Mildly alkaline basalts from Pavagadh Hill, India: Deccan flood basalts with an asthenospheric origin

Summary Of twelve flows at Pavagadh Hill, the two three-phenocryst-basalt flows with Mg#0.70 and Ni/MgO33 are the most primitive and perhaps as primitive as any basalts in the Deccan province. Scatter on variation diagrams and the occurrence of primitive flows at two different levels in the volcanic sequence implies that most rocks are probably not, strictly speaking, comagmatic. Nevertheless, mass balance calculations indicate a generalized differentiation scheme from primitive basalt to hawaiite that involved removal of olivine, augite, plagioclase and Fe-Ti oxides in the proportions 40:33:22:5 with 50% of the magma remaining. Crustal assimilation had a minimal effect on evolution of the basalts but rhyolites at the top of the volcanic sequence may have been produced by crustal melting following prolonged heat release from alkali basalt pooled along fault zones in the continental crust. Major element based calculations indicate that the most primitive basalts were generated by 7 to 10% melting of mantle peridotite. These low percentages of melting, typical of alkali basalts, are consistent with the steep slopes on chondrite-normalized REE diagrams. Low heavy REE concentrations point to residual garnet in the source region. Incompatible element concentrations (e.g. Rb, Ba, Zr, La) in Pavagadh basalts exceed those in Deccan tholeiitic basalts but are substantially lower than those reported for some other Deccan alkali basalts. Obviously Pavagadh basalts do not reflect the lowest percentages of melting and greatest amount of source region metasomatic enrichment attained in the Deccan province. Deccan tholeiitic and alkali basalts are largely characterized by low La/Nb ratios and high La/Ba ratios similar to those in oceanic island basalts. This indicates minimal involvement of the subcontinental lithospheric mantle in their petrogenesis. Comparison with continental mafic magma provinces where a subcontinental lithospheric mantle imprint is common indicates long periods of extension and/or melting of mantle lithosphere still hot from pre-extension subduction are more likely to produce magmas bearing the lithospheric imprint.

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Alkalische Basalte von Pavagadh Hill, Indien: Deccan-Flutbasalte von Astenosphärischer Herkunft

Zusammenfassung Im Gebiet von Pavagadh Hill, Indien, treten 12 Spät-Deccan und rhyolithische alkalibasaltische Ergüsse und Intrusiva auf. Variationsdiagramme zeigen, daß die Abfolge nicht komagmatisch ist. Zusammen mit Berechnungen der Massenbilanz unterstützen sie vielmehr ein Zwei-Stadienmodell für die Entstehung von Hawaiiten aus sehr primitiven (i.e. Mg#=Mg/(Mg+.(0.9*Fe_totat)) at.%0.70) Basalten. Olivin und Augit dominierten die frühe Fraktionierung während Augit vorherrschte als der Magmaanteil von 65% auf 50% sank. Die Entfernung von Plagioklas spielte bei der Differentiation nur eine geringe Rolle. Niedrige Th/Nb (0,2), Rb/Sr(<0,12) und K/NbVerhältnisse geben keine Hinweise auf signifikante Assimilation von Krustenmaterial. Die Seltene-Erd-Verteilungsmuster (SEE), niedrige Gehalte an schweren SEE sowie die Hauptelementspektren der Alkalibasalte weisen auf eine granatführende Ursprungsregion und auf einen Aufschmelzungsgrad von nur 7% bis 10% hin. Es gibt jedoch auch stärker alkalische (höhere Rb, Zr etc.) Deccanbasalte (i.e. Rajpipla). Die Assoziation von Deccanalkalibasalten, Rhyolithen und Störungszonen zeigt, daß letztere die Extraktion von Magma aus dem Mantel erleichterten und dazu führten, daß Magma aus Magmenkammern Krustenschmelzen (Rhyolithe) produzierte. Deccanbasalte tendieren zu hohen La/Ba und niedrigen La/Nb-Verhältnissen; dies weist auf eine asthenosphärische Herkunft hin, selbst wenn die Gesteine verhältnismäßig spät gebildet wurden (i.e. Pavagadh). Längere Perioden von Krustenextension oder von Subduktion, die der Extension vorhergeht, führt offensichtlich zur Entstehung von Magmen mit einer lithosphärischen Komponente.

     

云南金平晚二叠纪玄武岩特征及其与峨眉地幔柱关系——地球化学证据

分布于哀牢山－红河断裂带西南侧的金坪上二叠统玄武岩属于低钛拉斑玄武岩（LT）（Ti/Y＜500）。其地球化学特征总体与洋岛玄武岩（OIB）相似,根据其岩相学、主量元素,微量元素特征,将其划分为LT1和LT2两个地球化学亚类型,它们的分布和主要地球化学标志为：LT1分布于下部,高Mg^#(48-63),SiO2(50%-56%),高∑REE(118-145μg/g）、低Fe2O3（1．36％－1．63％）,Na2O(1.88%-3.17%),TiO2(1.37%-1.92%),高Th、U,低Nb,Ta和Sr负异常;LT2分布于上部,低SiO2(47%-56%),Sr强负异常,二者地球化学特征的差异是同一母岩浆经不同的分离结晶和同化混染作用的结果,金平与宾川峨眉山的化学地层学对比表明,金平LT1和LT2玄武岩与宾川峨眉山玄武岩下部的LT1、LT2十分相似,它们可能是同时,或在类似的环境下形成,金平玄武岩属于峨眉山大火山岩省的一部分,同为峨眉地幔柱早期活动的产物。新生代哀牢山－红河断裂的左滑剪运动导致了宾川与金平玄武岩的错位。     

The transition from alkali basalts to kimberlites: Isotope and trace element evidence from melilitites

Eighteen Cenozoic melilitite samples from Spain, France, West Germany and Czechoslovakia have been analyzed for major and trace elements (including REE) together with their Sr and Nd isotopic compositions. Leaching experiments produced significant shifts of their⁸⁷Sr/⁸⁶Sr ratio indicative of a contamination by a crustal component. Most samples fall within the Sr-Nd mantle array with ?_Nd values in the 1.5–6 range. These values are considered as minimum for the melilitite mantle source hence demonstrating its time integrated LRE depletion. The Ni and Cr contents of the samples are typical of primary magmas and exclude extensive crystallization of olivine and pyroxene in a closed system. However, the chemical relationships suggest that dilution of the liquids by mafic minerals of the conduits during their ascent has been important. The REE patterns show some variations which are interpreted by this dilution effect. Once normalized to Yb they are closely similar and perfectly distinguishable from those of alkali basalts and kimberlites. All of these rocks have Ce/Yb ratios which are high but distinctive for each rock type: 40 to 200 times the chondritic ratio for kimberlites, 20 to 30 for melilitites, 8 to 15 for alkali basalts. As contamination is likely to have modified somewhat the isotopic characteristics of most of these rocks, there is no overwhelming evidence that their source is chemically different. The Ba and Rb contents together with the REE patterns of the melilitites would constrain the degree of melting to be very small (<0.2%). The calculation of batch melting and steady zone refining models suggests that kimberlites, melilitites and alkali basalts may have been derived by equilibration of deep melts with different upper mantle levels characterized by decreasing garnet/clinopyroxene ratios. The strongly incompatible elements are enriched in the melt during its ascent by leaching of the wall rocks. For the steady zone refining model, the degree of melting concept loses its significance and the difficult requirement of extracting small liquid fractions from a molten source disappears. Within the frame of this model, the preenrichment of the kimberlite, melilitite and alkali basalts source in incompatible elements by metasomatic fluids is no longer necessary.     

Isotope-geochemical systematics of kimberlites and related rocks from the Siberian Platform

Using the ICP-MS method we have studied the isotope systematics of Sr and Nd as well as trace element composition of a representative collection of kimberlites and related rocks from the Siberian Platform. The summarized literature and our own data suggest that the kimberlites developed within the platform can be divided into several petrochemical and geochemical types, whose origin is related to different mantle sources. The petrochemical classification of kimberlites is based on persistent differences of their composition in mg# and in contents of indicator oxides such as FeO_tot, TiO₂, and K₂O. The recognized geochemical types of kimberlites differ from one another in the level of concentration of incompatible elements as well as in their ratios.Most of isotope characteristics of kimberlites and related rocks of the Siberian Platform correspond to the earlier studied Type 1 basaltoid kimberlites from different provinces of the world: Points of isotopic compositions are in the field of primitive and weakly depleted mantle. An exception is one sample of the rocks from veins of the Ingashi field (Sayan area), which is characterized by the Sr and Nd isotopic composition corresponding to Type 2 micaceous kimberlites (orangeites).The most important feature of distribution of isotopic and trace-element compositions (incompatible elements) is their independence of the chemical rock composition. It is shown that the kimberlite formation is connected with, at least, two independent sources, fluid and melt, responsible for the trace-element and chemical compositions of the rock. It is supposed that, when rising through the heterogeneous lithosphere of the mantle, a powerful flow of an asthenosphere-derived fluid provoked the formation of local kimberlite chambers there. Thus, the partial melting of the lithosphere mantle led to the formation of contrasting petrochemical types of kimberlites, while the geochemical specialization of kimberlites is due to the mantle fluid of asthenosphere origin, which drastically dominated in the rare-metal balance of a hybrid magma of the chamber.     

Nature and origin of eclogite xenoliths from kimberlites

Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.     

Composition and origin of Deccan basalts

The Deccan basalts are essentially composed of saturated tholeiitic lavas. However, undersaturated basalts, nephelinites, carbonatites, intermediate and acid differentiates have also been encountered in parts of western India (Upper Traps). Such unusual rocks are broadly aligned in two major rift zones in western India, the Narbada-Son and Cambay grabens and the faulted west coast. These rocks are younger than the earliest tholeiitic eruptions of central India (Lower Traps), but there are evidences of renewed eruptions of tholeiitic basalts in parts of western India. The earliest eruptions of Deccan basalts of quartz tholeiite composition have been derived by high degrees of partial melting of peridotite at moderate depth (37–41 km). The undersaturated basalts and nephelinites were possibly generated by low degrees of partial melting of garnet peridotite in the low velocity zone along the two tectonically active belts (rifts) in western India. The undersaturated basalts were formed prior to the break-up of Gondwanaland, when a concentration of pressure developed at the tectonically weak zones. The renewed eruptions of saturated tholeiites are thought to be post-tectonic, resulting from the release of stress when the tectonic events had ceased.     

Prospects of search for diamondiferous kimberlites in the northeastern Siberian Platform

The objects of study are Triassic hypabyssal diamondiferous kimberlites with an age of 220-245 Ma, containing macrocrysts of unaltered olivine. The latter are close in the time of formation to the main stage of intrusion of the Siberian Trap Province (252 Ma), which lasted less than 1 Myr. A comparative high-precision analytical study of the Ti, Ca, Cr, and Al impurity patterns in about 1000 olivine macrocryst samples with a forsterite content Fo = (100Mg/(Mg + Fe)) of 78 to 93 has demonstrated the effect of traps on the lithospheric composition. A comprehensive comparative study of diamonds from northern placers and Triassic kimberlites, including determination of their carbon isotope composition, was performed. Chromatography-mass-spectroscopic analysis of submicron fluid inclusions in diamonds from northern placers and kimberlites has shown predominant hydrocarbons of a wide range of compositions and subordinate contents of N₂, H₂O, and CO₂. These findings, together with the results of previous studies of subcalcic Cr-pyropes and diamonds found in the Lower Carboniferous gritstones of the Kyutyungde graben, lead to the conclusion that the Toluopka kimberlite field is promising for Paleozoic kimberlites. The results of comprehensive studies of diamonds and indicator minerals and U/Pb isotope dating of numerous detrital zircon samples from the basal horizon of the Carnian Stage (Upper Triassic) of the Bulkur site in the lower reaches of the Lena River suggest the presence of diamondiferous kimberlites within the northeastern Siberian Platform. The age of the probable primary diamond sources in the study area can be evaluated by an integrated U/Pb isotope dating of zircons, perovskites, and rutiles from the developed diamond placers and the basal horizon of the Carnian Stage.     

Petrochemical and thermodynamic evidence on the origin of kimberlites

The petrochemistry of kimberlites from Yakutia and Lesotho has been studied using a silicate melt model with the SiO₂, CO₂ and H₂O derivatives as the main anions.A model has been developed, according to which the dissolution of H₂O in an ultramafic melt results in orthosilicates (H₂SiC ₄ ^-2 , H₃SiO ₄ ^– , H₄SiO₄ etc.) rather than metasilicates, while the dissolution of CO₂ produces additional hydrocarbonate complexes. It suggests that at high PCO ₂ ¹ , and where the orthosilicic calcium salt clusters are likely to be present in the magma, the kimberlite melt can break down into carbonate and silicate liquids. Therefore, the composition of kimberlite magma will be determined by the H₂O/CO₂ ratio under the relatively constant fluid pressure. This can be seen from the distinct fluidrs trend in the H₂O-CO₂-SiO₂ diagram for the Yakutia and Lesotho diamond-bearing kimberlites. The H₂O/CO₂ ratio changes with the liquidus temperature along this trend (Perchuk and Vaganov 1977) which suggests that liquid immiscibility predominates over the simple CO₂ solubility in the melts of kimberlite composition. The well-known Boyd's diagrams for the equilibrium PT-conditions in peridotites have been applied along with new experimental data to natural Cpx and Opx, and the PT-parameters were correlated for peridotite inclusions in kimberlite pipes in Yakutia and Lesotho. The liquidus temperatures for the extrapolated area of these correlations gave depths (pressures) at which kimberlite magmas are formed (200–250 km).The hypothesis on SiO₂ partitioning between the melt and the fluid was used to calculate the composition of dry initial kimberlite which characterised the average mantle composition: SiO₂ — 45.12; TiO₂ — 2.49; Al₂O₃ — 3.58; Cr₂O₃ — 0.12; FeO — 9.32; MnO — 0.16; CoO — 0.11; MgO — 23.47; CaO — 13.44; Na₂O — 0.20; K₂O — 1.12; P₂O₅ — 0.69; S — 0.18; sum — 100 wt.%. This kimberlite is close to wehrlite in composition.     

Hydroxyl in olivines from mantle xenoliths in kimberlites of the Siberian platform

From a total of 335 olivine crystal grains, crystallographically orientated platelets and, where possible, parallelepipeds were prepared, chemically analysed by electron microprobe, examined under the polarisation microscope, and studied by polarised FTIR microscope-absorption-spectrometry in the _OH vibrational range, 3,000–3,800 cm^–1. The 335 crystal grains were extracted from 174 different specimens of Yakutian upper mantle material, including 97 xenoliths that represent all the rock types occurring in all the kimberlites of the Siberian shield. The other specimens were mega- and macrocrysts or inclusions in diamonds and garnets. Analysis of the polarised _OH-spectra allowed distinction between hydroxyl in non-intrinsic separate inclusions, NSI, and in isolated local or condensed extended defects, intrinsic to the olivines, ILD or CED, respectively. As the two latter types cannot be distinguished by vibrational spectroscopy, and as they are presumably interconnected by intracrystalline condensation reactions of the type n [ILD][CED]_n, we propose to symbolise them as [ILD/CED]. Of the total of 70 polarised _OH-bands that were found in the whole set, 17 are caused by NSI, 53 by [ILD/CED]. Total mean integrated _OH-band intensities, (̄_int)_total with ̄_int=(_||a+_||b+_||c)_int/3, were determined from the spectra. They yielded the contents of structurally unallocated water, using the recent calibration of the IR-method (Bell et al. 2003). The range is 0<wt. ppm (H₂O)_total<419. Olivines included in diamonds were found to be free of hydroxyl (detection limit of the single crystal IR-spectrometry, ca. 1 wt. ppm water). The total water contents of the different types of olivines increase in the sequence groundmass crystals < megacrysts < macrocrysts. NSI are: (1) Serpentine plus talc with _OH in the range 3,704–3,657 cm^–1, either polarised along a of the olivine matrix (Pbnm setting) or unpolarised. Approximately 232 olivines out of the 335 contain such NSI. Serpentine and talc occur mostly together, in rare cases one of them alone and if so, mostly talc. (2) Mg-edenite or Mg-pargasite occur rarely and with _OH at 3,709–3,711 cm^–1. NIS types (1) and (2) are presumably formed by metasomatic alterations of the host olivines, the orientated ones probably in the mantle, the unorientated ones during later stages. (3) The spectra of 23 olivine crystals, displayed specifically a _OH-band, polarised c>a>b, at 3,327–3,328 cm^–1, an energy typical of _OH in hydrous wadsleyite. We assume this phase to be present as NIS in the respective olivines, possibly as relic phase. (4) Weak bands between 3,175 and 3,260 cm^–1 polarised along c, are tentatively assigned to molecular water NSI with relatively strong hydrogen bonds to the matrix. We did not find larger clusters of molecular water, i.e. liquid-like water with its characteristic broad band centred at ca. 3,400 cm^–1. We did also not find any humite minerals as an NSI. Of the 53 _OH-bands intrinsic to olivine, the 29 most abundant and strong ones were subject to further analysis in terms of OH^–-bearing structural defects [ILD/CED]. Nearly all these bands are strongly polarised along a. Two bands at 3,672 and 3,535 cm^–1 are assigned to boron-related defects, [ILD/CED]_B. Five bands at 3,573, 3,563, 3,541, 3,524 and 3,512 cm^–1 are intensity-correlated and are assigned to Si-depleted titan-clinohumite-like defects, [ILD/CED]_(thl). The other, so far unidentified _OH of [ILD/CED] are suggested to originate from OH^– in different types of (Mg, Fe)-depleted defects recently predicted and discovered by TEM. These are called [ILD/CED]_(KWK). Eight mostly strong bands of them occur at energies higher than 3,573 cm^–1, [ILD/CED]_(KWK)-H, 13 strong to medium strong bands occur below 3,500 cm^–1, [ILD/CED]_(KWK)-L. Such intrinsic defects may occur alone, [ILD/CED]_(thl) and [ILD/CED]_(KWK)-H, or in different combinations with each other, [ILD/CED]_(KWK)-H+[ILD/CED]_(thl), [ILD/CED]_(KWK)-H+[ILD/CED]_(KWK)-L and [ILD/CED]_(KWK)-H+[ILD/CED]_(thl)+[ILD/CED]_(KWK)-L. Though there are indications that the occurrences of such types and combinations of the intrinsic OH^–-bearing defects in the olivines are related to the types and genetic peculiarities of their host rocks, straightforward and simple correlations do not exist. The reasons for this and also for the great number of varieties of intrinsic [ILD/CED] are discussed.Editorial responsibility: J. Hoefs

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Geochemistry and origin of the Mirny field kimberlites,Siberia

Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65–300). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t = 360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ɛNd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387–0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.

     

Hydroxyl defects in garnets from mantle xenoliths in kimberlites of the Siberian platform

A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v _OH vibrational range in search of the occurrence, energy and intensity of the v _OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the Siberian platform. The v _OH single-crystal spectra show either one or a combination of two or more of the following major v _OH bands, I 3645–3662 cm⁻¹, II 3561–3583 cm⁻¹, III 3515–3527 cm⁻¹, and minor bands, Ia 3623–3631 cm⁻¹, IIa 3593–3607 cm⁻¹. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO₂ > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v _OH spectra do not correspond to those of OH⁻ defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications of v _OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v _OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems I to III. Instead, they exhibit one weak, broad band (Δv _OH 200–460 cm⁻¹) near 3570 cm⁻¹, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of all v _OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between values below the detection limit of our single-crystal IR method, near 2 × 10⁻⁴ wt%, up to 163 × 10⁻⁴ wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role. Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of the graphite-pyrope facies. This latter result (2) may point to significantly lower f _H2O and f _O2 in the former as compared to the latter facies. Received: 25 November 1997 / Accepted: 9 March 1998     

The origin of island arc high-alumina basalts

A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al₂O₃ either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas. Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na₂O, MgO and Al₂O₃. These mismatches are easily explained if the HAB are plagioclase-accumulative. Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass). Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo₉₄, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with >10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with <7% MgO, but plagioclase nucleation is delayed by their low but significant (<1%?) H₂O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al₂O₃ contents (<17.5%). The majority of arc basalts, however, have Al₂O₃ contents in excess of 18%, reflecting plagioclase accumulation. We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (<10%) partial melting of peridotite at P<18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo_87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.     

Geochemistry and petrogenesis of basalts from the West Siberian Basin: an extension of the Permo–Triassic Siberian Traps, Russia

New major and trace element data for the Permo–Triassic basalts from the West Siberian Basin (WSB) indicate that they are strikingly similar to the Nadezhdinsky suite of the Siberian Trap basalts. The WSB basalts exhibit low Ti/Zr (50) and low high-field-strength element abundances combined with other elemental characteristics (e.g., low Mg#, and negative Nb and Ti anomalies on mantle-normalised plots) typical of fractionated, crustally contaminated continental flood basalts (CFBs). The major and trace element data are consistent with a process of fractional crystallisation coupled with assimilation of incompatible-element-enriched lower crust. Relatively low rates of assimilation to fractional crystallisation (0.2) are required to generate the elemental distribution observed in the WSB basalts. The magmas parental to the basalts may have been derived from source regions similar to primitive mantle (OIB source) or to the Ontong Java Plateau source. Trace element modelling suggests that the majority of the analysed WSB basalts were derived by large degrees of partial melting at pressures less than 3 GPa, and therefore within the garnet-spinel transition zone or the spinel stability field.

It seems unlikely that large-scale melting in the WSB was induced through lithospheric extension alone, and additional heating, probably from a mantle plume, would have been required. We argue that the WSB basalts are chemically and therefore genetically related to the Siberian Traps basalts, especially the Nadezhdinsky suite found at Noril'sk. This suite immediately preceded the main pulse of volcanism that extruded lava over large areas of the Siberian Craton. Magma volume and timing constraints strongly suggest that a mantle plume was involved in the formation of the Earth's largest continental flood basalt province.     

Composition and depth of origin of primary mid-ocean ridge basalts

Some workers have held that mid-ocean ridge basalts are fractionated from high pressure (15–30 kbar) picritic primary magmas whereas others have favored primary magmas generated at about 10 kbar with compositions close to those of mid-ocean ridge basalts. Of critical significance are presumed differences in composition between experimentally determined primary magmas and the least fractionated mid-ocean ridge basalts. To evaluate the significance of these differences, all based on electron microprobe analyses, we consider three sources of uncertainty: (1) analytical uncertainties for a single microprobe laboratory, (2) systematic interlaboratory analytical differences, and (3) real variations in the possible compositions of primary magmas that can be produced from a peridotite source at a given pressure. The first source of error is surprisingly large and can account for a substantial part of the total variation of normative quartz (hypersthene calculated as equivalent olivine and quartz) in FAMOUS basalts. The second is not as serious but remains undetermined for many laboratories. The third is potentially the largest but is not yet fully documented. The least fractionated FA-MOUS basalts have high mg numbers (70–73) compatible with derivation from the mantle by direct partial melting with little or no subsequent fractional crystallization. Because of the wide range of normative quartz content in these basalts, it appears necessary to consider them as representatives of multiple parental magmas. When all the sources of uncertainty are taken into account, we conclude that the experimental data by various investigators are all fairly consistent and favor derivation of the least fractionated mid-ocean ridge basalts by at most only a small amount of fractional crystallization from primary magmas having a wide range of normative quartz content and generated over a range of pressures from about 7–11 kbar. Contribution No. 420, Department of Geosciences, The University of Texas at Dallas     

A mantle plume origin for the Siberian traps: uplift and extension in the West Siberian Basin, Russia

The West Siberian Basin (WSB) records a detailed history of Permo-Triassic rifting, extension and volcanism, followed by Mesozoic and Cenozoic sedimentation in a thermally subsiding basin. Sedimentary deposits of Permian age are absent from much of the basin, suggesting that large areas of the nascent basin were elevated and exposed at that time. Industrial seismic and well log data from the basin have enabled extension and subsidence modelling of parts of the basin. Crustal extension (β) factors are calculated to be in excess of 1.6 in the northern part of the basin across the deep Urengoy graben. 1-D backstripping of the Triassic to Cenozoic sedimentary sequences in this region indicates a period of delayed subsidence during the early Mesozoic. The combination of elevation, rifting and volcanism is consistent with sublithospheric support, such as a hot mantle plume.

This interpretation accords with the geochemical data for basalts from the Siberian Traps and the West Siberian Basin, which are considered to be part of the same large igneous province. Whilst early suites from Noril'sk indicate moderate pressures of melting (mostly within the garnet stability field), later suites (and those from the West Siberian Basin) indicate shallow average depths of melting. The main region of magma production was therefore beneath the relatively thin (ca. 50–100 km) lithosphere of the basin, and not the craton on which the present-day exposure of the Traps occurs. The indicated uplift, widespread occurrence of basalts, and short duration of the volcanic province as a whole are entirely consistent with published models involving a mantle plume. The main argument against the plume model, namely lack of any associated uplift, appears to be untenable.     

Polygenetic sources of kimberlites, magma composition, and diamond potential exemplified by the East European and Siberian cratons

The petrological and geochemical characteristics of kimberlites from two Russian provinces of the northern East European craton (EEP) and the Siberian craton (SC) (especially the Yakutian diamondiferous province, YDP), and aphanitic kimberlites from the Jericho pipe (Canada) were compared for the elucidation of some aspects of the genesis of these rocks. The comparison of the EEP and YDP showed that they comprise identical rock associations with some variations in kimberlite composition between particular fields and regions, which are clearly manifested in the TiO₂-K₂O, TiO₂-(Y, Zr, HREE), SiO₂-MgO, SiO₂-Al₂O₃, MgO-Ni, MgO-CO₂, and MgO-H₂O diagrams and in variations in light element ratios (Li/Yb, Be/Nd, and B/Nb). The compositions of YDP kimberlites are confined mainly to quadrant III; i.e., their source was mainly the depleted mantle, whereas the compositions of EEP kimberlites fall within all four quadrants in the fields of both enriched and slightly depleted mantle reservoirs. The initial (¹⁴³Nd/¹⁴⁴Nd) _i ratio of kimberlites from the Yakutian collection is 0.5121–0.5126. The lead isotopic characteristics of the EEP and YDP kimberlites are similar to mantle values: ²⁰⁶Pb/²⁰⁴Pb of 16.19–19.14, ²⁰⁷Pb/²⁰⁴Pb of 15.44–15.61, and ²⁰⁸Pb/²⁰⁴Pb of 34.99–38.55. In the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram, part of the kimberlites, including those from the Botuobiya pipe, fall within the lower part of the field of group I kimberlites from southern Africa near the Pb isotopic composition of the depleted mantle. It was shown that the chemical compositions of the aphanitic kimberlites of the Jericho pipe (supposedly approaching the composition of primary magmas) are similar to those of some individual kimberlite samples from the YDP and EEP. It was supposed that the initial kimberlite melt arrived from the asthenosphere and was enriched in water and other volatile components (especially CO₂). During its ascent to the surface, the melt assimilated mantle components, primarily MgO; as a result, it acquired the compositional characteristics observed in kimberlites. Subsequent compositional modifications were related to diverse factors, including the type of mantle metasomatism, degree of melting, etc. We emphasized the importance of petrological and geochemical criteria (low contents of HREE and Ti in the rocks and a kimberlite source similar to BSE or EMI) for the estimation of the diamond potential of rocks.     

Incipient hydrothermal alteration of basalts and the origin of martian soil

The martian soil is a fine-grained regolith that is chemically basaltic in character with evidence for both gains and losses of volatile and mobile elements compared to martian basalt compositions. These chemical fractionations provide clues to geochemical processes on the surface of Mars. Geochemical processes contributing to the soil proposed in the past include the chemical and mechanical breakdown of rocks under surface conditions, the addition of volcanic aerosols containing S and Cl compounds, and the alteration of basaltic glass to palagonite. Our studies of terrestrial analogs suggest that hydrothermal alteration processes involving impact craters and volcanism could also contribute to the major element trends observed in martian soil. Data from Viking, Pathfinder, and the current MER missions consistently show that relative to basaltic martian meteorite compositions, the major element compositions of the soils are (1) depleted in the fluid-mobile element calcium, (2) generally similar or somewhat enriched in iron oxide and magnesium but MgO depleted compared to Gusev rocks, (3) locally variable in potassium, (4) possibly poorer in aluminum, and (6) very enriched in chlorine and sulfur. The major element trends, aside from the Cl and S enrichment, could be explained by the formation or addition of palagonite according to McSween and Keil (2000), but the missing CaO remains a problem. The chlorine and sulfur are probably derived from other processes such as volcanic aerosols and hydrothermal fluids. McSween and Keil (2000) also argued that hydrothermal alteration of basalts produce alteration trends that are inconsistent with the Mars soil, but this study concludes otherwise. We have used quantitative mass balance mixing models to investigate possible models involving mixtures of basaltic compositions with different types of alteration materials, including palagonite. We show that the Mars soil composition can be matched with a combination of unweathered basaltic martian meteorites with basaltic FeO-rich, CaO-poor alteration products. Palagonite is a possible, but not a necessary component of successful model mixtures. The hydrothermal alteration materials that form successful model mixtures are formed in low temperature, low water/rock ratio environments, and they can reproduce the required geochemical trends because they are poorer in CaO but not in FeO compared to their respective protoliths. These results argue that material altered by hydrothermal processes could be a plausible component of the soil, and that removal of CaO from the soil into some undiscovered reservoir after its formation is not required. The current soil on Mars, therefore, did not have to undergo an episode of in situ aqueous alteration but could represent a sink for materials that experienced aqueous processes in a different setting before erosion to form the soil. The soil can also represent a sink for mobile elements (e.g., S, Cl, and Br) derived from other sources such as volcanic aerosols and hydrothermal fluids.