Chondritic meteorites are made of primitive components that record the first steps of formation of solids in our Solar System. Chondrules are the major component of chondrites, yet little is known about their formation mechanisms and history within the solar protoplanetary disk (SPD . In addition, mineral 26 Al isochrons determined on the same chondrules show that their formation (i.e., fusion of their precursors by energetic events) took place from 0 Myr to ∼2 Myr after the formation of their precursors, thus showing in some cases a clear decoupling in time between the two events. The finding of a minimum bulk chondrule 26 Al isochron is used to constrain the astrophysical settings for chondrule formation. Either the temperature of the condensation zone dropped below the condensation temperature of chondrule precursors at ∼1.5 My after the start of the Solar System or the transport of precursors from the condensation zone to potential storage sites stopped after 1.5 My, possibly due to a drop in the disk accretion rate.Mg isotope analyses | MC-SIMS | HR-MC-ICPMS | chondrule history | short-lived 26 Al P rimitive meteorites (i.e., chondrites) are rocks that escaped melting and differentiation on their parent bodies. As a result, their components preserved a record of the mineralogy, chemistry, and isotopic compositions of the solids formed in the solar protoplanetary disk (SPD) before planetesimal formation. Viscous heating in the inner regions of the SPD (1) brought presolar dust and gas to temperatures higher than the sublimation point of most minerals, producing a gas that upon cooling produced the first Solar System solids by condensation. The relative uniformity of isotope ratios among many early Solar System materials (2) is testament to this phase of homogenization. The calcium−aluminum-rich inclusions (CAIs) that are found in primitive meteorites are considered to be formed from refractory precursors condensed at temperatures in the range from ∼1,400 K to ∼1,800 K (3). Chondrules are millimeter-size once-molten silicate spherules that comprise most of the mass (70-80%) of chondrites. They have compositions indicating that their formation took place from solid precursors condensed below 1,500 K (4, 5). The exact timing (duration, chronology) of (i) the condensation processes that produced the precursors of CAIs and of chondrules, and (ii) the processes that resulted in the formation of CAIs and chondrules, is still poorly known, although it is key to a better understanding of the complex origin and evolution of solids in the solar accretion disk.

Early solar materials bear a variety of isotopic anomalies that reflect compositional differences deriving from distinct stellar nucleosynthetic processes. As shown in previous studies, the stepwise dissolution with increasing acid strengths of bulk rock carbonaceous chondrites liberates Cr with both excesses and deficits in 53 Cr and 54 Cr relative to the terrestrial standard. The magnitude of the 54 Cr variations within a meteorite decreases in the sequence CI1 > CR2 > CM2 > CV3 > CO3 > CK4 and correlates with the degree of metamorphism of each carbonaceous chondrite class. This study shows that the Tagish Lake meteorite presents the highest excesses in 54 Cr ever measured in a bulk silicate phase. According to this study, the Tagish Lake meteorite is composed of the least re-equilibrated material known at this time. The magnitude of 54 Cr variation decreases now in the following sequence: Tagish Lake (ungrouped CI2) > Orgueil (CI1) > Murchison (CM2) > Allende (CV2). Moreover, this study shows that excesses in 53 Cr relative to Earth can be interpreted as representing the extent of aqueous alteration on meteorite parent bodies. Finally, the high 54 Cr anomalies measured in this meteorite make Tagish Lake one of the major targets to decipher the host of these anomalies.

We have determined ′ 26 MgDSM-3, the mass-independent variations in 26 Mg ∕ 24 Mg, of primitive, bulk meteorites to precisions better than ± 3 ppm (2se). Our measurements of samples from 10 different chondrite groups show ′ 26 MgDSM-3 that vary from −5 to 22 ppm. Our data define an array with a positive slope in a plot of ′ 26 MgDSM-3 against 27 Al ∕ 24 Mg, which can be used to determine (26 Al ∕ 27 Al)0, i.e. initial 26 Al ∕ 27 Al, and (′ 26 MgDSM-3)0, i.e. initial ′ 26 MgDSM-3. On such an isochron plot, the best fit of our new measurements combined with literature data implies (26 Al ∕ 27 Al)0 of (4.67±0.78)x10 −5 and (′ 26 MgDSM-3)0 of −31.6 ± 5.7 ppm (2se) for ordinary and carbonaceous chondrites, other than CR chondrites, which have anomalously low ′ 26 MgDSM-3. These parameters are within uncertainty of those defined by previous measurements of bulk calcium-, aluminium-rich inclusions (CAIs) that set canonical (26 Al ∕ 27 Al)0 ~ 5x10 −5. The most straightforward interpretation of all these observations is that

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Chemical weathering of silicate rocks is a key control on the long-term climate, via drawdown of atmospheric CO 2. Magnesium isotopes are increasingly being used to trace weathering, but are often complicated by several coincident fractionating processes. Here we examine Mg isotope ratios of waters stemming from beneath lava flows from the 2010 Eyjafjallajökull eruption. Travertine calcite was observed directly precipitating from these high-TDS (total dissolved solids) waters, and were also sampled. This system therefore provides the opportunity to study natural Mg isotope fractionation by calcite. Riverine δ 26 Mg increase from −2.37 to +0.43% with flow distance, as isotopically light travertine precipitates (δ 26 Mg = −3.38 to −3.94%). The solution Mg isotope ratios also co-vary with pH, calcite saturation indices and Sr/Ca ratios, strongly indicating that they are dominantly controlled by carbonate precipitation. Using experimental isotopic fractionation factors and the measured δ 26 Mg values, we can predict the compositions of the precipitated travertines that are within uncertainty of the directly measured travertines. Hence, in some systems, Mg isotopes can be used to quantify carbonate precipitation.

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The possibility of establishing an accurate relative chronology of early solar system events based on the decay of short-lived 26 Al to 26 Mg (half-life of 0.72 Myr) depends on the level of homogeneity (or heterogeneity) of 26 Al and Mg isotopes. However this level is difficult to constrain precisely because of the very high precision needed on the determination of isotopic ratios, typically of ± 5 ppm. In this study, we report for the first time a very detailed analytical protocol developed for high precision in situ Mg isotopic measurements