Gas‐melt interactions and their bearing on chondrule formation

Abstract

— Interactions between nebular gas and molten silicates or oxides could have played a major role in the formation and differentiation of the first solids formed in the solar system. In order to simulate such interactions, we set up a new experimental device in which isothermal condensation experiments have been conducted. Partially molten chondrule‐like samples have been exposed to high SiO(g) partial pressures, for intervals between 80 and 300 s and at temperatures ranging from 1600 to 1750 K. Results show that silica entering from the gas phase could be responsible for several textural and mineralogical features observed in natural chondrules. For instance, these experiments reproduce not only the mineralogical zonation of porphyritic olivine‐pyroxene chondrules with the peripheral location of pyroxenes, but also olivine resorption textures and the common poikilitical enclosure of olivines in pyroxenes. In the light of these similarities, we advocate that gas‐melt interactions through condensation are viable mechanisms for chondrule formation and hence may place severe constraints on the history of these primitive objects. In the nebula, high SiO(g) partial pressures could have been established by the volatilization of regions with high dust/gas ratio. A possible scenario for this stochastic thermal activity is the intense activity of the protosun in its young stellar object phase.

Silica‐rich igneous rims around magnesian chondrules in CR carbonaceous chondrites: Evidence for condensation origin from fractionated nebular gas

“…Evaporative loss of SiO 2 from chondrule melts had been used to explain the origin of type IA from type IB chondrules (e.g., Huang et al 1996;Sears et al 1996;Cohen et al 2000). We note, however, that the commonly observed increase in pyroxene/olivine ratio toward chondrule edges and the presence of PIR around olivine-rich chondrule cores in CR chondrites appear to be inconsistent with the evaporative SiO 2 loss from chondrule melts (see also Tissandier et al 2002). Based on the large scale Si/Mg variations throughout the Semarkona (LL3.0) type I chondrule suite which are uncorrelated with grain size, and hence the inferred degree of melting, Hewins (1997) also argued against volatilization of SiO 2 during chondrule formation.…”

“…6) are difficult (but not impossible) to reconcile with this scenario. Direct condensation of SiO (gas) into the chondrule melt (Tissandier et al 2002) may be required to explain these observations. This mechanism is consistent with the enrichment of bulk SIR in Ca, Al, and Ti as a signature of chondrule material (mesostasis) that interacted with gaseous SiO, and with the compositional trends in mesostases of the SIR and host chondrules (Fig.…”

“…2-6); and b) the near-equilibrium partitioning of Mn, Cr, and Na between high-Ca pyroxenes and mesostases in the silica-rich rims. Several mechanisms for the origin of the SIR can be considered: 1) reduction of ferromagnesian silicates in chondrule peripheries; 2) partial melting of the outer portions of the chondrules (Lofgren 2001); 3) fractional crystallization of the chondrules; 4) accretion of the silica-normative materials on the chondrule surfaces followed by subsequent melting and near-equilibrium crystallization (Krot et al 2000); and 5) direct condensation of SiO (gas) into chondrule melt (Tissandier et al 2002).…”

Partial melting of H6 ordinary chondrite Kernouvé: Constraints on the effects of reducing conditions on oxidized compositions

“…Enhanced volatilization in the sealed silica tube experiments likely reflects the longer run times, the lower oxygen fugacity, and reaction between the silica tube and graphite crucible to produce SiO 2 gas that reacted with the experimental charges (Tissandier et al 2002). It is clear that the tube volatilized and subsequently reacted with the melting olivine to create orthopyroxene.…”

Relationships between type I and type II chondrules: Implications on chondrule formation processes

“…Thiemens, 1996;Clayton, 2004;Chaussidon et al, 2008;Libourel and Chaussidon, 2011) and experimental (e.g. Yu and Hewins, 1998;Tissandier et al, 2002;Yu et al, 2003) observations strongly suggest that chondrules interacted with their gaseous surrounding environment during the period when they were molten.…”