Michael J. Bickle

Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites

The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to Fo93.6, implying eruption at around 1520°C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to Fo94.1, implying eruption at 1560°C. However, inferences of erupted liquids containing 32–33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580°C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18–20% MgO and erupted at circa 1400°C. If 1580°C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200°C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900°C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300°C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200–300°C temperature difference between present day jets and ambient mantle temperatures. An important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions.

Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya

Nd and Sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences. The potential of the technique is illustrated by this isotopic study of the Main Central thrust system of the Himalaya. Nd isotope data from the Garhwal Himalaya indicate that metasedimentary rocks from the Vaikrita Group (Nd = –14 to –19) correlate closely with those from the High Himalayan Crystalline Series, which constitutes the hanging-wall lithologies of the Main Central thrust. In contrast, metasedimentary rocks from the Munsiari Group (Nd = –23 to –28) show marked similarities to the Lesser Himalayan Series in the footwall of the Main Central thrust. Sr isotopes support the correlations in that the Vaikrita Group shows partial reequilibration at 500 Ma, whereas the Munsiari Group has not undergone Sr isotope homogenization since 1800 Ma. Thus, the Vaikrita thrust that juxtaposes these two formations is recognized as the Main Central thrust in Garhwal Himalaya. The thrust coincides, approximately, with the location of the kyanite isograd, confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the Main Central thrust. Along the Tons thrust (known locally as the Srinagar thrust) 50 km south of the Main Central thrust, low-grade quartzarenites with Nd-Sr isotope and trace element characteristics typical of Lesser Himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the High Himalayan Crystalline Series. The field relationships most probably result from out-of-sequence thrusting in which Lesser Himalayan Series rocks to the north were emplaced over low-grade equivalents of the High Himalayan Crystalline Series preserved in the external part of the orogen. This study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts.

Erosion and Exhumation in the Himalaya from cosmogenic isotope inventories of river sediments

The outward erosional flux is a key factor in the tectonic evolution of mountain belts and there is much debate about the feedbacks between tectonics, erosion and climate. Here we use cosmogenic nuclides (10Be and 26Al) analysed in quartz from river sediments from the Upper Ganges catchment to make the first direct measurements of large-scale erosion rates in a rapidly uplifting mountain belt. The erosion rates are highest in the High Himalaya at 2.7±0.3 mm/yr (1σ errors), fall to 1.2±0.1 mm/yr on the southern edge of the Tibetan Plateau and are 0.8±0.3 to <0.6 mm/yr in the foothills to the south of the high mountains. These relative estimates are corroborated by the Nd isotopic mass balance of the river sediment. Analysis of sediment from an abandoned terrace suggests that similar erosion rates have been maintained for at least the last few thousand years. The data presented here, along with data recently published for European river catchments, demonstrate that a log–linear relationship between relief and erosion rate holds over three orders of magnitude variation in erosion rate and between very different climatic and tectonic regimes. The erosion rate estimates from cosmogenic nuclides correlate well with exhumation rates calculated from previously published apatite fission track ages in the Indian Himalaya. This confirms that much of the exhumation in the Himalayan mountain chain is now balanced by erosion. However, exhumation rates calculated from high blocking temperature systems, such as 40Ar/39Ar in muscovite, imply lower exhumation rates. Rocks presently at the surface must have undergone a three- to six-fold increase in exhumation rate within the last few million years. We show how this could be explained either by climatic forcing of erosion rate changes or by tectonics. Published evidence for equally rapid changes of exhumation rate in the past and the probable diachroneity in the time at which the present exhumation rates accelerated imply that tectonics has moderated at least some of the change in exhumation rates.

Archean Greenstone Belts Are Not Oceanic Crust

Archean greenstone belts with a preponderance of mafic volcanic rocks, often preserved in tectonically complex sequences, are obvious candidates in the search for remnants of Archean ocean crust. We review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as Archean ophiolites and conclude, on the basis of basal unconformities, presence of xenocryst zircons, geochemical and isotopic evidence for crustal contamination, intrusive relationships with older basement and their internal stratigraphy, that none of these examples is derived from Archean oceanic crust. Thermal, tectonic, and isostatic constraints imply that Archean oceanic basins did exist and were covered with several kilometers of water. We consider it possible, indeed probable, that relict Archean oceanic crust is preserved in granite-greenstone terrains or other Archean tectonic settings but that an unequivocal Archean ophiolite has not yet been recognized.

Implications of melting for stabilisation of the lithosphere and heat loss in the Archaean

Abstract The increased depth and volume of melting induced in a higher temperature Archaean mantle controls the stability of the lithosphere, heat loss rates and the thickness of the oceanic crust. The relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions. The density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200°C or more hotter than modern upper mantle. If Archaean continental crust formed initially above oceanic lithosphere, the compositional density gradients may be sufficient to preserve a thick Archaean continental lithosphere within which the Archaean age diamonds are preserved. The amount of heat advected by melts at mid-ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases. Archaean tectonics could have been dominated by spreading rates 2–3 times greater than now and with mantle temperatures between ca. 1600°C and 1800°C at the depth of the solidus. Mid-ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form, protected from copious volcanism and high mantle temperatures.

Constraints on the exhumation and erosion of the High Himalayan Slab, NW India, from foreland basin deposits

Petrography, Sr–Nd isotope compositions and single-grain laser 40Ar–39Ar ages of detrital white mica from Early–Middle Miocene molasse of the Dharamsala and Lower Siwalik Formations of Northern India, dated by magnetostratigraphy, determine the sediment sources, their metamorphic grade and exhumation rates in the Himalayan palaeo-hinterland. Deposition of the Lower Dharamsala Member (21–17 Ma) occurred during the period of rapid isothermal decompression and crustal anatexis (24–18 Ma) of the metamorphic core. This episode of decompression is thought to be coeval with thrusting on the Main Central Thrust and normal faulting on the South Tibetan Detachment System. The sediment composition and detrital mica ages indicate erosion from the rapidly exhumed metamorphic slab of the High Himalayan Crystalline Series. Deposition of the Upper Dharamsala Member (17–13 Ma) and basal Siwalik Group (13–12.5 Ma) spanned the period in which thrusting transferred south from the Main Central Thrust. The sediment composition and detrital mica ages contrast strongly with those of the Lower Dharamsala, indicating erosion from sedimentary and low grade rocks. The isotopic composition indicates that these rocks were part of the High Himalayan Series unaffected by Tertiary metamorphism, i.e. from upper structural levels of the High Himalayan Slab. This suggests that a major reorganisation of the orogenic wedge occurred at 17 Ma involving forward propagation of the MCT and cessation of rapid exhumation of the metamorphic slab.

Strontium alteration in the Troodos ophiolite: implications for fluid fluxes and geochemical transport in mid-ocean ridge hydrothermal systems

New and published strontium isotope analyses from the Troodos ophiolite constrain fluid-solid exchange processes, and the magnitude and circulation paths of the hydrothermal fluids. The87Sr/86Sr profile reflects alteration in the recharge zone of an evolving hydrothermal system. Fluid-rock strontium isotope exchange in the upper ∼ 1.5 km of extrusive lavas was kinetically limited and seawater-derived fluids emitted from the base of this zone were buffered to87Sr/86Sr ratios between ∼ 0.7047 and 0.7059. In contrast, over the next ∼ 1 km depth interval of sheeted dykes and the uppermost plutonics,87Sr/86Sr values cluster about0.7054 ± 7 (2σ) and fluid flow is inferred to have been pervasive with near-equilibrium fluid-rock exchange. Quartz-chlorite and epidosite zones, the probable pathways of the concentrated, high-temperature upwelling fluids, have identical87Sr/86Sr ratios to adjacent diabase dykes. On Troodos a time-integrated fluid flux in excess of2.9 × 107 kg m−2 is required to transport the strontium isotope composition of ∼ 0.7054, set in the kinetically controlled exchange zone, through the ∼ 1 km of sheeted dykes and into the zones of concentrated upwelling. The uniformity of the87Sr/86Sr ratios in the diabase sheeted dykes and high-temperature epidosite and quartz-chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation. The inferred minimum time-integrated fluid flux of2.9 × 107 kg m−2 substantially exceeds that of∼ 5 × 106 kg m−2 inferred from thermal models of high temperature circulation, but is comparable with estimates of the hydrothermal flux from oceanic budgets of3He, Mg and87Sr. The high flux estimate for Troodos is consistent with the ophiolite venting fluids, with87Sr/86Sr elevated significantly above rock values, which contrasts with the near-MORB87Sr/86Sr ratios of fluids from active high-temperature vents at mid-ocean ridges and the87Sr/86Sr profile in DSDP hole 504B. This requires that either the modern systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the Troodos ophiolite differed from that in “normal” mid-ocean ridges and oceanic strontium budgets are balanced by another source. The lack of very elevated87Sr/86Sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems.

Advective-diffusive transport of isotopic fronts: An example from Naxos, Greece

Oxygen isotopic compositions on the margins of marble bands from kyanite-staurolite grade Miocene metamorphic rocks on the island of Naxos in the Aegean have been modified by infiltration of a fluid from underlying pelitic rocks. Detailed isotopic profiles across 1 m wide basal boundary layers fit, within error, the curves predicted for combined advective-diffusive modification of an initial step function in composition. This justifies the twin assumptions of constant porosity within the marbles and local (grain-scale) fluid-solid oxygen isotopic equilibrium on which the transport theory is based. The advective displacements give time integrated fluid fluxes of only 0.2 and 1.0 m3/m2 at the two higher grade localities sampled. Such volumes represent only a fraction of the fluid lost by devolatilisation of the underlying pelitic sequence. Flow regimes for two possible porosity structures can be derived from analysis of boundary layer shapes in terms of diffusive broadening and a gravitationally driven fluid flux. These are: (1) flow along a small (≈ 10−6) interconnected porosity along grain edges over a duration of ≈ 0.3Ma to 1Ma, or (2) flow in cracks with a larger porosity (≈ 10−4) but a much shorter time duration of ≈ 103 years. The time scale calculated for crack flow is that during which cracks were open to longitudinal diffusion. This may reflect the cumulative active time for intermittent deformation events.

Thin crust beneath ocean drilling program borehole 735B at the Southwest Indian Ridge

Abstract A wide-angle seismic experiment at the Atlantis II Fracture Zone, Southwest Indian Ridge, together with geochemical analyses of dredged basalt glass samples from a site conjugate to Ocean Drilling Program hole 735B has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735B. The measured Na8 composition of 3.3 ± 0.1 corresponds to a melt thickness of 3 ± 1 km, a result consistent with rare earth element inversions which indicate a melt thickness of between 1.5 and 4.5 km. The seismic crustal thickness to the north and south of the Atlantis Platform (on which hole 735B is located) is 4 ± 1 km, and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty. Beneath hole 735B itself, the Moho is at a depth of 5 ± 1 km beneath the seafloor. The seismic model suggests that, on average, about 1 km of upper crust has been unroofed on the Atlantis Platform. However, allowing for the inferred local unroofing of 2 km of upper crust at 735B, the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor, and is underlain by a 2–3 km thick layer of serpentinised mantle peridotite. The P-wave velocity of 6.9 km/s for the serpentinised peridotite layer corresponds to a 35 ± 10 vol% serpentine content. The Moho beneath hole 735B probably represents a serpentinisation front.

Continental Thermal and Tectonic Regimes during the Archaean

Metamorphic pressures estimated from mineral assemblages in Archaean high-grade terrains may be used to constrain geothermal regimes in the Archaean continental crust in two ways. First, they may be combined with temperature estimates to yield bounds on the heat flux during the thickening events that produced the metamorphism; the pressure-temperature data do not require any greater supply of heat to the base of the continental crust in the Archaean than is supplied at the present day. Second, the pressures alone may be used to infer the height of Archaean mountains; these mountains appear to have had elevations comparable with those of present-day mountain belts. It seems probable that the elevation of present-day belts is limited by the creep strength of the lithosphere and, in view of the strong temperature dependence of creep in earth materials, this second observation also implies that Archaean continental thermal regimes were similar to present-day ones. These observations, together with the common interpretation of komatiite melting temperatures as being representative of Archaean upper mantle temperatures, are inconsistent with earth thermal histories based on parameterized convection calculations. The force per unit length of orogenic belt required to maintain the elevation contrasts implied by the pressure estimates is similar in magnitude to present-day driving forces for plate motion.