Noble gases in ultra-high-pressure eclogites and peridotites of the CCSD core
Final Report Abstract
Major results of noble gas analyses of eclogites from the CCSD project are: All eclogites contain a clear crustal noble gas component. This crustal component consists of different amounts of atmospheric noble gases with varying element composition and by radiogenic (4He*, 40Ar*), nucleogenic (21Ne*, 22Ne, 131Xe*) and fissiogenic (131-136Xe fiss) isotopes produced within the crust in time by nuclear decay reactions. Noble gas components with similar characteristics had been observed previously only for crustal rocks from the KTB drill hole and for polycrystalline diamonds with eclogitic affinity. Thus our results may contribute an important missing link between crustal rocks subjected to variable degrees of high p,T metamorphism. The clinopyroxenes separated from the host eclogite appear to host a lower proportion of atmospheric noble gases than compared to their respective whole rocks suggesting minerals other than clinopyroxene contain a higher proportion of atmospheric noble gases. In addition, the amounts of gas in general and specifically atmospheric contributions tend to be somewhat larger in both unit 4 eclogites compared with both unit 6 eclogites. We relate this observation to rock- meteoric water interaction that had been already documented in oxygen isotopes. This interaction might be also mirrored in the well-definedcorrelation between Kr/Ar and Xe/Ar ratios in unit 4 eclogites, in opposite to unit 6 eclogites for which this correlation is somewhat blurred. 241±0 and 276±2 Ma 39Ar-40Ar ages of phengites from unit 4 are in excess to the age of the ultra-high-pressure event determined by U-Pb and Lu-Hf methods (218-223 Ma), though phengite formation should be coeval. This might be caused by the presence of pure crustal 40Ar* in the fluids from which phengites were formed. In this case obtained 'isochrons' simply reflect late mixing of atmosphere-type argon with an essentially atmospheric argon-free purely radiogenic component, but having no geo-chronologic meaning. Two splits of biotite from MH-20 (unit 6) have 39Ar-40Ar ages of about 1100 Ma, an age datum unsupported by other methods so far within this region (except Lu-Hf model ages). Again, an excess 40Ar component might have been present during biotite formation. However, we cannot rule out a geologic meaning of this age, documenting the collisional history of the local crustal section which is not visible by other dating methods anymore. This would require more data for support.
Publications
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2007. Intercalibration of 40Ar-39Ar age standards NL25, HB3gr
hornblende, GA-1550, SB-3, HD-B1 biotite and BMus/2 muscovite. Chemical Geology, Vol. 242. 2007, Issues 1–2, pp. 218–231.
Schwarz W.H., Trieloff M.
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2007. Noble gases in ultra-high pressure eclogites and peridotites of the CCSD core. Abstracts ICDP Kolloquium,2007, Potsdam.
Schwarz W.H., Trieloff M., Altherr R.
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2008. Noble gases and phengite 4Ar/39Ar ages in ultra-highpressure
eclogites of the CCSD core. Abstracts ICDP Kolloquium, Hannover. 2008, pp. 117-118.
Schwarz W.H., Trieloff M.
