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