Leg 209 Summary: Processes in a 20-km-Thick Conductive Boundary Layer beneath the Mid-Atlantic Ridge, 14°–16°N

Oceanic Core Complex Development in Modern and Ancient Oceanic Lithosphere: Gabbro‐Localized versus Peridotite‐Localized Detachment Models

“…In contrast, the detachment-fault surfaces of the Atlantis Bank (SWIR) and Kane (MAR) core complexes are dominated by gabbroic and troctolitic rocks and fault schists (Karson and Lawrence 1997;MacLeod et al 1998;Arai et al 2000;Kinoshita et al 2001;Matsumoto et al 2002), leading to models of core complex development that emphasize moderate levels of magmatism during detachment faulting (Dick et al 1992Cannat et al 1997;Karson 1999;MacLeod et al 2003;Ildefonse et al 2007). However, recent deep drilling of two core complexes thought to have developed during reduced magma supply, the Atlantis Massif and the 15Њ45ЈN, has revealed long sections (∼1400 and ∼200 m, respectively) of gabbroic rocks in the footwalls of both massifs Kelemen et al 2007). The recovery of gabbroic rocks in these drill cores has prompted revision of the amagmatic models of OCC development (Ildefonse et al 2007), leading to new models that suggest that core complexes form when 30%-50% of the total extension is accommodated by magmatic accretion, requiring significant magma intrusion into the footwalls of OCCs during detachment faulting (Tucholke et al 2008).…”

“…A few OCCs have been explored with both seafloor sampling and deep drilling via the Ocean Drilling Program (ODP) or the Integrated Ocean Drilling Program (IODP): the Atlantis Bank, Southwest Indian Ridge (SWIR); the Kane inside corner high, 23Њ30ЈN, Mid-Atlantic Ridge (MAR); the 15Њ45ЈN core complex, MAR; and the Atlantis Massif, MAR (e.g., Karson and Lawrence 1997;Blackman et al 2006;Kelemen et al 2007). Of these core complexes, the Atlantis Bank (SWIR) and Atlantis Massif (MAR) OCCs are the sites of the deepest ODP and IODP drill holes, respectively, and they have been sampled by more submersible dives than any other drilled core complexes ( fig.…”

IODP Expedition 335: Deep Sampling in ODP Hole 1256D

“…1c vs. 1d; Dick, 1989;Cannat et al, 1995Cannat et al, , 2004Cannat et al, , 2009Chen and Phipps Morgan, 1996;Dick et al, 2003;Escartin et al, 2008). Although insights on formation of intrusive crust at detachment-dominated, slow-spread lithosphere have been obtained (e.g., Holes 735B, U1309D, and other sites; ODP Legs 118, 153, 176, 209, and IODP Expeditions 304-305;Dick et al, 2000;Ildefonse et al, 2007;Kelemen et al, 2007), the thermal regime and the melt supply and delivery in these settings differ significantly from those of the axial zone in fast-spreading lithosphere. Detailed understanding of the relatively uniform mechanisms operating at fast spreading ridges would provide a vital benchmark against which heterogeneous accretion on slow-spreading ridges could be compared.…”

Mantle Sulfides and their Role in Re–Os and Pb Isotope Geochronology

“…If estimates of the bulk silicate Earth Pb isotope composition are correct (i.e., broadly chondritic for U, Th, and Pb; e.g., Palme and Jones 2003) and that the U/Pb ratio was set at ~ 4.50 Ga when core formation ended (Kleine and Rudge 2011), then there must be a reservoir of unradiogenic Pb. Observations of Pb isotopes in both bulk-rock peridotite and individual BMS grains suggest that the mantle may represent one of the reservoirs for unradiogenic Pb based upon isotopic measurements of peridotites and their minerals (e.g., Malaviarachchi et al 2008;Burton et al 2012;Warren and Shirey 2012), and predictions based upon trace element depletion (Godard et al 2005;Kelemen et al 2007;Hanghøj et al 2010). …”