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doi:10.2204/iodp.proc.345.114.2014

Gabbroic rock

During Expedition 345, 1 gabbronorite sample from Hole U1415E, 4 clinopyroxene oikocryst-bearing troctolite samples and gabbro samples from Holes U1415I and U1415J, 9 orthopyroxene-bearing olivine gabbro and olivine-bearing gabbronorite samples from Holes U1415H–U1415J and U1415P, 10 troctolite samples from Holes U1415J and U1415P, and 20 olivine gabbro and gabbro samples from Holes U1415J and U1415P were selected for chemical analyses (Table T1). Except for the gabbronorite in Hole U1415E, all samples are characterized by their significantly less evolved composition compared to gabbroic rock previously collected along the Northern Escarpment of the Hess Deep Rift (Gillis, Mével, Allan, et al., 1993; Pedersen et al., 1996; Natland and Dick, 1996; Hanna, 2004; Kirchner and Gillis, 2012). The rocks overlap in composition with the most primitive gabbro and troctolite sampled along the EPR (Pito Deep; Perk et al., 2007) (Figs. F1, F2; see also “Inorganic geochemistry” for each hole). The main chemical signatures of the different gabbroic rocks sampled at Site U1415 and of the alteration that affected the lithologic intervals are summarized below.

Hole U1415E gabbronorite

Hole U1415E gabbronorite has low Mg# (73) and low Cr (123 ppm) and Ni (67 ppm) concentrations and plots at the most primitive end of the field of gabbro and gabbronorite previously sampled at Hess Deep. These compositions indicate that this sample crystallized within an evolved magmatic system, late in a MORB crystallization sequence.

Site U1415 primitive gabbroic series

Olivine gabbro and gabbro

The olivine gabbro and gabbro sampled in Holes U1415J and U1415P have compositions typical of primitive gabbroic rocks (e.g., Cannat, Karson, Miller, et al., 1995; Godard et al., 2009; Perk et al., 2007). They have high Mg# (79–87) and Ca# (100 × cationic Ca/[Ca + Na]; 77–92), high Ni contents (130–570 ppm), and low TiO₂ (0.1–0.3 wt%) and incompatible lithophile element (e.g., Y < 11 ppm) contents. Six samples are distinguished by a significantly higher Cr content (1500–2550 ppm) compared to neighboring gabbro. These high concentrations may reflect the occurrence of a Cr-rich minor phase (e.g., Cr-spinel) in these samples (see “Igneous petrology” in the “Hole U1415P” chapter [Gillis et al., 2014]).

Clinopyroxene oikocryst-bearing troctolite and gabbro

Although they are very different texturally, clinopyroxene oikocryst-bearing troctolite and gabbro sampled in Holes U1415I and U1415J are similar in composition to Site U1415 olivine gabbro in that they have high Mg# (80–85) and Ca# (82–86), high Ni concentrations (150–330 ppm), and low TiO₂ (<0.2 wt%) and trace element (e.g., Y < 6 ppm) contents.

Orthopyroxene-bearing olivine gabbro and olivine-bearing gabbronorite

Orthopyroxene-bearing olivine gabbro has primitive compositions similar to neighboring gabbro and olivine gabbro in that it is characterized by high Mg# (79–87) and Ca# (73–92) and low TiO₂ (0.1–0.3 wt%) and trace element (e.g., Y < 6 ppm) contents. Gabbro has high Ni (150–460 ppm) and Cr (222–1000 ppm) contents typical of primitive oceanic gabbro (e.g., Fig. F1).

Troctolite

Troctolite overlaps in composition with gabbro but has on average a more primitive composition with high Mg# (81–89) and Ca# (79–98), high Ni (260–1500 ppm) and Cr (365–1100 ppm) contents, and low TiO₂ (<0.1 wt%) and incompatible lithophile element (e.g., Y < 3 ppm) contents. The most primitive troctolite sampled in Hole U1415P has a composition that overlaps the field of impregnated mantle peridotite from the Hess Deep Rift and elsewhere (Fig. F1). However, in contrast to the olivine-rich troctolite sampled at Atlantis Massif (IODP Expedition 304/305 Site U1309), these samples are low in Ni relative to their high Mg#, which suggests that they were formed by a dominantly cumulate process. Our data provide the first chemical characterization of these rock types in the Hess Deep area.

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