Vs. Kamenetsky et al., Parental basaltic melts and fluids in eastern Manus backarc Basin: implications for hydrothermal mineralisation, EARTH PLAN, 184(3-4), 2001, pp. 685-702
The eastern Manus Basin is an actively forming backarc extensional zone beh
ind the New Britain Island are, which hosts a number of submarine volcanic
edifices and hydrothermal fields. Isotopic and trace element geochemical ch
aracteristics of the edifices are comparable with those of the adjacent sub
aerial New Britain are, and differ significantly from those of MORE-like la
vas on and near the Manus Spreading Ridge in the central part of the basin.
Fractional crystallisation dominates magma evolution from primitive basalt
s to andesites, dacites and rhyodacites in the eastern Manus Basin, but sev
eral lineages with differing trace element enrichment have been delineated.
Melt inclusions within olivine phenocrysts (Fo(82-92)) Of two representati
ve east Manus basalts, respectively, with modest (0.2 wt%) and high (0.8 wt
%) potassium contents, host ubiquitous CO2-bearing vapour bubbles, denoting
presence of an immiscible fluid phase at early stages of crystallisation.
Bubbles often carry precipitate phases whose abundance is broadly proportio
nal to the bubble size reaching a maximum in fluid bubbles with little or n
o melt. Among the precipitates, detected by laser Raman spectroscopy and ED
S-scanning electron microscopy, carbonates are common and include magnesite
, calcite, ankerite, rhodochrosite and nahcolite (NaHCO3). Gypsum, anhydrit
e, barite, anglesite, pyrite, and chalcopyrite have also been found. Some a
morphous precipitates recrystallise after bubbles are opened to Na-Ca carbo
nates, halite and Na-K-Ca alumine-silicates. Copper abundances decrease fro
m basalt to dacite across the eastern Manus fractionation spectrum, whereas
Pb behaves as an incompatible element, increasing to highest values in the
dacites. Zinc abundance reaches maximum concentrations in andesite, and de
creases during further fractionation. Loss of Cu especially from the fracti
onating magmas, in the absence of immiscible sulphide liquid, strongly impl
ies metal partitioning into CO2-H2O fluid, which is degassed significantly
during magma fractionation. Hydrothermal fluids in the PACMANUS system may
carry a direct contribution of the magmatic metal-bearing fluid, exsolved f
rom the crystallising are-like magmas at this immature backarc basin, and a
re able to transport and concentrate major amounts of ore metals, particula
rly Cu. (C) 2001 Elsevier Science B.V. All rights reserved.