From a study of first-transition series trace elements and low-partition-coefficient trace elements or so-called hygromagmatophyle elements (Treuil and Varet, 1973; Treuil and Joron, 1976), we will try to define the relative importance of the three fundamental parameters- mantle source composition, extent of partial melting, and crystal fractionation-which bear on the formation of basalts recovered during Leg 45.We relate the presented data to the results of shipboard study (Chapters 7 and 8, this volume) and then give a tentative interpretation, taking into account our present knowledge of the behavior of trace elements during magmatic processes of FAMOUS and Leg 37 results. [NOT CONTROLLED OCR]