Annotated record and chemical analyses of a polygenetic manganese nodule (slab) from the Scott Plateau, Indian Ocean (core VA-16-13KD)

A large manganese nodule (manganese slab) was dredged from 2100 m on the Scott Plateau by R.V. Valdivia in 1977. It is an irregular ellipsoid, with a maximum dimension of 28 cm, parallel to the sea floor. Chemical analyses show that Mn and Fe proportions are comparable, and total Ni + Cu + Co content averages 0.7%. The nodule has a complex growth history which started with radial upward growth leading to coalescing into a continuous crust. The crust was coated with horizontal layers. After fracturing and infilling of cracks with calcareous sediment, further layers encased the nodule.

Trace element geochemistry of zircons frim in situ ocean lithoshere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ~ 0.7, a SiO2 ~ 1.0, pressure ~ 2 kbar); intra-sample variation is typically ~60-15°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680-740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.

(Table 1) Paleomagnetic of reoriented core pieces of IODP Hole 304-U1309D

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the "rolling-hinge" model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.