Composition of Chondrules and Matrix and Their Complementary Relationship in Chondrites

Complementary chemical and isotopic relationships between chondrules and matrix have the potential to distinguish between categories of chondrule forming mechanisms, e.g., exclude all mechanisms that require different reservoirs for chondrules and matrix. The complementarity argument is, however, often misunderstood. Complementarity requires different average compositions of an element or isotope ratio in each of the two major chondrite components chondrules and matrix, and a solar or CI chondritic bulk chondrite ratio of the considered elements or isotopes. For example, chondrules in carbonaceous chondrites typically have superchondritic Mg/Si ratios, while the matrix is subchondritic. Another example would be the Hf/W ratio, which is superchondritic in chondrules and subchondritic in matrix. We regard these ratios to be complementary in chondrules and matrix, because the bulk chondrite has solar Mg/Si and Hf/W ratios. In contrast, Al/Na ratios are also different in chondrules and matrix, but the bulk is not solar; therefore, Al/Na does not have a complementary relationship. A number of publications over the past decade have reported complementary relationships for many element pairs in different types of chondrites. Recently, isotopic complementarities have also been reported. A related, though different, argument can be made for volatile depletion patterns in chondrules and matrix, which can then also be considered as being complementary. The various models for chondrule formation require either that chondrules and matrix formed from a single (i.e., common) parental reservoir, or that chondrules and matrix formed in separate regions of the protoplanetary disk and were later mixed together. As chondrules and matrix have different compositions, mixing of these two components would result in a random bulk chondrite composition. The observation of complementary chondrule–matrix relationships together with a CI chondritic, element or isotope ratio is unlikely to be the result of a random mix of chondrules and matrix. It seems much more likely that chondrules and matrix formed in a single reservoir with initially CI chondritic element or isotope ratios. Incorporation of different minerals in chondrules and matrix together with volatile element depletion of the entire reservoir then resulted in chondrule-matrix complementarities and bulk chondrite volatile depletion. This excludes any chondrule formation mechanism that requires separate parental reservoirs for these components. Any chondrule forming mechanism must explain complementarity. Chondrules and matrix must have formed from a common reservoir.