Executive summary: "Mantle Frontier" workshop

The workshop on “Reaching the Mantle Frontier: Moho and Beyond� was held at the Broad Branch Road Campus of the Carnegie Institution of Washington on 9–11 September 2010. The workshop attracted seventy-four scientists and engineers from academia and industry in North America, Asia, and Europe.Reaching and sampling the mantle through penetration of the entire oceanic crust and the Mohorovi�ić discontinuity (Moho) has been a longstanding goal of the Earth science community. The Moho is a seismic transition, often sharp, from a region with compressional wave velocities (Vp) less than 7.5 km s-1 to velocities ~8 km s-1. It is interpreted in many tectonic settings, and particularly in tectonic exposures of oceanic lower crust, as the transition from igneous crust to mantle rocks that are the residues of melt extraction. Revealing the in situ geological meaning of the Moho is the heart of the Mohole project. Documenting ocean-crust exchanges and the nature and extent of the subseafloor biosphere have also become integral components of the endeavor. The purpose of the “Mantle Frontier� workshop was to identify key scientific objectives associated with innovative technology solutions along with associated timelines and costs for developments and implementation of this grandchallenge

Recognizing detachment-mode seafloor spreading in the deep geological past.

Large-offset oceanic detachment faults are a characteristic of slow- and ultraslow-spreading ridges, leading to the formation of oceanic core complexes (OCCs) that expose upper mantle and lower crustal rocks on the seafloor. The lithospheric extension accommodated by these structures is now recognized as a fundamentally distinct “detachment-mode” of seafloor spreading compared to classical magmatic accretion. Here we demonstrate a paleomagnetic methodology that allows unequivocal recognition of detachment-mode seafloor spreading in ancient ophiolites and apply this to a potential Jurassic detachment fault system in the Mirdita ophiolite (Albania). We show that footwall and hanging wall blocks either side of an inferred detachment have significantly different magnetizations that can only be explained by relative rotation during seafloor spreading. The style of rotation is shown to be identical to rolling hinge footwall rotation documented recently in OCCs in the Atlantic, confirming that detachment-mode spreading operated at least as far back as the Jurassic

Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge)

Microbathymetry data, in situ observations, and sampling along the 138200N and 138200N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high-angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the alongextension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension-parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 138200N OCC, and gabbro and peridotite at 138300N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 138300N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 138200N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution

Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites : in situ ion probe analysis of primary Os-rich PGM

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 245 (2006): 777-791, doi:10.1016/j.epsl.2006.03.021.In-situ 187Os/188Os ratios are determined on Os-rich platinum-group minerals in podiform chromitites both in the Proterozoic ophiolite, Eastern Desert, Egypt, and in the Phanerozoic Oman ophiolite. Because they have very low Re/Os, these primary minerals reflect the initial 187Os/188Os ratios of their parental magmas. The platinum-group minerals (PGM) in the central Eastern Desert chromitites exhibit sub-chondritic to chondritic 187Os/188Os ratios, 0.1226 on average, which is lower than the primitive upper mantle evolution trend of a comparable age. Those of the southern Eastern Desert chromitites have more radiogenic Os, with supra-chondritic 187Os/188Os ratio of about 0.1293 on average, which could be due to crustal contamination. The three chromitite types in the northern part of the Oman ophiolite are almost indistinguishable in terms of their 187Os/188Os ratios; they have overlapping values ranging from sub-chondritic to supra-chondritic ratios. The PGE-rich, mantle chromitite samples have a wide range of 187Os/188Os ratio from 0.1230 up to 0.1376, with an average of 0.1299. The values of the PGE-poor mantle chromitites overlap in their 187Os/188Os ratios with PGE-rich chromites, but are less variable and have a significantly higher average ratio. The Moho transition zone (MTZ) chromitites are highly variable in the 187Os/188Os ratio, ranging from 0.1208 up to 0.1459. The wide range of 187Os/188Os ratios, from 0.1192 to 0.1459, in platinum-group minerals in Egyptian and Oman ophiolites can be attributed to the diversity of origin of their podiform chromitites. The Os-isotope data combined with spinel chemistry indicate that the way involved in podiform chromitite formation was not substantially different between the Proterozoic ophiolite of Egypt and the Phanerozoic ophiolite in northern Oman. The Os-isotope compositions of the mantle chromitites in the Proterozoic ophiolite of Egypt clearly suggest crustal contamination. The heterogeneity of 187Os/188Os ratios combined with the spinel chemistry and high PGE contents of the PGE-rich chromitite in the Oman ophiolite may give reliable evidence for high degree partial melting at a supra-subduction zone setting. Crustal contamination from the subducted slab, and assimilation of previously altered, lower crustal gabbro, may have contributed to the high Cr# spinel and radiogenic Os characteristics in chromitite formed in the mantle section and along the Moho transition zone, respectively.Kelemen and Hanghøj were supported in this project by US National Science Foundation grants OCE-9819666 and OCE-0118572

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): B07103, doi:10.1029/2010JB007931.Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45° rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge