Isotope silicon

Brown LE, Clayton DD (1992) Silicon isotopic composition in large meteoritic SiC particles and Na origin of Ne. Science 258 970-972... 

ISOTOPES There are 21 isotopes of silicon, three of which are stable. The isotope Si-28 makes up 92.23% of the element s natural abundance in the Earth s crust, Si-29 constitutes 4.683% of all silicon found in nature, and the natural abundance of Si-30 is merely 3.087% of the stable silicon isotopes found in the Earth s crust. 

Silicon isotope ratios have been generally measured by fluorination (Douthitt 1982 Ding et al. 1996). However, the method is time consuming and potentially hazardous, therefore, more recently MC-ICP-MS techniques have been introduced (Cardinal et al. 2003 Engstrom et al. 2006). Determinations with SIMS have been carried out by Robert and Chaussidon (2006). Very recently, Chmeleff et al. (2008) have shown that a UV-femtosecond laser ablation system coupled with MC-ICP-MS gives S Si and 5 °Si-values with very high precision. 

In ocean water distinct °Si gradients with depth exist (Georg et al. 2006) surface waters are relatively rich in °Si whereas deep waters are more depleted in °Si, which is due to a silicon isotope fractionation during the uptake by organisms... 

Silicon isotope variations in the ocean are caused by biological Si-uptake through siliceous organisms like diatoms. Insofar strong similarities exist with C-isotope variations. Diatoms preferentially incorporate Si as they form biogenic silica. Thus, high 5 °Si values in surface waters go parallel with low Si-concentrations and depend on differences in silicon surface water productivity. In deeper waters dissolution of sinking silica particles causes an increase in Si concentration and a decrease of 5 °Si-values. 

De La Rocha C (2003) Sihcon isotope fractionation by marine sponges and the reconstruction of the silicon isotope composition of ancient deep water. Geology 31 423 26 De La Rocha CL, De Paolo DJ (2000) Isotopic evidence for variations in the marine calcium cycle over the Cenozoic. Science 289 1176-1178... 

Engstrom E, Rodushkin I, Baxter DC, Ohlander B (2006) Chromatographic purification for the determination of dissolved silicon isotopic compositions in natural waters by high-resolution multicollector inductively coupled mass spectrometry. Anal Chem 78 250-257 Epica community members (2004) Eight glacial cycles from an Antarctic ice core. Nature 429 623-628... 

Georg RB, Reynolds BC, Frank M, Halhday AN (2006) Mechanisms controlhng the silicon isotopic compositions of river water. Earth Planetary Sci Lett 249 290-306 Gerdes ML, Baumgartner LP, Person M, Rumble D (1995) One- and two-dimensional models of fluid flow and stable isotope exchange at an outcrop in the Adamello contact aureole, Southern Alps, Italy. Am Miner 80 1004-1019... 

Reynolds BC, Frank M, HaUiday AN (2006) Silicon isotope fractionation during nutrient utilization in the North Pacific. Earth Planetary Sci Letters 244 431 143 Rice DD, Claypool GE (1981) Generation, accumulation and resource potential of biogenic gas. Am Assoc Petrol Geol Bull 65 5-25... 

Robert F, Chaussidon M (2006) A paleotemperature curve for the Precambrian oceans based on silicon isotopes in cherts. Nature 443 969-972... 

Titanium isotopic data for mainstream silicon carbide grains versus 829Si. The correlation between excesses of minor titanium isotopes and minor silicon isotopes most likely reflects galactic chemical evolution. The offset of the S50Ti trend to pass above the solar composition probably reflects 5-process nucleosynthesis in the parent stars, which most strongly affects 50Ti. Data from Huss and Smith (2007) and references therein. 

Re-assessment of silicon isotope reference materials (IRMM-018 and NBS 28 isotope standard) has been performed using both high resolution multiple ion collector ICP-MS and gas mass spec-... 

Van den Boom et al.150 have reported on the determination of silicon isotope ratio measurements in silicate materials by MC-ICP-MS (at a mass resolution of 2500 to resolve isobaric interferences) after sodium hydroxide sample digestion and purification of silicon. 829Si and 830Si have been determined for several silicon isotope standard reference materials. A precision for 830Si of 0.18-0.41 %o was achieved. Precise and accurate measurements of isotope ratios on transient signals by HPLC-MC-ICP-MS for nuclear application was performed by Giinther-Leopold et al.151... 

From the isotopic decomposition of normal silicon one finds thatthe mass-29 isotope, 29Si, is the second most abundant of the Si isotopes 4.67% of all Si. This makes it 19.7 times less abundant than the most abundant of the silicon isotopes, namely 28Si, which constitutes 92.23% of natural Si. Considering that one million silicon atoms is the common cosmochemical standard in solar-system matter,... 

In stellar regions where the s process is occurring, the large cross section of the important reaction 33S + n -> 3°Si + 4He directs the capture flow through sulfur back into silicon. This is significant for the silicon isotopes (see Nucleosynthesis origin in 3°Si). 

The positive particle produced along with the silicon isotope is a positron, a particle identical to an electron except that it has a positive charge. When a positron collides with an electron, both are annihilated and a relatively large amount of energy is produced. 

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