[PPS09-P04] Alminium26-Magnesium26 dating of chondrule in ordinary chondrite NWA7936(L3.15) by NanoSIMS
Keywords:ordinary chondrite, Al-Mg chronology, thermal metamorphism, relative ages, NWA7936, NanoSIMS
Meteorites are important samples for understanding the history of early solar system. Their structure and age would give us valuable information about early solar system. The undifferentiated meteorites,whose parent bodies haven't experienced differentiation processes,may have the oldest information before planetesimal formation.
In our study,we studied ordinary chondrites which are undifferentiated meteorites and found on the earth.They,however,generally show some degree of thermal metamorphism.For chondrites with petrologic type(i.e.,the degree of thermal metamorphism,increase from 3 to 6)>4.0,only information after thermal metamorphism would be obtained. We can study the formation ages of chondrules or chondrites with petrologic type 3. Even this group,however,is affected by thermal metamorphism to some extent,and further important to know the relationship between the reliable Al-Mg age and the degree of thermal metamorphism.In this study,we conducted Al-MG dating of chondrules in an ordinary chondrule(L3.15)NWA7936,which haven't been well studied,trying to examine reliability of the chronometer.
We chosed 26Al-26Mg dating method.Half life period of 26Al is 0.7My. We plotted δ26Mg against 27Al/24Mg,where the slope of the correlation line givesthe 26Al/27Al initial ratio.We compare the obtained 26Al/27Alinitial ratios of chondrules with that of CAIs(5.0×10-5,whose absolute age is known to be 4567.3 My). Then we compare the present results with those of privious works.
Before the SIMS analysis,we analyzed the chondrules using SEM-EDS at the department of Earth and Planetary Science,the University of Tokyo and selected the areas with high Al/Mg ratios >30,suitable for an isochron plot. The analyzed chondrules consist mostly of olivine phenocrysts with mesostasis including small anorthite grains,and minor amount of metals. We applied the 26Al-26Mg dating method for threechondrules using NanoSIMS at Atmosphere and Ocean Research Institute,the University of Tokyo. We analyzed four points in each chondrules.Primary ion beam is 16O-,2μm in diameter.with an intensity of 200pA.Secondary ions of 27Al++,26Mg+,25Mg+,and,24Mg+ were measured simultaneously with EMs.We measured 100 cycles for each point.The time of 1 cycle was 25s.
The three chondrules show clear excesses in 26Mg and we can draw 'isochrons' passing through the origin. The obtained 26Al/27Al initial ratios are 1.33±0.41,1.67±0.94,and 1.10±0.71(×10-5),respectively. Relative ages are 1.34(+0.38,-0.27),1.11(+0.83,-0.45),and 1.52(+1.04,-0.50)(Ma),respectively. The three ages are identical within erros. If we assume a single formation ages for chondrules in this meteorite,we may obtain weighted average of 1.34 (+0.30,-0.23)Ma after formation.
We compared the present results owith those of privious work. The initial 26Al/27Al ratios of chondrules are consistently in the range of (0.5 to 2.0)×10-5 for chondrites with petrographic types of 3.0 to 3.3, and this range most likely corresponds to the time when chondrules formed. On the other hand, meteorites with petrographic types of >3.4 tend to show much lower values of initial 26Al/27Al ratio, suggesting that the values probably show the timing of thermal metamorphism.
The present results show identical formation ages for three chondrules in NWA7936 within uncertainties. The previous data for Semarkona(LL3.0), however, suggests that formation ages of chondrules in the same chondrules in the same chondrite may have true variation (1 to 2 My). Hence, the present results may be fortuitous,and we may find variations in chondrules ages for NWA7936 if we increase the number of analyses If 1-2My difference exists in the chondrule formation ages in a single chondrite, it may require a long-lasting(for a few Mys) mechanism for the formation of chondrules. Further studies about the formation mechanisms of chondrules are required for better understanding of the evolutionary history of the protoplanetary disk.
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