wAbstract
@This study reports detailed stable Cr, Sm-Nd and Rb-Sr isotope
data for a ca. 1.9 Ga old subaerial weathering profile at Schreiber
Beach, Ontario, Canada, from which detailed major and trace element
signatures and Β18O values were previously reported.
The weathering profile developed on Neoarchean (`2.7 Ga) pillow
basalts and is unconformably overlain by the Paleoproterozoic
(`1.88 Ga) Gunflint Chert and basal conglomerates. This stratigraphy
suggests that the basalts were uplifted and subaerially weathered
prior to deposition of the Gunflint Formation.
@The aim was to investigate the behaviour of chromium during ancient
weathering processes and to contribute to the question whether
or not stable isotope fractionation accompanied the release of
chromium from the weathering rocks at times when atmospheric oxygen
was supposedly high enough (in a period following the Great Oxygenation
Event (GOE) `2.45 Ga ago and prior to the deposition of the Gunflint
cherts at `1.82 Ga). These iron-rich cherts belong to one of the
last regionally extensive banded iron formations (BIFs) deposited
before the `1 billion year-long Mesoproterozoic period during
which a drastic change in ocean chemistry prevented deposition
of BIFs.
@The gradual textural, mineralogical, and geochemical changes
from unweathered basalts to strongly weathered hematite-bearing
basalts with increasing stratigraphic height are associated with
shifts in the chromium isotope compositions. The Β53Cr
value of unweathered pillow basalt cores (-0.19}0.02ρ, 2Π) is
within the range of mantle inventory values, whereas weathered
brown to green basalts (soils), exhibiting up to 30% lower Cr
concentrations compared to unweathered pillow cores, are isotopically
lighter (Β53Cr = -0.35}0.11ρ). In contrast,
red, hematite-rich basalts and hyaloclastites underlying the brown
to green basaltic soils, with the highest Β18O
enrichment in the profile, are isotopically heavier (Β53Cr
= +0.05}0.15ρ). Rb-Sr isotope data of weathered basalts define
a correlation line with a slope corresponding to an age of 1574}24
Ma (MSWD = 13) which we interpret to reflect a diagenetic event
accompanied by alkaline metasomatism during subsidence/burial
of the Gunflint basin. The non-correlation of K enrichment factors
with Β53Cr, Β18O and other
major and trace elements in the weathered rocks indicates that
alkaline metasomatism did not affect the geochemical signatures
produced during in situ subaerial weathering. Instead, correlations
between Β53Cr and Β18O, and
between Β53Cr, Ce/Ce*, U and
V, indicate coupled mobilization of the redox sensitive elements
during oxidative processes and their partial re-deposition at
depth from high Β18O freshwaters. Partial re-precipitation
of heavy Cr at depth can be explained on the basis of the weak
positive correlation between Β53Cr and Fe enrichment
factors; it is seen as a consequence of the reduction of mobile
Cr(VI) and co-precipitation as mixed Fe(III)-Cr(III) oxhydroxides
during contemporaneous oxidation of the Fe2+-bearing
groundwaters at depth, a process similar to the one governing
the deposition of most Precambrian BIFs. Redistribution of certain
elements during the weathering process, in this case REEs, is
furthermore indicated by a correlation line in a Sm-Nd isochron
diagram defined by the Schreiber Beach data with a slope corresponding
to an age of 1.93}0.19 Ga (MSWD = 26), an age which is compatible
with the direct geological constraints for the timing of the paleosol
formation at this locality.
Keywords: Chromium isotopes; Paleosol; Paleoproterozoic weathering
profile; Red soil; Gunflint Formation; Trace elements; Oxygen
isotopesx
1. Introduction
2. Geological relationships
@2.1. Regional geology
@2.2. The Schreiber Beach profile
3. Samples
4. Analytical methods
@4.1. Separation of Cr
@4.2. Separation of Sm-Nd and Rb-St isotopes
@4.3. TIMS measurements
@4.4. Major and trace elements, and oxygen isotopes
@4.5. SEM (scanning electron microscope)-BSE (backscatter electron)
analyses
5. Results
@5.1. Cr isotopes and Cr concentrations
@5.2. Rb-Sr isotopes
@5.3. Sm-Nd isotopes
@5.4. Element enrichment/depletion and inter element/isotope relationships
6. Discussion
@6.1. Is the Schreiber Beach profile a weathering profile?
@6.2. Cr mobility recorded in the weathering profile
@6.3. Alkaline metasomatism versus in situ weathering
@6.4. Implications for the use of Cr isotopes as a tracer for
oxidative weathering
7. Conclusions
Acknowledgements
References