Whether the platinum group elements (PGE) can be taken as the indicators of extraterrestrial materials is a very important and interesting scientific problem. It is discussed on the basis of systematic investigation a...Whether the platinum group elements (PGE) can be taken as the indicators of extraterrestrial materials is a very important and interesting scientific problem. It is discussed on the basis of systematic investigation and study of a great amount of related literature. The following conclusions can be obtained: (ⅰ) extraterrestrial impact event can cause the PGE anomaly; conversely, the PGE anomaly may not represent the existence of extraterrestrial impact event, because the PGE anomaly can be caused by many terrestrial events (e.g. volcanic activity); (ⅱ) the PGE anomaly, especially the global PGE anomaly can inspire us to think it from extraterrestrial event, but it may not be as useful as previously thought as unambiguous identifiers of large extraterrestrial impact event in the earth’s history.展开更多
Information on abundance and distribution of platinum\|group elements (PGE) in peridotite from ophiolite is relevant to understand the nature and evolution of the upper mantle. The ophiolite suite outcropping along th...Information on abundance and distribution of platinum\|group elements (PGE) in peridotite from ophiolite is relevant to understand the nature and evolution of the upper mantle. The ophiolite suite outcropping along the famous suture zone of Yarlung Zangbo River, Tibet, has attracted wide attention of geologists both in China and abroad. The Dagzhuka ophiolite of in the suture zone is thought to display a complete ophiolitic sequence. The peridotite from the Dazhuka ophiolite is composed of fairly fresh or unaltered lherzolite, harzburgite and minor dunite which are the upper mantle residuum formed by 15%～35% partial melting. In this paper we have mainly studied the abundance and distribution of PGE in the harzburgites and lherzolites. The harzburgites and lherzolites preserve total PGE contents of 28 37×10 -9 ～50 67×10 -9 respectively higher than Primitive mantle or the peridotite from some typical ophiolites and Alpine. They all have fractionated chondrite\|normalized PGE patterns with positive slopes from Ir to Pd (Fig.1), and Pd/Ir=1 13～2 40>1, Pd/Rh=2 23～7 49>1, and Pd/Pt=0 26～1 16 (average 0 67) higher than Primitive mantle (1 11, 2 35, 0 57) or CI\|chondrite (1 01, 2 73, 0 53). Consequently, the Dagzhuka peridotite are PGE\|enriched, but otherwise possess residual characteristics arising from a minimum of 15% partial melting. It is suggested that mantle residuum by partial melting have low total PGE contents, fractionated chondrite\|normalized PGE patterns with negative or slightly flat slopes from Ir to Pd, and Pd/Ir<1. But, the total PGE contents, chondrite\|normalized PGE patterns and Pt/Ir, Pd/Rh and Pd/Pt values of the Dagzhuka peridotite are completely inconsistent with a residual origin. Partial melting would have partitioned all sulfide in the source into the melt. In fact, no sulfide or silicate melt remained in the Dagzhuka peridotite. Therefore, PGE in Dagzhuka peridotite are not present in sulfides. The PGE\|enrichment and fractionation of the Dagzhuka peridotites seem to arise from 展开更多
文摘Whether the platinum group elements (PGE) can be taken as the indicators of extraterrestrial materials is a very important and interesting scientific problem. It is discussed on the basis of systematic investigation and study of a great amount of related literature. The following conclusions can be obtained: (ⅰ) extraterrestrial impact event can cause the PGE anomaly; conversely, the PGE anomaly may not represent the existence of extraterrestrial impact event, because the PGE anomaly can be caused by many terrestrial events (e.g. volcanic activity); (ⅱ) the PGE anomaly, especially the global PGE anomaly can inspire us to think it from extraterrestrial event, but it may not be as useful as previously thought as unambiguous identifiers of large extraterrestrial impact event in the earth’s history.
文摘Information on abundance and distribution of platinum\|group elements (PGE) in peridotite from ophiolite is relevant to understand the nature and evolution of the upper mantle. The ophiolite suite outcropping along the famous suture zone of Yarlung Zangbo River, Tibet, has attracted wide attention of geologists both in China and abroad. The Dagzhuka ophiolite of in the suture zone is thought to display a complete ophiolitic sequence. The peridotite from the Dazhuka ophiolite is composed of fairly fresh or unaltered lherzolite, harzburgite and minor dunite which are the upper mantle residuum formed by 15%～35% partial melting. In this paper we have mainly studied the abundance and distribution of PGE in the harzburgites and lherzolites. The harzburgites and lherzolites preserve total PGE contents of 28 37×10 -9 ～50 67×10 -9 respectively higher than Primitive mantle or the peridotite from some typical ophiolites and Alpine. They all have fractionated chondrite\|normalized PGE patterns with positive slopes from Ir to Pd (Fig.1), and Pd/Ir=1 13～2 40>1, Pd/Rh=2 23～7 49>1, and Pd/Pt=0 26～1 16 (average 0 67) higher than Primitive mantle (1 11, 2 35, 0 57) or CI\|chondrite (1 01, 2 73, 0 53). Consequently, the Dagzhuka peridotite are PGE\|enriched, but otherwise possess residual characteristics arising from a minimum of 15% partial melting. It is suggested that mantle residuum by partial melting have low total PGE contents, fractionated chondrite\|normalized PGE patterns with negative or slightly flat slopes from Ir to Pd, and Pd/Ir<1. But, the total PGE contents, chondrite\|normalized PGE patterns and Pt/Ir, Pd/Rh and Pd/Pt values of the Dagzhuka peridotite are completely inconsistent with a residual origin. Partial melting would have partitioned all sulfide in the source into the melt. In fact, no sulfide or silicate melt remained in the Dagzhuka peridotite. Therefore, PGE in Dagzhuka peridotite are not present in sulfides. The PGE\|enrichment and fractionation of the Dagzhuka peridotites seem to arise from
