Spinifex Textures Research Articles

Archaean flow-top alteration zones formed initially in a low-temperature sulphate-rich environment

The 3,500–3,400 Myr Onverwacht Group in the southern part of the Barberton Greenstone Belt, South Africa, includes up to 12 km of komatiitic and tholeiitic lavas containing minor sedimentary and felsic volcanic units1. Komatiitic lavas showing spinifex textures, chrome spinels and high Ni and Cr contents are common throughout the sequence2. The rocks have been subject to polyphase Archaean deformation and low-grade metasomatism2–4. At several positions within the Onverwacht Group (Fig. 1), the tops of komatiitic lavas underlying chert beds are marked regionally by 1–50-m thick zones of silicification, carbonatization and K2O enrichment (Fig. 2). Because these zones have been variously interpreted to represent the felsic tops of mafic-to-felsic volcanic cycles, weathering horizons and shear zones, their origin is germane to any interpretation of the stratigraphy, structure and evolution of the Barberton belt. Here we present evidence that they formed inlarge part by low-temperature, near-surface alteration of komatiitic lavas during intervals of volcanic quiescence. They do not mark fault zones and cannot be used to subdivide the greenstone belt sequence into a series of fault-bounded structural slices.

Emplacement and cooling of komatiite lavas

We propose that komatiite lavas were emplaced as turbulent flows, accompanied by vigorous forced convection with cooling rates often in excess of hundreds of °C h−1. They melted and assimilated up to 10% of the ground over which they flowed, forming deep channels. Nickel sulphide mineralization may have resulted from incorporation of sulphur-rich sediment. After emplacement, high cooling rates persisted, resulting in spinifex textures due to compositional convection.

Ultramafic flows of the Crixas greenstone belt, Goias — Brazil

Abstract The Crixas greenstone belt, together with the Guarinos and Pilar belts, has a basal, intermediate and an upper unit consisting of extrusive and intrusive ultramafic, mafic and sedimentary rocks, respectively. The Crixas belt, the main object of this study, has a basal unit formed of peridotitic and pyroxenitic lava flows intercalated with iron formation, chert, graphite schist, mafic and acid tuffs, and intrusive bodies of pyroxenite and gabbro. The ultramafic unit becomes thicker towards the south end of this belt and it is there that the flows are best exposed, particularly along the Alagadinho creek, where detailed mapping was carried out. Three types of ultramafic flows were recognized: flows with olivine spinifex texture in the central zone, flows with polyhedral jointing at the base, clinopyroxene spinifex texture towards the top and flows without spinifex, but showing polyhedral jointing throughout. The flows are composed of olivine, clinopyroxene, glass and opaques. Greenschistfacies metamorphism, together with deformation, have obliterated part of the primary volcanic textures and structures in these rocks, and partially changed olivine to serpentine and chlorite, clinopyroxene to tremolite, and glass to a very fine chlorite. Preliminary chemical work suggests that the ultramafic rocks are komatiites, characterised by MgO > 20%, CaO/Al 2 O 3 > 1 and low TiO 2 , Na 2 O and K 2 O; these compositions coupled with the presence of spinifex texture indicate an origin from an ultramafic liquid. The flows from Crixas represent the first recognition of spinifex-textured komatiites in the central-western part of Brazil.

REE geochemistry of komatiites and associated rocks from Piumhi, southeastern Brazil

Komatiitic rocks with spinifex textures have recently been found in two areas in Brazil: (1) in the vicinity of Piumhi, State of Minas Gerais and (2) in the Goias Central Massif. This adds to the notion of a world-wide occurrence of komatiites in most, if not all, Precambrian shields. This report documents major element compositions and rare earth abundances in komatiites and associated rocks from the Piumhi Massif, of probable Archean age. The komatiites are found to have flat heavy REE and somewhat enriched light REE distribution patterns. Basaltic komatiites have near-chondritic Al 2O 3/TiO 2 ratios. However, their low CaO/Al 2O 3 ratios (∼ 0.6) suggest that these rocks might have lost CaO by alteration, or might have undergone clinopyroxene fractionation. Modal calculations indicate that the LREE enrichment more likely reflects mantle source characteristics, and is also consistent with the LREE-enriched pattern in peridotic komatiite. Komatiites and associated rock types including tholeiites, minor andesites, sedimentary sequences and granitic intrusions in the Piumhi Massif are entirely comparable with those of typical Archean greenstone—granite terrains. However, the preliminary results of SmNd isotopic analyses suggest that they were emplaced in the Proterozoic, ∼ 1.84 Ga ago (Appendix).

The petrogenesis of the Lac Guyer komatiites and basalts and the nature of the komatiite-komatiitic basalt compositional gap

A sequence of ultramafic rocks in the Lac Guyer Archean greenstone belt exhibit brecciated flow tops, pillow structures, and spinifex textures testifying to their volcanic origin. Massive, spinifex-textured and differentiated flows in the sequence have the chemical characteristics of peridotitic komatiite, with MgO ranging from 19–25 wt.%. Associated pillowed flows have compositions that straddle the conventional boundary between komatiite and komatiitic basalt with MgO contents ranging from 16 to 19 wt.% MgO and are best termed pyroxenitic komatiites. Unlike other komatiitic occurrences, the peridotitic and pyroxenitic komatiites at Lac Guyer constitute a continuous chemical spectrum with no evidence of population minimum near 18 wt.% MgO. The contrasting behaviour of highly compatible elements, such as Ni and Cr, versus incompatible elements, such as Zr, indicate that this compositional spectrum was produced by a variation in the extent of partial melting (10–40%) of a garnet lherzolite source in the Archean mantle. The pyroxenitic komatiites represent liquids produced during lower (10–20%) degrees of melting during which garnet remained in the mantle residue. However, a change in slope in the distribution of Zr vs. Y between the pyroxenitic and the peridotitic komatiites indicates that garnet was completely consumed at the more extensive degrees of melting which produced the peridotitic komatiites. The Lac Guyer volcanic rocks display a population minimum at 15 wt.% MgO separating komatiitic magmas whose compositions are controlled by partial melting from basalts whose composition is controlled by crystal fractionation. The population minimum near 18 wt.% MgO which is taken as the boundary between komatiite and komatiitic basalt may have a similar origin.

Petrology and shock metamorphism of Pampa del Infierno chondrite

Pampa del Infierno, an L6 chondrite, displays strong evidence of impact metamorphism. Rare chondrules and two types of dark-colored clasts occur in a light-colored matrix. Granular clasts are similar in mineralogy and chemistry to the host meteorite, but display shock metamorphic features, produced mainly by deformation, such as mosaicism, undulatory extinction, and fracturing. Partial melting in the granular clasts is manifested by the presence of selvages of mafic glass with troilite-iron eutectic intergrowths around remnants of low-Ca pyroxene and plagioclase glass with skeletal poikilitic inclusions of olivine. Clasts with spinifex texture are believed to have crystallized from a supercooled, impact-generated, ultramafic melt of the host chondrite or a chondritic source of similar composition. The light-colored matrix mainly displays evidence of shock metamorphism under subsolidus conditions as manifested by kinking and deformation twinning in pyroxene; high-pressure phase transitions of olivine and low-Ca pyroxene to ringwoodite and majorite, respectively; and lineation that still preserves the deformation features in the different mineral phases. Pertinent shock-wave data used to interpret the metamorphic history of the Pampa del Infierno chondrite suggest metamorphism by impact at a minimum peak pressure greater than 300 kbar.

Komatiites: An early precambrian phenomenon

Using the presently accepted definition, komatiites are ultramafic volcanic rocks characterised by bladed and/or skeletal olivine texture (spinifex texture). Where textural evidence is absent,the geochemical definition is equivocal and as a result there is considerable ambiguity, particularly among the basaltic types. If the added control of Al 2O 3/TiO 2 ⩽ 25 is added to the definition, this eliminates rocks of the boninite group and also the low-Ti basalts found in some ophiolite complexes. The bulk of komatiites occur in the Archaean and the most obvious explanation is that the eruption of high (up to 30%) MgO melts was controlled by an enhanced geothermal gradient. There is some evidence to suggest that the high CaO/Al 2O 3 ratio characteristic of some komatiites (e.g. Barberton) is found only in those greenstone belts older than 3.0 b.y.

Archaean and Lower Proterozoic units in Central Brazil

The following main units have been recognized as Archaean basement rocks in central Brazil: a) High-grade granite-gneiss terrain composed predominantly by granodioritic and tonalitic gneisses; these rocks are cut by amphibolite dykes and present several nucleii locally surrounded by an array of Archaean greenstone belts, b) Greenstone belts presenting basal komatiitic ultramafic-mafic lavas with abundant spinifex textures, followed by basalts and chemical sediments such as cherts, limestones and graphitic pelites, that are often interbedded with acid and intermediate volcanics; metamorphism is within greenschist facies. Granulite mobile belt, including the granulitized mafic-ultramafic complexes of Barro Alto, Niquelândia and Brava. The Lower Proterozoic is represented by: a) metasedimentary sequences overlying the granite-greenstone terrains, with several tin bearing granite intrusions. b) Gabbro-anorthosite layered plutons and their associated mafic volcanics which are followed by acid and intermediate volcanics and ocean floor sediments. c) Mafic-ultramafic layered intrusions in the granulite terrain.

Geochemical characteristics of the Crixás greenstone belt, Goiás, Brazil

Geochemical data (major elements and Ni, Cr, Co, Cu, Pb. Zn, V, and Zr) are presented for the Crixas Greenstone Belt (Goias, Brazil). The chemical composition of the ultramafic flows, displaying spinifex texture, as well as that of the pillow-lava structured metabasalts of Crixas, indicate a komatiitic nature. The granitic archean rocks show a potassium enrichment (progressive decrease of the Na/K ratio) during their evolutionary history. Iron formation (Lower Metasedimentary Unit) and metachert intercalated with tale and chlorite schists (Upper Ultramafie Unit) exhibit gold and arsenium anomalies. In both sequences the highest gold concentrations are located in the hinge line region of folds.

Archaean ultramafic rocks with relict spinifex textures in the Umanak area, central West Greenland

Conspicuous olivine and serpentine textures were found in a number of ultramafic bodies in the Umanak area while it was being mapped on a scale of 1:20000. The rocks in the area mapped by the writer are mainly a suite of quartzo-feldspathic gneisses (Pulvertaft et al., this report) in which there are distinct amphibolite-dominated supracrustal horizons which enable the major structures of the area to be recognised. The structure which dominates the area is a large recumbent/rec1ined fold plunging ESE at a low angle and closing to the NNE. Isoclinal folds belonging to an earlier phase, and late open folding have also been recognised in the area.

Geochemistry of some ultramafic komatiite lava flows from the Barberton Mountain Land, South Africa

Thin ultramafic flows from the Komati Formation in the Barberton Greenstone Belt show striking textural variations similar to those found in some ultramafic flows from Munro Township, Ontario and Western Australia. The spinifex textured A2 zone has been divided into an A2r zone consisting of short randomly oriented olivine blades with vesicles in the interblade areas and an A2p zone made up of booklets of olivine blades forming a ‘thermotactic’ spinifex texture. The composition of the A2r zone is considered to be representative of the initial ultramafic liquid, while the variation in the composition of the A2p zone reflects the evolving residual liquid composition as the flow crystallized. Detailed modelling of major and trace element data obtained from a 3.1 m thick flow demonstrates that olivine fractionation and accumulation after extrusion probably generated the observed compositional range (25.7–31.7 wt.% MgO). The Ni content of olivine cores analysed from the B zone of this flow are consistent with these crystals having existed in equilibrium with a liquid composition represented by the random spinifex textured A2r zone. A DCr (Cr % in olivine/Cr % in ‘liquid’) of 0.5 found for these olivine crystals suggests that the high Cr content of olivines from Archaean ultramafic lavas is due to the very high Cr content of the liquids from which they crystallized. The textural and chemical data are combined to develop a cooling and crystallization model for this flow.

Tertiary or Mesozoic komatiites from Gorgona Island, Colombia: Field relations and geochemistry

An exceptional occurrence of ultramafic lavas within the volcanic member of the Mesozoic (or younger) Gorgona Igneous Complex represents the first known komatiites of post-Precambrian age. Gorgona komatiites are virtually unaltered and display typical spinifex textures, with 7–10 cm long plates of olivine (Fo 88 to 91) surrounded by acicular aluminous augite, subordinate plagioclase (An 56 to 78), basaltic glass, and two spinel phases. The MgO contents of the komatiites range from 15 to 22 wt.%. Sr and Nd isotopic compositions are indicative of depletion of incompatible elements in the mantle source region, as is the case for “normal” mid-ocean ridge basalts. The komatiites are low in total REE abundances and extremely depleted in LREE. They represent primary melts generated by high degree of partial melting of the mantle. Eruption temperatures are estimated at 1,450° to 1,500° C.

Lavas ultrabásicas da unidade basal do greenstone belt de Crixás (GO): uma nova classe de rochas ultrabásicas no Estado de Goiás

The basal unit of the Crixas Greenstone is constituted of peridotitic and pyroxenitic flows intercalated with iron formation, chert, maficand acid tuffs and intrusive bodies of piroxenite and babbro. At the south end of the Crixas belt, the ultrabasic unite becomes thicker and the flows are best exposed, particulary along the Alagadinho Creek where detailed mapping was carried out. Three types of ultrabasic flows were recognized: flows with spinifex texture (olivine) in the medium zone; flows with polyedral jointing and spinifex texture (clinopyroxene) in the upper zone; flows with polyhedral jointing. The ultrabasic flows are composed of olivine, clinopyroxene, glass and opaques. The metamorphism of greenschist facies, together with deformation, have destroyed most of the primary volcanic textures and structures and changed partially olivine to and chlorite, clinopyroxene to tremolite-actinolite and glass to very fine chlorite. A preliminary chemical work sugests that these ultrabasic rocks belong to the komatiitic suite, characterized by the ratio CaO/Al 2 O 3 ≥ 1 and low TiO 2 , Na 2 O and K 2 O. The identification in the greenstone of Crixas, of komatiitic peridotite, which represents a new of ultrabasic rock in the Central Western Part of Brazil, implies a reevaluation of the serpentine belt and ·alpine type ultramafic rocks of that region.

PETROLOGIA DAS LAVAS ULTRABÁSICAS E BÁSICAS DA SEQUÊNCIA VULCÂNO-SEDIMENTAR MORRO DO FERRO. FORTALEZA DE MINAS (MG)

In this work the petrology of the Archean Morro do Ferro volcano-sedimentary sequence is being defined. The sequence is located in southwest part of the Sao Francisco Craton in the vicinity of the Fortaleza de Minas Village. The basal part of the Greenstone belt of Morro do Ferro is constituted of ultramafic flows with spinifex texture intercalated with aluminous tuffs, and iron formation. The medium part is formed of massive basaltic flows, now represented by tremolite-chlorite-epidote schist associated with reworked volcanic rocks, iron formation and talc schist lenses. The petrological studies indicated a fractionation trend wich starts with rocks of komatiitic afiliation (peridotite, pyroxenite, basalt) to rocks of tholeiitic composition (basalt). The fractionation is caracterized by a decrease in MgO content, a constant CaO/Al 2 O 3 ration, that reflects the olivine crystalization in the initial stages, followed by clinopiroxene and plagioclase.

Palaeogene komatiites from Gorgona Island

UNMETAMORPHOSED mafic and ultramafic rocks overlain by Neogene oceanic sediments are exposed on Gorgona Island which is a remnant of the Coastal Cordillera along the Pacific margin of Colombia1. Gorgona is one of the rare places in the world where young pyroxenitic komatiites exhibiting typical quenched spinifex textures (Fig. 1) occur. The spinifex rocks form lenticular bodies (? lava flows) within cumulate ultramafics and troctolitic gabbros and are closely associated with diabases and tholeiitic pillow basalts1. We show here that in textures, rock and mineral chemistry, the Gorgona spinifex rocks are almost identical with the Archean peridotitic to pyroxenitic komatiites from Munro Township (North-east Ontario)2. For this reason the term komatiites is used for the Palaeogene rocks, although one could call them high Mg-tholeiites.

GEOLOGIA DA RAIZ DE UM GREENSTONE BELT NA REGIÃO DE FORTALEZA DE MINAS. MINAS GERAIS

The aim of this study is to define the volcano-sedimentary sequence of Morro do Ferro, located in Fortaleza de Minas, as a part of a larger greenstone belt. The volcano-sedimentary sequence, in contact with orthogneisses, migmatites, cataclastic rocks of the sialic basement is unconformably overlied by the phillytes and marbles of the Araxa Group. The basal unit of the sequence, Morro do Niquel Unit, is composed of ultramafic flows with spinifex texture intercalated with chemical sediments and aluminous tuffs. Over it is the middle unit called Corrego Salvador, composed of basaltic rocks associated with acid tuffs and chemical sediments. The upper unit, Morro do Ferro, is exclusively constituted of sedimentary rocks, phillytes, marbles, iron formation, and muscovite-chlorite schist and quartz-chlorite schist which may represent reworked volcanic rocks. The volcano-sedimentary belt is intensively tectonized having tight vertical isoclinal N-W folds. There are indications of three fases of deformation. The regional metamorphism is of greenschist facies with choritoid and chlorite-albite-epidote-tremolite parageneses in basic rocks. Potentially the ultramafic flows are the most favorable rocks to have mineralization, Ni-Cu massive sulfides.

Ultramafic rocks of the Widgiemooltha-Norseman area, Western Australia: Petrological diversity, geochemistry and mineralisation

Mineral exploration of the Widgiemooltha-Norseman region of Western Australia has located massive and disseminated nickel sulphide mineralisation. This paper discusses the geological setting of the nickel sulphide mineralisation with reference to the stratigraphy, structure and metamorphism of the ultramafic sequence and spatially associated rocks. The amphibolite facies metamorphosed ultramafic rocks of the Widgiemooltha area are compared with greenschist metamorphosed ultramafic rocks at Eundynie, which exhibit excellently preserved pseudomorphs of primary igneous textures. Some 2000 ultramafic rock samples were analysed for 13 element/oxides and divided into four major mineralogically and texturally distinct groups. The data were analysed statistically by determination of means and standard deviations and multiple regression analyses. The data suggest differentiation of a magma at depth followed by a vast outpouring of lava to produce picritic-peridotitic rocks. The flows crystallised with upper spinifex zones and lower euhedral-olivine zones in varying proportions dependent on magma composition. The development of spinifex texture is compositionally controlled. This texture is not developed in rocks with MgO contents greater than 26% (± 2%). The comparison of the ultramafics of the Widgiemooltha and Eundynie areas indicates that talc-carbonate formation and serpentinization have modified primary igneous textures and geochemistries, resulting in the production of a diverse group of rock types. Such post-magmatic alteration processes in association with structural elements are considered important in the upgrading of nickel sulphide mineralisation.

Thick, layered peridotite–gabbro lava flows in Munro Township, Ontario

Two unusually thick (120 m) lava flows in Munro Township, Ontario, are differentiated from peridotite at the base to gabbro near the top. One flow is komatiitic and contains the following units (from top to bottom): ultramafic flow-top breccia (19.5% MgO); olivine and pyroxene spinifex-textured rocks (17–10% MgO); gabbro (7% MgO); pyroxene cumulates (10–25% MgO); olivine cumulates (32–35% MgO); and a basal border zone. The composition of the lava at the time of eruption is believed to be represented by the ultramafic flow-top breccia. Following eruption, this lava differentiated to produce ultramafic cumulate rocks, and liquid of quartz-normative basaltic composition that solidified as gabbro.The other flow is tholeiitic and has a picritic hyaloclastite flow top (14% MgO) underlain successively by an aphanitic pyroxenite zone (but no spinifex texture), a gabbro zone, a thick clinopyroxene-rich cumulate unit, and a thin basal veneer of cumulate olivine.

Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario

Munro Township, in the Archean Abitibi greenstone belt of northeast Ontario, contains volcanic and hypabyssal rocks of two magma series: (1) an Fe-rich tholeiitic series of basaltic to dacitic lava flows (3–10 m thick), layered peridotite-pyroxenite-gabbro flows (120 m thick), and layered sills (700 m thick); (2) an ultramafic-mafic komatiitic series, comprising discrete lava flows of peridotitic to andesitic composition (1–17 m thick), layered peridotite-gabbro flows (120 m thick), and layered sills (500 m thick). The komatiitie lavas form a succession about 1000 m thick that is both underlain and overlain by thicker successions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic, and basaltic komatiites. The most ultramafic are peridotitic cumulates rich in forsteritic olivine (Fo89–94), at the bases of flows and sills. Many less mafic peridotitic komatiites (MgO: 20–30 per cent), which typically form the upper parts of flows and the marginal parts of small intrusions, exhibit spinifex textures indicative of their formation from ultrabasic liquids. Pyroxenitic komatiites (MgO: 12–20 per cent) also may contain olivine, but are dominated by clinopyroxene, usually in spinifex textures. Basaltic komatiites (MgO <12 per cent) are composed mainly of clino-pyroxene and plagioclase or devitrified glass, rarely in spinifex texture and more commonly in equigranular textures. Peridotitic komatiite with roughly 30 per cent MgO appears to represent a parental liquid from which the more ultramafic komatiites formed by accumulation of olivine, and the less mafic types were derived by fractionation of olivine, joined and finally succeeded in later stages by clinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristics of the komatiites from the Barberton Mountain Land, South Africa (Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al2O3 ratios that distinguish the Barberton rocks. The Munro komatiites are identical in this respect to ultramafic volcanic rocks in Australia, Canada, Rhodesia, and India. It is proposed that the definition of the term komatiite be broadened so that it includes all members of this ultramafic-mafic rock series, not only those from Barberton Mountain Land. The proposed criteria are: (1) highly ultramafic compositions in noncumulate lavas; (2) unusual volcanic structures such as spinifex texture and polyhedral jointing; (3) low Fe/Mg ratios at given Al2O3 values or high CaO/Al2O3 ratios; low TiO2 at given SiO2; and high MgO, NiO, and Cr2O3.

Ultramafic and related mafic rocks; their classification and genesis with special reference to the concentration of nickel sulfides and platinum-group elements

Ultramafic and related mafic rocks can conveniently be classified in terms of the tectonic environment in which they were emplaced. Among those bodies emplaced in an orogenic environment one can distinguish synvolcanic bodies, both intrusive and extrusive, emplaced before the peak of orogenesis; alpine bodies (ophiolites) emplaced during orogenesis; and Alaskan-type complexes emplaced in the waning stages of orogenesis. Large stratiform complexes (Bushveld type), plateau-type sills (Dufek type), and medium- and small-sized picritic intrusions are characteristic of a cratonic environment. Synvolcanic bodies belong to two distinct magma suites: (1) the highly magnesian komatiitic suite, characterized by spinifex-textured ultramafic flows, a low FeO/(FeO + MgO) ratio for a given Al 2 O 3 content, and relatively low TiO 2 contents; and (2) a less magnesian tholeiitic suite, lacking spinifex texture and having a higher FeO/(FeO + MgO) ratio and TiO 2 content.The Alaskan complexes differ from each of the other economically interesting types in the alkalic nature of their magma (Irvine, 1974) and therefore the total absence of orthopyroxene in the rocks. Stratiform complexes such as Bushveld and Stillwater are at the other end of the spectrum. They have a high normative orthopyroxene to clinopyroxene ratio and crystallize in this order: olivine, orthopyroxene, and plagioclase plus augite or plagioclase and then augite. Magmas responsible for some of the other important bodies fall between these two end members, although they are all tholeiitic and contain normative orthopyroxene.Platinum-group element concentrations occur as discrete platinum-group minerals or as solid solutions in sulfides, arsenides, and sulfarsenides associated with synvolcanic bodies, stratiform complexes, and picritic intrusions. Alpine and Alaskan-type rocks contain only very low grade concentrations of sulfides, but platinum-group element concentrations occur as placer deposits derived from Alaskan-type bodies and, to a lesser extent, from alpine bodies.In the sulfide ores of the Merensky, Pechenga, Sudbury, and Noril9sk deposits, the Pt/(Pt + Pd) ratio decreases systematically with an increasing Cu/(Cu + Ni) ratio. Since the latter ratio is thought to increase with progressive differentiation of the host silicate magma, it is suggested that the trend of the decreasing Pt/(Pt + Pd) ratio is also related to differentiation. Ores associated with komatiites are major exceptions to the trend exhibited by the tholeiites, the komatiite ores having both a low Cu/(Cu + Ni) and a low Pt/(Pt + Pd) ratio. It is suggested that this is due to the fact that komatiites originate at unusually great depths in the mantle, depths at which mantle sulfides (rich in Pd) have accumulated and are hence incorporated in the magmas.