Previous   |    Next

doi:10.2204/iodp.proc.345.114.2014

Comparison to previously sampled oceanic gabbro

The main geochemical characteristics of Site U1415 gabbroic rocks are consistent with formation as a cumulate sequence from a common parental MORB melt, with troctolite representing the most primitive end-member of this sequence. Site U1415 gabbroic rocks appear to constitute a suite of gabbroic rocks of which the gabbronorite sampled in Hole U1415E represents the most evolved end-member. The rocks overlap in composition with the most primitive of slow- and fast-spread gabbroic rock sequences (Figs. F1, F2). These primitive geochemical signatures seem, however, to be contradictory with orthopyroxene (as much as 5%) in the primary mineral assemblage of the olivine gabbro sampled in Holes U1415I, U1415J, and U1415P. In MORB crystallization series, orthopyroxene is expected to crystallize from evolved melts. The presence of orthopyroxene in the primitive gabbroic sequence sampled at Site U1415 suggests that it was formed in a more complex magmatic system.

A single sample of primitive gabbroic rock containing high-Mg# orthopyroxene was sampled along the southern slope of the intrarift ridge in the Hess Deep area (Coogan et al., 2002). The Hess Deep sample was interpreted as indicating that a fraction of the melts that formed the lower crust interacted with the mantle during melt extraction and therefore was not undersaturated in orthopyroxene, as are typical MORB parental melts. High-Mg# depleted gabbronorite has also been observed in Oman ophiolite (Boudier et al., 2000) and at Deep Sea Drilling Project Site 334 on the Mid-Atlantic Ridge (Nonnotte et al., 2005), where they were interpreted as resulting from the contamination of MORB parental melts by water. Further studies will be carried out on shore to decipher the petrogenetic processes leading to the formation of the orthopyroxene-bearing primitive gabbro sampled during Expedition 345.

As noted above, Site U1415 gabbroic rocks have compositions similar to Pito Deep gabbro and troctolite (Perk et al., 2007). High-Mg# plutonic rock from Pito Deep has been sampled between 300 and 700 m beneath the sheeted dike complex (Perk et al., 2007), including a single layered specimen. Site U1415 gabbroic rock is drilled from a deeper oceanic crustal section and display extensive modal layering and primitive geochemistry. In the last decades, countless studies of layered intrusions have cast light on a wide variety of layer-forming mechanisms that influence the cumulate geochemistry (e.g., Holness and Winpenny, 2009; Meyer et al., 2009). However, modal layering in MORB cumulates from fast-spreading ridges, as well as their actual composition, remains nearly unknown, and it is worthwhile to note that fast-spreading plutonic crust remains undersampled compared to slow-spreading crust, as illustrated in Figures F1 and F2.

Top of page   |    Previous   |    Next