Ian H. Campbell

Stirring and structure in mantle starting plumes

Simple arguments show that ascending thermal plumes will entrain their surroundings as the result of coupling between conduction of heat and laminar stirring driven by the plume motion. In the initial stages of ascent of a plume fed by a continuous buoyancy flux (a starting plume) the plume consists of a large buoyant head followed by a narrow vertical conduit. Laboratory experiments reported here show that the spherical head entrains ambient material as it rises, while the axial conduit carries hot source material to the stagnation point at the cap of the plume, from where it spreads laterally into thin laminae. We develop an analysis of the effects of entrainment on the structure and dynamics of starting plumes. The analysis predicts that under conditions appropriate to the earths mantle large volumes of cooler lower mantle will be stirred into the head of a plume by the time it reaches the top of the mantle if it originates at the core-mantle boundary. The result is a major cooling and enlargement of the head. Source material ascending in the trailing conduit will undergo little contamination or cooling until the conduit is deflected from the vertical by large scale shear associated with plate motion. This plume structure explains the close association of high-temperature melts (komafiites or picrites) with more voluminous, lower temperature basalts in Archaean greenstones and modem continental flood basalt provinces: the picrites can be produced by melting in the hot axial conduit and the basalts from the cooler bulk of the head. More generally, we put forward stirring in plumes as one plausible mechanism contributing to compositional heterogeneity in hotspot melts. The predicted diameter of plume heads originating at the core-mantle boundary is - 1000 km and this is expected to enlarge to -2000 km when the plume collapses beneath the lithosphere. This result is in excellent agreement with the observed extent of volcanism and uplift associated with continental flood volcanism. It also provides support for the hypothesis that at least some plumes originate at the core-mantle boundary.

Synchronism of the Siberian Traps and the Permian-Triassic Boundary

Uranium-lead ages from an ion probe were taken for zircons from the ore-bearing Norilsk I intrusion that is comagmatic with, and intrusive to, the Siberian Traps. These values match, within an experimental error of �4 million years, the dates for zircons extracted from a tuff at the Permian-Triassic (P-Tr) boundary. The results are consistent with the hypothesis that the P-Tr extinction was caused by the Siberian basaltic flood volcanism. It is likely that the eruption of these magmas was accompanied by the injection of large amounts of sulfur dioxide into the upper atmosphere, which may have led to global cooling and to expansion of the polar ice cap. The P-Tr extinction event may have been caused by a combination of acid rain and global cooling as well as rapid and extreme changes in sea level resulting from expansion of the polar ice cap.

Mantle Plumes and Continental Tectonics

Mantle plumes and plate tectonics, the result of two distinct modes of convection within the Earth, operate largely independently. Although plumes are secondary in terms of heat transport, they have probably played an important role in continental geology. A new plume starts with a large spherical head that can cause uplift and flood basalt volcanism, and may be responsible for regional-scale metamorphism or crustal melting and varying amounts of crustal extension. Plume heads are followed by narrow tails that give rise to the familiar hot-spot tracks. The cumulative effect of processes associated with tail volcanism may also significantly affect continental crust.

Interaction of mantle plume heads with the Earth's surface and onset of small‐scale convection

We investigate the behavior of a spherical blob of buoyant fluid as gravity forces it toward either a rigid horizontal boundary or a free surface. The diapir fluid is assumed much less viscous than the ambient fluid. This fundamental problem is the simplest unsteady model relevant to the ascent of hot plumes of buoyant material toward Earths surface or the base of the lithosphere and closely models the heads of starting plumes. As the diapir approaches the boundary, it collapses in the vertical and spreads horizontally while a layer of the surrounding mantle is slowly squeezed out from betweeen the diapir and the surface. Experimental results for the thinning and lateral spreading of the bouyant fluid, and for the thinning of the squeeze layer, are given for both the case of a rigid, nonslip boundary and that of a free surface. These are compared with similarity scaling laws based on a balance between the bouyancy of the diapir and viscous stresses in the diapirs surroundings. We also observe that the squeeze layer can become gravitationally unstable, leading to a bifurcation from convection on the scale of the original plume to convection on scales much smaller than the diapir. The vertical exchange on smaller horizontal scales enables the plume to more rapidly approach the boundary. At the time instability occurs the diapir has spread to roughly twice its initial diameter. Application of these results, and previous results from surface uplift, to the plumes responsible for continental flood basalts is subject to knowledge of the local value of upper mantle viscosity. If this is taken to be 3×1020 Pa s, most uplift takes place ovcer approximately 5 m.y. Eruption of voluminous basalts will not take place until at least 5–10 m.y. after the surface has reached its maximum elevation. If small-scale instabilities do develop within mantle plume heads, they may be an essential mechanism allowing the top of the plume to ascend to the shallow depths required for extensive melting. It may also explain the observation of Hooper (1990) that volcanism in the Deccan and Columbian Plateau begins before the onset of crustal extension.

A two-stage model for the formation of the granite-greenstone terrains of the Kalgoorlie-Norseman area, Western Australia

Abstract Accurate dating of granites and greenstones from the Kalgoorlie-Norseman area shows that there is considerable overlap in time between the formation of granites and greenstones. However, within any given area, the oldest granites are always younger than the oldest greenstones. It is suggested that the long-lived (100 Ma) thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting, leading to the production of granites. The time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust. Anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma. Where this happens the basaltic magma ponds at the base of the crust, supplying additional heat and accelerating the melting process. Eventually the granitic melt layer becomes unstable and rises within the crust. Areas intruded by granite become topographic highs, with topographic lows forming above areas where light melt has been withdrawn from the lower crust. When the topographic highs protrude above sea-level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones. Thus the change from a basalt-dominated lower greenstone succession to a sediment and felsic volcanic-dominated upper whitestone sequence, seen in many granite-greenstone terrains, results from the intrusion of granitic magma into the upper crust. The crust of the Kalgoorlie-Norseman area formed in two distinct stages, separated by a time break of up to 700 Ma. The first stage involved the formation of primitive crust, probably above subduction zones. This primitive crust was characterized by low Rb/Sr and high Sm/Nd ratios. The second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents. This led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas. Melting reset the Rb/Sr, Sm/Nd and U/Pb systems, giving the impression that the second event, which occurred at ∼ 2.7 Ga, was a major crust-forming event. It was, in fact, a crustal reworking process although some basalt was added to the crust at this time. Although the two-stage history of the Kalgoorlie-Norseman area is clearly seen in the lead isotopes, and in zircons, it is not readily discernible from the Rb Sr and Sm Nd systematics, due to mantle-like ratios of those elements in the primitive crust.

A re-evaluation of the olivine-spinel geothermometer

The Irvine olivine-spinel geothermometer, as formulated by Jackson (1969), appears to yield magmatic temperatures when applied to plutonic rocks such as the Stillwater Complex but Evans and Wright (1972) have demonstrated that it gives temperatures in excess of 2,000 ° C when applied to volcanic assemblages. A re-evaluation of the geothermometer has shown that more realistic temperatures can be obtained for volcanic rocks by using a different free energy value of FeCr2O4 in the formulation. The revised geothermometer gives temperatures in the range 1,100–1,300 ° C for samples from Kilauea and 500–800 ° C for basic plutonic rocks from layered intrusions, indicating that Mg and Fe2+ have re-equilibrated at subsolidus temperatures in these intrusions as suggested by Irvine (1965). This theory was tested by heating uncrushed natural samples from layered intrusions to magmatic temperatures for periods ranging from two days to four weeks. The result was a marked increase in the Mg/Fe2+ ratio in the spinels and a decrease in the Mg/Fe2+ ratio in the olivines, confirming that considerable subsolidus re-equilibration had taken place in the unheated samples.

Some problems with the cumulus theory

Abstract There are a number of features in layered intrusions which do not appear to be consistent with the cumulus theory. The most important of these are: (1) Rhythmic layering in plagioclase-pyroxene cumulates from some intrusions (e.g. Jimberlana and Stillwater) may be inverted with the light plagioclase-rich cumulates at the base of the layer and heavy pyroxene-rich cumulates at the top. (2) Hydraulic sorting in rhythmically layered cumulates is often poor, suggesting that chemical and not mechanical processes control the distribution of minerals in this type of layered sequence. (3) The textures of cumulates from the steeply dipping marginal zone of Jimberlana, which cannot have formed by gravity settling, are indistinguishable from those found in the flatly dipping central layers of the intrusion. Similarly, it is difficult to explain the cumulate textures in the ‘overturned’ marginal layered series of Jimberlana. (4) Igneous layering, including cross-bedding, graded bedding and trough banding, has been described in layered sequences with near-vertical primary dips. (5) Calculations and experiments with a centrifuge furnace suggest that plagioclase cannot sink in Fe-rich tholeiitic liquids. (6) The settling of crystals as individual grains assumes that nucleation is homogeneous. This is unlikely. It is probable that heterogeneous and self-nucleation are the dominant nucleation mechanisms during the formation of cumulates. This gives rise to two possibilities. Firstly, that most grains nucleate against pre-existing settling crystals to form composite grains which gravitate to the floor of the chamber. Secondly, that the cumulus grains nucleate in situ at the temporary floor of the magma chamber.

Zircon xenocrysts from the Kambalda volcanics: age constraints and direct evidence for older continental crust below the Kambalda-Norseman greenstones

Abstract The Hangingwall Basalt at Kambalda, Western Australia, contains zircons that have been shown by ion microprobe analyses to have very high U and Th contents and a wide variety of crystallization ages. Nearly all of these zircons certainly are xenocrysts; a few might relate to intrusive veinlets. The age of the youngest xenocrysts, 2693 ± 50Ma(2 σ) , shows that the eruptive age of the basalt cannot exceed 2743 Ma. This confirms that the apparent Sm Nd isochron giving 3200 Ma [1,2] for Kambalda mafic and ultramafic rocks is a mixing-line [2] between unrelated components enriched and depleted in light rare earth elements. Mixing probably occurred at depth by erosion of 3200–3500 Ma old felsic crust from the walls of the HWB conduits. The zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern Yilgarn Block. The latter implies that the Kalgoorlie-Norseman greenstone sequences were formed in a continental rather than an oceanic environment.

Two ages of porphyry intrusion resolved for the super-giant Chuquicamata copper deposit of northern Chile by ELA-ICP-MS and SHRIMP

Zircon U-Pb ages measured in situ by excimer laser ablation–inductively coupled plasma–mass spectrometry (ELA-ICP-MS) and verified by sensitive high-resolution ion microprobe (SHRIMP) on ore-bearing felsic porphyries from the Chuquicamata porphyry copper-molybdenum deposit, northern Chile, identify two discrete igneous events. The volumetrically dominant East porphyry has an age of 34.6 ± 0.2 Ma, whereas the Bench and West porphyries yield ages of 33.3 ± 0.3 Ma and 33.5 ± 0.2 Ma, respectively. The age of the East porphyry is indistinguishable from a Re-Os age for early molybdenite mineralization (35 Ma) and the oldest reported 40 Ar- 39 Ar ages for hydrothermal alteration, confirming a genetic link with mineralization. Previous geological studies and 40 Ar- 39 Ar and Re-Os geochronology identify two main hydrothermal events: high-temperature potassic alteration with chalcopyrite at 33.4 ± 0.3 Ma followed by lower temperature quartz- sericite alteration with pyrite at 31.1 ± 0.3 Ma. The ages of the West and Bench porphyries match the ages for potassic alteration. Younger quartz-sericite alteration may reflect an additional fourth intrusion concealed at depth. The anomalously large size of Chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic-hydrothermal systems.

The effects of temperature, oxygen fugacity and melt composition on the behaviour of chromium in basic and ultrabasic melts

Chromite was equilibrated with two natural basic liquids and one natural ultrabasic liquid at temperatures and oxygen fugacities appropriate to geological conditions. The experiments were designed to document changes in mineral and glass compositions between the iron-wustite and nickel-nickel oxide buffers, with special emphasis on conditions along quartz-fayalite-magnetite. The Cr contents of the melts at chromite saturation increase strongly with increasing temperature and with decreasing oxygen fugacity. A relationship is described which accounts for the compositional dependence of the partitioning of Cr between spinels and silicate melts by considering the exchange of FeCr2O4 component between the crystalline and melt phases. Interpretation of the data in terms of this exchange suggests that Cr3+ in metaluminous melts occurs in octahedrally coordinated sites, and that it does not depend on charge-balancing by monovalent cations. In this model, Cr3+ is proposed to behave like network-modifying Al3+ and Fe3+, i.e., the excess aluminum and ferric iron which do not participate in tetrahedrally coordinated matrix or network-forming complexes. The results can also be applied to the problem of the formation of massive chromitites of great lateral extent in basic layered intrusions. The data are consistent with a model in which the crystallization of chromite is initiated through magma mixing, in combination with the rapid heat loss associated with periodic influxes of magma into a chamber. An alternative model, in which chromite crystallization is initiated by repeated fluctuations in oxygen fugacity, is possible only if the magma fO2 is not controlled by an oxygen buffer such as QFM.