Oxygen Isotopic Compositions in a Plagioclase-Olivine Inclusion from Ningqiang Similar to Those in Al-rich Chondrules

We report the petrology and oxygen isotopic composition,using a Cameca Nano SIMS 50L ion microprobe,of a plagioclase-olivine inclusion,C#1,found in the Ningqiang carbonaceous chondrite.In addition to major phases(plagioclase,spinel and olivine),C#1 is also surrounded by a pyroxene rim(64 vol%Ca-rich and 36 vol%Ca-poor pyroxenes).On a three-isotope oxygen diagram,δ^(17)O vs.δ^(18)O,the compositions of individual minerals analyzed in C#1 fall along the carbonaceous chondrite anhydrous mineral line(CCAM),and oxygen isotopic compositions in C#1 show significant variability in δ^(18)O and δ^(17)O.The oxygen isotopic compositions of the pyroxene rim minerals are similar to those of the other host minerals,which suggests that the rim likely formed from the same melting process as the host.The rim is considered to have formed as a result of interaction between an ^(16)O-poor gas and a melt.Some spinel grains are typically ^(16)O-rich and likely of relict origin,which is similar to ^(16)O-rich Ca-,Al-rich inclusions,which are probably a precursor of C#1.The inclusion then likely melted in an ^(16)O-poor region where chondrules form,accompanied by oxygen isotope exchange with an ^(16)O-poor gas.Some anorthite,pyroxene and spinel might have undergone fluid-assisted thermal metamorphism on the Ningqiang chondrite parent body.The oxygen isotope data and evolution of the C#1 plagioclase-olivine inclusion are similar with those of Al-chondrules in chondrites.

A Study of Chondrules in the Boxian Meteorite from Anhui Province, China

The types of chondrules in the Boxian meteorite have been determined Electron microprobe nalvsis ofvarious textural types of chondrules in the meteorite has revealed the composition ofglassy chondrules and oftheir glassy matrix,Also presented in this paper are the related explanations.In addition.the genesis,texturaldiversity and evolution of the chondrules as well as the olivine girdles observed in the chondrules arediscussed.

In situ SIMS oxygen isotope analyses: Evidence for the formation of aluminum-rich chondrules from ordinary chondrites

Aluminum-rich chondrules （ARCs）, which share mineralogic and chemical properties with both Ca, Al-rich inclusions （CAIs） and ferromagnesian chondrules, play an important role in revealing their temporal and petrogenetic relationships. In this work, seven ARCs were found in three ordinary chondrites GRV 022410 （H4）, GRV 052722 （H3.7） and Julesburg （L3.6）. They contain bulk Al2O3 - 17%-33% and exhibit igneous textures composed of olivine, high- and low-Ca pyroxene, plagioclase, spinel and glass. In situ SIMS analyses show that ARCs have oxygen isotopic compositions （δ18O=-6.1‰-7.1‰; δ17O= -4.5‰-5.1‰） close to ferromagnesian chondrules but far more depleted in 160 than CAIs （δ18O=-40‰; δ17O=-40‰）. Most ARCs plot close to the terrestrial mass fractionation （TF） line, and a few between the TF and carbonaceous chondrite anhydrous mixing （CCAM） lines. Plagioclase, nepheline and glass suffered O-isotopic exchanges during the metamorphism processes in the parent body. Spinel, olivine and pyroxene represent the primary O-isotopic compositions of ARCs, and define a fitted line with a slope of- 0.7±0.1. Compared with the results of previous studies, shallower slope as well as more depleted 160 compositions further demonstrates that ARCs in ordinary chondrites are not a simple mixing product of ferromagnesian chondrules and CAIs. Instead, they probably experienced higher-degree oxygen isotope exchange with a δ6O-poor nebular gas reservoir during multiple melting episodes.

Insights into chondrule formation process and shock-thermal history of the Dergaon chondrite (H4-5)

The Dergaon fall represents a shock-melted H4-5 （S5） ordinary chondrite which includes at least ten textural varieties of chondrules and belongs to the high chondrule-matrix ratio type. Our study reveals that the chondrules are of diverse mineralogy with variable olivine-pyroxene ratios （Type Ⅱ）, igneous melt textures developed under variable cooling rates and formed through melt fractionations from two different melt reservoirs. Based on the experimental analogues, mineralogical associations and phase compositions, it is suggested that the Dergaon chondrules reflect two contrasting environments： a hot, dust-enriched and highly oxidized nebular environment through melting, without significant evapora- tion, and an arrested reducing environment concomitant with major evaporation loss of alkali and highly volatile trace elements. Coexistence of chlorapatite and merrillite suggests formation of the Dergaon matrix in an acidic accretionary environment. Textural integration and chemical homogenization occurred at - I atmospheric pressure and a mean temperature of 765 ℃ mark the radiogenic thermal event. Equilibrated shock features （olivine mosaicism, diaplectic plagioclase, polycrystalline troilite） due to an impact-induced thermal event reflect a shock pressure 〉45 GPa and temperature of 600 ℃. By contrast, the local disequilibrium shock features （silicate melt veins comprising of olivine crystallites, troilite melt veins and metal droplets） correspond to a shock pressure up to 75 GPa and temperature 〉950 ℃.

A Ca-Al-RICH CHONDRULE IN BOXIAN CHONDRITE

The Boxian meteorite (LL3-4), fallen in October 1977, possesses low degree of thermal metamorphism and good chondrule texture. A Ca-Al-rich chondrule （CA chondrule） was found in this chondrite. This chondrule is ca. 0.36 mm in diame ter. There is only one light red euhedral spinel crystal, ca. 34％ in volume of this chondrule, while prismatic fassaite crystals occupy ca. 29％ and only one small crystal of olivine and light brown intergranular glass of the above minerals, ca. 37％ （Fig. 1）.