Malcolm Hole

A modified Archie's law for two conducting phases

Abstract Many types of mixing model are used widely within the earth sciences to determine the electrical properties of porous media consisting of solid and fluid phases of known conductivities, volume fractions and distributions (i.e. phase connectivities). Most models are valid for two or more conducting phases. However, of the simple models only Archie’s law includes a variable term, the Archie cementation exponent m, that describes the connectivity of the phases. Unfortunately, Archie’s law is only valid for one conducting phase distributed within a non-conducting phase, which makes it inapplicable in instances where the rock matrix has a significant conductivity such as for clay-rich rocks and in calculations involving partial melting. More complex models exist which account for multiple conducting phases and control over phase conductivity. We have adapted the conventional Archie’s law to produce a simple modified Archie’s law that can be used with two conducting phases of any conductivity and any volume fraction, while retaining the ability to model variable connectivities within those phases that result from variations in their distribution. The modified model has two exponents (m and p) that describe the connectivity of each of the two phases. The exponents are related through an equation that depends also on the volume fractions of the two phases. The modified and the conventional versions of Archie’s law have been tested on a granular analogue porous medium with a conducting matrix and a pore space saturated with a range of saline fluids with different salinities and conductivities. The new model describes the experimentally determined electrical behaviour of the system extremely well, improving greatly on the conventional Archie’s law.

Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic

Abstract This work provides new palinspastic palaeofacies reconstructions of SW Gondwana incorporating rotation of a Falkland/Malvinas microplate. We discuss the implications of this for the tectonic evolution of the southern South Atlantic and hence for the regional hydrocarbon potential. Existing Gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents. In most continental reconstructions, the Falkland/Malvinas Plateau was assumed to be a rigid fragment of pre-Permian South American crust. However, it has been suggested, on the basis of palaeomagnetic data, that the Falkland/Malvinas Islands were rotated by ∼180° after 190 Ma. This rotation hypothesis has been successfully tested on the basis of Devonian stratigraphy and palaeontology, Permian stratigraphy and sedimentology and Late Palaeozoic and Early Mesozoic structure, making it unlikely that the plateau behaved as a rigid structure during breakup. We have explored the consequences of accepting this hypothesis for the tectonic evolution of SW Gondwana by compiling new palaeogeographic maps for the Permian–Cretaceous of the southern Atlantic area. To achieve a realistic close fit, we have devised a pre-rift proxy for the ocean–continent boundary for the South Atlantic. In order to produce the best fit, it is necessary to subdivide South America into four plates. The consequences of this are far-reaching. Our work suggests that although sedimentary basins were initiated at different times, three major tectonic phases can be recognised; in regional terms these can be thought of as pre-, syn- and post-rift. During the pre-rift time (until the Late Triassic), the area was dominated by compressional tectonism and formed part of the Gondwana foreland. The Falkland/Malvinas Islands lay east of Africa, the Falkland/Malvinas Plateau was ∼33% shorter and Patagonia was displaced east with respect to the rest of South America, in part along the line of the Gastre Fault System. Potential source facies are dominantly post-glacial black shales of Late Permian age deposited in lacustrine or hyposaline marine environments; these rocks would also be an effective regional seal. Sandstones deposited in the Late Permian would be dominantly volcaniclastic with poor reservoir qualities; Triassic sandstones tend to be more mature. There was significant extension from about 210 Ma (end-Triassic) until the South Atlantic opened at about 130 Ma (Early Cretaceous). In the early syn-rift phase, extension was accompanied by strike-slip faulting and block rotation; later extension was accompanied by extrusion of large volumes of lava. Early opening of the South Atlantic was oblique, which created basins at high angle to the trend of the ocean on the Argentine margin, and resulted in microplate rotation in NE Brazil. Intermittent physical barriers controlled deposition of Upper Jurassic–Cretaceous anoxic sediments during breakup; some of these mudrock units are effective seals with likely regional extent. During crustal reorganisation, clastic sediments changed from a uniform volcaniclastic provenance to local derivation, with variable reservoir quality. In the late rift and early post-rift phase, continental extension changed from oblique to normal and basins developed parallel to the continental margins of the South Atlantic. This change coincides with the main rifting in the Equatorial basins of Brazil and the early impact of the Santa Helena Plume. It resulted in widespread development of unconformities, the abandonment of the Reconcavo–Tucano–Jatoba rift and the end of NE Brazil plate rotation, which remained attached to South America. There was extensive deposition of evaporites, concentrated in (but not restricted to) the area north of the Rio Grande Rise/Walvis Ridge. Widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements. Our main conclusion is that the regional hydrocarbon potential of the southern South Atlantic has been constrained by the tectonic evolution.

Tectonic controls on the geochemical composition of Cenozoic, mafic alkaline volcanic rocks from West Antarctica

Cenozoic, mafic alkaline volcanic rocks throughout West Antarctica (WA) occupy diverse tectonic environments. On the Antarctic Peninsula (AP), late Miocene-Pleistocene (7 to <1 Ma) alkaline basaltic rocks were erupted <1 to 45 million years after subduction ceased along the Pacific margin of the AP. In Marie Byrd Land (MBL), by contrast, alkaline basaltic volcanism has been semi-continuous from 25–30 Ma to the present, and occurs in the West Antarctic rift system. Together, these Antarctic tectono-magmatic associations are analogous to the Basin and Range, Sierran, and Coast Range batholith provinces. Unlike the western US, however, basaltic rocks throughout WA have uniform geochemical characteristics, with especially narrow ranges in initial87Sr/86Sr (0.7026–0.7035),143Nd/144Nd (0.51286–0.51299), and La/Nb (0.6–1.4) ratios, suggesting very limited liput from “old” subcontinental lithosphere or crustal sources during magma genesis. However, there are significant differences in the relative and absolute abundances of the LILE (large-ionlithophile elements), and these divide WA into two provinces. Basalts from the AP region have unusually high K/Ba and K/Rb ratios (50–140 and 500–1500 respectively) and marked Ba depletion (Ba/Nb=2.5–8.0; Ba ppm 66–320) relative to MBL basalts, which have LILE distributions within the range for OIB (ocean-island basalt) (K/Ba <50, Ba/Nb 5–20). This geochemical contrast is accompanied by a three-fold increase in the age range of volcanic activity and a three orders of magnitude increase in the volume of eruptive products, within MBL. The regional differences in geochemistry, and in the volume and duration of volcanic activity, are best explained by a plume-related origin for MBL basalts, whereas alkaline magmatism in the AP is causally related to slab window formation following the cessation of subduction. Plume activity has alreadybeen proposed to explain tectonic doming and associated spatial patterns of volcanism in MBL. Most MBL geochemical traits are shared by the volcanic rocks of the western Ross Sea, suggesting that a large plume head underlies the West Antarctic rift system. The uniformity of basalt compositions throughout WA and the entire rift system suggest uniformly minimal extension throughout this region during late Cenozoic time. Differences in crustal thicknesses can be explained by early Cenozoic or pre-Cenozoic extension, but restraint on extension is suggested by the size of the region and the implied size of the plume. The c. 95% encirclement of the Antarctic plate by mid-ocean ridges and transforms restrains extension on a regional scale, leading to nonadiabatic plume rise and correspondingly little decompression melting.

Role of subduction-plate boundary forces during the initial stages of Gondwana break-up: evidence from the proto-Pacific margin of Antarctica

Abstract In the West Antarctic sector of Gondwana, early stages of break-up are associated with the large Antarctic-Karoo-Tasman basalt province. Formation of this within-plate province was synchronous with active margin tectonics and development of both a proto-Pacific margin magmatic suite along the Antarctic Peninsula and the extensive Tobífera volcanic suite associated with the Rocas Verdes marginal basin system of southern South America and South Georgia. Extension, concurrent with subduction and oceanward migration of the magmatic focus, resulted in a broad extensional province in a back-arc and intra-arc-setting. High geothermal gradients and basalt underplating caused crustal melting on the east coast of the Antarctic Peninsula and formation of bimodal basalt-rhyolite suites. Large-ion lithophile element enriched initial rifting magmas were succeeded, at least in part of the Rocas Verdes basin, by early drift magmas of transitional chemistry and then by entirely asthenospheric MORB magmas representing lithospheric rupture and sea-floor spreading. A plate interaction model is proposed for the initial stages of Gondwana break-up relating the broad zone of mantle melting to a reduction in subduction-plate boundary forces. The change from Gondwanide compression to lithospheric extension in the Jurassic may be linked to a change from shallow to steeply dipping subduction, and to a slowing of subduction rates caused by a change in plate boundary zone parameters. A possible reduction of compressive boundary stresses may have enabled unconfined, overthickened Permo-Triassic crust to extend because of gravitational instability, thus facilitating break-up. We suggest that break-up was not plume-related, but was due to variations in the regional stress field associated with changing plate-boundary forces. The continental crust was placed under tension with substantial lithospheric thinning and decompression melting of an enriched mantle source forming the broad linear zone of within-plate magmatism. The presence of a plume beneath the Karoo province may have thermally weakened the lithosphere and induced local rifting, contributing to, but not causing the eventual separation of East and West Gondwana.

Integrated whole-rock trace element geochemistry and heavy mineral chemistry studies; aids to the correlation of continental red-bed reservoirs in the Beryl Field, UK North Sea

Correlating continental red-bed successions in the sub-surface is a common problem for the hydrocarbon industry. These successions are typically barren of fauna and often monotonous, leading to non-diagnostic wire-line log signatures. A high-resolution, high precision study of detrital garnet chemistry within Triassic reservoir sandstones from the Beryl Field of the North Sea failed to subdivide the sequence satisfactorily. However, the whole-rock concentrations of immobile trace elements such as Zr, Nb and Cr can be shown to be controlled primarily by the abundances of the heavy minerals zircon, rutile and chrome-spinel, respectively. The chemistry of detrital rutile and chrome spinel varies widely within any one sample, implying that the whole-rock concentrations of Nb and Cr are also a function of the chemistry of these heavy minerals. Having calibrated a type well with a detailed mineralogical and geochemical study, it was possible to correlate between wells using whole-rock geochemical cross-plots.

Trace-element and isotopic characteristics of small-degree melts of the asthenosphere: Evidence from the alkalic basalts of the Antarctic Peninsula

Abstract Miocene-Recent continental alkalic basalts were erupted along the Antarctic Peninsula as a result of decompressional melting of the asthenosphere caused by the formation of slab-windows beneath the continental margin following the cessation of subduction. The basalts appear not to be related to a period of major lithospheric attenuation, nor were they formed as a result of the influence of a mantle plume. They exhibit strong trace-element and isotopic affinities with OIB, Sr- and Nd-isotope compositions ranging from 0.70269 to 0.70343 and 0.512863 to 0.51300, respectively, similar to the composition of HIMU OIB. However, new Pb-isotope analyses show that 206 Pb 204 Pb ratios (18.79–19.28) fall within the range for E-type MORB with Δ8 4 and Δ7 4 varying from −28 to +26 and from +1 to +10, respectively. Δ8 4- values , Sr-isotope ratios and some LILE/HFSE ratios exhibit negative covariations with La n Yb n and Nb/Y ratios implying some control of degree of partial melting on geochemical composition. Nb/U ratios (14–40) are considerably lower than most OIB and MORB. The basalts also have unusually low absolute abundances of Rb and Ba and high K/Ba and K/Rb ratios (50–140 and 400–1500, respectively). Correlated PbSrNd isotope and trace-element behaviour suggests that the asthenosphere from which these basalts were derived was subjected to multiple melt extraction/depletion events. One period of melt extraction was ancient (∼ 1.7 Ga) and similar to that affecting MORB source mantle, and was followed by a more recent (?Mesozoic) event. This more recent event resulted in increased U/Pb, U/Nb and U/Th ratios and further depletion in ultra-incompatible element such as Rb and Ba, causing high K/Rb and K/Ba ratios in the erupted lavas. This implies that the asthenosphere beneath the Antarctic Peninsula is heterogeneous on a small scale. Small-degree melts are capable of sampling geochemically, and possibly mineralogically, distinct mantle domains from larger-degree melts. During larger degrees of partial melting, the scale of melting approaches the scale of heterogeneity and integration of melts from different geochemical domains occurs.

Integrated two-dimensional lithospheric conductivity modelling in the pyrenees using field-scale and laboratory measurements

Abstract Recent magnetotelluric (MT) studies have shown that the lower crust in the Pyrenees contains a high conductivity zone consistent with a subducting continental slab, whose conductivity is 0.33 S/m. Partial melting has been interpreted to be the most plausible explanation for this high conductivity. Here we report a two-dimensional conductivity model of the lithosphere by integrating field-scale and laboratory determinations of the conductivity of continental crustal and mantle rocks. The laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt conductivity when temperature, pressure and lithology are known. Consequently, we have also calculated the density, lithostatic pressure, and several alternative temperature profiles for use in the model from gravity, seismic and thermal field data. These can be used with a prescribed melt fraction to predict the electrical conductivity at depth, which can be compared with the MT conductivity data. Alternatively, the laboratory data can be combined with the MT conductivity data to predict the melt fraction at depth. The primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models; (i) Waff’s model/Hashin–Shtrikman (HS) upper bound, (ii) HS lower bound, (iii) parallel layers, (iv) perpendicular layers, (v) random melt areas, and (vi) a modified Archie’s law that takes account of the presence of two conducting phases. The modelling results indicate that a good match to the MT data can be obtained along the whole profile by the influence of pressure, temperature and the fluid phase with the only exception being the subducted slab, where a minimum of 4.7% melt fraction is necessary to explain the data.

The generation of continental flood basalts by decompression melting of internally heated mantle

Modeled primary magma compositions for flood basalts from the Central Atlantic magmatic province and the Ferrar large igneous province (Antarctica) require mantle potential temperatures ( T P) of 1450 ± 50 °C, with melting taking place over a pressure range of ∼2.0 GPa to <0.5 GPa. This range of T P is consistent with internal warming of mantle insulated by continental lithosphere, and does not necessarily require a mantle plume. The substantial crustal attenuation needed to allow decompression melting was facilitated by plate boundary forces that accompanied continental breakup. However, continental fragmentation and the plate boundary forces necessary to drive low- T P rift-related magmatism may still require the action of hot mantle plumes.

Low-temperature aqueous mobility of the rare-earth elements during sandstone diagenesis

Abstract Diagenetic francolite (carbonate fluor-apatite) occurs as overgrowths on detrital apatite grains in sandstones from the Lower Jurassic Statfjord Formation of the North Sea. The francolite overgrowths are considerably more enriched in the rare-earth elements (up to 1 wt % Ce2O3), Sr and F than the detrital cores. These elements were carried in aqueous solution, probably in the form of complex ligands involving organically sourced carbon and halogens. It is possible that reported aberrant neodymium isotope model ages within the Statfjord Formation are the result of mobilization and relative fractionation of Sm and Nd during diagenesis, rather than a result of changes in provenance.

Large 230Th-excesses in basalts produced by partial melting of spinel lherzolite

Abstract Excesses of 230 Th over 238 U in mid-ocean ridge basalts (MORB) require that the mantle source region preferentially retains U over Th during partial melting. Based on existing mineral-melt partitioning data, 230 Th excesses are widely cited as evidence that partial melting beneath ridges begins within the garnet stability field, at pressures over 2.8 GPa. However, recent experimental and theoretical studies of U–Th partitioning show that melting in the presence of aluminous mantle clinopyroxene may also generate 230 Th -excess. In order to try to distinguish between these models we sought basalts with independent constraints on their depth and extent of partial melting. We report data from alkali basalts from the Antarctic Peninsula whose tectonic setting indicates that they formed by 230 Th -excesses, which would conventionally be ascribed to the involvement of garnet. Instead we show that the trace element signature and isotopic data can be reconciled with partial melting involving residual aluminous–clinopyroxene within the spinel stability field. These Antarctic Peninsula basalts provide the first observational evidence that significant 230 Th -excesses can be produced by partial melting of spinel lherzolite and challenge the perceived importance of garnet in MORB petrogenesis.