Kump et al EPSL 2005.pdf - Bryn Mawr College

L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662 655
generally exceeds the Fe concentration such that Fe
is titrated out of the vent fluid by the precipitation of
FeS and FeS 2 [2,3]. Walker and Brimblecombe [4]
pointed out, however, that in the absence of seawater
sulfate, vent fluids would be Fe dominated simply
because basalt has an abundance of Fe-minerals
relative to S-minerals.
In a similar vein, Canfield [5] suggested that the
end of BIF deposition at ~1.8 Ga resulted from the
accumulation of sulfate in the ocean following the rise
in atmospheric oxygen abundance at ~2.3 Ga. In
Canfield’s model, microbial reduction of this sulfate
led to the development of sulfidic conditions in the
deep sea that severely reduced the concentration of
ferrous iron because of the insolubility of FeS. An
alternative explanation, consistent with Walker and
Brimblecombe’s model, is that abiogenic sulfide associated
with hydrothermal venting of Paleoproterozoic
sulfate-rich seawater-derived fluids led to the
precipitation of iron-sulfide minerals, creating conditions
unfavorable for BIF production (with Fe/
H 2 Sb1). According to either model, the return of
BIF deposition during the Neoproterozoic bSnowball
EarthQ episodes would have been more related to the
depletion of seawater sulfate during the ice-covered
interval than to the establishment of oceanic anoxia
(the original argument put forth by Kirschvink [6] and
Beukes and Klein [7]); anoxia alone does not affect
the Fe/H 2 S ratio of vent fluids nor does it allow for
the buildup of Fe because of the high concentrations
of H 2 S that develop.
In this paper we explore the Walker and Brimblecombe’s
model [4] using available thermodynamic
data for seawater equilibria at elevated temperatures
and pressures. Assuming the absence of seawater
sulfate, we find that Precambrian vent fluids were
considerably more reducing, with ratios of Fe/H 2 S
that exceeded unity. Low hydrostatic pressures during
Snowball Earth episodes of the Neoproterozoic and
for much of the Archean, if mid-ocean ridge depths
were shallower then than now [1], may have further
enhanced Fe concentrations.
2. Constraints on vent fluid composition
After nearly 20 years of experimentation and theoretical
development, we now have a clearer understanding
of the effects of temperature and pressure on
the chemistry of mid-ocean ridge hydrothermal systems
[8,9]. The composition of fluids collected from
black smoker chimneys can be explained for the most
part in terms of fluid–mineral equilibria, established at
elevated temperatures and pressures in subseafloor
reaction zones near magma chambers below the
chimneys. These solutions then rise convectively to
the sea floor, mixing with cold seawater, precipitating
insoluble minerals, and in some cases, separating into
vapor and brine [10,11].
2.1. Effect of seawater sulfate on vent fluid
composition
In sharp contrast with modern hydrothermal systems,
low dissolved sulfate in the ocean [5,12,13]
would likely have rendered Precambrian hydrothermal
systems more reducing, enhancing dissolved Fe
concentrations in coexisting vent fluids. We can illustrate
this by means of a phase diagram for a portion of
the FeO–Fe 2 O 3 –H 2 S–SiO 2 –CaO–H 2 O–HCl system at
400 8C, 400 bars (Fig. 1). Reaction of basalt/gabbro or
even more reducing protolith with a fluid lacking
sulfate would permit the inherent redox capacity of
the rock to buffer the fluid resulting in relatively high
H 2 /H 2 S ratios, consistent with constraints imposed by
ferrous iron-bearing phases, such as might be approximated
by the fayalite–magnetite–pyrrhotite-bearing
system. Vent fluids impacted by magmatic degassing
effects provide an indication of this. For example,
hydrothermal vent fluids from 98 to 108 N, East
Pacific Rise and the Endeavour segment of the Juan
de Fuca Ridge, indicate high H 2 /H 2 S ratios and high
H 2 concentrations in the immediate aftermath of subseafloor
magmatic intrusions [14]. Although uncertainties
exist in terms of temperature, pressure and
phase separation effects, the high H 2 in particular is
an indicator of distinctly reducing conditions associated
with vapor release at the magmatic–hydrothermal
interface [14,15]. In all cases, however, time series
observations reveal sharp decreases in dissolved H 2
and H 2 S, with changes in H 2 greater than H 2 S [14],
suggesting more oxidizing conditions. Indeed, even
modest rock–seawater interaction can be expected to
cause the fluid to achieve saturation with respect to
anhydrite, assuming coexistence of plagioclase feldspar
and quartz (Fig. 1), which is in good agreement