Data report: microstructure of chilled margins in the sheeted dike complex of IODP Hole 1256D

Abstract

Chilled margins were recovered from the sheeted dike complex (SDC) of superfast (>200 mm/y)-spreading East Pacific Rise-spread crust during drilling of Integrated Ocean Drilling Program Hole 1256D on the Cocos Plate. The chilled margins contain stretched spherules, oriented plagioclase laths, grain-size segregation, and color banding. These rheomorphs locally crosscut veins but are elsewhere crosscut by veins. Electron microprobe investigations found that the chilled margins comprise dispersed micrometerscale minerals and veins including chlorite, actinolite, quartz, anhydrite, sphene, calcite, sphalerite, K-feldspar (adularia and/or orthoclase), magnetite, pyrite, and chalcopyrite. Though many of these phases are present throughout the SDC, anhydrite and calcite have not been previously recognized >100 m below the SDClava transition zone, and, with one exception, K-feldspar has not been previously identified in Hole 1256D core. Microstructures include quartz clasts surrounded by anhydrite, K-feldspar veins and clasts cut or surrounded by chilled margin material, and lenses of ductily deformed sphene. Some of the crosscutting relationships and distribution of mineral phases could be explained by hydrothermal alteration that occurred roughly simultaneously with dike intrusion. IntroductionDuring Integrated Ocean Drilling Program (IODP) Expedition 312, the structural geology team was impressed by the microstructure of chilled margins recovered from the sheeted dike complex (SDC) of the Pacific crust drilled in Hole 1256D. Microstructures and crosscutting relationships within and around the chilled margins led the structure team to postulate that the fracturing and hydrothermal alteration of the chilled margins occurred more or less during dike intrusion. If so, the microstructures are evidence for weakening of the crust by elevated fluid pressure (Harper and Tartarroti, 1996;Umino et al., 2008) and provide a mechanism for the generation of hydrothermal plumes at the seafloor by diking (Delaney et al., 1998).The purpose of this data report is to summarize shipboard observations of the SDC chilled margins and to present new, higher resolution data on the microstructures, mineralogy, and compositions of the dike margins. We review shipboard hand sample and thin section studies and present new backscatter electron images (BSEI)

Magma flow directions in the sheeted dike complex at superfast spreading mid-ocean ridges: Insights from IODP Hole 1256D, Eastern Pacific

“…Nevertheless, we used the direction of K max axes as an indicator of flow direction, particularly where these were measured close to the chilled margins, similar to previous studies despite the differences in the shape of the AMS ellipsoid. We used the orientation of K max axes to track the paleo‐flow direction because we noted that the long axes of silicate minerals are subparallel to the chilled margins in most instances (Figure ) [ Hayman et al ., ]. This arrangement of silicate minerals provided a framework on which opaque minerals grew, effectively tracking the direction of magma flow either as a primary feature preserved during dike intrusion or as an indirect indicator of flow [ Stacey , ; Hargraves et al ., ].…”

“…Inspection of cores and thin sections revealed the presence of several Fe-rich opaque mineral phases in the SDC, corresponding mostly to (titano)magnetite and minor amounts of ilmenite, sphene, pyrite, and rare chalcopyrite [Teagle et al, 2006;Durand et al, 2007;Hayman et al, 2009;Kr asa et al, 2011]. The strong magnetic behavior of (titano)magnetite crystals is expected to exert a major influence on, and largely control, the K-T curves and calculated Curie temperatures.…”

“…Nonetheless, several samples containing chilled margins and igneous contacts were recovered, 25 of which were oriented in the “core reference frame” (the vertical axis is known). Fifteen of these 25 samples contained igneous contacts with dips of 70°–90° and several others with dips <70° to <50° [ Hayman et al ., ]. Additionally, a total of 68 individual dike margins were inferred from Formation MicroScan (FMS) images from 1061 to 1320 mbsf [ Tominaga et al ., ].…”

“…The chilled margins are glassy to cryptocrystalline aphyric basalts that have quenched against cryptocrystalline to fine‐grained, massive basaltic hosts [ Expedition 309 and 312 Scientists , ; Teagle et al ., ; Wilson et al ., ; Hayman et al ., ] (Figure a). Within the SDC, chilled margins were considered boundaries of “igneous units” of different compositions and/or igneous textures [ Expedition 309 and 312 Scientists , ] (Figure ).…”

Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D

“…Secondary minerals along dike margins associated with high-temperature hydrothermal discharge fluids include chlorite ± epidote ± pyrite ± chalcopyrite ± quartz. Low-temperature minerals that may be associated with recharge fluids include anhydrite ± calcium carbonate ± chlorite-smectite ± K-feldspar (Hayman et al, 2009). The downhole distribution of these minerals along dike margins shows no trend with depth, with both recharge and discharge minerals present throughout, although the relative abundance of each mineral varies (Supplementary material D).…”