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doi:10.2204/iodp.proc.335.203.2016

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 335 (April–May 2011) was the fourth scientific drilling expedition to Hole 1256D (after Ocean Drilling Program Leg 206 and IODP Expeditions 309 and 312) and aimed to reach ocean crust Layer 3. Hole 1256D is located at 6.736°N, 91.934°W, in the eastern equatorial Pacific Ocean in ~15 Ma crust that formed at the East Pacific Rise at a superfast spreading rate (>200 mm/y) (Wilson, 1996). This hole currently represents the only continuous section of fast-spread upper oceanic crust ever drilled in modern oceans.

The previous expedition (312) deepened the hole to 1507.1 meters below seafloor (mbsf) and reached the sheeted-dike/gabbro transition zone for the first time (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006). Expedition 335 aimed to deepen the hole into the cumulate gabbros, a unit poorly known at present-day fast-spreading ridges (see the “Expedition 335 summary” chapter [Expedition 335 Scientists, 2012a]). However, owing to collapse events during drilling operations and to the toughness of recrystallized rocks located at the sheeted-dike/gabbro transition zone (i.e., granoblastic basalts), only five cores with <15% recovery were drilled to 1521.6 mbsf during Expedition 335. Nevertheless, a unique and huge collection of samples (~50 kg), including large cobbles (up to 4.5 kg), centimeter-sized pebbles, and very fine to coarse-grained sand and gravels were recovered during cleaning operations (see the “Expedition 335 summary” chapter [Expedition 335 Scientists, 2012a]).

Sand and gravels correspond to fine-grained cuttings coming from various depths in the borehole since the first drilling expedition (Leg 206) and recovered in the junk baskets associated with the drill bit (see “Operations” in the “Expedition 335 summary” chapter [Expedition 335, Scientists, 2012a]). They represent an integrated sampling of the entire hole from the lava sequence to the plutonic section. Even if the depth from which the cuttings fell is unknown, their study is a good opportunity to make a comprehensive review of the various lithologies present in the hole.

During Expedition 335, a petrographic study was initiated on fine-grained cuttings coming from two fishing runs (i.e., pipe deployment recovering cuttings during hole cleaning operations). Six grain-mount thin sections were grain counted, and the lithology of each grain was identified to establish the nature of the fine-grained cuttings (Fig. F4 in the “Site 1256” chapter [Expedition 335 Scientists, 2012c]). The present report completes this preliminary work by studying sand and gravels from more fishing runs and by undertaking electron microprobe analyses on these cuttings. However, a different petrographic approach than the one employed on board the ship was applied: (1) we conducted a more detailed petrographic identification of the grains in order to study the lithologic diversity of the cuttings and (2) we applied a method using the area of the grains instead of the number of grains to estimate the abundance of each lithology in order to reduce underestimations related to grain size variations. From mineral composition, we also conducted thermometry and oxybarometry calculations on the cuttings to estimate the temperature and redox conditions prevailing at different levels in the hole.

This study provides a unique, significant, and robust database linking the lithology, mineral composition (~4400 analyses), temperature, and oxidation state of the upper crust at a modern fast-spreading ridge and will be useful (1) for comparison with other drilled and dredged oceanic sites and with fossil-exposed mid-ocean ridges (ophiolites) and (2) for understanding the magmatic, metamorphic, and hydrothermal processes occurring at fast-spreading ridges.

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