Robin E. T. Hill

The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia

Abstract Komatiites in the western part of the Norseman-Wiluna greenstone belt in the Yilgarn Block of Western Australia display a wide variety of volcanic facies, ranging from very thin differentiated Munro Township-type flow units through to very thick olivine-rich cumulate flow units containing high proportions of adcumulate dunite. Cumulate flow units have been mapped in detail in the Agnew-Wiluna segment of the Norseman-Wiluna Greenstone Belt. In the Yakabindie and Mt. Keith areas, thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet-like sequences of finer grained olivine orthocumulates. They show fine-scale internal layering, broad-scale cryptic layering and upper fractionated sequences containing harrisites, pyroxene bearing cumulates and in some cases gabbroic derivatives. Low grade disseminated sulphide mineralisation occurs within the dunite lenses. In the stratigraphically equivalent Kathleen East area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows. A similar relationship is also seen further south, where the Perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows. These field relationships indicate an extrusive origin for the adcumulate dunite bodies. The Walter Williams Formation, in the west-central part of the Norseman-Wiluna Belt, is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent. It consists of a central sheet-like body of adcumulate dunite, underlain and overlain by olivine orthocumulates. The adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite. At its northernmost extent, the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros, dolerites and pyroxene spinifex-textured material, interpreted as a periodically replenished lava lake sequence. Both the lenticular dunite bodies of the Agnew-Wiluna Belt and the sheet-like dunite body of the Walter Williams Formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates. The adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows. Flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates. Field evidence from Perseverance, and the general geometry of the dunite lenses, suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low-melting felsic volcanic substrates. The textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion. Dunite sheets form close to the eruption sites, and dunite lenses as a result of channellisation further away from the vent, or close to the vent in the situation where the substrate is non-refractory and thermal erosion can take place readily. Kambalda-type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic, and Munro-type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions, or close to the vent of very small ones. Waning eruption rates leads to proximal facies being overridden by distal ones, a common observation in komatiite sequences. The size of thermal erosion channels requires very rapid eruption rates comparable to those in Phanerozoic flood basalt terrains.

Geochemistry of komatiites from Forrestania, Southern Cross Province, Western Australia: Evidence for crustal contamination

The 2.9 Ga-old Forrestania greenstone belt is intermediate in character between typical > 3.0 Ga and ~ 2.7 Ga belts in containing both Al-depleted and Al-undepleted komatiites. The extensive nickel exploration database on the belt provides a unique opportunity to examine geochemical variations in the context of volcanological evolution. Within each of the six komatiitic units in the greenstone belt there is a transition from komatiites (18–28 wt.% MgO in the liquid) to high- and low-magnesium komatiitic basalts (5–18 wt% MgO) with stratigraphic height, and much of the chemical variance within the komatiitic suite can be explained in terms of the addition or fractionation of olivine. There are, however, aspects of the incompatible-element signatures which must be attributed to other factors. The more primitive komatiites from the lower ultramafic units are Al-depleted (Al2O3/TiO2 ~ 10.5–13.0) and depleted in both light and heavy REE (LaSmn ~ 0.5, GdYbn ~ 1.5). Komatiites from the uppermost ultramafic units are variably less depleted in Al (Al2O3TiO2 ~ 16.5–20.5). This is interpreted to reflect decreasing retention of garnet in the source region of the Forrestania komatiites over time, and possibly a decrease in the depth of melting. Within the Al-depleted suite there are also anomalous enrichments in SiO2, FeO, TiO2 and Zr, which correlate with regional variations in the composition of substrate metasedimentary rocks (banded iron formation versus felsic tuff). Coupled with field evidence for trough structures at the base of lava channel sequences, these chemical anomalies provide strong evidence for supracrustal thermal erosion by komatiite lavas. The Forrestania data indicate that 2.9 Ga-old komatiites were generated over a range of melting conditions spanning those typical of older and younger Archaean sequences, and that contamination by supracrustal thermal erosion can influence komatiite chemistry on a regional scale.

The physical volcanology of Archaean komatiite sequences from Forrestania, Southern Cross Province, Western Australia

Abstract The Archaean supracrustal sequence at Forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km. Lithologies range from olivine cumulates, which crystallised at the interface between substrate and flowing lava, to spinifex-textured rocks, which underwent rapid crystallisation within ponded lava lobes. Characteristic associations of different rock types are interpreted in terms of variations in eruption rate, which presumably reflect proximity to the vent or to a major lava river. Thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment, where lava flow was continuous and eruptive velocities were high. Sheeted bodies of olivine cumulate formed from voluminous, turbulent flow of lava over relatively refractory substrates, such as basalt and oxide-facies banded iron-formation. Ribbon-shaped olivine-cumulate bodies formed in an equally dynamic environment within major lava channels, where the substrate was less refractory and consisted of quartz-rich sedimentary rocks which were susceptible to thermal erosion. Distributary lava channels in more distal settings are represented by thinner ribbon-like accumulations of olivine mesocumulate to orthocumulate. Unusual poikilitic pyroxene-bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks. Pyroxene- and plagioclase-bearing cumulates are present at the top of other channel sequences and reflect late-stage ponding and in situ differentiation of komatiitic lava within the channel. The channel-facies rocks are flanked by sequences comprising multiple, thin, spinifex-textured flow-units, which formed from the episodic overflow of lava from the distributary channels. Similar rocks also occur stratigraphically above many of the thick olivine-cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle.

The role of fluids in the metamorphism of komatiites, Agnew nickel deposit, western Australia

The Agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy. Olivine + tremolite + chlorite + cummingtonite ±enstatite assemblages occur at the margin of the ultramafic lens, giving way to olivine + anthophyllite, olivine + talc and olivine + antigorite assemblages successively inwards. These rocks are interpreted as having crystallized from komatiitic lavas, and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates. The relative modal abundances of metamorphic olivine, tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith. Peak metamorphic conditions are estimated at 550° C, based on garnet-biotite thermometry, at a maximum pressure of 3 kb. This temperature falls within the narrow range over which metamorphic olivine may co-exist with enstatite, anthophyllite, talc or antigorite depending upon the fugacity of water in the metamorphic fluid. The observed mineralogical zonation is therefore attributed to infiltration by CO2-rich fluids, generated by decarbonation of talc-carbonate rocks formed during pre-metamorphic marginal alteration of the ultramafic lens. Metamorphic fluids were essentially binary mixtures of water and CO2, with minor H2S having a maximum partial pressure less than 1 percent of total pressure. Enstatite-bearing assemblages formed in the presence of CO2-rich fluids at fluid: rock volume ratios close to one, while anthophyllite, talc and antigorite bearing assemblages formed in the presence of progressively more water-rich fluids at progressively lower fluid-rock ratios.

Poikilitic chromfite in komatiitic cumulates

SummaryChromite is a widespread accessory mineral in olivine-rich cumulates derived from komatiitic lavas. The distribution and crystal habit of chromfite is related to the degree of differentiation of the parent magma as reflected in the composition of cumulus olivine. Cumulates with olivine forsterite content greater than 93 mol percent typically contain no chromfite at all, while chromfite forms clusters of disseminated euhedral grains in cumulates with forsterite less than 91 mol percent. In the forsterite 91–93 interval, chromite may develop lobate interstitial habits. In the Six Mile Well ultramafic complex in the Yakabindie region of the Agnew-Wiluna greenstone belt, and in other olivine-rich komatiitic units within this belt, chromfite shows well developed poikilitic textures enclosing olivine.The absence of intercumulus silicate phases in these rocks and the low normative chromfite content of the parent magma make it impossible for these chromfite grains to have crystallised from intercumulus trapped liquid. These rocks must be cotectic chromite-olivine adcumulates, owing their unusual texture to differing relative rates of nucleation and growth of chromfite and olivine, crystallising togetherin situ. This observation provides further evidence for a cumulus origin for oikocrysts in layered intrusions, and casts doubt on the usefulness of cumulus terminology.ZusammenfassungChromit ist in Olivin-reichen Kumulaten, die von komatiitischen Laven abstammen, ein häufiges akzessorisches Mineral. Die Verteilung und der Kristallhabitus von Chromit hängt vom Grad der Differentiation des Stammagmas, der in der Zusammensetzung der Kumulus-Olivine zum Ausdruck kommt, ab. Für Kumulate mit Olivinen mit einem Forsterit-Gehalt von über 93 mol.% ist es charakteristisch, daß sie überhaupt keinen Chromit enthalten, während Chromit in Kumulaten mit Olivinen mit weniger als 90 mol.% Forsterit Cluster aus fein verteilten, idiomorphem Körnern bildet. Im Intervall Forsterit 90–93 kann Chromit lobate, interstitielle Formen ausbilden. Im ultramafischen Komplex Six Mile Well in der Yakabindie-Region des Agnew-Wiluna-Grünsteingürtels und in anderen Olivin-reichen komatiitischen Einheiten in diesem Gürtel zeigen die Chromite gut entwickelte poikilitische Texturen mit eingeschlossenen Olivinen.Da silikatische Interkumulus-Phasen in diesen Gesteinen fehlen und der normative Chromitgehalt des Stammagmas niedrig ist, können diese Chromitkörner nicht aus einer Interkumulus-Schmelze auskristallisiert sein. Diese Gesteine müssen daher kotektische Chromit-Olivin-Adkumulate sein, die ihre ungewöhnliche Textur den variierenden relativen Keimbildungs- und Wachstumsraten von Chromit und Olivin, die gemeinsam in situ kristallisierten, verdanken. Diese Beobachtung stellt einen weiteren Beweis für die Entstehung von Oikokristallen als Kumulusphase in geschichteten Intrusionen dar und läßt die Zweckmäßigkeit der Kumulus-Terminologie als zweifelhaft erscheinen.

Using automated digital image analysis to provide quantitative petrographic data on olivine–phyric basalts ☆

Abstract A knowledge of the distribution of crystal sizes and shapes can prove critical to an understanding of the emplacement and cooling history of igneous rocks. We outline here a relatively automated method, which uses standard image processing techniques to acquire size, shape and modal data for olivine phenocrysts and voids in a suite of picritic Hawaiian basalts. The technique requires nothing more than a petrographic microscope fitted with a digital camera, and a PC which will run off-the-shelf image processing software. The method could be adapted to a variety of petrographic problems and its use need not be restricted to the study of igneous rocks.

Partial melting and recrystallization of Archeaan komatiites by residual heat from rapidly accumulated flows

Spinifex-textured komatiites at Honeymoon Well, Western Australia, show evidence of partial melting and recrystallization of original igneous textures. Their textures and mineral compositions differ markedly from those typical of komatiites. Spinifex olivine plates are bent and broken, while interstitial space between spinifex and cumulus olivine is occupied by polygonal aggregates of clinopyroxene, orthopyroxene, minor olivine and plagioclase. Similar granular pyroxene-plagioclase aggregates occur as diffuse veins cutting spinifex zones and cumulate zones of the flows and, in places, form the matrix to a breccia containing corroded fragments of spinifex rock. Thermometry based on the two pyroxene assemblages yields temperatures of 1055° to 1141° C, just below the low-pressure komatiite solidus. Mineral compositions are different from those of typical komatiites: clinopyroxenes are Al-poor and Cr-rich, olivines are unusually iron-rich and depleted in Cr and Ca, and the low-Ca pyroxene is bronzite rather than the more typical pigeonite. We interpret these observations as the results of thermal metamorphism, partial remelting and subsequent slow crystallization of originally normal spinifex-textured komatiite flows. The rocks in question occupy a 40–70 m interval sandwiched between two olivine-rich units: an underlying 90 m-thick olivine adcumulate layer, forming part of the cumulate zone of a basal 160 m-thick flow, and an overlying 1 km-thick extrusive body composed mostly of olivine mesocumulate and adcumulate and capped in turn by spinifex-textured flows. Thermal modelling shows that a sinusoidal temperature profile of cool flow tops and hot flow centres would exist within this sequence shortly after eruption. Conductive thermal relaxation of this profile could reheat spinifex zones to the extent of inducing partial melting and textural reconstitution. Such reheating is largely dependent on the time interval between the emplacement of successive flows. Calculations suggest that at Honeymoon Well the emplacement interval must have been of the order of 10 years or less. Textural reconstitution may have contributed to the development of the thick orthocumulate sequences characteristic of komatiites in the Agnew-Wiluna belt.