Calcium-aluminum-rich inclusions recycled during formation of porphyritic chondrules from CH carbonaceous chondrites

We report on the mineralogy, petrography, and O-isotope compositions of similar to 60 Ca, Al-rich inclusions (CAIs) incompletely melted during formation of porphyritic chondrules from the CH metal-rich carbonaceous chondrites and Isheyevo (CH/CB). These include (i) relict polymineralic CAIs in porphyritic chondrules, (ii) CAIs surrounded by chondrule-like igneous rims, (iii) igneous pyroxene-rich and Type C-like CAIs, and (iv) plagioclase-rich chondrules with clusters of relict spinel grains. Al-26-Mg-26 systematics were measured in 10 relict CAIs and 11 CAI-bearing plagioclase-rich chondrules. Based on the mineralogy, the CH CAIs incompletely melted during chondrule formation can be divided into grossite-rich (n = 13), hiboniterich (n = 11), spinel +/- melilite-rich (n = 33; these include plagioclase-rich chondrules with clusters of relict spinel grains) types. Mineralogical observations indicate that these CAIs were mixed with different proportions of ferromagnesian silicates and experienced incomplete melting and gas-melt interaction during chondrule formation. These processes resulted in partial or complete destruction of the CAI Wark-Lovering rims, replacement of melilite by Na-bearing plagioclase, and dissolution and overgrowth of nearly end-member spinel by chromium-and iron-bearing spinel. Only two relict CAIs and two CAIbearing chondrules show resolvable excess of radiogenic Mg-26; the inferred initial Al-26/Al-27 ratios are (1.7 +/- 1.3) X 10(-6), (3.7 +/- 3.1) x 10(7), (1.9 +/- 0.9) X 10(6) and (4.9 +/- 2.6) X 10(6). There is a large range of Delta O-17 among the CH CAIs incompletely melted during chondrule formation, from similar to-37%(infinity) to similar to-5%(infinity); the unmelted minerals in individual CAIs, however, are isotopically uniform and systematically O-16-enriched relative to the host chondrules and chondrule-like igneous rims, which have Delta O-17 ranging from similar to-7%(infinity) to similar to+ 4%(infinity). Most of the CH CAIs incompletely melted during chondrule formation are mineralogically and isotopically similar to the CH CAIs surrounded by Wark-Lovering rims and apparently unaffected by chondrule melting. The mineralogy and O-isotope compositions of the CH CAI-bearing chondrules are similar to those of the CH porphyritic chondrules without relict CAIs. We conclude that CH porphyritic chondrules formed by incomplete melting of isotopically diverse solid precursors, including mineralogically and isotopically unique CAIs commonly observed only in CH chondrites. Therefore, the CH porphyritic chondrules must have formed in a distinct disk region, where the CH CAIs were present at the time of chondrule formation. Because most CH CAIs avoided chondrule melting, we infer that chondrule formation was highly localized. These observations preclude formation of CH porphyritic chondrules by splashing of molten planetesimals, by impact processing of differentiated planetesimals, and by large scale nebular shocks, e.g., shocks driven by disk gravitational instabilities or by X-ray flares. Instead, they are consistent with small-scale chondrule-forming mechanisms proposed in the literature, such as nebular processing of dust balls by bow shocks and by current sheets. (C) 2016 Elsevier Ltd. All rights reserved.