Silicified seafloor contribution to TTG formation: insights from zircon O and Si isotopes

The silicon and oxygen isotope compositions of Precambrian cherts: A record of oceanic paleo-temperatures?

Proglacial rivers have been shown to have distinctive silicon (Si) isotope compositions, providing new insights into the mechanisms controlling Si cycling in the subglacial environment and suggesting terrestrial Si isotope exports may have varied between glacial and interglacial periods. However, Si isotope data are currently limited to a small number of glacial systems in the northern hemisphere, and it is unclear how compositions might vary across a spectrum of glacial influence. Using Chilean Patagonia as a unique natural laboratory, we present Si isotope compositions of 0.45 μm filtered (fSi), 0.02 μm filtered (DSi), and reactive amorphous (ASi) fractions from 40 river catchments with variable glacial cover and explore the key controls on Si cycling. The 0.45 μm filtered glacier-fed river samples displayed isotopically light compositions and a positive linear correlation with upstream glacial cover. This relationship was controlled by the inclusion of an isotopically light colloidal-nanoparticulate (0.02–0.45 μm) silicate phase that was only present in glacier-fed rivers and dominated Si budgets in these catchments. This phase was predominately composed of feldspars and its lability in seawater is uncertain, representing a significant unknown in resolving glacial Si isotope exports from this region. When the colloidal-nanoparticulates were removed from solution by ultra-filtration, the resultant DSi isotope compositions of glacier-fed catchments were not isotopically distinct from some non-glacial rivers and exhibited no clear relationship with glacial cover. The colloidal-nanoparticulate concentration of other weathering-sensitive elements (Li, Mg, Ba, Sr) also showed a linear relationship with glacial cover, suggesting that their isotopic compositions could be affected in a similar manner. These findings highlight the benefit of size-fractionated sampling and the need for more research to understand the lability of colloidal-nanoparticulate species, especially in glacier-fed rivers. Finally, we explore the controls on river ASi isotope compositions and show how including these reactive particulate phases is critical to quantifying terrestrial Si isotope budgets, both in Patagonia and other global regions.

O, Mg, and Si isotope distributions in the complex ultrarefractory CAI Efremovka 101.1: Assimilation of ultrarefractory, FUN, and regular CAI precursors

A combined in situ oxygen, silicon isotopic and fluid inclusion study of a chert sample from Onverwacht Group (3.35 Ga, South Africa): New constraints on fluid circulation

Differences between mono-generic and mixed diatom silicon isotope compositions trace present and past nutrient utilisation off Peru

Abstract. The stable isotope composition of benthic sponge spicule silica is a potential source of palaeoceanographic information about past deep seawater chemistry. The silicon isotope composition of spicules has been shown to relate to the silicic acid concentration of ambient water, although existing calibrations do exhibit a degree of scatter in the relationship. Less is known about how the oxygen isotope composition of sponge spicule silica relates to environmental conditions during growth. Here, we investigate the vital effects on silica, silicon and oxygen isotope composition in a carnivorous sponge, Asbestopluma sp., from the Southern Ocean. We find significant variations in silicon and oxygen isotopic composition within the specimen that are related to unusual spicule silicification. The largest variation in both isotope systems was associated with the differential distribution of an unconventional, hypersilicified spicule type (desma) along the sponge body. The absence an internal canal in the desmas suggests an unconventional silicification pattern leading to an unusually heavy isotope signature. Additional internal variability derives from a systematic offset between the peripheral skeleton of the body having systematically a higher isotopic composition than the internal skeleton. A simplified silicon isotope fractionation model, in which desmas were excluded, suggests that the lack of a system for seawater pumping in carnivorous sponges favours a low replenishment of dissolved silicon within the internal tissues, causing kinetic fractionation during silicification that impacts the isotope signature of the internal skeleton. Analysis of multiple spicules should be carried out to "average out" any artefacts in order to produce more robust downcore measurements.

Twenty-four composite samples of the fine-grained matrix of glacial diamictites deposited from the Mesoarchaean to Palaeozoic have been analysed for their silicon isotope composition and used to establish, for the first time, the long-term secular Si isotope record of the compositional evolution of upper continental crust (UCC). Diamictites with Archaean and Palaeoproterozoic Nd model ages show greater silicon isotope heterogeneity than those with younger model ages (irrespective of depositional age). We attribute the anomalously light Si isotope compositions of some diamictites with Archaean model ages to the presence of glacially milled banded iron formation (BIF), substantiated by the high iron content and Ge/Si in these samples. We infer that relatively heavy Si isotope signatures in some Palaeoproterozoic diamictites (all of which have Archaean Nd model ages) are due to contribution from tonalite-trondhjemite-granodiorites (TTGs), evidenced by the abundance of TTG clasts. By the Neoproterozoic (with model ages ranging from 2.3 to 1.8 Ga), diamictite Si isotope compositions exhibit a range comparable to modern UCC. This reduced variability through time is interpreted as reflecting the decreasing importance of BIF and TTG in post-Archaean continental crust. The secular evolution of Si isotopes in the diamictites offers an independent test of models for the emergence of stable cratons and the onset of horizontal mobile-lid tectonism. The early Archaean UCC was heterogeneous and incorporated significant amounts of isotopically light BIF, but following the late Archaean stabilisation of cratons, coupled with the oxygenation of the atmosphere that led to the reduced neoformation of BIF and diminishing quantities of TTGs, the UCC became increasingly homogeneous. This homogenisation likely occurred via reworking of preexisting crust, as evidenced by Archaean Nd model ages recorded in younger diamictites.

Condensation of fallout glasses in the Hiroshima nuclear fireball resulting in oxygen mass-independent fractionation

Silicon isotope fractionation during nutrient utilization in the North Pacific

Stable silicon isotopes of groundwater, feldspars, and clay coatings in the Navajo Sandstone aquifer, Black Mesa, Arizona, USA

Silicon (Si) isotope composition and Si distribution among different rice plant organs and different parts of rice leaf at maturity were studied, which may provide new insights into the mechanism of Si accumulation in plants and biogeochemical Si cycle. An isotope ratio mass spectrometer (IRMS) was used to examine Si isotope fractionation by rice plant grown in a hydroponic system. The observed 30Si-depletion (about 0.3‰) of whole plant relative to external nutrient solutions suggested biologically mediated Si isotope fractionation occurred during uptake. However, it was not possible to judge the Si uptake mechanism with the data. For δ30Si variation within plant, there was a consistent increasing trend from lower to upper tissues (stem < leaf < husk < grain; leaf sheath < leaf blade base <leaf blade middle < leaf blade top). The phenomenon, reflecting kinetic isotope effects, could be explained that isotope fractionation during Si deposition in rice plant was a Rayleigh-like behavior. The range (−2.7‰ to 2.3‰) of δ30Si variation among rice plant tissues in present experiment exceeded that (−1.7‰ to 2.5‰) of phytoliths observed previously in continents, which would enhance understanding the role of phytoliths on globe Si isotope balance.

Silicon isotopic fractionation in marine sponges: A new model for understanding silicon isotopic variations in sponges

Silicon isotope and trace element constraints on the origin of ∼3.5 Ga cherts: Implications for Early Archaean marine environments

Iron and silicon isotope fractionation in silicate melts using first-principles molecular dynamics

Oxygen, silicon, and Mn-Cr isotopes of fayalite in the Kaba oxidized CV3 chondrite: Constraints for its formation history