科研之友
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摘要
The noble gas geochemistry of different types of terrestrial diamond including coated stones, alluvial diamonds, framesites, carbonados and impact diamonds yields a wealth of information on the sources of volatiles responsible for diamond formation. We present an illustrated compilation of published analyses of noble gases in different types of natural diamond. Noble gases in diamond record primary signatures from the mantle indicative of their integrated sources, and, the contribution of different metasomatic agents including subducting fluids and kimberlitic melt sampled during diamond growth. In addition, they show evidence of secondary processes such as resorption. Most data are available for coated stones, which trap abundant volatile-rich microscopic inclusions in their rims. While the coated stones are dominated by Mid-Oceanic-Ridge-Basalt (MORB) type noble gas signatures, the other diamond types contain predominantly crustal and atmospheric components although some mantle gases may be present, the latter indicated in elevated Ne-20/Ne-32 and/or Xe-129/Xe-132 ratios relative to atmospheric values. Some alluvial diamonds have very high He-3/He-4 that may represent the presence of a solar component trapped during their formation, but are just as likely to be the result of cosmogenic He-3 implantation during their prolonged residence at the Earth's surface. Oceanic-Island-Basalt (OIB) type noble gases occur in nanometer sized inclusions in metamorphic diamond from Kazakhstan, yet their significance as a fingerprint of mantle processes is not fully understood. Implanted noble gases occur near the outer surfaces of individual crystals, and are generally not a major hindrance for the study of mantle signatures, except for polycrystalline diamond like framesites with small grain size. Some diamonds including the polycrystalline carbonados, are dominated by crustal noble gases with no discernible mantle component evidenced by very low He-3/He-4 and Ne-20/Ne-22 ratios, and very high Ne-21/Ne-22 and Xe-131,Xe-134,Xe-136/Xe-132 ratios. In many diamonds, variations in both concentration and isotopic composition within samples from the same geographical location require complex diamond growth. For example, coated stones of Zaire trap noble gases from multiple sources and different generations of diamond growth. Thus noble gas studies have the potential to record major processes during the complex growth histories of natural diamond and also to provide valuable information about the subcontinental mantle. Noble gas signatures may be affected by diffusive losses, notably in some framesites.
Lastly, we discuss the future trend and scope of noble gas studies in diamond combining noble gas analyses with other elements including trace elements and halogens, and in situ Ar-40-Ar-39 age determinations to constrain the entrapment and diamond growth age. A key to greater understanding lies in systematic pre-analytical sample characterisation and treatment. This includes core-rim separation for coated stones, removal of the outer 25 lam to remove implanted gases, analysis of fluid inclusion density, and characterised heterogeneity using optical microscope to identify resorption and alteration.
- 出版日期2013-11
