Texture of chondrules

• Main Author: Hiroko Nagahara
• Format: Report
• Language: English
• Published: Geological Institute, University of Tokyo 1983
• Subjects: Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=1552; http://id.nii.ac.jp/1291/00001552/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=1552&item_no=1&attribute_id=18&file_no=1

Textures of chondrules in the unequilibrated ordinary chondrites have been summarized on the basis of the processes of their formation inferred from the results of previous experiments. The chondrules studied are classified by their textures into the following four groups : (1) those formed by complete melting of the precursor materials, (2) those formed by incomplete melting of the precursor materials, (3) "lithic fragments" or "lithic chondrules" formed through low temperature heating of the precursor materials, and (4) complex chondrules formed by accretion of two chondrules or by squeezing out the liquid. Groups (1) and (2) are further classified into different subgroups depending on their texture and constituent minerals. The subgroups in group (1) include barred olivine chondrules, radial pyroxene chondrules, and glassy chondrules. In group (2), the subgroups are porphyritic olivine and pyroxene chondrules, porphyritic olivine chondrules, and porphyritic pyroxene chondrules. The porphyritic olivine and pyroxene chondrules include the following specific types : normal porphyritic olivine and pyroxene ones without relict minerals, chondrules with relict minerals, chondrules which are Ca- and Al-rich, which have been intensively heated, and which are metal-rich. The conditions of formation of these different chondrules have been estimated by comparing them with the experimentally reproduced textures. Group (1) is interpreted to have been heated at aboveliquidus temperatures, about 1500℃, and cooled at rates of (10)^4-(10)^2°/hr, with most suitable rates of (10)^3°/hr for barred olivine chondrules, and (10)^2-1°/hr, with most suitable rates of several tens°/hr for radial pyroxene chondrules. Group (2) was heated at subliquidus temperatures (probably between 1200-1000° and 1500℃, and cooled at various rates, relatively rapidly for porphyritic olivine chondrules and slowly for euhedral tabular pyroxene. Group (3) without spherical form was heated near or below solidus temperatures.