Primitive or undifferentiated meteorites (chondrites) date back to the origin of the Solar System, and thus preserve a record of the physical and chemical processes that occurred during the earliest evolution of the accretion disk surrounding the young Sun. The oldest Solar System materials present within these meteorites are millimetre- to centimetre-sized calcium-aluminium-rich inclusions (CAIs) and ferromagnesian silicate spherules (chondrules), which probably originated by thermal processing of pre-existing nebula solids. Chondrules are currently believed to have formed approximately 2-3 million years (Myr) after CAIs (refs 5-10)--a timescale inconsistent with the dynamical lifespan of small particles in the early Solar System. Here, we report the presence of excess (26)Mg resulting from in situ decay of the short-lived (26)Al nuclide in CAIs and chondrules from the Allende meteorite. Six CAIs define an isochron corresponding to an initial (26)Al/(27)Al ratio of (5.25 +/- 0.10) x 10(-5), and individual model ages with uncertainties as low as +/- 30,000 years, suggesting that these objects possibly formed over a period as short as 50,000 years. In contrast, the chondrules record a range of initial (26)Al/(27)Al ratios from (5.66 +/- 0.80) to (1.36 +/- 0.52) x 10(-5), indicating that Allende chondrule formation began contemporaneously with the formation of CAIs, and continued for at least 1.4 Myr. Chondrule formation processes recorded by Allende and other chondrites may have persisted for at least 2-3 Myr in the young Solar System.

Long- and short-lived radioactive isotopes and their daughter products in meteorites are chronometers that can test models for Solar System formation. Differentiated meteorites come from parent bodies that were once molten and separated into metal cores and silicate mantles. Mineral ages for these meteorites, however, are typically younger than age constraints for planetesimal differentiation. Such young ages indicate that the energy required to melt their parent bodies could not have come from the most likely heat source-radioactive decay of short-lived nuclides ((26)Al and (60)Fe) injected from a nearby supernova-because these would have largely decayed by the time of melting. Here we report an age of 4.5662 +/- 0.0001 billion years (based on Pb-Pb dating) for basaltic angrites, which is only 1 Myr younger than the currently accepted minimum age of the Solar System and corresponds to a time when (26)Al and (60)Fe decay could have triggered planetesimal melting. Small (26)Mg excesses in bulk angrite samples confirm that (26)Al decay contributed to the melting of their parent body. These results indicate that the accretion of differentiated planetesimals pre-dated that of undifferentiated planetesimals, and reveals the minimum Solar System age to be 4.5695 +/- 0.0002 billion years.

With a half-life of ∼0.73 Myr, the 26 Al-to-26 Mg decay scheme is ideally suited for dating meteorites and planetary processes that occurred in the first 5 Myr of the Solar System [1]. While useful, other short-lived chronometers like the 53 Mn-53 Cr and 182 Hf-182 W systems suffer from several disadvantages as compared to the 26 Al-26 Mg system. Firstly, the 53 Mn-53 Cr and 182 Hf-182 W systems may be variably affected by volatility-driven fractionation events as well as coupled core-forming and silicate melting processes. Secondly, the relatively long half-lives of 53 Mn and 182 W (3.7 and 9 Myr, respectively) hinder resolution of <<1 Myr age information from these systems. In contrast, both Al and Mg are refractory and lithophile elements (i.e. insensitive to metal-silicate fractionation events) and, coupled with the short half-life of 26 Al, means that the 26 Al-26 Mg chronometer is likely to be the most useful tool for precisely constraining the timing of silicate melting on differentiated planetesimals.

We report on new field observations from Icelandic lava flows that have the same surface morphology as many Martian flood lava flows. The Martian flood lavas are characterized by a platy‐ridged surface morphology whose formation is not well understood. The examples on Mars include some of the most pristine lava on the planet and flows >1500 km long. The surfaces of the flows are characterized by (1) ridges tens of meters tall and wide and hundreds of meters long, (2) plates hundreds of meters to kilometers across that are bounded by ridges, (3) smooth surfaces broken into polygons several meters across and bowed up slightly in the center, (4) parallel grooves 1–10 km long cut into the flow surface by flow past obstacles, and (5) inflated pahoehoe margins. The Icelandic examples we examined (the 1783–1784 Laki Flow Field, the Búrfells Lava Flow Field by Lake Myvatn, and a lava flow from Krafla Volcano) have all these surface characteristics. When examined in detail, we find that the surfaces of the Icelandic examples are composed primarily of disrupted pahoehoe. In some cases the breccia consists of simple slabs of pahoehoe lava; in other cases it is a thick layer dominated by contorted fragments of pahoehoe lobes. Our field observations lead us to conclude that these breccias are formed by the disruption of an initial pahoehoe surface by a large flux of liquid lava within the flow. In the case of Laki, the lava flux was provided by surges in the erupted effusion rate. At Búrfells it appears that the rapid flow came from the sudden breaching of the margins of a large ponded lava flow. Using the observations from Iceland, we have improved our earlier thermal modeling of the Martian flood lavas. We now conclude that these platy‐ridged lava flows may have been quite thermally efficient, allowing the flow to extend for >100 km under a disrupted crust that was carried on top of the flow.