eruption. Estimated bulk volatile
contents of the two most vesicular vent samples are high
(1.9+-0.1 wt % H2O and 5.4+-0.4 CO2) and are interpreted to have
formed by closed system degassing. Estimated bulk volatile
contents in four other vesicular vent samples are lower 1.3+-0.2
wt % H2O and 2.0+-0.4 wt % CO2), and these samples and interpreted
to have lost essentially all exsolved gas during eruption and
flow. Forward degassing models can predict the observed range in
dissolved H2O and CO2 contents, calculated vapor compositions,
and vesicularity as a function of SiO2. The models involve open
to closed system degassing of an H2O-CO2 vapor phase from melts
initially having H2O/P2O5=3 and CO2H2O=1-4 mass. Cl
concentrations (400-1360 p.p.m) in glasses correlate with
concentrations of nonvolatile, incompatible elements.
Concentrations of noble gases measured on bulk glass samples are
low compared with mid-oceanic ridge basalt (MORB). The low
concentrations result mainly from extensive vapor exsolution from
the magma. The helium isotopic ratios for gases released from
vesicles are similar to MORB values [6.8-8.5 times the air ratio
(RA)], whereas those released from glasses are lower than MORB
values as a result of in situ decay of U and Th. The S contents
(0.11-0.22 wt %) of most of the alkali olivine basaltic and
basanitic glasses are sufficient to saturate the silicate melt
with immiscible Fe-S-O liquid at the T and P of eruption and
quenching. However, two vesicular samples appear to hae lost some
dissolved S owing to eruptive degassing. Magmatic oxygen
fugacitites estimated from Fe3+/[Sigma]Fe range from
[Delta]FMQ=-0.8 to +0.7, with the nephelinitic glasses being more
oxidizing than the less alkalic glasses. We infer that the mantle
source region for the North Arch magmas was homogeneous with
respect to Fe3+/[Sigma]Fe and that melting occurred in the
absence of graphite or CH4- rich fluid. The effect of variable
partial melting on magmatic oxygen fugacity may be a common
feature of Hawaiian volcanism. These complex data pint to a
simple result, namely that parental magma compositions can be
derived by variable extents of melting of a homogeneous source
followed by olivine crystallization and degassing at 400 bar. If
the parental liquids are produced by 1.6-9.0% partial melting
(+-20% relative), then mantle volatile contents are estimated to
be 525+-75 p.p.m. H2O, 1300+-800 p.p.m. CO2 and 30± p.p.m. Cl.
Pages: 911 - 939
Part of the OUP Journal of Petrology WWW serviceCopyright Oxford University Press, 1997