The petrogenesis of peridotitic komatiites: Evidence from high-pressure melting experiments

The compositions of liquidus olivines and orthopyroxenes of natural specimens of spinifex- and quench-textured peridotitic komatiites from the Belingwe Greenstone Belt, Rhodesia have been determined for pressures between 10 and 40 kbars. In conjunction with the chemical variation exhibited by these peridotitic komatiites it is concluded that the more magnesian lavas cannot be derived by concentration of olivine phenocrysts, fractional crystallisation or equilibrium partial melting. The peridotitic komatiites could evolve by polybaric assimilation and complete melting of garnet lherzolite into an initial liquid containing about 24% MgO.     

Heat loss from the earth: A constraint on Archaean tectonics from the relation between geothermal gradients and the rate of plate production

The models suggested for the oceanic lithosphere which best predict oceanic heat flow and depth profiles are the constant thickness model and a model in which the lithosphere thickens away from the ridge with a heat source at its base. The latter is considered to be more physically realistic. Such a model, constrained by the observed oceanic heat flow and depth profiles and a temperature at the ridge crest of between 1100°C and 1300°C, requires a heat source at the base of the lithosphere of between 0.5 and 0.9 h.f.u., thermal conductivities for the mantle between 0.005 and 0.0095 cal cm⁻¹ °C⁻¹ s⁻¹ and a coefficient of thermal expansion at 840°C between 4.1 × 10⁻⁵ and 5.1 × 10⁻⁵ °C⁻¹. Plate creation and subduction are calculated to dissipate about 45% of the total earth heat loss for this model. The efficiency of this mechanism of heat loss is shown to be strongly dependent on the magnitude of the basal heat source. A relation is derived for total earth heat loss as a function of the rate of plate creation and the amount of heat transported to the base of plates. The estimated heat transport to the base of the oceanic lithosphere is similar to estimates of mantle heat flow into the base of the continental lithosphere. If this relation existed in the past and if metamorphic conditions in late Archaean high-grade terrains can be used to provide a maximum constraint on equilibrium Archaean continental thermal gradients, heat flow into the base of the lithosphere in the late Archaean must have been less than about 1.2–1.5 h.f.u. The relation between earth heat loss, the rate of plate creation and the rate of heat transport to the base of the lithosphere suggests that a significant proportion of the heat loss in the Archaean must have taken place by the processes of plate creation and subduction. The Archaean plate processes may have involved much more rapid production of plates only slightly thinner than at present.     

Plate tectonics in the Eastern Alps

The weight of the geological evidence, which includes the recognition of a late Cretaceous paired metamorphic belt, suggests that a southward dipping subduction zone existed in the Eastern Alps. On this basis a new plate tectonic model is presented for the post-Palaeozoic evolution of that orogen.     

Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges

Exhumation of the Himalayan-Tibetan orogen is implicated in the marked rise in seawater ⁸⁷Sr/⁸⁶Sr ratios since 40 Ma. However both silicate and carbonate rocks in the Himalaya have elevated ⁸⁷Sr/⁸⁶Sr ratios and there is disagreement as to how much of the ⁸⁷Sr flux is derived from silicate weathering. Most previous studies have used element ratios from bedrock to constrain the proportions of silicate- and carbonate-derived Sr in river waters. Here we use arrays of water compositions sampled from the head waters of the Ganges in the Indian and Nepalese Himalaya to constrain the end-member element ratios. The compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two-component mixing of silicate and carbonate-derived components and lie on a plane in multicomponent composition space. Key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes Na = 0 for carbonate and constant Ca/Na for silicate. The fractions of Sr derived from silicate and carbonate sources are then calculated by mass-balance in Sr-Ca-Mg-Na composition space. Comparison of end-member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low-Mg trace calcite in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation. Alternatively, composition-dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation-ratio diagrams. However the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained. Silicates are calculated to provide ∼50% of the dissolved Sr flux from the head waters of the Ganges assuming that discrepancies between Ca-Mg-Na covariation and the silicate rock compositions arise from addition of trace calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent secondary calcite deposition, this estimate would be increased. Moreover, when ⁸⁷Sr/⁸⁶Sr ratios of the Sr inputs are considered, silicate Sr is responsible for 70 ± 16% (1σ) of the ⁸⁷Sr flux forcing changes in seawater Sr-isotopic composition. Since earlier studies predict that silicate weathering generates as little as 20% of the total Sr flux in Himalayan river systems, this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated.     

Early Himalayan exhumation: Isotopic constraints from the Indian foreland basin

Nd- and Sr-isotopic compositions of Palaeogene foreland basin sediments are used to provide insights into early Himalayan evolution, particularly the timing of exposure of high ⁸⁷Sr/⁸⁶Sr units, erosion of which may have caused the late Tertiary increase in oceanic Sr-isotopic ratios. During the late Palaeocene–early Eocene, erosion was from mixed sources including suture zone rocks. Exhumation of the High Himalaya was occurring by the time of deposition of alluvial sediments after mid-Oligocene times and this source has dominated Himalayan sediments from at least this time until the present day. The transition is interpreted to reflect exhumation of 'basement rocks' of the Indian plate, when the High Himalaya became a sufficient topographic barrier to separate suture zone rocks from the foreland basin. The marked rise in seawater ⁸⁷Sr/⁸⁶Sr from 40 Ma is consistent with the erosion of a Himalayan source with a high ⁸⁷Sr/⁸⁶Sr ratio.     

The correlation of reaction and isotope fronts and the mechanism of metamorphic fluid flow

 Infiltration of a metabasite sill from Islay, Scotland by an H₂O-CO₂ fluid caused (1) modification of δ¹⁸O and (2) carbonation at the sill margins. Maps of δ¹⁸O and reaction progress were constructed from a 20 × 47.7 metre sample grid across the sill. The grid consisted of 300 samples, spaced at m, dm and cm intervals, many of which were analysed for both δ¹⁸O and reaction progress. The δ¹⁸O was determined by laser fluorination of whole rock silicate powders and reaction progress was determined by rapid field-based measurement of % calcite (“fizz-o-meter”, Skelton et al. 1995). Reaction and isotope fronts outlined tube-like features that emanate from the sill margin and discrete nodes that, although detached from the sill margin in two dimensions, are thought to represent sections through similar tubes in three dimensions. We envisage that these protrusions are the fossil record of metamorphic “fluid pathways” whereby fluid permeated the sill. Isotope and reaction fronts are found to correlate spatially as predicted by a modified form of the chromatographic equation which describes this envisaged geometry, that is where isotopic and reactive transport in the fluid phase are facilitated by advection along specific fluid pathways and transverse diffusion in the surrounding rock. These fluid pathways consist of bundles of anastomosing grain boundary channels or micro-cracks, which are thought to propagate through transient cyclic infiltration, reaction, porosity enhancement and fracturing. This mechanism is self-perpetuating and accentuates random perturbations at the sill margin to form the observed tubes. We argue that this is the earliest stage of the infiltration process which has affected metabasites of the SW Scottish Highlands and that subsequent shear deformation of the reacted rims of these pathways, has caused their re-orientation and juxtaposition to form the reacted sill margins described by Skelton et al. (1995). Received: 17 February 1998 / Accepted: 6 December 1999     

A 3500 Ma plutonic and volcanic calc-alkaline province in the Archaean East Pilbara Block

Variably foliated, predominantly granodioritic plutonic rocks from the northern part of the Shaw Batholith in the east Pilbara Archaean craton are dated at 3,499±22 Ma (2σ errors) by a whole-rock Pb-Pb isochron. These rocks intrude the surrounding greenstone sequence, and their age is indistinguishable from that sequence. High strain grey gneisses which occupy much of the western and southern Shaw Batholith are chemically and isotopically similar to the North Shaw suite and are inferred to have been derived from this suite by tectonic processes. Felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc-alkaline volcanic and plutonic province at ～3,500 Ma. This volcanic and plutonic suite is similar to modern calc-alkaline suites on the basis of major element, rare earh element and most other trace element contents. The Archaean suite contrasts with modern equivalents only in having lower concentrations of HREE and higher concentrations of Ni and Cr. The average composition of the North Shaw suite is similar to that of Archaean gneiss belts for most elements and is consistent with the previously formulated hypothesis that the Shaw Batholith is transitional to the upper crustal level of a high-grade gneiss belt. Enrichment of the gneissic crust in the Shaw Batholith in alkali and heat-producing elements is inferred to have taken place by both igneous and hydrothermal processes over a protracted time interval. Late- and post-tectonic adamellite and granite melts intrude the gneissic rocks and there is isotopic evidence consistent with the gneisses being substantially enriched in Rb by pegmatite injection at ～3,000 Ma.     

An experimental evaluation of cleaning methods for fossil ostracod Mg/Ca and Sr/Ca determination

To evaluate the effect of sample preparation on the trace element composition of ostracod shells, fossil shells subjected to various cleaning steps were analyzed by ICP-AES. The variations in ostracod Mg/Ca ratios reveal that fossil ostracod samples cleaned by ultrasonic methanol yield lower values than those by other methods, including increasingly rigorous reductive cleaning. In our sample sets, shell Mg/Ca decreases ∼ ∼1.5 mmol/mol during clay removal. Sample Sr/Ca results do not seem to vary significantly under the equivalent cleaning steps. For ostracod analysis, pre-treatment using multiple methanol ultrasonic cleaning is suggested to be used for gaining an appropriate result.     

Determination of time-integrated metamorphic fluid fluxes from the reaction progress of multivariant assemblages

An expression is derived for the calculation of time-integrated metamorphic fluid fluxes in two or more dimensions in rocks undergoing multivariant reactions under conditions of varying pressure, temperature and angle of flow. This calcuation requires knowledge of mineral assemblages, modes and compositions, which are obtained from isobaric T-X _CO2 pseudosections constructed using the program THERMOCALC and compared with those observed in east central Vermont. THERMOCALC is capable of reproducing peak mineral assemblages, modes, compositions and the observed reaction sequences within the system KCaNaFMASCH for two kyanite grade pelitic carbonate rocks from a Barrovian style regional metamorphic terrain in east central Vermont, U.S.A. Calculation of fluid fluxes for decarbonation reactions under conditions of horizontal, layer-parallel flow produces time-integrated fluid flux figures of the order of 10⁸ moles m⁻². Allowance for possible cross-layer flow from adjacent dehydrating pelites reduces this figure significantly, with episodic cross-layer fluxes of the order of 10⁵ moles m⁻² being capable of driving the observed decarbonation. Chlorite bearing carbonate protoliths would have initially dehydrated with increasing temperature, a process requiring down-temperature fluid flow to produce the assemblages currently observed. Received: 28 January 1998 / Accepted: 20 September 1998