Chondrules are millimeter to sub-millimeter size silicate spherules that formed during localized and transient high-temperature events in the early solar system. Although it is not yet understood how chondrules formed, recent studies have provided important clues in understanding the physical conditions of chondrule formation. In this paper I review recent developments in studies of chondrules to provide constraints on high-temperature processes during the evolution of protoplanetary disks, including: (1) the timing and duration of the chondrule-forming high-temperature period, based on long-lived and short-lived chronometers; (2) the thermal history of chondrule melts during each chondrule-forming event, as constrained from laboratory experiments and petrological and isotopic studies of chondrules, including shock-wave heating as a plausible heat source for chondrule formation; and (3) sources for shock waves and chondrule-forming regions in the protoplanetary disk. Given our current knowledge, I provide the following possible answers to the above four points: (1) the period of chondrule formation began shortly after the formation of calcium-aluminum-rich inclusions and lasted for a few million years; (2) chondrule precursors were heated at a rate of > 104 K/h in the temperature range of ∼ 1400-1600 K, and melt droplets were cooled from a peak temperature of ∼ 1800-2200 K at a rate of ∼ 5-1000 K/h; (3) high-velocity shock waves (> 20 km/s) in a low-density gas region (< 1019 particles/m3) may be appropriate for localized transient heating events associated with chondrule formation; and (4) X-ray flares from the young Sun in its T-Tauri stage might be the source of the shock waves.
The Bajawa Cinder Cone Complex consists of at least 78 cinder cones, which can be grouped into five morphometric ages. The oldest group, Bajawa 01, is dated between 0.53 and 0.73 Ma. The groups Bajawa 02, 03, and 04 have an age range from 0.41 Ma to 0.51 Ma, 0.32 Ma to 0.40 Ma, and 0.22 Ma to 0.31 Ma, respectively; the youngest cinder cone group, Bajawa 05, is aged between 0 and 0.20 Ma. Three batches of magma have contributed to the long eruptive episode of the complex. Magma mixing and fractional crystallization were important processes in differentiation. Magma mixing may have been caused by recharge of a mafic contributor to form a mixed magma in a temperature range of 836 °C-978 °C and at a pressure of less than 1.25 kbar before eruption.
During the Late Paleocene, at least five ignimbrite units were emplaced from the Yakutinskaya caldera complex in Primorye, Russia. The erupted ignimbrites show two distinct chemical cycles, believed to represent the “high”-silica and “low”-silica parts of the compositionally zoned magma chamber. Two petrographically distinctive types of rhyolites are distinguishable in each chemical cycle, based on their phenocryst chemistry and silica content: (1) “low”-silica rhyolites with mineral assemblages of quartz, sanidine, plagioclase, ferrohypersthene, ferroaugite (Ca41Mg21Fe38), biotite, and hornblende, and (2) “high”-silica rhyolites with a similar mineral assemblage to “low”-silica, but containing more Fe-rich clinopyroxene (Ca44Mg2Fe54) and biotite, and a with lower phenocryst abundance. This difference is related to the variation in chemical composition and temperature of the magma in the zoned magma chamber for each eruption cycle. Rb-Sr mineral-rock isochron ages show that the ignimbrites erupted between 59.7 ± 1.6 and 54.8 ± 2.6 (2σ) Ma (Late Paleocene), and initial 87Sr/86Sr ratios are distinct in the different ignimbrite units. The “high”-silica rhyolites show the highest 87Sr/86SrI ratios (0.70810-0.70738), whereas “low”-silica rhyolites show lower 87Sr/86SrI ratios (0.70659-0.70724). The compositional zoning of the single magma chamber can be explained by the large-scale mass transport in the liquid phase due to roofward migration and concentration of volatile species.