There are no confirmed microfossils older than 3,500 million years (Myr) on Earth, probably because of the highly metamorphosed nature of the oldest sedimentary rocks 1 . Therefore, studies have focused almost exclusively on chemical traces and primarily on the isotopic composition of carbonaceous material,which has led to controversies regarding the origin of isotopically light reduced carbon 2 . Schists from the approximately 3,700-Myr-old Isua supracrustal belt in southwest Greenland contain up to 8.8 wt% graphitic carbon that is depleted in 13 C, and this depletion has been attributed to biological activity 3,4 . However, because non-biological decarbonation reactions and Fischer-Tropsch-type synthesis 5 can produce reduced Page 1 of 11 carbon with similar isotopic compositions, non-biological interpretations are possible 2 . Apatite with graphite coatings, within iron formations from the ca. 3,830-Myr-old Akilia supracrustal belt in southwest Greenland has been interpreted as the metamorphosed product of biogenic matter 6 , supported by the presence of biologically important heteroatoms within the graphite 7,8 . However, it has been suggested that some graphite in the Akilia iron formations was deposited by metamorphic fluids 8,9 . This latter interpretation is echoed in the Nuvvuagittuq supracrustal belt (NSB) by the presence of poorly crystalline, fluid-deposited graphite that coats apatite 10 , demonstrating that some apatite-graphite occurrences are produced abiotically during fluid remobilization and high-grade metamorphism.The NSB in northeastern Canada represents a fragment of the Earth's primitive mafic oceanic crust.The NSB is composed predominantly of basaltic metavolcanic rocks (Extended Data Fig. 1 Data Fig. 2). The presence of well-preserved, 20-3,000-µm chalcopyrite crystals within the NSB jasper and carbonate iron formations (Extended Data Fig. 3a) demonstrate the lack of postdepositional oxidation.Most NSB rocks were subjected to upper amphibolite-facies metamorphism around 2,700 Myr ago 14,20 . Here we describe parts of the NSB that were less affected by deformation (Supplementary Table 4) and focus on sites where metamorphic grade appears not to have exceeded lower amphibolite facies 17 . This setting is evidenced by local outcrops in the southwestern margins of the belt that preserve primary chert, diagenetic calcite rhombohedra with poikilitic textures, and minerals of low metamorphic grade such as euhedral stilpnomelane and minnesotaite in chert that lack pseudomorphic retrograde textures.

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Mineralogical and geochemical data for ca. 1720 Ma Si-Fe-Mn seafl oor-hydrothermal sedimentary rocks (exhalites) near the Jones Hill Zn-Cu-Pb-Ag-Au volcanogenic massive sulfi de (VMS) deposit, northern NewMexico, provide valuable insights into the redox state of late Paleoproterozoic deep seawater. Distal exhalites ~1200 m south of the deposit form beds 0.5-2 m thick composed of interlayered iron formation and metachert. The iron formation consists mostly of quartz and magnetite, and includes 0.3-3-cm-thick laminae of fi ne-grained garnet-quartz rock, which in places contains as much as 9.4 wt% MnO that resides chiefl y in spessartine-rich garnet (coticule). Shale-normalized rare earth element data for an unaltered, low-Al quartz-magnetite iron formation show no Ce anomaly, which rules out fully oxic deep waters during exhalative mineralization. The garnet-quartz rocks and coticules mostly have small positive Ce anomalies, which are larger for calculated detrital-free compositions, thus precluding deposition in anoxic waters. Signifi cant amounts of ferric iron are inferred for protoliths of the iron formation, based on the presence of abundant magnetite laminae, and of magnetite inclusions in cores of the spessartine garnets. Protoliths of the garnet-quartz rocks and coticules probably consisted largely of clays and Fe-Mn oxyhydroxides. Together these mineralogical and geochemical data suggest that the Jones Hill exhalites were deposited from deep seawater having low concentrations of dissolved O 2 corresponding to suboxic conditions, and not the sulfi dic conditions proposed for late Paleoproterozoic deep seawater by other workers. Exhalites associated with Cu-rich VMS deposits, when effects of alteration and detrital components are considered, can be important proxies for evaluating the evolving redox state of ancient deep oceans. on 23 July 2009 geosphere.gsapubs.org Downloaded from Paleoproterozoic seafl oor-hydrothermal Si-Fe-Mn exhalites Geosphere,

The Trondheim Region ophiolites in the Mid-Norwegian Caledonides represent variably tectonized ophiolite fragments. We present high-precision thermal-ionization mass spectrometry and secondary-ion mass spectrometry (SIMS) U-Pb zircon dates, whole-rock geochemical and Sm-Nd data and Lu-Hf zircon analyses that permit the timing and nature of various stages in the evolution of the ophiolite to be elucidated. Plagiogranite intrusions dated at 487 and 480 Ma have relatively juvenile Nd and Hf isotopic compositions (1 Nd(t) ¼ 6.3, 1 Hf(t) ¼ 8.2-12.4). Geochemical data indicate a subduction-zone influence, suggesting formation in an oceanic back-arc setting. At 481 Ma, a granitoid body with a relatively strong unradiogenic Nd and Hf isotopic composition (1 Nd(t) ¼ 22.6 to 24.0, 1 Hf(t) ¼ 3.8-6.4) and subduction-zone geochemical signature intruded the ophiolite. We interpret this stage to reflect the formation or migration of an oceanic arc above a subduction zone influenced by continentally derived sediments. At c. 475-465 Ma, a greenstone-dominated conglomerate and volcaniclastic sequence was deposited on the eroded ophiolite, indicating obduction between about 480 and 475 Ma. At c. 468-467 Ma, the deformed ophiolite and its sedimentary cover was intruded by trondhjemite dykes and shoshonitic volcanic rocks with intermediate Nd and Hf isotopic compositions (1 Nd(t) ¼ 3.0-3.9, 1 Hf(t) ¼ 4.4-10.2). We interpret this magmatism to reflect subduction-polarity reversal and establishment of a magmatic arc at the continental margin shortly after obduction.Supplementary material: Whole-rock geochemistry, Sm-Nd isotopic data, SHRIMP U-Pb zircon, TIMS U-Pb zircon and Lu-Hf isotopic data are available at http://www.geolsoc.org. uk/SUP18689 A complete ophiolite is a segment of oceanic lithosphere tectonically exposed on land by obduction. The word ophiolite is derived from the Greek word 'ophio' for snake and 'lith' for stone, referring to the commonly green-coloured spilites and serpentinites that make up many ophiolites. Dense oceanic lithosphere usually subducts underneath more buoyant continental lithosphere; however, under certain circumstances relatively small sheets of young, still hot and buoyant oceanic lithosphere can be overthrust on to continental lithosphere. Such occurrences are important, not only for providing access to the earlier history of an ocean basin, which is usually obliterated by subduction, but also because their tectonomagmatic history charts important episodes in the destruction of an ocean.