New Pulse Of Magma Research Articles

Platinum-Group Element Geochemistry and Magma Evolution of the Mount Hagen (Papua New Guinea) Magmatic System

Abstract Mount Hagen is a mid-Pleistocene stratovolcano located in the Papua New Guinea Highlands. It was selected for this study because of (1) its close location to several giant Cu–Au porphyry deposits in the region; (2) its high-K calc-alkaline character (absarokite–shoshonite) and (3) its wide compositional range (2 to 12 wt % MgO). Whole-rock geochemistry, petrography and QEMSCAN observations are consistent with fractional crystallisation of the evolving magma being controlled by the phases now represented by the megacrysts in the lavas: olivine and clinopyroxene at high MgO and hornblende and plagioclase at low MgO. However, the role of plagioclase is enigmatic. Although it is an abundant megacryst phase in the low-MgO samples, plagioclase has little or no influence on fractional crystallisation, suggesting that it may be an antecryst. The appearance of plagioclase megacrysts does not slow the rate of increase in Al2O3 or Sr with decreasing sample MgO and the low-MgO samples do not have Eu anomalies. At ~5.5 wt % MgO, there are significant changes in the system that are inconsistent with simple fractional crystallisation. These changes include hornblende and plagioclase replacing olivine and clinopyroxene as the principal megacryst phases, a sharp rise in platinum-group elements (PGE) concentrations, the brief reappearance of high Fo olivine megacrysts and reversed zoning in the plagioclase megacrysts. These changes are interpreted to have resulted from a new pulse of magma entering the system, with higher water, MgO, SiO2 and PGE concentrations than the original parent magma. Scatter in the PGE data is attributed to the presence of micronuggets in all samples, including the most mafic samples. We conclude that the magma system became sulphide saturated during an early stage in its fractionation history, probably before leaving the crust to deep crustal magma chamber, and then became undersaturated as a consequence of decreasing pressure as magma ascended into a mid-crustal magma chamber. The early saturation episode had a pronounced influence on PGE concentration but had little affect Cu and Au, due to their lower partition coefficient into sulphides. The magma became sulphide saturated again in the crustal magma chamber at ~8.5 wt % MgO, this time affecting all chalcophile elements. Given the early episodes of sulphide saturation, depletion in Au in the Mount Hagen magma system and the absence of a known porphyry system, it is unlikely that Mount Hagen produced economic porphyry mineralisation.

A note on the erosive nature of potholes in the Bushveld Complex

Abstract We describe an impressive ~55 m high outcrop from the Pilanesberg Platinum Mine open pit, located in the North-Western Bushveld Complex. The outcrop exposes the complete two-dimensional structure of three Merensky Unit potholes that cut several metres down into the underlying footwall anorthosites. The transgressive field relationships are interpreted to have resulted from thermochemical erosion of the footwall rocks by new pulses of magma replenishing the chamber and resulting in incremental growth of the Bushveld Complex.

Thermal model of successive dike injections and implications for the development of intraplate volcanoes

Temperatures in the root zones of volcanoes play a critical role in the development and persistence of shallow-level magmatic reservoirs in the crust. Here, we present a 1D thermal model allowing evaluation of the thermal impact of magma travelling in conduits to the surface on the root zone of a volcano. This thermal model has been developed to better understand the formation of a vertical intrusion located in the root zone of a dismembered Miocene volcano on Fuerteventura, Canary Archipelago. This intrusion, named PX1, constitutes an almost pure amalgamation of dikes of either clinopyroxenitic or gabbroic composition. Both types of dikes display cumulate textures and are interpreted as resulting from the protracted crystallization of a mafic magma. The formation of clinopyroxenitic, in contrast to gabbroic dikes, requires that the residual melt was extracted at high temperature (>1050°) to avoid plagioclase crystallization.Simulations of multiple dike injections show that the temperature in the root zone increases significantly with the addition of dikes, but the maximum temperature reached in the system depends on the duration of magma flow in the conduits and the time interval between dike injections (i.e., repose period). Active flow is the critical parameter that distinguishes instantaneous dike injection from a magmatic conduit. Without significant magma flow (>1 month), high-temperature conditions (>1000 °C) cannot be maintained in the pluton unless dikes are very thick and the repose period is extremely small. On the other hand, magma flow times of one to several months, combined with short time intervals between dike injections (<25 years), which are conditions comparable to those recorded for historical eruptions of oceanic island volcanoes, allow the production and preservation of temperatures above the plagioclase liquidus for significant durations, as required to generate clinopyroxenitic dikes such as those observed in the PX1 pluton.Persistent high temperature in the vicinity of magma conduits limits the differentiation of melts in transit to the surface, providing a potential explanation for why lavas of mafic to intermediate composition predominate in intraplate volcanoes such as Fuerteventura or Fogo Island (Cape Verde Archipelago). In extreme cases, when temperatures over 1000–1050 °C in the central part of the feeding zone are maintained for years, the remaining magma in the conduit does not solidify but is preserved in a mushy state. New pulses of magma would not be able to cross this zone but would rather amalgamate in the incipient magma reservoir. The present model differs from previous models of sill intrusion in that magmas do not need to pond at depth to create a reservoir but merely supply heat while travelling to the surface. Depending on the time interval between dike injections and the duration of magma flow through the crust, magma rising in vertical conduits could directly feed the volcanic edifice or could lead to the formation of magma reservoirs. This process may explain why some volcanoes erupt mafic or differentiated magmas during distinct periods of activity.

4D seismic tomography of Nevado del Ruiz Volcano, Colombia, 2000–2016

To track and analyze temporal changes in the volcanic system of Nevado del Ruiz, Colombia, repeated three-dimensional seismic tomography (4D seismic tomography) for P and S wave velocities (Vp and Vs, respectively) during the period 2000–2016 was performed. P and S wave arrival time data were classified on the basis of changes in volcanic activity into five time periods: 2000–2006, 2007–2009, 2010–2012, 2013–2014, and 2015–2016. Our results showed clear changes in 2013–2014 and 2015–2016, periods of heightened volcanic activity after an episode of unrest in 2010. Regions of low Vp, low Vs, and high Vp/Vs (>1.9) were found beneath and northwest of the active crater at depths of 2–3 km below sea level. The enhancement of those anomalies suggests that a new pulse of magma arrived in the volcanic system at that time. Regions with low Vp/Vs values (1.3–1.6) in the upper part of the volcano, at depths from −3 to −2 km, are interpreted as the result of vapor accumulation within highly fractured zones of the hydrothermal system. The ratios of fractional change of Vp and Vs in 2015–2016 suggest that partial melting zones were located beneath the active crater at depths of −2 to 0 km. Our results show that the volcanic system of Nevado del Ruiz has experienced changes associated with the input of new magma, which ascended and interacted with preexisting magma bodies and led to the recent emplacement of a small dome at the bottom of the active crater. We combined our results and evidence from previous studies to propose a model of the Nevado del Ruiz plumbing system from 1985 to 2016.

Trace element geochemistry of magnetite from the giant Beiya gold-polymetallic deposit in Yunnan Province, Southwest China and its implications for the ore forming processes

The Beiya gold–polymetallic deposit is one of the largest gold deposits in China and is considered to be a typical porphyry-skarn system located in the middle of the Jinshajiang–Ailaoshan alkaline porphyry metallogenic belt. Massive magnetite is widespread in the Beiya ore district but its genesis is still the subject of debate. Five representative magnetite types are present in the Beiya deposit, namely magmatic magnetite (M1) from the ore-related porphyry, disseminated magnetite (M2) from the early retrograde alteration, massive magnetite (M3) from the early quartz-magnetite stage, massive magnetite (M4) from the middle quartz-magnetite stage and magnetite (M5) from the late quartz-magnetite stage. Compared with the M1 magnetite, the magnetites from stages M2 to M5 are depleted in Ti, Al and high field strength elements, implying a hydrothermal origin, distinct from the magmatic accessory magnetite in the ore-related porphyry (M1). The concentrations of cobalt in the hydrothermal magnetites decrease gradually from M2 to M5, and can be used to discriminate the magnetite types. The Al+Mn and Ti+V contents of the successively precipitated magnetite grains (M2–M5) suggests that the ore forming temperature decreased from M2 to M4, but increased from M4 to M5, possibly as the result of a new pulse of magma entering the chamber, which may have triggered the gold mineralization. The V content in the hydrothermal magnetite suggests that the oxygen fugacity increased from M2 to M4 but decreased as soon as the sulfides entered the system (M5).

Lithological and geochemical constraints on the magma conduit systems of the Huangshan Ni-Cu sulfide deposit, NW China

Magmatic Ni-Cu sulfide deposits in northern Xinjiang, China, are associated with small mafic-ultramafic complexes, with the sulfide ores generally occurring in ultramafic rocks. The Huangshan deposit (up to 65 Mt of ore at 0.49% Ni and 0.31% Cu), one of the largest magmatic Ni-Cu deposits in northern Xinjiang, is composed of a layered sequence of lower websterite, lower lherzolite, websterite, norite-gabbro, gabbro, diorite, and gabbronorite, with sulfide mineralization mainly found in the lower lherzolite, lower websterite, and websterite. Systematic variations of the major oxides and trace elements suggest that the rocks of the Huangshan deposit are fractionated from the same parental magma, with the sharp contact and discontinuous trends of major oxide contents between different lithologies implying intrusion of four distinct stages of magma from a single deep-seated staging chamber. The reversals in olivine Fo contents and major oxides in the lower lherzolite were the result of inhomogeneity in olivine within the lower chamber. The Se/S ratios (63.1∼150 × 10−6) and the negative correlation between Se/S and δ34S (0.63∼2.42‰) of the sulfide ores suggest that a large contribution of crustal S caused the sulfide segregation. The sulfides in the lower lherzolite have lower Cu contents (1386–2200 ppm) and Cu/Pd ratios (2.31 × 105–1.36 × 106) relative to those in the mineralized lower websterite (Cu = 2300 to 18,700 ppm, and Cu/Pd = 6.65 × 105 to 2.73 × 106). A positive correlation between Pd/Ir and Ni/Ir for the vein-textured sulfides in the lower websterite likely reflects fractionated sulfides picked up by a new pulse of magma. In contrast, the restricted range of Pd/Ir ratios indicates that the PGE contents of the disseminated sulfides in the lower lherzolite resulted from reaction between the sulfides and new pulses of S-undersaturated magma.

PGE geochemical constraints on the origin of the Ni-Cu-PGE sulfide mineralization in the Suoi Cun intrusion, Cao Bang province, Northeastern Vietnam

The Permian (266–262 Ma) Suoi Cun intrusion in the Song Hien Rift Zone (NE Vietnam) consists of a sulfide-bearing mafic-ultramafic unit and a sulfide-free mafic unit. The Cu-Ni-PGE mineralization is represented by disseminated sulfides throughout the sulfide-bearing unit containing ~0.5 wt.% Ni, ~0.05 wt.% Cu, and ~0.2 ppm PGE. The sulfide schlieren have a limited distribution and contain ~2.6 wt.% Ni, ~0.5 wt.% Cu, and ~2.6 ppm PGE. The Suoi Cun rocks containing disseminated sulfides display moderately fractionated mantle-normalized PGE patterns with positive Pd and negative Ru anomalies. In contrast, the sulfide schlieren show enrichment in Ru with lower contents of other PGE except Pd. The low Cu/Pd ratios (1,385–11,529) throughout the intrusion indicate that all sulfides were separated from a PGE-undepleted magma as a result of a single sulfide segregation event. We suggest that sulfides segregated from Mg-rich basaltic magmas in a deep-seated magma chamber due to crustal contamination with country rocks. Then, the sulfide liquid along with early crystallizing olivines and Cr-spinels were pushed out upwards into an upper magma chamber by new pulses of magma. Two processes were important for understanding the PGE distribution: 1) fractionation of the sulfide liquid gave rise to PGE distribution observed in the disseminated ore and, 2) the interaction of oxidized silicate melts with the sulfide liquid was the responsible for the low PGE contents in the sulfide schlieren due to PGE transfer from the oxidized sulfide liquid to the silicate melt.

THE UPPER ZONE OF THE BUSHVELD COMPLEX AT ROOSSENEKAL, SOUTH AFRICA: GEOCHEMICAL STRATIGRAPHY AND EVIDENCE OF MULTIPLE EPISODES OF MAGMA REPLENISHMENT

Drill core from a fence of five boreholes located in the vicinity of Roossenekal, in the eastern limb of the Bushveld Complex, South Africa, provides complete coverage of what is probably one of the thickest and most well-developed sections of the Upper Zone. The lowermost portions of the Rooiberg Group felsites that overlie the Upper Zone, and the uppermost portion of the Upper Main Zone, which forms the footwall, were also intersected. Field relationships from floor-attached domes (eastern limb) and gap areas (western limb) reveal a marked angular unconformity at the base of the Upper Zone. The Upper Zone represents a discrete intrusive event and should not be interpreted as a continuum, or the fractionated residue of the underlying components of the Rustenburg Layered Suite (RLS). A new subdivision of the Upper Zone into five subunits (A through E) is proposed on the basis of the cumulus mineralogy and whole-rock geochemistry, the latter based on samples at an average vertical spacing of 20 m throughout the column at Roossenekal. The separation of subzones A and B is based on our geochemical data, as is the identification of subzone E, but recognition of subzones C (olivine) and D (apatite) is broadly consistent with the detailed mapping and stratigraphic column presented by Von Gruenewaldt (1973). Subunits A through D reveal macro-layering for which the intrusion is so well known. More than 40 individual Ti-magnetite layers were identified in the drill-core, which together with the numerous anorthosite layers, are the most prominent feature of this part of the column. The principal lithology is a somewhat monotonous sequence of ferrogabbro or magnetite-olivine gabbro. The absence of Ti-magnetite layers and anorthosite from subzone E, which is dominated by diorite with <5 modal % Ti-magnetite is significant. The whole-rock geochemical data presented here is not consistent with a closed system fractionation hypothesis for subunits A through D, despite this being widely accepted in recent contributions. An upward differentiation trend is preserved only in subunit E. The incremental build up of cumulates from successive pulse of magma is not only consistent with our views on the underlying layered cumulates (periods of closed system differentiation are rare) but is also consistent with field relationships, including each of the principal limbs representing individual sill-like features. The Upper Zone crystallized from batches of iron-rich tholeiitic magma derived from a deep staging chamber. Magma was injected as near-lateral flows, probably in a southward direction in the eastern limb. The introduction of evolved magma into a shallow crustal environment resulted in the early crystallization of Fe-Ti oxides. Ti-magnetite layers crystallized from oxide-rich slurries intruded near the floor of the chamber. Fe-Ti oxide pipes formed from slurries forced downward in areas of structural weakness. The regular, albeit step-like, upward change in composition of the Ti-magnetite layers identified by Molyneux (1974) and others is ascribed to mixing of crystal-rich magma slurries with increasingly differentiated resident magma: the lowermost layers (and pipes) contain the highest concentrations of V2O5. The spatial association of Ti-magnetite layers and anorthosite is emphasized and we suggest the latter formed as a consequence of disequilibrium partial melting of earlier formed silicate cumulates due to heat associated with new pulses of magma.

On the onset and evolution of the Ross-orogeny magmatism in North Victoria Land — Antarctica

An extensive geochemical (major-, trace-element and oxygen isotope) and geochronological (U/Pb geochronology on zircon) characterisation of Ol-bearing ultramafic–mafic cumulates and gabbro-diorites recently discovered in the southwestern region of the Wilson Terrane (North Victoria Land — Antarctica) was undertaken in order to constrain source characteristics, magma evolution and emplacement history in this sector of the Cambro–Ordovician Ross Orogeny. Textural and chemical data indicate disequilibria among mineral phases of these Ol-bearing cumulate rocks. Corroded Cr-rich clinopyroxene with high Mg# (0.82) and Ca-rich plagioclase likely represent xenocrysts from an old magmatic system, and are not in equilibrium with the host cumulate assemblage (Ol + Opx + Amp + Bt + Pl). The calculated liquid in equilibrium with the xenocrysts (“melt 1”) exhibits exceptionally high La/Yb ratios and Th–U concentrations, suggesting a strong sediment influx in the mantle source, possibly from the subducted slab. Because of the extensive fractional crystallisation (Fo 70; Mg# = 76) and crustal contamination (e.g., δ 18O Opx = 7.13–7.47‰) the trace element composition of the liquid in equilibrium with the cumulate assemblage (“melt 2”) does not reveal the nature of the mantle source not the differentiation processes. The gabbroic parental liquid for the main cumulate assemblage was not produced by assimilation and fractional crystallisation (AFC) of the equilibrium “melt 1” calculated from the xenocrystic paragenesis. Rather, it likely represents a new pulse of magma originated from a different mantle source, which then evolved through AFC into gabbro-diorites. Zircon U–Pb dating of one gabbro-diorite yields a crystallisation age of 489 ± 4 Ma. Zircon in the Ol-bearing cumulates yielded two ages populations at 521 ± 2 Ma and 502 ± 3 Ma, which we interpret to represent the actual age difference between the two magmatic systems. This age difference suggests that poorly differentiated melts with adakite-type signature intruded the crust prior to the generation of the large volumes of gabbroic and dioritic magmas. These early pulses represent up to now the oldest proof of subduction related mantle melts in North Victoria Land and predate the diffuse igneous activity dominated by intermediate- to felsic products.

ENDOGENOUS AND EXOGENOUS PLUTONS: THE INFLUENCE OF EMPLACEMENT STYLE ON CONTAMINATION OF GRANITIC MAGMA

Whether granitic magmas can detach and digest fragments of wallrock or mingle with other magmas depends in large part on the style of magma emplacement. Many composite plutons grow by relatively small pulses of magma, some of which may reach their solidus before injection of the next pulse. Composite “nested” plutons may grow as new pulses of magma invade and displace their predecessors, or “sheeted” plutons may grow as new magma follows contacts between wallrock and older intrusions. Contrary to a common assumption, the innermost parts of a composite intrusion are not necessarily the last to be emplaced or the most fractionated. Emplacement kinematics of some plutons and lava domes are similar. Lava domes grow in two ways. Endogenous domes inflate as new lava ascends and ponds within a cooler lava crust, and exogenous domes accrete as new lava ascends, breaches the lava crust, and spreads over the exterior. Exogenous growth of sheeted plutons offers less opportunity for magma mingling and mixing, but allows more wallrock assimilation (contamination) than endogenous growth of nested plutons.

Geology and geochronology of the Spirit Mountain batholith, southern Nevada: Implications for timescales and physical processes of batholith construction

The Spirit Mountain batholith (SMB) is a ∼ 250 km 2 composite silicic intrusion located within the Colorado River Extensional Corridor in southernmost Nevada. Westward tilting of 40–50° has exposed a cross-section from the roof through deep levels of the batholith. Piecemeal construction is indicated by zircon geochronology, field relations, and elemental geochemistry. Zircon U/Pb data (SHRIMP) demonstrates a ∼ 2 million year (17.4–15.3 Ma) history for the SMB. Individual samples contain zircons with ages that span the lifetime of the batholith, suggesting recycling of extant zircon into new magma pulses. Field relations reveal several distinct intrusive episodes and suggest a common injection geometry of stacked horizontal sheets. The largest unit of the SMB is a gradational section (from roof downward) of high-silica leucogranite through coarse granite into foliated quartz monzonite. Solidification of this unit spans most of the history of the batholith. The 25 km × 2 km leucogranite was emplaced incrementally as subhorizontal sheets over most or all of the history of this section, suggesting repeated fractional crystallization and melt segregation events. The quartz monzonite and coarse granite are interpreted to be cumulate residuum of this fractionation. Age data from throughout this gradational unit show multiple zircon populations within individual samples. Subsequent distinct intrusions that cut this large unit, which include minor populations of zircons that record the ages of earlier events in construction of the batholith, preserve a sheeted, sill-on-sill geometry. We envision the SMB to have been a patchwork of melt-rich, melt-poor, and entirely solid zones throughout its active life. Preservation of intrusion geometries and contacts depended on the consistency of the host rock. Zircons recycled into new pulses of magma document remobilization of previously emplaced crystal mush, suggesting the mechanisms by which evidence for initial construction of the batholith became blurred.

Petrology and Geochemistry of Mafic-Ultramafic Fragments from the Aguablanca Ni-Cu Ore Breccia, Southwest Spain

Aguablanca (southwest Spain) is the first economic Ni-Cu-(PGE) deposit found in southern Europe. Two features make it an unusual example of magmatic sulfide ore: it is related to the development of an Andean-type continental magmatic arc, and it is hosted by a subvertical magmatic breccia. The structural style and the geodynamic context of the deposit contrast with most plutonic Ni-Cu-PGE deposits elsewhere, which occur at specific levels of layered mafic intrusions in rift environments. The Ni-Cu deposit is hosted by the Aguablanca intrusion, a mafic body composed of gabbronorite and minor quartz-diorite, gabbro, and norite. Sulfides are concentrated in a gabbronorite matrix along a subvertical (dip of 70°–80° N), funnel-like magmatic breccia that contains barren or slightly mineralized ultramafic-mafic cumulate fragments. Modal compositions of the fragments reflect a wide variety of rock types, including peridotite (hornblende-rich werhlite, dunite, and hornblende-rich harzburgite), pyroxenite (ortho- and clinopyroxenite), gabbro (gabbro, gabbronorite, and hornblende gabbro), and anorthosite. The primary silicate assemblage includes olivine (Fo91–Fo79), orthopyroxene (Mg no. 0.85–0.73), clinopyroxene (Mg no. 0.93–0.62), plagioclase (An99–An38), amphibole (Mg no. 0.87–0.68) and phlogopite (Mg no. 0.89–0.64). The wide range of rock types and the Fe-enrichment trends in the primary ferromagnesian silicates suggest magmatic differentiation processes from the parent melts, with the fragments representing different stages of cumulate formation. The ore-bearing breccia contains both semimassive and disseminated sulfides in the gabbronorite matrix. Textures vary between meso- and orthocumulate, and the rock-forming magmatic silicates are orthopyroxene (Mg no. 0.83–0.74), clinopyroxene (Mg no. 0.89–0.78), plagioclase (An50-An77), and intercumulus amphibole (Mg no. 0.86–0.70), phlogopite (0.84–0.69) and minor quartz. The gabbronorite in the matrix of the breccia is petrographically and chemically very similar to that of the unmineralized parts of the main Aguablanca intrusion and exhibits a similar differentiation trend, suggesting that the matrix of the ore-bearing breccia and the unmineralized rocks belong to a same magmatic suite. The local presence of mafic-ultramafic fragments in the barren Aguablanca intrusion supports this suggestion. The presence of highly Ni depleted olivine, whole-rock Cu/Zr ratios below 1, and the local occurrence of disseminations of magmatic sulfides in the peridotite fragments point to sulfide segregation before and/or during the formation of the peridotite cumulates. Mantle-normalized incompatible trace element patterns of the fragments along with published sulfur isotope data are consistent with crustal contamination, suggesting that addition of crustal sulfur from pyrite-bearing black slates led to sulfide saturation. These results support a model in which sulfides segregated and settled during the differentiation of an unexposed mafic-ultramafic complex, now sampled as fragments in the breccia, whereas the overlying silicate magma, most probably fed by successive fresh magma injections, underwent fractional crystallization, giving rise to this cumulate sequence. The emplacement of the ore breccia took place at temperatures above the (monosulfide solid solution (mss) solidus but below the olivine and pyroxene solidus, likely owing to the explosive injection of a new pulse of magma into the chamber, which mingled with the sulfide liquid and disrupted the overlying cumulate sequence. As a consequence, fragments reached their current position in the breccia, injected along with the sulfide and the silicate melts, which subsequently formed the sulfide-rich gabbronorite.

The Kap Gustav Holm Tertiary Plutonic Centre, East Greenland

The plutonic centre is exposed on the narrow headland of Kap Gustav Holm in East Greenland. It was emplaced into the hinge zone of a major crustal flexure that developed during initial rifting between Greenland and northwest Europe. The earliest intrusion is a funnel-shaped tholeiitic gabbro that was emplaced into the developing flexure, probably about 55 Ma ago. The intrusion comprises a Basal Contact Zone of gabbro 100m thick, showing syn-plutonic deformation and recrystallization; a Lower Zone of massive olivine gabbro 250–1500 m thick; and an Upper Zone of layered gabbro with a minimum thickness of 500–700 m. All are cut by sheets of gabbro pegmatite. Abrupt reversals in cryptic and phase layering suggest that the gabbro intrusion is the result of several injections of magma. The gabbro intrusion was tilted steeply seawards before emplacement of plutons and sheets of monzonite, syenite and granite about 50 Ma ago. The main monzonite body forms a vertical cylinder 2.5 km in diameter, with saucer-shaped mineral-graded layering and abundant volcanic inclusions. Regular stratigraphic variation in the composition of plagioclase and augite with structural height is interrupted by an abrupt reversal, suggesting emplacement of a new pulse of magma. Syenite forms ring dykes and partial ring dykes of two distinct compositions: an older subalkaline syenite related to the monzonite and gabbro, and a younger mildly peralkaline syenite. Granite forms a small composite pluton. Abundant syn-plutonic bodies of microdioite are associated with monzonite, syenite and granite intrusions.

The generation and shape of feeder dykes from mantle sources

The criterion for fracture propagation suggests that the primary magma generated in the mantle ascends in a stepwise fashion. Initially a pulse of magma forms a dyke above the mantle source and ascends for some distance, whereafter it stops. A subsequent pulse of magma joins the first one, and the ascent of the accumulating body of magma continues for some distance each time a new pulse of magma reaches the body of magma situated in the top of the dyke. In this manner a dyke is formed that may extend either partly or the whole way from the mantle source to the surface. This model may explain the large pressures, up to 2000 bars, that prevail during basaltic eruptions. The calculated shape of the ascending pulses of magma suggests that their heights and widths are similar.

Turbulent mixing between fluids with different viscosities

Fluid dynamic processes associated with the injection of new pulses of magma into magma chambers have received increased attention recently1–3 especially chambers replenished from below with hotter, denser magma of more primitive composition3,4. The results of several laboratory analogue experiments3,4 have shown that the dynamical effects of crystallization following replenishment can be quite different when the two layers have similar, or very different, viscosities. The experiments reported below have examined the effect of viscosity differences on the filling process itself. When a turbulent ‘fountain’ of fluid of low viscosity v1 is injected upwards into a less dense fluid of higher viscosity (v2), the two fluids may mix thoroughly or not at all, depending on the relative magnitudes of an input Reynolds number Re1 = wd/v1 and the viscosity ratio v2/v1 where w is the mean velocity and d is the diameter of the input. The criterion for mixing can be expressed alternatively as wd/v2 > k, a constant, in agreement with a more general theoretical argument which is also outlined here. When applied to magma chambers, our results imply that basaltic magmas will mix readily, but that a basaltic inflow will mix with a silicic host magma only with great difficulty.

A Model for the Origin of the Platinum-Rich Sulfide Horizons in the Bushveld and Stillwater Complexes

The sulfides of the Merensky Reef and the UG–2 chromite seam of the Bushveld Complex and the H.P. Reef of the Stillwater Complex have concentrated platinum group elements (PGE) with remarkable efficiency. Attempts to model these sulfides have exposed two major problems: (i) very high Nernst distribution coefficients (D) are required and (ii) a mass balance problem. The second problem, that of mass balance, stems from the first. If the D value is high, the sulfides must reach equilibrium with a large mass of silicate melt in order to take full advantage of this high D value. In practice R, the silicate/sulfide mass ratio, must be greater than D. It is suggested that the Merensky Reef, UG–2 and H.P. Reef were formed by major new pulses of picritic magma. Factors which control the value of R depend on the injection process and on the mechanism of sulfide liquation. During the early stages of fractional crystallization, when olivine and bronzite are the fractionating phases, the density of the magma in the chamber decreases with increased fractionation. A new pulse of magma entering the chamber will be denser than the magma in the chamber and will spread out across the floor. If the new pulse enters the chamber with sufficient momentum it will initially jet upwards into the overlying magma then fall back on itself when negative buoyancy overcomes the jet's initial momentum. During jetting the new pulse mixes with the host magma to form a hybrid melt which collects at the floor of the chamber, producing a gravity stratified layer with warm primitive magma overlain by cooler, more fractionated magma. On cooling the gravity stratified zone breaks up into a number of double diffusive convection cells. Once plagioclase becomes a cumulus phase the density of a tholeiitic magma increases with increased fractionation. Eventually the density of the magma in the chamber becomes greater than that of the parent magma. A new pulse entering the magma chamber at a stratigraphic level above the density crossover point will rise as a plume to the top of the chamber. During ascent the new pulse will mix turbulently with the host magma to produce a density stratified hybrid layer at the top of the chamber. This time hot primitive magma will overlie cooler, more fractionated magma. Magma mixing is a process that can produce sulfide liquation. The turbulence associated with either jets or plumes can produce a very high R value. The actual value of R is probably controlled by the size of the new pulse; the larger the pulse the higher the R value. If sulfide liquation is temperature-controlled, stratigraphic factors and the relative densities of the two magmas may have an important influence on R. The most favorable condition for producing a high value for R is when a new pulse of hot magma, slightly lighter than the host magma, enters a density stratified chamber. Under these conditions, the new pulse will rise to its own density level, then spread laterally to form a density stratified intermediate layer which will lose heat rapidly to the cooler overlying host magma. If the density difference between the original magmas was small, the density stratification forming within the hybrid layer will destabilize as it cools, allowing any immiscible sulfides which form to equilibrate with a large volume of silicate melt. If the density difference is large, cooling will not destabilize the density stratification and the sulfides will have a low R value.

A Study of Macro-Rhythmic Layering and Cumulate Processes in the Jimberlana Intrusion, Western Australia. Part I: The Upper Layered Series

The upper layered series of the Jimberlana Intrusion rests unconformably on the lower layered series and has resulted from a major new pulse of magma which entered the magma chamber during the final stages of crystallization of the lower series. All parameters which vary systematically with fractionation are sharply reversed at the contact between the two layered series. The lower portion of the ultramafic zone of the upper layered series is composed of a repeated macro-rhythmic succession of olivine cumulates, bronzite-olivine cumulates, and bronzite cumulates. A detailed investigation was made of this sequence so that the contacts between the units could be compared with the contact between the upper and lower layered series. Cu, Ni, Cr, P, and U were measured on a whole rock basis; the Mg/Mg÷Fe ratios of the pyroxenes and olivines, the Cr content of pyroxenes and the Ni content of olivines were determined by the electron microprobe. Each of the macro-rhythmic units is associated with a reversal in mineral variation and in the Ni-Cr trends and has a zone of high sulphide values at its base. These features are also found at the contact between the upper and lower layered series and, if the upper series is due to a new influx of magma, it follows that the macro-units are also due to multiple injection. This conclusion is strongly supported by the size of the reversals which are too large to be explained by the alternative hypotheses to multiple injection suggested by Jackson (1961) and Wager (1959). The anomalous sulphide-rich zones at the contacts between the upper and lower layered series and between the macro-rhythmic units are thought to have formed from a narrow supercooled zone which developed for a brief period at the bottom of the magma chamber following each new magma pulse. The olivines from these sulphide-rich zones are depleted in Ni, suggesting that Ni has been scavenged by the sulphides. This is only possible if the olivine grains crystallized with the sulphides in the restricted supercooled zone at the bottom of the magma chamber.