Canyon Diablo Meteorite Troilite

If an internal isochron cannot be generated, a model age can be determined from the measured 207pb /206Pb of the sample and the assumed initial lead isotopic ratios. For studies of the early solar system, this initial lead composition is assumed to be that measured in troilite (FeS) from the Canyon Diablo meteorite. Troilite is a uranium-free mineral and its host meteorite formed very early in the history of the solar system. Because the U/Pb ratio of the solar system is low, the lead incorporated into the troilite should not have evolved significantly from the initial composition in the solar system. 

Atmospheric nitrogen (AIR) Vienna Standard Mean Ocean Water (VSMOW) Canyon Diablo Meteorite Troilite (CDT)... 

Ag2S was mixed with cuprous oxide and converted to SO2 by heating at 900 C (2). BaSC>4 was directly converted to SO2 by thermal decomposition in quartz at 1600 C (3). Product SO2 was freed from water and COo> before isotopic measurement on a Micromass 602D mass spectrometer. S/ S ratios are reported to a precision of 0.2%o relative to troilite from the Canyon Diablo meteorite using the normal 8 S%o notation. 

For stable sulphur isotopes, the standard is an iron sulphide mineral (troilite) from the Canyon Diablo meteorite. It is known as CDT (Canyon Diablo troilite) and equation 1 becomes ... 

The U (uranium)-Th (thorium)-Pb (lead) isotopic system represents three independent decay schemes and is a powerful but complex tool with which to unravel the history of the Earth s mantle (Text box 3.2). During planetary accretion U and Th are refractory, lithophile elements and will reside in the mantle. Pb on the other hand is a volatile and chalcophile/ siderophile element and may in part, be stored in the core. Initial U and Th concentrations are derived from chondritic meteorites, and initial Pb isotope compositions are taken from the iron-sulfide troilite phase in the Canyon Diablo meteorite. The initial bulk Earth U/Th ratio was 4.0 0.2 (Rocholl Jochum, 1993). 

Standards The international sulfur isotope scale is based on a troilite sample from the Canyon Diablo meteorite, termed CDT. The International Atomic Energy Agency (IAEA) in Vienna has promoted the use of a more accurate sulfur isotope scale, V-CDT, to replace CDT because of isotopic inhomogenities of +0.4(i in the original sample. A new Ag2S standard, IAEA S-1, has a defined value of -0.301 V-CDT. 

For many years the reference standard commonly referred to is sulfur from troilite of the Canyon Diablo iron meteorite (CDT). As Beaudoin et al. (1994) have pointed out, CDT is not homogeneous and may display variations in " S up to 0.4%o. Therefore a new reference scale, Vienna-CDT or V-CDT has been introduced by an advisory committee of IAEA in 1993, recommending an artificially prepared Ag2S (IAEA-S-1) with a S Svcdt of —0.3%o as the new international standard reference material. 

Figure I. Canyon Diablo iron No, 34,6050, American Meteorite Laboratory, The dark inclusions are nodules of graphite and troilite.

Isotopic composition of sulphur S is determined usually by the ratio (4 stable isotopes - S, and - are known). This ratio is most stable in meteorites. That is why as standard is used the sulphur isotopic composition in troilite (FeS) from the meteorite in the Canyon Diablo Troilite (CDT) where the ratio is 1/22.22. 

Canyon Diablo Troilite (FeS from an iron meteorite)... 

Sulphur 32 Q 16 95.02 22.220 CDT. Troilite of Diablo Canyon meteorite. MacNcuncua md Thode. 1950... 

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