Magmatic Processes in the Uppermost Mantle at Oceanic Spreading Centers

Abstract

The magmatic processes responsible for oceanic crust generation at spreading centers, which are considered in the next chapter, depend on the mechanisms of melt extraction from the underlying mantle diapirs. The latter process was examined in chapter 7 where was shown that, on the basis of data from ophiolites, the uppermost mantle is a critical zone, corresponding in ophiolites to the transition zone between ultramafic and mafic sections. In particular, it was proposed in § 7.5 that a dynamic pressure prevailing in this zone could retain melt inside the peridotites and locally create conditions for melt percolation in an undercompacted medium, thus making a continuous transition to the crustal magma chamber. The magmatic processes operating in the deep crust below a ridge are hence coupled with those occurring in the mantle diapir, by and through this transition zone. This justifies a detailed discussion of the origin of its typical formations, dunites, wehrlites and, occasionally, chromite pods. Descriptions of typical transition zones can be found for the two main ophiolite types in § 3.3.3 for HOT and in § 4.2.3 and 4.3.3 for LOT. LOT transition zones share many common characteristics with HOT. As seen in table 8-I, the tabular dunite bodies, rich in melt impregnation, which were described in Trinity are comparable with those of HOT transition zones. In Trinity, it has been noted that the foliation, which is systematically steep in LOT, rotates to become parallel to the Moho a few tens of meters below it (fig. 4.12), a feature common in all observed HOT situations. In contrast, table 8-I also shows that neither the wehrlite sills and intrusions nor the chromite pods, so common in HOT transition zones, have been reported in LOT.