Chromium isotope systematics in three mantle-derived domains of Central European Variscides: Relationship between δ53Cr values and progressive weathering of serpentinized ultramafic rocks
Abstract
Most chromium (Cr) entering the ocean originates from continental weathering of ultramafic rocks. Recent data indicate that (i) mantle-derived rocks are characterized by homogeneous δ(53)Cr values of approximately -0.12%o; (ii) some serpentinized peridotites, serpentinites and weathered serpentinites contain isotopically heavier Cr than mantle protoliths; and (iii) Cr transported by rivers is isotopically variable, but positive δ(53)Cr values predominate.
There is a need to identify solid materials that, in sufficient volumes, store the complementary isotopically light Cr. We studied δ(53)Cr systematics in three mantle domains belonging to the Paleozoic Central European orogenic belt.
These domains are characterized by contrasting P-T histories. At two of six study sites, a 30-m deep drilling was carried out to assess spatial gradients in Cr concentrations ([Cr]) and δ(53)Cr values toward the weathering front.
The studied rocks, comprising serpentinized peridotites with preserved mantle signature, serpentinites, carbonatized serpentinites, and hornblendites, were characterized by a variable degree of weathering. Spinel in these rock types contained up to 33 wt% Cr but represented less than 1% of the rock volume; chromite was not detected in any of the samples.
Silicates, such as pyroxene, serpentine, hornblende and chlorite, with an average [Cr] of 0.24 wt.% were likely the predominating potential Cr source for the environment. Despite the known higher weathering rates of Cr-silicates compared to Cr-spinelides, no systematic trends toward lower [Cr] and higher δ(53)Cr values were detected upcore.
The overall range of whole-rock δ(53)Cr values was from -0.25 to 0.14%o (mean of -0.09 +- 0.01%o). There was no relationship between whole-rock δ(53)Cr values and indices of alteration/weathering, such as water content, loss on ignition, and leaching of alkalies.
We did not observe progressive removal of isotopically light Cr during continental weathering of serpentinized mantle-derived rocks that had been previously reported from altered and hydrated ultramafic rocks at some other sites. Our data suggest that such Cr isotope fractionation, if present, may be associated with earlier hydrothermal alteration rather than with weathering.
Mean δ(53)Cr values of soils were indistinguishable from the underlying ultramafic rocks, indicating only small sensitivity of Cr isotope composition to pedogenesis.