Chondrites in the CB/CH-like carbonaceous chondrite Isheyevo:Evidence for various chondrule-forming mechanisms and multiple chondrule generations

摘要

The recently discovered metal-rich carbonaceous chondrite Isheyevo consists of Fe,Ni-metal grains,chondrales,heavily hydrated matrix lumps and rare refractory inclusions.It contains several lithologies with mineralogical characteristics intermediate between the CH and CB carbonaceous chondrites;the contacts between the lithologies are often gradual.Here we report the mineralogy and petrography of chondrules in the metal-rich (~70 vol%) and metal-poor (~20vol%) lithologies.The chondrules show large variations in textures [cryptocrystalline,skeletal olivine,barred olivine,porphyritic olivine,porphyritic olivine-pyroxene,porphyritic pyroxene],mineralogy and bulk chemistry (magnesian,ferrous,aluminum-rich,silica-rich).The porphyritic magnesian (Type I) and ferrous (Type II) chondrules,as well as silica-and Al-rich plagioclase-bearing chondrules are texturally and mineralogically similar to those in other chondrite groups and probably formed by melting of mineralogically diverse precursor materials.We note,however,that in contrast to porphyritic chondrules in other chondrite groups,those in Isheyevo show little evidence for multiple melting events;e.g.,relict grains are rare and igneous rims or independent compound chondrules have not been found.The magnesian cryptocrystalline and skeletal olivine chondrules are chemically and mineralogically similar to those in the CH and CB carbonaceous chondrites Hammadah al Hamra 237,Queen Alexandra Range 94411 (QUE94411) and MacAlpine Hills 02675 (MAC02675),possibly indicating a common origin from a vapor-melt plume produced by a giant impact between planetary embryos;the interchondrule metal grains,many of which are chemically zoned,probably formed during the same event.The magnesian cryptocrystalline chondrules have olivine-pyroxene normative compositions and are generally highly depleted in Ca,Al,Ti,Mn and Na;they occasionally occur inside chemically zoned Fe,Ni-metal grains.The skeletal olivine chondrules consist of skeletal forsteritic olivine grains overgrown by Al-rich (up to 20 wt% A12O3) low-Ca and high-Ca pyroxene,and interstitial anorthite-rich mesostasis.Since chondrules with such characteristics are absent in ordinary,enstatite and other carbonaceous chondrite groups,the impact-related chondrule-forming mechanism could be unique for the CH and CB chondrites.We conclude that Isheyevo and probably other CH chondrites contain chondrules of several generations,which may have formed at different times,places and by different mechanisms,and subsequently accreted together with the heavily hydrated matrix lumps and refractory inclusions into a CH parent body.Short-lived isotope chronology,oxygen isotope and trace element studies of the Isheyevo chondrules can provide a possible test of this hypothesis.