Roger Hart

The Jamestown Ophiolite Complex, Barberton mountain belt: a section through 3.5 Ga oceanic crust

The mafic to ultramafic rocks of the Barberton greenstone belt, South Africa, form a pseudostratigraphy comparable to that of Phanerozoic ophiolites. This Archaean complex, referred to here as the Jamestown Ophiolite Complex, consists of a high temperature tectono-metamorphic peridotite overlain by an intrusive extrusive igneous section, which in turn is capped by a chert-shale sequence. There is a complete range from komatiitic to tholeiitic compositions within single intrusive units. Crustal contamination and magma mixing is evident from field and geochemical data. Pillow structures, 40Ar/39Ar ages and oxygen isotope analysis suggest that hydrothermal interaction with the Archaean ocean severely hydrated and chemically altered the entire simatic section during its formation. As a consequence, only a ‘ghost’ igneous geochemistry is preserved. This regional open-system alteration may have increased the MgO content of the igneous rocks by as much as 13%, and the most primitive liquids, from which the extrusive sequence evolved, were ‘picritic’ in character. Rocks with a komatiitic chemistry were derived during crystal accumulation from picritic-crystal mushes (predominantly olivine-clinopyroxene) and/or by metasomatism during one or more subsequent episodes of hydration-dehydration. In contrast to Phanerozoic ophiolites, the Jamestown complex is relatively thin (≦3 km), which implies that locally at least the ca 3.5 Ga oceanic crust was also thin. This is consistent with the regionally extensive metasomatic alteration, and is compatible with theoretical and experimental models predicting higher Archaean heat transfer from the mantle concentrated within Archaean oceans.

Earth's earliest continental lithosphere, hydrothermal flux and crustal recycling

The Kaapvaal craton in southern Africa and the Pilbara craton of northwestern Australia are the largest regions on Earth to have retained relatively pristine mid-Archaean rocks (3.0–4.0 Ga). The Kaapvaal craton covers about 1.2×106 km2, and varies in lithospheric thickness between 170 and 350 km. At surface, the craton can be subdivided into a number of Archaean sub-domains; some of the subdomains are also well defined at depth, and local variations in tomography of the lithosphere correspond closely with subdomain boundaries at surface. The Archaean history of the Kaapvaal craton spans about 1 Gyr and can be conveniently subdivided into two periods, each of about the same length as the Phanerozoic. The first period, from circa 3.7-3.1 Ga, records the initial separation of the cratonic lithosphere from the asthenosphere, terminating with a major pulse of accretion tectonics between 3.2 and 3.1 Ga, which includes the formation of “paired metamorphic belts”. This period of continental growth can be compared to plate tectonic processes occurring in modern-day oceanic basins. However, the difference is that in the mid-Archaean, these oceanic processes appear to have occurred in shallower water depths than the modern ocean basins. The second period, from circa 3.1-2.6 Ga, records intra-continental and continental-edge processes: continental growth during this period occurred predominantly through a combination of tectonic accretion of crustal fragments and subduction-related igneous processes, in much the same way as has been documented along the margins of the Pacific and Tethys oceans since the Mesozoic. The intra-oceanic processes resulted in small, but deep-rooted continental nucleii; the first separation of this early continental lithosphere could only have occurred when the mean elevation of mid-oceanicridges sank below sea-level. Substantial recycling of continental lithosphere into the mantle must have occurred during this period of Earth history. During the second period, at least two large continental nucleii amalgamated during collisional processes which, together with internal chemical differentiation processes, created the first stable continental landmass. This landmass, which is known to have been substantially bigger than its present outline, may have been part of the Earths first supercontinent. The oldest known subdomains of the craton include the oceanic-like rocks of the Barberton greenstone belt. The comagmatic mafic-ultramafic rocks (3.48–3.49 Ga) of this belt represent a remnant of very early oceanic-like lithosphere (known as the Jamestown Ophiolite Complex), which was obducted, approximately 45 Ma after its formation, onto a volcanic arc-like terrain by processes similar to those which have emplaced modern ophiolites at convergent margins of Phanerozoic continents. The early metamorphic history, metamorphic mineralogy, oxygen isotope profiles and degree of hydration of the 3.49 Ga Jamestown Ophiolite Complex are similar to present day subseafloor hydrothermal systems. The ratio of ΔMg to ΔSi for hydrothermally altered igneous rocks, both present day and Archaean, are remarkably uniform at −5(±0.9) and the same as that of hydrothermal fluids venting on the present-day East Pacific Rise. This observation suggests that the process of Mg exchange for Si in hydrothermal systems was commonplace throughout Earths history. The chemistry of vent fluids and hydrothermally altered igneous rocks was combined with an inventory of 3He in the mantle to model Earths total hydrothermal flux. An Archaean flux (at 3.5 Ga) of about 10 times present day was accompanied by a correspondingly greater abundance of Mg(OH), SiO2, carbonate and FeMn metasomatic rock types as well as massive sulphides. Assuming a constant column of seawater since the Archaean, the average residence time of seawater in the oceanic crust was 1.65−8.90×105 years in the Archaean. Assuming that 3He and heat are transported from the mantle in silicate melts in uniform proportions, the model stipulates that accretion of oceanic crust decreased from about 3.43−6.5×1017 g/yr to a present-day rate of 0.52−0.8×1017 g/yr, with a drop in heat flow from 1.4−2.6×1020 cal/yr to 2.1−3.2×1019 cal/year. The total amounts of SiO2 and Fe mobilised in marine hydrothermal systems since 3.5 Ga is less than their masses in the present exosphere reservoirs (crust, hydrosphere, atmosphere). The total amounts of Mg, K, CO2, Ca and Mn are greater than their respective masses in exosphere reservoirs; therefore, they must have been recycled into mantle. The total mass of recycled hydrothermal components is small compared to the mass of the mantle. The flux of volatiles in hydrothermal systems is large compared to their volume in the atmosphere suggesting that the CO2 and O2 budgets of the atmosphere have been influenced by hydrothermal processes, especially in the Archaean.

Preferential formation of the atmosphere–sialic crust system from the upper mantle

IN the past decade, extensive studies of the rare gas content of mantle derived rocks have led to several models of Earth degassing, all of which assume that the argon of the atmosphere degassed from the entire mantle. We give evidence here to support the concept of concurrent transfer of elements to the atmosphere and sialic crust preferentially from the upper mantle. This concept accounts for the following observations: (1) the 40Ar concentrations in the rapidly quenched glassy margins of deep sea pillow basalts are too low for many estimates of the K concentration of the mantle1,2; (2) the 40Ar/36Ar ratio of rocks derived from the deep mantle is similar to the atmospheric 40Ar/36Ar ratio3–7; (3) the upper mantle under oceanic ridges is depleted in K and other large-ion-lithophile elements. A schematic representation of the upper mantle depletion process is given in Fig. 1 which shows that partial melting associated with spreading ridges and Benioff zones exclusively in the upper mantle is responsible for the accretion of continents and associated degassing to the atmosphere.

The closed-system approximation for evolution of argon and helium in the mantle, crust and atmosphere

The atmosphere formed by the outgassing of the depleted mantle, leaving a remnant of non-degassed mantle which forms the source of some ocean-island basalts such as Hawaii and Iceland. The 40Ar in the atmosphere degassed from 50% to 90% of the mantle possibly synchronous with sea-floor spreading, ocean-ridge hydrothermal activity and continent formation. The bulk of the degassing ended 1.2–1.8 Ga ago. The similarity of the 40Ar36Ar ratio between the atmosphere and non-degassed mantle suggests both have approximated closed systems. On the other hand, more than 99% of He outgassnd from the mantle has been lost to space from the upper atmosphere. Portions of the oceanic crust and mantle contaminated by atmospheric noble gases are distinguished from non-degassed mantle by this He depletion.