PHOSPHATE‐SULFIDE ASSEMBLAGES AND Al/Ca RATIOS IN TYPE‐3 CHONDRITES

Phosphate‐sulfide assemblages are common constituents in type‐3 carbonaceous and ordinary chondrites. CV3 chondrites contain assemblages of pentlandite‐merrillite and troilite‐merrillite as well as isolated grains of Ca‐pyroxene; CO3 chondrites contain troilite‐merrillite (± schreibersite) as well as isolated grains of plagioclase; and, H‐L‐LL3 chondrites contain troilite‐merrillite (± metal), troilite‐chlorapatite, and metal‐chlorapatite. The phosphate‐bearing assemblages probably formed in the following manner: (1) metal grains with significant P formed in the nebula at high temperatures; (2) Schreibersite exsolved and crystallized at metal grain boundaries during cooling; (3) some metal grains were sulfurized at lower temperatures by H2S; (4) the metal‐schreibersite and sulfide‐schreibersite assemblages accreted rims of finegrained silicates; and, (5) the Schreibersite reacted with Ca, O and Cl from these silicates to form merrillite and chlorapatite. The reported bulk compositions of chondritic constituents have ***CI‐normalized Al/Ca ratios >1, whereas whole‐rock ratios are unfractionated. Even though the phosphate‐bearing assemblages and isolated grains of Ca‐bearing silicates are ubiquitous in type‐3 chondrites, they are insufficiently abundant to lower the Al/Ca ratios of the major chondritic components to those of the whole‐rocks. It seems probable that some of the analytical data are incorrect; bulk compositions determined by microprobe may yield erroneously high Al/Ca ratios if samples are analyzed with a broader electron beam than used for analyzing the standards. We recommend analyzing standards and samples with the same size beam.