A review of the occurrence, form and origin of C-bearing species in the Khibiny Alkaline Igneous Complex, Kola Peninsula, NW Russia

Lithos 85 (2005) 93 – 112 www.elsevier.com/locate/lithos

A review of the occurrence, form and origin of C-bearing species in the Khibiny Alkaline Igneous Complex, Kola Peninsula, NW Russia V.A. Nivina, P.J. Treloarb,*, N.G. Konoplevaa, S.V. Ikorskya a

Geological Institute, Kola Science Centre RAS, 14, Fersman Str., Apatity 184209, Russia b CEESR, Kingston University, Penrhyn Road, Kingston-upon-Thames, KT1 2EE, UK Received 15 March 2004; accepted 11 March 2005 Available online 7 July 2005

Abstract The Khibiny Complex hosts a wide variety of carbon-bearing species that include both oxidized and reduced varieties. Oxidised varieties include carbonate minerals, especially in the carbonatite complex at the eastern end of the pluton, and Nacarbonate phases. Reduced varieties include abiogenic hydrocarbon gases, particularly methane and ethane, dispersed bitumens, solid organic substances and graphite. The majority of the carbon in the Khibiny Complex is hosted in either the carbonatite complex or within the so-called bCentral ArchQ. The Central Arch is a ring-shaped structure which separates khibinites (coarsegrained eudialite-bearing nepheline-syenites) in the outer part of the complex from lyavochorrites (medium-grained nephelinesyenites) and foyaites in the inner part. The Central Arch is petrologically diverse and hosts the major REE-enriched apatite– nepheline deposits for which the complex is best known. It also hosts zones with elevated hydrocarbon (dominantly methane) gas content and zones of hydrothermally deposited Na-carbonate mineralisation. The hydrocarbon gases are most likely the product of a series of post-magmatic abiogenic reactions. It is likely that the concentration of apatite-nepheline deposits, hydrocarbon gases and Na-carbonate mineralisation, is a function of long lived fluid percolation through the Central Arch. Fluid migration was facilitated by stress release during cooling and uplift of the Khibiny Complex. As a result, carbon with a mantle signature was concentrated into a narrow ring-shaped zone. D 2005 Elsevier B.V. All rights reserved. Keywords: Abiogenic; Hydrocarbons; Fischer–Tropsch synthesis; Igneous; Kola peninsula

1. Introduction

* Corresponding author. E-mail addresses: [email protected] (V.A. Nivin), [email protected] (P.J. Treloar). 0024-4937/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2005.03.021

The nepheline-syenites, foidolites and carbonatites of the Devonian Khibiny pluton, in the Kola Peninsula of NW Russia, contain an array of rare minerals as well as economically viable apatite–nepheline

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deposits (Dudkin, 1993; Kogarko et al., 1995). However, it has long been known that the pluton contains unusually high levels of hydrocarbon gases (HCG) (Petersilie, 1958, 1964). The presence of these gases has a number of practical implications and it is therefore important to understand their origin and distribution. It remains debatable by what means these reduced gases evolved in association with oxidised silicate rocks (Kogarko, 1977; Marakushev et al., 1998) although recent models, reviewed below, suggest that HCG evolution might be the result of abiogenic sub-solidus reactions, possibly of a Fischer–Tropsch reaction (Potter et al., 1998). However, the potential productivity of the reaction remains uncertain. The presence of the HCG, however formed, raises a number of questions and possibilities. It is unclear how homogeneously the HCG are distributed through the complex. What is the total volume of gas stored within the complex? How do abiogenically produced hydrocarbon gases contribute to the Earth’s carbon cycle? Are methane and hydrogen continually released to the atmosphere through the roofs of the Khibiny and Lovozero plutons and if so to what extent does this influence the origin of a negative anomaly in the ozone layer recorded above the central part of the Kola peninsula (Syvorotkin, 2002)? In addition, naturally combustible, and potentially explosive hydrocarbon gases, are released into mines that work the apatite deposits hosted by the Khibiny pluton (Nivin, 1991; Nivin et al., 1991, 2001a). Changes in gas chemistry and gas-dynamic indices have been shown to document changes in the geo-mechanical state of a rock mass and may therefore be used as predictive indicators for, often catastrophic, rock bursts in the underground workings (Nivin and Belov, 1993; Nivin et al., 2001a). It therefore remains important to develop a comprehensive picture of the distribution of the HC gases. Carbon is present within the Khibiny alkaline complex not just in the form of various HC gases. It is also present in a reduced state in dispersed bitumens as well as in the relatively rare form of solid biogenic matter and, even more rarely, as graphite. A significant part of the total amount of carbon present in the complex is held in an oxidized state in the carbonatite complex located in the eastern part of the massif. Besides its petrological importance, the carbonatite

body is of interest because of the unusual Ba–REE– Sr mineralization that it hosts (Dudkin et al., 1984; Zaitsev et al., 1998; Dudkin, 2001). Carbon is also held in the form of rare Na-carbonate minerals which are commonly located in veins (Khomyakov, 1985). The significance of the Na-carbonate-bearing veins is that their distribution may be linked to zones of increased HCG content, and thus their presence may be used as a tracer for predicting zones of high gas pressure that may result in rock bursts during mining (Ivanyuk et al., 1996). In this paper we explore a process-driven linkage between strain release during uplift-driven exhumation, hydrocarbon gas generation and the distribution of C-bearing species. The main aim is to demonstrate that, if this linkage is sound, the recognition of zones of Na-carbonate mineralisation in alkaline igneous intrusions might be used to identify reservoirs of abiogenic hydrocarbons. A secondary aim is to review, briefly, the form, composition and occurrence of the varied C-bearing species within the Khibiny complex. Published data are used here together with unpublished data collected by members of the Geological Institute of the Kola Scientific Centre.

2. Composition and distribution of C-bearing species The Khibiny pluton is late Devonian (377–362 Ma) in age (Kramm et al., 1993; Bayanova, 2004) and was emplaced at the contact between Archaean gneisses and a Proterozoic sedimentary–volcanic succession. A deep mantle plume was probably instrumental in driving Paleozoic alkaline magmatism in the Kola province (Arzamastsev et al., 2001; Tolstikhin et al., 1999, 2002). Geochemical, and especially isotopic, data indicate a depleted mantle source for the parental magmas of the Khibiny intrusion and related apatite deposits, and only limited contamination with crustal fluids (Kramm and Kogarko, 1994; Kogarko, 1999; Tolstikhin et al., 1999, 2002; Arzamastsev et al., 2001; Nivin et al., 2001b). The pluton has an asymmetric structure with different rock types occurring in concentric zones (Zak et al., 1972; Galakhov, 1975; Kostyleva-Labuntsova et al., 1978; Arzamastsev, 1994; Kogarko et al., 1995). It is mainly composed of nepheline-syenites (Fig. 1) and

V.A. Nivin et al. / Lithos 85 (2005) 93–112

95

N

5.0

0

5.0

10.0 km

hydrothermal veins dykes of alkaline & alkaline-ultramafic rocks explosion pipes (after Ivanyuk et al. 2002) carbonatites alkaline syenites apatite-nepheline rocks

melteigite-urtites rischorrites nepheline syenites (khibinites, lyavochorrites and foyaites) fine-grained nepheline syenites & xenoliths of alkaline-ultramafic rocks distribution pattern of occluded gases (after Ikorsky 1977) locations of the most intense release of free gas

Fig. 1. Schematic geological map of the Khibiny complex. The circular zone of melteigite–urtites and rischorrites marks the location of the Central Arch complex.

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is separated into two, approximately equal, parts by a ring-like structure known as the Central Arch. The external part of the complex is largely constituted of massive and trachytoidal khibinites (coarse-grained, eudialite-bearing, foyaite-like nepheline-syenites). The internal part of the pluton consists mainly of foyaites with a marginal zone of lyavochorrites (medium-grained nepheline-syenites). Rocks of the Central Arch include urtites, ijolites, melteigites, rischorrites (massive, coarse-grained kalsilite-bearing nepheline-syenites with up to 11% K2O) and juvites (nepheline-rich syenites intermediate between rischorrites and urtites). Rocks within the Central Arch show a symmetric zonation with apatite–nepheline rocks in the core flanked by melteigites–urtites, then rischorrites and finally K–Na nepheline-syenites (khibinites on the outer side and lyavochorrites and foyaites on the inner side). The nepheline-syenites are strongly albitised close to, and on either side of, the Central Arch (Tikhonenkov, 1963; Zak et al., 1972; Galakhov, 1975). The main apatite–nepheline deposits are located within the Central Arch complex. From the NW to the SE these include the Partomchorr, Kuelpor, Snezhnyi Tsirk, Kukisvumchorr, Yukspor, Apatitovyi Tsirk, Rasvumchorr, Eveslogchorr, Koashva, Vuonnemiok, H’orpakhk and Oleniy Ruchey deposits (Fig. 1). A number of hypotheses for their origin have been suggested. These include crystallisation of phosphate rich magmas, crystallization of P-bearing nephelinesyenite or apatite–nepheline melts, differentiation of an ijolite–urtite melt or metamorphism of ijolite– urtites or relic alkaline effusive rocks (Galakhov, 1975; Kostyleva-Labuntsova et al., 1978; Dudkin, 1993; Kogarko, 1999 and references therein). The majority of the late pegmatite and hydrothermal veins, as well as explosion pipes and dykes of alkaline and alkaline–ultrabasic rocks are also found within, or close to, the Central Arch (Arzamastsev et al., 1988; Ivanyuk et al., 2002). Table 1 lists the various forms of C-bearing species present in the Khibiny complex. The majority of the carbon is held within the carbonatite complex which is located near the eastern margin of the pluton where the Central Arch pinches out (Fig. 1). It may thus be linked to the Central Arch. The core of the complex is about 1 km in diameter, and is composed of calcite– albite, calcite–biotite, aegirine–calcite–biotite rocks

Table 1 C-bearing species of the Khibiny pluton State of carbon

Occurrence forms

Reduced

Hydrocarbon occluded and free (fracture) gases Dispersed bitumens Solid organic matter Graphite Carbonates of Ca, Mn, Fe, Mg (carbonatite complex) Veined Na-carbonates Hypergene Na-carbonates

Neutral Oxidised

and albite–calcite, biotite–calcite and mangano-calcite carbonatites. The core is surrounded by stockworks of calcite–albite rocks, carbonatites with Ba–REE–Sr mineralization, phoscorites and carbonate-bearing zeolites with individual veins up to 20 m wide (Dudkin et al., 1984; Zaitsev, 1996; Zaitsev et al., 1997; Dudkin, 2001). Na-carbonate minerals, including shortite and tuliokite, are also present (Snyatkova et al., 1984). The host foyaites are intensively carbonatised. The carbonatite complex which, according to borehole data, extends to a depth of at least 1600 m, is cut by numerous dykes and veins of lamprophyres and alkaline porphyries (Dudkin et al., 1984). A distinctive feature of the Khibiny carbonatites is the widespread development of REE mineralization (Zaitsev et al., 1997; Dudkin, 2001). The Khibiny carbonatites differ from other carbonatites within the Kola alkaline province in that they have a higher gas-content (c. 21 cm3 kg 1 as opposed to 2.3 cm3 kg 1) with enhanced levels of HC gases in the gas phase (Dudkin et al., 1984; Ikorsky et al., 1992). When compared with the host nepheline-syenites, they have higher (8.3 cm3 kg 1 as opposed to 1.1 cm3 kg 1) concentrations of CO2 in their fluid inclusion population as well as greater levels of dispersed bitumens (Dudkin et al., 1984; Pripachkin et al., 1985). The isotopic composition of carbon in the Khibiny carbonatites with d 13C between 7.8 and 1.8x (Fig. 2) is similar to that of carbonatites elsewhere on Earth. Together with Rb–Sr isotope data (Kramm et al., 1993), this suggests synchronous extraction of the parental carbonatite and nephelinesyenite magmas but from different mantle sources (Kramm and Kogarko, 1994). Geochemical data suggest partial crustal contamination of the carbonatites (Zaitsev et al., 1997).

V.A. Nivin et al. / Lithos 85 (2005) 93–112 0

C13, ‰VPDB

in the Kukisvumchorr apatite–nepheline deposit suggests the paragenetic sequence: microcline–aegirine– natrolite–calcite–barite–donnayite-(Y)–mackelveyite(Y)-solid organic species (Ivanyuk et al., 1996). This implies that the REE-bearing and then the Na-bearing carbonates both crystallized late in the

1 2 3 4 5

-2

-4

Table 2 Carbonate phases described from the Khibiny complex (after Yakovenchuk et al., 1999)

-6

-8

-10 4

97

8

12 18O,

16

20

24

28

‰ VSMOW

Fig. 2. Carbon and oxygen isotope composition of carbonate phases from the Khibiny pluton, after Borschevsky et al., 1987; Khomyakov, 1995; Zaitsev, 1996; Zaitsev et al., 2002.) 1: Ca, Mn, Fe and Mg-bearing carbonates from the carbonatite complex. 2: REE-bearing carbonates from the carbonatite complex. 3: Na and Ca-bearing carbonate phases from hydrothermal veins of the Central Arch. 4: (3) and Na-bearing carbonate phases from hydrothermal veins of the Central Arch. 5: hypergene carbonates from hydrothermal veins of the Central Arch.

Soda-carbonate mineralization is pervasively developed in rocks of the Central Arch complex, but is most intensively developed along its southeast flank and close to the carbonatite complex (Khomyakov, 1995). Below the weathering zone, at depths of 0.2 to 2 km, boreholes and underground workings cut zones of soda-carbonate mineralisation up to hundreds of meters thick. These zones contain veins of carbonate minerals (Table 2), which include Na-carbonates, the most important of which are natrite, thermonatrite, pirrsonite (Fig. 3), trona and natron (Khomyakov, 1995). Also present within these zones are Na-carbonate minerals (shortite, tuliokite) and Na-carbonate–phosphate minerals (sidorenkite, bronshtedtite), as well as calcite, aragonite, ankerite, rhodochrosite, siderite, strontianite, Ba-carbonates (witherite, barytocalcite), and La, Ce and Y-bearing carbonates (burbankite, carbocernaite, ancylite, zhonghuacerite-(Ce), donnayite-(Y), ewaldite, mackelveyite-(Y)) (Khomyakov, 1995; Ivanyuk et al., 1996; Yakovenchuk et al., 1999). There is a regular increase in the intensity and variety of carbonate mineralisation towards the apatite–nepheline ore bodies (Ivanyuk et al., 1996). Detailed analysis of carbonate-bearing hydrothermal veins

Mineral name

Formula

Ancylite-(Ce) Ancylite-(La) Ankerite Aragonite Barentsite Barytocalcite Bastna¨site-(Ce) Burbankite Calcite Carbocernaite Cebaite-(Ce) Cerussite Cordylite-(Ce) Dawsonite Dolomite Donnayite-(Y) Eitelite Ewaldite Ferrotychite Gaylussite Hydrocerussite Khanneshite Kukharenkoite-(Ce) Kutnohorite Mackelveyite-(Y) Magnesite Malachite Nahcolite Natrite Natron Parisite-(Ce) Petersenite-(Ce) Pirssonite Rhodochrosite Shortite Siderite Strontianite Synchysite-(Ce) Thermonatrite Trona Tuliokite Tundrite-(Ce) Vaterite Witherite

SrCe [CO3]2(OH)d H2O SrLa[CO3]2(OH)d H2O CaFe2+[CO3]2 Ca[CO3] Na7Al[CO3]2[HCO3]2F4 Ba7Ca[CO3]2 Ce[CO3]F (Na,Ca)3(Sr,Ba,Ca,REE)3[CO3]5 Ca[CO3] (Na,Ca)(Sr,Ce,Ba) [CO3]2 Ba3Ce2[CO3]5F2 Pb[CO3] NaBaCe2[CO3]4F NaAl[CO3](OH)2 CaMg[CO3]2 Sr3NaCaY[CO3]6d 3H2O Na2(Mg,Fe2+)[CO3]2 Ba(Ca,Na,Y0[CO3]2d nH2O Na6Fe2[SO4] [CO3]4 Na2Ca[CO3]2d 5H2O Pb3[CO3]2(OH)2 Na3(Ba,Sr,Ca,Ce)3[CO3]5 Ba2Ce[CO3]3F CaMn[CO3]2 Ba3NaCaY[CO3]6d 3H2O Mg[CO3] Cu2[CO3](OH)2 NaH[CO3] Na2[CO3] Na2[CO3]d 10H2O CaCe2[CO3]3F2 Na4Ce2[CO3]5 Na2Ca[CO3]2d 2H2O Mn[CO3] Na2Ca2[CO3]3 Fe2+[CO3] Sr[CO3] CaCe[CO3]2F Na2[CO3]d H2O Na3[CO3][HCO3]d 2H2O Na6BaTh[CO3]6d 6H2O Na3Ce4Ti2[SiO4]2[CO3]3O4(OH)d 2H2O Ca[CO3] Ba[CO3]

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1 mm

Fig. 3. SEM image of pirssonite crystals from a thermonatrite– sodalite–microcline vein in urtites of the Kukisvumchorr apatite deposit.

sequence. The Na-carbonate veins show a wider range in both carbon and oxygen isotope compositions than the carbonatite complex (Fig. 2). Along with the hypogene Na-carbonate mineralisation is a widespread hypergene mineralisation characterised mainly by Na-bearing hydro-carbonates (Ivanyuk et al., 1996; Goryainov et al., 1998; Khomyakov, 1995). They have the lowest d 13C and highest d 18O ratios of all the Khibiny carbonate phases (Fig. 2). At many localities within the Central Arch, particularly within the apatite–nepheline deposits, Narich zones (Fig. 4) form structures with geometries typical of percolation clusters. These are morphologically similar to nepheline–apatite and pegmatite stockworks (Goryainov et al., 1998; Ivanyuk et al., 2002). Percolation theory is grounded in the work of Broadbent and Hammerslay (1957) who described the

rischorrites

urtites

apatite-nepheline rocks

soda mineralization

100 m

1m

4 cm

Fig. 4. Schematic map of part of the Rasvumchorr-bearing carbonate phases from hydrothermal veins of the Central Arch apatite deposit showing the concentration of clusters of Na-carbonate mineralisation in rischorrites adjacent to zones of apatitite–nepheline ore. (after Goryainov et al., 1998). Images show the stockwork appearance of the clusters.

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filtration of liquid or gas through a porous medium. The approach that they outlined is applicable to many fields and is especially relevant to studies of the development of dendritic and skeletal crystals and the flow of fluids through fracture systems (Balberg et al., 1991; Goryainov and Ivanyuk, 2001a,b). Percolation clusters follow the laws of fractal geometry and therefore tend to be self similar (Sadovsky and Pisarenko, 1991). The Khibiny soda carbonates occur in percolating clusters of veins with a sub-horizontal orientation, a geometry typical of precipitation from a liquid diffusing through a porous medium (Sokolov, 1986). Although they show a wider range, the carbon isotope chemistry of the Na-carbonates (d 13C = 9.6 to 0.9x; Borschevsky et al., 1987; Khomyakov, 1995) is similar to that of the carbonatite complex. The gas phase in the Khibiny massif is represented by two morphological types. Occluded gas (OG), is enclosed in isolated micro-cavities of minerals, mainly in fluid inclusions, and free or fracture gas (FG) fills fracture systems, mainly micro-fractures of varying extent, opening and connectivity (Ikorsky et al., 1992). OG can be extracted from rocks only by crushing or heating to high temperatures. Two crushing techniques have been used: vacuum ball mill and mercury pump (Petersilie and So¨rensen, 1970; Ikorsky et al., 1992) and original vibrating chamber connected to a gas chromatograph (Ikorsky, 1999).

99

FG can be spontaneously released through opening of a fracture system by erosion, drilling or mining. Usually a portion of the FG remains in micro-fractures for some time after opening of a reservoir. Such residual free gases (RFG) can be extracted during thermovacuum degassing of a sample, such as a borehole core, and thus can be used to characterize the FG composition and content (Ikorsky et al., 1992; Nivin et al., 2001a). The main component of the gas phase in the Khibiny nepheline-syenites, foidolites and apatite ores is methane. This occurs together with small amounts of molecular hydrogen, and Cn Hn + 2 species up to pentane, and nitrogen. Minor components include helium, argon and non-saturated hydrocarbons (alkenes). Small amounts of CO and CO2 may be present although the latter is prevalent in the gas phase hosted within the carbonatites (Dudkin et al., 1984; Ikorsky et al., 1992). Sampled intrinsic FGs are commonly diluted by air to a greater or lesser extent (Table 3). Both OG and FG contain the same gaseous components although in different ratios (Table 4). In particular, FGs contain, on average, considerably higher contents of hydrogen, ethane and helium. The carbon isotope composition is heavier in the OG, where d 13CC1–C5 and d 13CCH4 varies from 13.2 to 3.2x, than in the FG where it varies from 19.3 to 6.5x (Galimov and Petersilie, 1967; Galimov,

Table 3 Selected samples of free gas–air mixtures from boreholes within the Central Arch zone Sample

29372 9-b 288P452 50P269 50P450 PP4 138Yu520 1253 - 16 1288 - 2

Location1 KO EV EV AT AT AT AT AT YU YU KU KU ST

Reference3

Vol.% CH4

C2H6

C2H4

C3–C25

H2

He

CO2

N2

O2

61.88 2.45 75.00 0.085 5.11 28.25 64.65 84.08 0.12 76.2 7.7 75.3 0.15

3.46 0.29 8.06 0.025 0.41 1.87 3.64 7.72 0.068 5.2 0.93 n.a. n.a.

n.a. n.a. n.d. 0.00013 0.00006 0.019 n.d. n.a. 0.0075

0.284 0.38 0.19 0.00319 0.02905 0.13056 0.2372 0.71 0.00142

0.00096 n.a. n.a.

0.06307 n.a. n.a.

3.80 0.45 17.6 0.0027 0.41 3.9 17.9 3.07 0.8 13.9 4.7 11.8 5.05

n.a. 0.01 0.314 0.0007 0.046 0.039 0.64 n.a. 0.0007 n.a. 0.19 n.a. 0.028

0.60 0.15 n.d. 0.011 n.d. n.d. n.d. 0.90 0.12 n.a. n.d. n.a. n.a.

n.a. 80.6 1.1 78 74.2 53 8.9 n.a. 78.3 4.3 67.7 12.3 73.9

6.80 18.7 0.1 21 19.4 10.5 2.2 1.6 19.9 0.4 17 0.60 19.1

[1] AD [2] AD AD AD AD [1] AD [3] AD [1] AD

Notes. 1 Apatite–nepheline deposits: KO—Koashva, EV—Eveslogchorr, ATS—Apatitovyi Tsirk, YU—Yukspor, KU—Kukisvumchorr, ST— Snezhnyi Tsirk; 2 C3H8 + C3H6 + iC4H10 + nC4H10 + aC4H8 + hC4H8 + iC5H12 + C5H12; 3 [1]—Pripachkin, Kamenev, 1968; [2]—Ikorsky et al., 1992; [3]—Petersilie, 1964; AD—authors’ data; n.a.—not analysed; n.d.—not detected.

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Table 4 Mean chemical composition of the gas phase in the Khibiny massif (calculated on the basis of an airless mixture of components) Gas type OG FG

Vol% CH4

H2

C2H6

He

CH4/H2

CH4/C2H6

93 74

4 20

2.7 5

0.05 0.8

23.3 3.7

34.4 14.8

OG: occluded gas, FG: fracture gas.

1975; Voitov, 1977; Voytov, 1992). d 13C decreases in the order CH4–C2H6–C3H8. Potter, unpublished observed an increase in d 13CCH4 in OG from 11.44 to 3.28x and from 25.33 to 12.34x during step heating of samples from 400 to 700 8C. The lower values of d 13C in this case could result from contamination with dispersed bitumens and solid organic matter. The gas phase is not distributed homogeneously through the pluton. The OG content in rocks and minerals varies from b 0.1 to 150 ml/kg. The greatest variability is recorded in urtites and rischorrites of the Central Arch. Analysis of the available data shows that, on a regional scale, the highest HC gas contents are found in nepheline-syenites close to the Central Arch, especially in areas characterised by albitisation

(Fig. 5; Tikhonenkov, 1963; Ikorsky, 1977; Ikorsky et al., 1992). On a more local scale, gas contents are higher in rocks immediately above and below apatite– nepheline ore zones than in the ores themselves (Figs. 6, and 7; Ikorsky and Fanygin, 1986; Ikorsky et al., 1992). A similar relationship is sometimes observed in nepheline-syenites cut by pegmatites (Ikorsky, 1967). All of the significant discharges of FG recorded from the Khibiny complex are from within the Central Arch zone (Ikorsky et al., 1992; Nivin et al., 2001a). Enhanced FG release has been observed within the Central Arch in boreholes drilled on surface and in underground workings. Enhanced FG concentrations have not been found in boreholes drilled outside the Central Arch. Petersilie (1964) examined gas compositions in friable Quaternary deposits on the Khibiny massif and country rocks through use of shallow (1– 1.5 m) holes. More than 600 holes, preliminarily sealed with clay in their upper part, were sampled. Methane (up to 0.169 vol.%), hydrogen and heavier HCG were found in many of the holes. Reinterpretation of these data shows inconsistency between mean HCG concentrations in subsurface (FG) and occluded gases in underlying alkaline rocks. More important is that the highest concentrations are in zones above

3

HCG, cm /kg

50

25

0

TO INTRUSION CENTRE

massive khibinites

lyavochorrites

trachytoid khibinites

trachytoid foyaites

rischorrites

massive foyaites

melteigite-urtites

zones of intensive albitization in nepheline syenites

Fig. 5. Generalized distribution of occluded gases along a combined radial cross-section through the Khibiny massif (after Ikorsky et al., 1992). High HC gas concentrations are spatially linked to zones of albitisation.

V.A. Nivin et al. / Lithos 85 (2005) 93–112

a)

SW

600

NE

0

-600 m

NW

b)

SE

NW

SE

600

0

-600

-1000 m

average occluded gas concentrations 10-15cm3/kg

apatite-nepheline ores prospecting drill holes

15-30 cm3/kg >30 cm3/kg <10 cm3/kg

Fig. 6. HC gas concentrations in gas-saturated zones in longitudinal (a) and transverse (b) sections through productive zones of the Oleniy Ruchey apatite deposit (after Ikorsky and Fanygin, 1986). The highest gas concentrations are generally spatially linked with the apatite–nepheline ores.

rocks of the Central Arch and the pluton margins. To amplify these data we have recently initiated a new study of the subsoil atmosphere. In the course of a pilot study, we sampled subsoil atmospheres along a profile crossing the Khibiny massif from SW to NE using 0.5–0.7 m deep shot-holes. Chromatographic analysis shows measurable CH4 contents from 0.00020 to 0.0144 vol.% in only 11 of 140 samples. These 11 are located over the CA structure as well as over khibinites immediately outside of the CA as over carbonatised foyaites near the carbonatite plug. CO2

101

concentrations of 0.1 to 1.0 vol.% were found in most shot-holes using a portable Riken-18 gas-analyser. The highest CO2 concentrations were recorded over the pluton margin, the CA and the carbonatite complex. These preliminary data suggest that much of the pluton does not carry significant CH4 contents. In general the highest levels of RFG in borehole cores and high initial flow rates of FG in shot-holes drilled in underground mines, are recorded from dry zones which contain no free water in the fractures, have high enhanced OG contents (Figs. 7 and 8) and have well developed systems of interconnecting micro-cracks. These micro-fractured blocks of FGsaturated gas are probably surrounded by zones of micro-fractures with poor connectivity otherwise they would contain a greater volume of free water. The variable distribution of CH4 and H2 in mine atmospheres suggests, in the absence of a forced ventilation system, a low rate of diffusional outflow of FG gas which is variable in both space and time. Evidence for rapid discharge, such as short term blowouts of terrestrial gas and fluid from boreholes, intensive gas filtration from fracture systems into shotholes and the presence of gas bubbles within water, are rarely observed. As a rule, intensity of gas release is sharply reduced within the first minutes or hours after opening a reservoir. Gas discharge rates within single shot-holes with a depth of 2 m and diameter of 40 mm do not exceed 5 l/min (Nivin et al., 2001a). However, more often discharge rates are two orders of magnitude less than this. The duration of gas release varies from several days to more than a year and is probably dependant on both the size of the local reservoir and its communication with other parts of the pluton through gas-filled micro-fractures. In the Khibiny complex reduced carbon is also present in the form of micron-sized particles of dispersed bitumens, as well as in larger particles of various solid organic species (Petersilie, 1964; Zezin, 1968; Florovskaya et al., 1968; KostylevaLabuntsova et al., 1978; Pripachkin et al., 1985; Ivanyuk et al., 1996). The total content of reduced carbon in the rocks averages 0.04 wt.% and can be as high as 0.1 wt.% in ijolite–urtites hydrated during post-magmatic cooling and up to 2.1 wt.% in minerals in pegmatites and hydrothermal veins. The concentration of dispersed bitumens (a chloroform extract) varies from 0.0001 up to 0.01 wt.% with the maximum

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a) RFG 10 5 0

b) RFG

OG 0 10 30

210m

4

2

0 0

c) OG 20 40

1178m

d)

RFG 3

OG

1 0

0

10

RFG 20

6

311m

3

0

0 30 60

160m

rischorrites

ijolites

urtites

lyavochorrites

juvites

apatite-nepheline ores

e) OG

OG 0

30

60

301m friable Quaternary deposits

3

Fig. 7. Distribution (cm /kg) of occluded (OG) and residual free gases (RFG) in core from boreholes in apatite–nepheline deposits. Figures under the columns indicate the borehole length. Boreholes: a — #477 from the Yukspor deposit; b and c — #1258 and #1266, respectively, from the Kukisvumchorr deposit; d and e — #1667 and #1666, respectively, from the Kuelpor deposit. There are no recorded RFG data from hole (e).

values in altered ijolite–urtites, as well as in rischorrites and carbonatites (Florovskaya et al., 1968; Pripachkin et al., 1985). Segregations of concentrated organic carbon (Fig. 9), reaching 5 cm and more in size, are represented by anthraxolite, kerite, impsonite, albertite, asphaltite, asphalt and maltha. They are found in pegmatites and hydrothermal rocks of the Central Arch (Zezin, 1968; Florovskaya et al., 1968; Ivanyuk et al., 1996) and in carbonatites (V. Yakovenchuk, pers. comm.) and in most cases were precipitated during the last stages of hydrothermal activity (Florovskaya et al., 1968; Ivanyuk et al., 1996). d 13C values from solid organic carbon are between 10.8 and 31x (Zezin, 1968). Graphite, which is the neutral form of carbon, occurs in albitized nepheline-syenites and in pegmatite and hydrothermal veins, both as radial aggregates (Fig. 10) and in a dispersed state (Florovskaya et al., 1968; Kostyleva-Labuntsova et al., 1978). This graphite has d 13C values of 21 to 19x (Zezin, 1968).

3. The evolution of HC-bearing gaseous species in the Khibiny pluton As they occur together in many rocks (Figs. 7 and 8) and have similar chemical compositions (Table 4) it is possible that the free and occluded HC gases were generated synchronously with each other in individual alkaline rock units and that they have a common origin (e.g., Ikorsky et al., 1992). Although this assumption may be generally valid, it is possible for the FG and OG gases to have different sources. For instance, FG could have been generated within, and then released from, deeper parts of the intrusion and then trapped within fracture systems in higher levels. One way for this to happen is if gas originally trapped as FG was subsequently released due to seismic opening of a fracture system. This gas could then have migrated through the pluton before becoming trapped in a second fracture system, probably at a higher level. In addition, if gas migrates consistently upward

V.A. Nivin et al. / Lithos 85 (2005) 93–112

103

a) 1

0

0

b) 100

0

40m

1

0

FG flow rate, ml/min

Methane of OG, ml/kg

100

0 distribution of occluded (methane) gas flow rate of shot-hole free gas

apatite-nepheline ores urtites

rischorrites

Fig. 8. Relationship between rock type and the two main morphological gas types in the Apatite Tsirk deposit (a level + 530 m, b level + 470 m). The highest OG concentrations and FG flow rates are in urtites immediately below the apatite–nepheline horizon.

through the pluton it is likely that there are a number of zones in which the FG is a temporary resident on its way to the surface. Seismic activity could result in opening of a fracture system, migration of FG from that system and its collection in secondary fracture opened during the seismic event.

A number of general models have been cited for HC gas evolution in alkaline complexes. These include transport of organic carbon in fluids from surrounding volcano-sedimentary units into the alkaline plutons; in situ, sub-solidus, late-stage or post-magmatic abiogenic formation of HC gases; or prolonged, long term flow of deeply-derived, abiogenic gases that

3

1

2

3 Fig. 9. SEM image showing segregations of a solid organic carbon compound (black) in a hydrothermal vein within the Kukisvumchorr apatite–nepheline deposit. 1 — tuliokite, 2 — carbon compound, 3 — pirssonite.

Fig. 10. Radial axial growths of graphite and aegirine from an aegirine–albite vein in khibinites, (eudialite-bearing foyaite-like nepheline-syenites) from Mt. Takhtarvumchorr, in the southern part of the Khibiny massif.

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isotope chemistry. Potter and Konnerup-Madsen (2003) argued that the C and H isotope data are inconsistent with a biogenic origin. They summarised d 13CCH4 and dDCH4 data from a variety of hydrocarbon-bearing fluids in alkaline igneous rocks. The d 13CCH4 data range from 1.0 to 44.9x although the majority are in the range of 20 to 28x. For the Khibiny pluton they range from 3.2 to 12.8x (Table 5). In the Ilı´maussaq intrusion the d 13CCH4 data show a restricted range of 1.0 to 7.0x. These data do not fall within the mantle field ( 3 to 9x), so a direct mantle origin is unlikely, although the stable existence of methane and other hydrocarbons in the mantle has been suggested by Sugisaki and Mimura (1994) and Zubkov (2001) among others. In the absence of a direct mantle origin two possibilities remain for sub-solidus evolution of the HC gases. One is respeciation of C–O–H gases (Konnerup-Madsen, 2001). The other involves a Fischer– Tropsch reaction (of the type CO2 + 4H2 X CH4 + 2H2O) which involves interaction between magmatic minerals and a reduced late-magmatic fluid phase (Fig. 11). Abundant textural evidence from the Khibiny and Lovozero plutons, summarised in Potter et

possibly continues to the present day (see Ref. in Potter and Konnerup-Madsen, 2003). In the particular case of the HC gases of the Khibiny and nearby Lovozero complexes a number of models have been proposed. The two complexes have similar gas occurrences, fluid inclusion petrographies, and chemical and isotopic compositions. Proposed models include: magmatic assimilation from country rocks followed by thermogenic alteration of organic matter, or later (post-magmatic) hydrocarbon migration from organicbearing country rocks (Lyutkevich, 1967; Kravtsov, 1968); prolonged, long term flow of deeply derived abiogenic gases that possibly continues to the present day (Voitov, 1975; Kravtsov and Grigoruk, 1972); abiogenic HCG formation in alkaline melt before or during magmatic crystallization (Petersilie, 1964; Karzhavin and Vendillo, 1970) or at sub-solidus late-and/or mainly post-magmatic stages of evolution of these complexes (Ikorsky, 1977; Kogarko, 1977; Kogarko et al., 1987; Ikorsky et al., 1992; Nivin et al., 1995; Potter et al., 1998, 1999, 2004; Potter, unpublished; Nivin, 2002). An abiogenic, post-magmatic, sub-solidus hypothesis for hydrocarbon gases in alkaline rocks is based largely on textural data, fluid inclusion analysis and

Table 5 Carbon isotope data for gas components from the Khibiny pluton Rock/mineral Occluded gases Carbonatite (a) Khibinite (b) Khibinite (b) Urtite (b) Ijolite (b) Nepheline (c) Alkaline feldspar (b) Eudialite (b) Eudialite (b) Eudialite (c) Aenigmatite (c) Free gases Ijolite–urtite (b) Ijolite–urtite (c) Ijolite–urtite (c) Rischorrite ? (d) Rischorrite ? (d) Rischorrite ? (d)

d 13CC1–C5

d 13 CH4 (x)

d 13 C2H6 (x)

d 13 C3H8 (x)

d 13 CO2 (x) 7.3

13.2 3.2 12.8 8.4 11.4 –

9.1 24.5

25.7 26.0

4.3 14.6 8.4 7.9

8.1 – 6.1

4.6

19.3 –

11.8 6.5 10.6 11.2 16.5 –

8.5 +10.6 14.2

11.7 23.9 15.6 7.7 24.0 –

a: Potter, unpublished; b: Galimov and Petersilie; c: Galimov, 1975; d: Voitov et al., 1990.

12.1

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H2O (Crustal fluid)

105

CO2 (Magmatic fluid)

H2 (Newly generated)

Hydrothermal mineral alterations

Fischer-Tropsch reaction CO2+4H2= CH4+2H2O

H2O

CH4 - H2O (Immiscible fluid)

Separation of CH4 and other hydrocarbons Fig. 11. A possible model for hydrocarbon formation in agpaitic magmatic complexes (after Potter et al., 1999).

al. (1998, 2004), supports a post-magmatic origin for the HC gases through a Fisher–Tropsch reaction. These textures relate to breakdown of anhydrous phases and their replacement by hydrous phases together with the production of hydrogen gas which feeds the next stage of the Fischer–Tropsch reaction. In particular, fluid inclusion planes are morphologically linked to hydrated minerals and their breakdown products (Potter et al., 1998, 2004; Potter, unpublished). The H2 which triggers the Fischer–Tropsch reaction could be produced as a result of hydration of phases such as aegirine, magnetite, cancrinite and zeolites, or replacement of nepheline (Potter et al., 1998, 1999, 2004). Potter et al. (2004) list balanced equations which model these reactions and which supercede the reactions listed in Potter et al. (1998), not all of which were balanced. Evidence for the Fischer–Tropsch reaction is also based on microther-

mometric and Laser Raman analysis of fluid inclusion suites, document the presence of methane and higher hydrocarbons, and the mineral textures spatially linked to those suites (Ikorsky, 1977; Kogarko, 1977; Kogarko et al., 1987; Ikorsky et al., 1992; Potter et al., 1998, 1999; Potter, unpublished; Nivin, 2002). The respective volumes of HC gases present in alkaline igneous complexes, including Khibiny, is also indicative of an abiogenic, rather than a biogenic, origin. A plot of log-normalised abundances of hydrocarbon species for HC-bearing fluids from alkaline igneous intrusions shows a log-linear decrease in concentration with increasing C-number (Potter et al., 2004, Fig. 6). This Schulz–Florey distribution is very different from that shown by biogenic hydrocarbons with show little change in abundance with increasing C-number.

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The isotopic composition of methane and the higher order hydrocarbon compounds are persuasive evidence for abiogenic genesis of the HC gases Potter and Konnerup-Madsen (2003). Biogenically produced gases tend to have much more depleted CH4 signatures than abiogenically produced gases (see Potter and Konnerup-Madsen (2003) and references therein). For instance, values of d 13CCH4 of N 13 for gases in the Khibiny complex are far less depleted than those commonly recorded in biogenic methane. In addition, Potter and Konnerup-Madsen (2003) showed that, for abiogenically produced gases, d 13C values become depleted with increasing carbon number (Cn). This is the case for CH gases analysed from a variety of alkaline complexes including Khibiny and Lovozero (see Fig. 4 in Potter and Konnerup-Madsen, 2003). Depletion of d 13C values with increasing carbon number (Cn) in HCG from alkaline igneous complexes (Galimov and Petersilie, 1967; Galimov, 1975; Voitov, 1977, 1992; Konnerup-Madsen et al., 1988; Nivin et al., 1995; Potter and Konnerup-Madsen, 2003) strongly implies an abiogenic origin as a result of simple hydrocarbon species polymerization as polymerisation of simple hydrocarbon species will lead to preferential enrichment of 12C to 13C with increasing Cn. This contrasts with organic cracking of complex organic species which gives an inverse d 13C distribution. Equilibrium and non-equilibrium fractionation of mantle derived carbon with an initial isotopic ratio of d 13C~5x during Fischer–Tropsch synthesis, together with chromatographic isotope separation during fluid movement (Bottinga, 1969; Galimov, 1988; Gutsalo and Plotnikov, 1990; Deines, 2002), can both explain the different isotopic compositions of the carbon species. Fractionation of d 13C isotopes during Fischer– Tropsch synthesis in a relatively closed system is well defined. Initial conversion of a primary magmatic CO2 fluid to CH4 with subsequent partitioning will fractionate the CH4 to a lighter d 13C signature. Latestage CH4, associated with total conversion of CO2, will have d 13C signatures closer to the initial magmatic value. Therefore, a wide range of d 13CCH4 values can be produced during CH4 generation (Voytov, 1992). The majority of CH4 found in igneous rocks show signatures between 20 and 28x, indicating fractionation from the initial magmatic fluid at temperatures between 300 and 400 8C (Bottinga, 1969). The initial origin of the carbon remains

uncertain. Available isotopic and geochemical data suggest that interaction between subducted oceanic lithosphere and an ascending mantle plume may be a feature of alkaline and carbonatitic magmatism in general (Ray et al., 1999; Kogarko and Khain, 2001) and of the Kola Province in particular (Sorokhtin et al., 1996; Dauphas and Marty, 1999; Matsumoto et al., 2001). Carbon, and other volatile components, may have been incorporated into the system solely through recycling of the subducted plate and the sediments carried by it. Given the isotopic signature of carbon within the Khibiny complex it is more likely that it represents a mixture of subducted and primary mantle carbon. Gas production through Fischer–Tropsch synthesis relates primarily to the evolution of occluded gas. CO2 present as a reactant was a residual late-stage magmatic fluid. Occluded HC gases were trapped in fluid inclusions at less than 2 kbar pressure and between 200 and 400 8C (Potter et al., 1998, 2004), although Kogarko (1999) suggests temperatures as low as 150 8C. A similar origin for HC gases has been suggested for the Strange Lake peralkaline granite (Salvi and Williams-Jones, 1997). CH gases in fluid inclusions in the Ilimaussaq intrusion closely resemble the Khibiny and Lovozero OG in chemistry, isotope composition and occurrence and were probably trapped at a late magmatic stage (Konnerup-Madsen, 1988, 2001; Konnerup-Madsen et al., 1988) although these workers prefer an origin through respeciation of a C–O–H gas phase. However, Markl et al. (2001) suggest that some of the methane fluid phases could have a high temperature magmatic origin. In contrast to the Ilimaussaq intrusion (Markl and Baumgartner, 2002), post magmatic features are widely developed in the Khibiny and Lovozero plutons. The origin of the free gas is less clear than that of the occluded gas which is most likely of a postmagmatic abiogenic origin. Although the FG contain the same chemical components as the OG, they have a subtely different chemical composition (Table 4). On the basis of the differences in chemistry, the distribution patterns of the OG and FG, the greater mobility of the FG and the relationship between gas content and the mechanical state of a rock mass (Nivin and Belov, 1993; Nivin et al., 2001a), as well as the continuing seismic activity of the massif (Kuz’min et al., 1994; Kozyrev et al., 1996), we suggest that the FG proba-

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bly contains a variable mixture of HC gases derived from a variety of sources. These include: gas trapped in fractures and along cleavage planes later or at the same time as the OG was trapped in fluid inclusions; gas that has diffused through the fracture network from deeper parts of the pluton and which may have been chemically and isotopically modified during diffusion; HCG with a composition modified by reaction with fresh fracture surfaces (Tsarev, 1988) and release of OG from fluid inclusions opened during stress.

4. Migration and localization of C-bearing species in the Khibiny complex The data outlined above suggest that the various abiogenic C-bearing species contained within the Khibiny Complex share a common origin, although each may have reacted differently with the mineral system or with externally derived fluids. The features common to all species include a, probably deep, carbon source as well as a single driving mechanism for their distribution within the Complex. By contrast, the presence of low temperature organic hydrocarbon species (hopanes and steranes) within the Khibiny complex is probably the result of incorporation of biogenic carbon into the complex through mixing with meteoritic water and/or the operation of redox reactions at a late stage of epi-magmatic mineralisation. The introduction of meteoric waters into the magmatic complex at this, and other, stages of its evolution is documented by oxygen and argon isotopic data (Tolstikhin et al., 1985; Nivin et al., 2001b; Ul’yanov et al., 2001; Nivin and Ikorsky, 2002). Recent and current gas mobility in the Khibiny complex is probably a function of local stress release resulting from changes in the regional stress pattern (Nivin et al., 2001a). There is evidence that Na-carbonate mineralization is also, at least in part, the result of stress changes within the massif. This suggests a possible relationship between the HC gases and soda mineralisation (Ivanyuk et al., 1996; Goryainov et al., 1998). There is a qualitative correlation, based on repeated observations particularly within the apatite– nepheline deposits of the Central Arch, between Nacarbonate mineralisation and enhanced levels of HC gas. Hypergene Na-carbonate mineralization is most commonly found in those core samples from the

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Central Arch that have between 40 and 120 cm3/g occluded HCG content (Ivanyuk et al., 1996; Goryainov et al., 1998). This suggests a possible relationship between HC gases and soda mineralisation. Dudkin and Mazukhina (2001) experimentally described a possible genetic link between these two species. Hypogene Na-carbonate mineralisation has been attributed to its derivation from bendogenic melt-solutionsQ (Khomyakov, 1995) or to precipitation from low-temperature hydrothermal solutions together with solid organic compounds, (Ivanyuk et al., 1996). Evolution of hypergene sodium carbonates has been attributed to a number of mechanisms: interaction between nepheline and F-and CO2-bearing underground or surface waters (Dorfman et al., 1967); redeposition of hypogene Na-bearing minerals (Khomyakov, 1995); reaction between a water saturated HC gas phase with villiaumite and nepheline (Dudkin and Mazukhina, 2001); addition of CO2 to hydrated peralkaline silicates (Khomyakov, 1995); and reaction between CO2 and agpaitic minerals, particularly nepheline, undergoing decomposition as a result of stress release (Goryainov et al., 1998). These data all suggest a linkage between C mobility, the stress state of the rocks and Na-rich mineralisation. Na-carbonate mineralisation can occur very quickly. There are a number of documented observations of incipient mineralisation developing in core samples newly collected from boreholes drilled into stressed rock units. Initially homogenous with no sign of mineralisation, within a few months the cores develop a penetrative network of microfractures, the result of stress release, and become coated with a thin white rind of Na-carbonate crystals. A number of studies have suggested that the structure and formation of the Central Arch can be viewed in the context of percolation theory (Ivanyuk et al., 1996, 2002; Goryainov et al., 1998; Goryainov and Ivanyuk, 2001a,b). Key features of this analysis are that the Khibiny pluton is cone shaped, underwent continuous unroofing over a long period with an uplift rate of 1.0– 1.5 mm per year (Kuz’min et al., 1994) and is characterised by a non-equilibrium mineral system. Uplift and exhumation encouraged decompression-related dilatancy of internal structures which resulted in the petrological system being saturated with endogenic energy. Transfer of this excess energy, driven by long-term thermal and exhumational relaxation of the intrusion, was focussed through the Central Arch zone

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which is marked by magmatic apatite–nepheline orebodies, numerous dykes and pegmatites, hydrothermal vein networks, epi-and post-magmatic Na-carbonates and solid biogenic carbon species, as well as by zones with enhanced HCG content. Pegmatites in the Khibiny complex form a heterogeneous suite of coarse-grained magmatic rocks that crystallised at a late stage from a volatile-rich magma. The main pegmatite assemblages include: arfvedsonite–(aegirine, lepidomelane)–microcline–microperthite; microcline–microperthite; aegirine–diopside–nepheline; nepheline–aegirine–diopside; aegirine (arfvedsonite, lepidomelane)–microcline; aegirine (arfvedsonite, lepidomelane)–adularia; aegirine–zeolite. They often show evidence for hydration and metasomatism (Kostyleva-Labuntsova et al., 1978). The pegmatites occur in close spatial association with hydrothermal veins and it is considered that both were emplaced during the final stages of magma crystallisation. In its simplest sense, the energy flow can be characterized in terms of large scale fluid and heat fluxes. It is possible that there remains a residual mantle, or deep crustal, contribution related to the after effects of Devonian plume activity. If so, mantle or deep crustal derived hydrogen could be an effective heat transporting agent (c.f. Letnikov and Dorogokupets, 2001). Energy produced by strain relaxation within the Central Arch during exhumation could also have contributed to the total energy budget. Thus the Central Arch can be viewed as a long-lived zone of tensional stress that accommodated fluid percolation, including that of meteoric water, and within which gas migration was localized and focussed. Over-pressurized gas and fluid percolating through the Central Arch resulted in the accumulation of HC gases as well as the localised precipitation of Na-carbonate minerals within hydrothermal vein systems. Fluid and heat fluxes, and hence, the rate of mineral formation and hydrocarbon generation, would have decreased as the rate of uplift and strain release decreased. At the same time the upper limit of the zone of HCG generating mineral reactions would have descended relative to the pluton roof. Today, only low-temperature hypergene Na-carbonate clusters are being formed. However, free micro-fracture gases are being redistributed due to local stress changes and there may remain some infiltration into upper levels of the pluton of HC gas recently formed in deeper parts of the pluton.

During unloading, the rocks would have experienced decompressive stress and strain as well as temperature decrease. This is most clearly marked in a change in nepheline structure that involves its ordering and partial release of Na, Fe and Si (Ikorsky, 1981; Goryainov and Ivanyuk, 2001a,b). This element mobility, driven by decompression and enhanced by fluid mobility, resulted in localised growth of aegirine together with the development of zones of widespread albitisation of feldspathoid-bearing assemblages (Nivin, 2002). It is probable that emplacement of late-stage pegmatites and hydrothermal veins occurred at this time. There is a clear spatial relationship between pegmatites, hydrothermal veins, zones of alteration and zones of enhanced HCG content, all of which are located near, and within, the Central Arch structure of melteigite–urtites and rischorrites. At a microscopic level, the same process of Na mobility is preserved in nepheline crystals from rocks adjacent to the Central Arch. In these rocks aegirine crystals nucleated on nepheline rims. That this nucleation was driven by strain is shown by the disappearance of strain lamellae within nepheline crystals being replaced by aegirine (Ikorsky, 1981). Fluid inclusions containing HC gases are often developed adjacent to the aegirine (Ikorsky, 1981; Potter, unpublished) implying that strain release was accompanied by fluid access.

5. Conclusions The C-bearing species of the Khibiny alkaline intrusion include both oxidised and reduced varieties. The former include various carbonate forms that include calcite, dolomite and a wide range of sodacarbonate species. The latter include hydrocarbon gases, dispersed bitumens, solid organic minerals and a minor amount of graphite. Isotopic data suggest that, with the exception of the organic carbon, all of these probably have a common mantle source. The HC gases have a post-magmatic abiogenic evolution and probably evolved through a Fischer–Tropsch type synthesis. Most of the carbon within the complex, including that contained within the carbonatite complex, is spatially linked to the ring-like Central Arch structure. This structure is dominated by melteigite– urtites, rischorrites and apatite–nepheline rocks. Gas abundance data suggest that the highest levels of both

V.A. Nivin et al. / Lithos 85 (2005) 93–112

occluded and free HC gases are located within and near the Central Arch. This zone is abundantly fractured and is marked also by the presence of enhanced levels of apatite and Na-carbonate mineralisation. The margins of these zones coincides with zones of albitisation. Zones of enhanced gas presence are also characterised by rocks that contain structurally wellordered nepheline. The increase in ordering of the nepheline structure clearly correlates with an increase in gas content. The location of abiogenically produced hydrocarbon gases and associated bitumens together with pegmatitic and hydrothermal veins, dykes of alkaline and alkaline–ultramafic rock and stockwork shaped clusters of Na-carbonate mineralisation can be explained using percolation theory. The cone-shape of the Central Arch together with its decompression during uplift and exhumation, concentrated extensional strains which permitted flow of fluid and heat. The structure of the Central Arch thus represents a percolating cluster through which endogenic energy is discharged permitting the formation and localisation of a variety of C-bearing species.

Acknowledgements Financial support for this study was provided by INTAS (award 01-0244) and the Division of Geology, Geophysics, Geochemistry and Mining Sciences of the Russian Academy of Science (State contract No.-1127). The paper has benefited from positively critical reviews from Dr Frances Wall and Dr Gregor Markl. We acknowledge discussions with Joanne Potter, Andrew Rankin as well as Professor P.M. Goryainov, Drs G.Yu. Ivanyuk and V.N. Yakovenchuk.

References Arzamastsev, A.A., 1994. Unique Paleozoic Intrusions of the Kola Peninsula. In: Mitrofanov, F.P. (Ed.), KSC RAS, Apatity. Arzamastsev, A.A., Bea, F., Glaznev, V.N., Arzamastseva, L.V., Montero, P., 2001. Kola alkaline province in the Paleozoic: evaluation of primary mantle magma composition and magma generation conditions. Rus. J. Earth Sci. 3 (1) (http://rjes. agu.org/v03/tje01054/tje01054.htm]). Arzamastsev, A.A., Kaverina, V.A., Polezhaeva, L.I., 1988. Dyke Rocks of the Khibiny Massif and its Framing. KSB USSR Academy of Sciences, Apatity (in Russian).

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Balberg, I., Berkowitz, B., Drachsler, G.E., 1991. Application of a percolation model to flow in fractured hard rocks. J. Geophys. Res. 96B, 10015 – 10021. Bayanova, T.B., 2004. Ages of Reference Geological Complexes in the Kola Region and Duration of Magmatism Processes. Nauka, St.-Petersburg, p. 174 (in Russian). Borschevsky, Yu. A., Borisova, S.L., Medvedovskaya, N.I., Amosova, Kh.B., Stepanova, N.A., 1987. Isotope features of minerals and rocks of the Khibiny-Lovozero complex and some aspects of a problem of their genesis. Zapiski Vsesoyuznogo Mineralogicheskogo Obschestva 61, 532 – 540. Bottinga, Y., 1969. Calculated fractionation factors for carbon and hydrogen isotope exchange in the system calcite–carbon dioxide–graphite–methane–hydrogen–water vapor. Geochim. Cosmochim. Acta 33, 49 – 64. Broadbent, S.R., Hammerslay, J.M., 1957. Percolation processes. 1. Crystals and mazes. Proc. Camb. Philos. Soc. 53, 629 – 641. Dauphas, N., Marty, B., 1999. Heavy nitrogen in carbonatites of the Kola Peninsula: a possible signature of the deep mantle. Science 286, 2488 – 2490. Deines, P., 2002. The carbon isotope geochemistry of mantle xenoliths. Earth-Sci. Rev. 58, 247 – 278. Dorfman, M.D., Goroschenko, J.G., Sikorskaya, E.K., Dorfman, A.N., 1967. About interaction of nepheline with water solutions of fluoric sodium and carbon dioxide gas. Geokhimiya 7, 786 – 795 (in Russian). Dudkin, O.B., 1993. Huge concentrations of phosphorus in Khibiny. Geol. Ore Depos. 35 (3), 195 – 204 (English translation: Birmingham: Interperiodica 1994, 3, 1–9). Dudkin, O.B., 2001. Geochemistry of carbonatite from the Khibiny pluton and its place among similar rocks. Geochem. Int. 39, 711 – 715. Dudkin, O.B., Mazukhina, S.I., 2001. Natural hydrocarbon gas: a possible reason for soda discharge from alkaline rocks of the Khibiny massif. Trans. (Dokl.) Russ. Acad. Sci. Earth Sci. Sect. 380, 867 – 869. Dudkin, O.B., Minakov, F.V., Kravchenko, M.P., Kravchenko, E.V., Kulakov, A.N., Polezhaeva, L.I., Pripachkin, V.A., Pushkarev, Yu.D., Ryungenen, G.I., 1984. Khibiny Carbonatites. Kola Branch of USSR Academy of Sci., Apatity (in Russian). Florovskaya, V.N., Zezin, R.B., Ovchinnikova, L.I., Pikovsky, Yu.I., Teplitskaya, T.A., 1968. Diagnostics of Organic Substance in Rocks and Minerals of a Magmatic and Hydrothermal Origin. Nauka, Moscow (in Russian). Galakhov, A.V., 1975. Petrology of the Khibiny Alkaline Massif. Nauka, Leningrad (in Russian). Galimov, E.M., 1975. Carbon Isotopes in Oil–Gas Geology. NASA (TT F-682). Galimov, E.M., 1988. Carbon geochemistry. Geochem. Int. 25, 94 – 115. Galimov, E.M., Petersilie, I.A., 1967. About carbon isotope composition hydrocarbon gases and CO2 in alkaline igneous rocks of the Khibiny, Lovozero and Ilimaussaq massifs. Dokl. Akad. Nauk USSR 176, 914 – 917 (in Russian). Goryainov, P.M., Ivanyuk, G.Yu., 2001a. Self-Organization of Mineral Systems. GEOS, Moscow (in Russian).

110

V.A. Nivin et al. / Lithos 85 (2005) 93–112

Goryainov, P.M., Ivanyuk, G.Yu., 2001b. Energetic percolation resource of new ideas in geotectonics. Vestnik Voronezhskogo Universiteta. Geologiya 11, 7 – 22 (in Russian). Goryainov, P.M., Ivanyuk, G.Yu., Yakovenchuk, V.N., 1998. Tectonic percolation zones in the Khibiny massif: morphology, geochemistry, and genesis. Izv., Phys. Solid Earth 34, 822 – 827. Gutsalo, L.K., Plotnikov, A.M., 1990. About fractionation of carbon isotopes in methane and carbon dioxide of hydroterms. Geokhimiya 12, 1677 – 1687 (in Russian). Ikorsky, S.V., 1967. Organic Substance in Minerals of Igneous Rocks (by the Example of the Khibiny massif). Nauka, Leningrad (in Russian). Ikorsky, S.V., 1977. About regularities of distribution and time of accumulation of hydrocarbon gases in rocks of the Khibiny alkaline massif. Geokhimiya 11, 1625 – 1634 (in Russian). Ikorsky, S.V., 1981. Type features of the composition and structural state of nepheline. Geochem. Int. 17, 37 – 47. Ikorsky, S.V., 1999. Equipment for analysis of microincluded gases extracted from rocks and minerals by crushing. Terra Nostra, 99/6: ECROFI XV — Abstracts and Program European Current Research On Fluid Inclusions, Potsdam, June, 21–24, 1999, pp. 153 – 154. Ikorsky, S.V., Fanygin, A.S., 1986. About zonal distribution of hydrocarbon gases in Oleniy Ruchey apatite deposit in Khibiny. Dokl. Akad. Nauk Ukr. SSR 290, 683 – 686 (in Russian). Ikorsky, S.V., Nivin, V.A., Pripachkin, V.A., 1992. Gas geochemistry of endogenic formations. Nauka, St. Petersburg (in Russian). Ivanyuk, G.Yu., Yakovenchuk, V.N., Goryainov, P.M., 1996. Main features of formation of carbonates in hydrothermal veins of Kukisvumchorr deposit. Zap. Vseross. Mineral. Obsestva 125, 9 – 23 (in Russian). Ivanyuk, G.Yu., Yakovenchuk, V.N., Goryainov, P.M., Konopleva, N.G., Pakhomovsky, Ya.A., Krivovichev, S.V., 2002. The natural mineralogical autoclave. Geology and Mineral Resources of the Kola Peninsula 2. MUP bPolygraphQ, Apatity, pp. 91 – 103 (in Russian). Karzhavin, V.K., Vendillo, V.P., 1970. Thermodynamic equilibrium and conditions for existence of hydrocarbon gases in a magmatic process. Geochem. Int. 7, 797 – 803. Khomyakov, A.P., 1985. New perspective type of soda raw material. Natural Soda and Dawsonite Manifestations in USSR. Nauka, Moscow, pp. 62 – 79 (in Russian). Khomyakov, A.P., 1995. Mineralogy of Hyperagpaitic Alkaline Rocks. Oxford Scientific Publications, Clarendon Press, Oxford. Kogarko, L.N., 1977. Genetic Problems of Agpaitic Magmas. Nauka, Moscow (in Russian). Kogarko, L.N., 1999. Problems of the genesis of giant apatite and rare metal deposits. Geol. Ore Depos. 4, 351 – 366. Kogarko, L.N., Kononova, V.A., Orlova, M.P., Woolley, A.R., 1995. Alkaline Rocks and Carbonatites of the World: Part 2. Former USSR. Chapman and Hall, London. 230 pp. Kogarko, L.N., Khain, V.E., 2001. Alkaline magmatism in the earth’s history: a geodynamic interpretation. Trans. (Dokl.) Russ. Acad. Sci. Earth Sci. Sect. 377A, 359 – 361.

Kogarko, L.N., Kosztolanyi, Ch., Ryabchikov, I.D., 1987. Geochemistry of the reduced fluid in alkali magmas. Geochem. Int. 24, 20 – 27. Konnerup-Madsen, J., 1988. Abiogenic hydrocarbon gases associated with alkaline igneous activity. Mem. Geol. Soc. India 11, 13 – 24. Konnerup-Madsen, J., 2001. A review of the composition and evolution of hydrocarbon gases during solidification of the Ilimaussaq alkaline complex, South Greenland. Geol. Greenl. Surv. Bull. 190, 163 – 170. Konnerup-Madsen, J., Kreulen, R., Rose-Hansen, J., 1988. Stable isotope characteristics of hydrocarbon gases in the alkaline Ilimaussaq complex, South Greenland. Bull. Mineral. 111, 567 – 576. Kostyleva-Labuntsova, E.E., Borutsky, B.E., Sokolova, M.N., Shlyukova, Z.V., Dorfman, M.D., Dudkin, O.B., Kozyreva, L.V., Ikorsky, S.V., 1978. Mineralogy of the Khibiny Massif. Nauka, Moscow (in Russian). Kozyrev, A.A., Panin, V.I., Maltsev, V.A., Timofeyev, V.V., 1996. Experience of prediction and prevention of rock bursts in the Khibiny apatite mines, Russia. In: Aubertin, M., Hassani, F., Mitri, H. (Eds.), Rock Mechanics. A.A. Balkema, Rotterdam, pp. 639 – 644. Kramm, U., Kogarko, L.N., 1994. Nd and Sr isotope signatures of the Khibiny and Lovozero agpaitic centres, Kola Alkaline Province. Russ. Lithos 32, 225 – 242. Kramm, U., Kogarko, L.N., Kononova, V.A., Vartiainen, H., 1993. The Kola alkaline province of the CIS and Finland: precise Rb– Sr ages define 380–360 Ma age range for all magmatism. Lithos 30, 33 – 44. Kravtsov, A.I., 1968. Geological Conditions of Gas Presence in Coal, Ore and Non-Metalliferous Mineral Raw Deposits (Monograph), p. 330. Moscow (in Russian). Kravtsov, A.I., Grigoruk, V.N., 1972. Geology and geochemistry of natural gases in the Khibiny apatite–nepheline deposits. Izvestiya Vuzov. Geol. Prospect. 3, 90 – 96 (in Russian). Kuz’min, I.A., Kremenetskaya, E.O., Tryapitsyn, V.M., 1994. The Khibiny Earthquakes in November–December 1993. KSC RAS, Apatity (in Russian). Letnikov, F.A., Dorogokupets, P.I., 2001. The role of superdeep fluid systems of the Earth’s core in endogenic geological processes. Trans. (Dokl.) Russ. Acad. Sci. Earth Sci. Sect. 378, 500 – 502. Lyutkevich, Ye.M., 1967. On bitumen and gas shows of Kola Peninsula and nature of original matter. Genesis of Oil and Gas. Nedra, Moscow, pp. 255 – 261 (in Russian). Marakushev, A.A., Paneyakh, N.A., Rusinov, V.L., Pertsev, N.N., Zotov, I.A., 1998. Petrological model of formation of giant ore deposits. Geol. Ore Depos. 40, 211 – 227. Markl, G., Baumgartner, L., 2002. pH changes in peralkaline latemagmatic fluids. Contrib. Mineral. Petrol. 144, 331 – 346. Markl, G., Marks, M., Schwinn, G., Sommer, H., 2001. Phase equilibrium constraints on intensive crystallization parameters of the Ilimaussaq complex, South Greenland. J. Petrol. 42, 2231 – 2257. Matsumoto, T., Chen, Y., Matsuda, J., 2001. Concomitant occurrence of primordial and recycled noble gases in the Earth’s mantle. Earth Planet. Sci. Lett. 185, 35 – 47.

V.A. Nivin et al. / Lithos 85 (2005) 93–112 Nivin, V.A., 1991. The main principles and measures of gas-safe work at the uderground mines of the bApatitQ corporation. Gorn. Zh. 8, 34 – 36 (in Russian). Nivin, V.A., 2002. Gas concentrations in minerals with reference to the problem of the genesis of hydrocarbon gases in rocks of the Khibiny and Lovozero massifs. Geochem. Int. 40, 883 – 898. Nivin, V.A., Belov, N.I., 1993. Correlations between gas-geochemical and geomechanical parameters of alkali rocks at ore deposits. Geochem. Int. 30, 34 – 40. Nivin, V.A., Ikorsky, S.V., 2002. Some genetic features of the Lovozero rare-metal deposits (NW Russia) as it follows from noble gas (He, Ar) isotope abundances. Deep-Seated Magmatism, Magmatic Sources and the Problem of Plumes, pp. 230 – 253 (Vladivostok). Nivin, V.A., Ikorsky, S.V., Pushkin, M.G., 1991. Gas presence in deep levels of apatite deposits (Khibiny). Izvestiya VUZov. Geol. i Razved. 6, 54 – 58 (in Russian). Nivin, V.A., Devirts, A.L., Lagutina, Ye.P., 1995. The origin of the gas phase in the Lovozero massif based on hydrogen-isotope data. Geochem. Int. 32, 65 – 71. Nivin, V.A., Belov, N.I., Treloar, P.J., Timofeyev, V.V., 2001a. Relationships between gas geochemistry and release rates and the geomechanical state of igneous rock massifs. Tectonophysics 336, 233 – 244. Nivin, V.A., Ikorsky, S.V., Kamensky, I.L., 2001b. Noble gas (He, Ar) isotope evidence for sources of Devonian alkaline magmatism and ore formation related within the Kola province (NW Russia). Alkaline Magmatism and the Problems of Mantle Sources, pp. 177 – 188 (Irkutsk). Petersilie, I.A., 1958. Hydrocarbonic gases and bitumens if igneous massifs in the central part of the Kola Peninsula. Dokl. Akad. Nauk Ukr. SSR 122, 1086 – 1089 (in Russian). Petersilie, I.A., 1964. Geology and Geochemistry of Natural Gases and Disperse Bitumens of Some Geological Formations of the Kola Peninsula. Nauka, Moscow (in Russian). Petersilie, I.A., So¨rensen, H., 1970. Hydrocarbon gases and bituminous substances in rocks from the Ilimaussaq alkaline intrusion, South Greenland. Lithos 3, 59 – 76. Potter, J., 2000. The Characterisation and Origin of Hydrocarbons in Alkaline Rocks of the Kola Alkaline Province. Unpubished PhD. Thesis. Kingston University. Potter, J., Konnerup-Madsen, J., 2003. A review of the occurrence and origin of abiogenic hydrocarbons in in igneous rocks. In: Petford, N., Mccaffrey, K.J.W. (Eds.), Hydrocarbons in Crystalline Rocks, Geological Society London, Sp. Publ., vol. 214, pp. 151 – 763. Potter, J., Rankin, A.H., Treloar, P.J., 1999. The relationship between CH4 and CO2 inclusions and Fe–O–S mineralization in intrusions of the Kola Alkaline Province. In: Stanley, C.J., et al. (Eds.), Mineral Deposits: Processes to Processing 1. Balkema, Rotterdam, pp. 87 – 90. Potter, J., Rankin, A.H., Treloar, P.J., 2004. Abiogenic Fischer– Tropsch synthesis of hydrocarbons in alkaline igneous rocks; fluid inclusion, textural and isotopic evidence from the Lovozero complex, N.W. Russia. Lithos 75, 311 – 330. Potter, J., Rankin, A.H., Treloar, P.J., Nivin, V.A., Ting, W., Ni, P., 1998. A preliminary study of methane inclusions in alkaline

111

igneous rocks of the Kola igneous province, Russia: implications for the origin of methane in igneous rocks. Eur. J. Mineral. 10, 1167 – 1180. Pripachkin, V.A., Kamenev, E.A., 1968. Prediction of gas seeps under exploitation of mineral deposits in igneous rocks. Sovetskaya Geologiya 9, 123 – 129 (in Russian). Pripachkin, V.A., Pavlova, M.A., Galakhova, T.N., Volokhova, T.S., Malashkina, V.T., 1985. Bitumens of Khibiny carbonatites. Dokl. Akad. Nauk Ukr. SSR 281, 1424 – 1426 (in Russian). Ray, J.S., Ramesh, R., Pande, K., 1999. Carbon isotopes in Kerguelen plume-derived carbonatites: evidence for recycled inorganic carbon. Earth Planet. Sci. Lett. 170, 205 – 214. Sadovsky, M.A., Pisarenko, V.F., 1991. Self-similarity in geophysics. Priroda 1, 13 – 23 (in Russian). Salvi, S., Williams-Jones, A.E., 1997. Fischer–Tropsch synthesis of hydrocarbons during sub-solidus alteration of the Strange Lake peralkaline granite, Quebec/Labrador, Canada. Geochim. Cosmochim. Acta 61, 83 – 99. Snyatkova, O.L., Pronyagin, N.I., Markitakhina, T.M., Khomyakov, A.P., 1984. Carbonatite complex of the Khibiny massif and prospects of finding in the massif of economic accumulations of natural soda. Izvestiya AN SSSR. Seriya Geologicheskaya 11, 124 – 128 (in Russian). Sokolov, I.M., 1986. Dimensions and other geometric indicators in flowing theory. Uspekhi physicheskikh nauk 150, 221 – 254 (in Russian). Sorokhtin, O.G., Mitrofanov, F.P., Sorokhtin, N.O., 1996. Origin of Diamonds and Diamond Presence Prospects in East Part of the Baltic Shield. KSC RAS, Apatity (in Russian). Sugisaki, R., Mimura, K., 1994. Mantle hydrocarbons: abiotic or biotic. Geochim. Cosmochim. Acta 58, 2527 – 2542. Syvorotkin, V.L., 2002. Deep Degassing of the Earth and Global Catastrophes. Geoinformtsentr, Moscow (in Russian). Tikhonenkov, I.P., 1963. Nepheline Syenites and Pegmatites of the Khibiny Massif and a Role of the Postmagmatic Phenomena in their Formation. Leningrad, Moscow (in Russian). Tolstikhin, I.N., Kamensky, I.L., Sharkov, I.V., Dudkin, O.B., Pripachkin, V.A., 1985. Isotopes of light noble gases in carbonatites of the Kola Peninsula. Kola Sci. Center Russ. Acad. Sci. 42 pp. (in Russian). Tolstikhin, I.N., Kamensky, I.L., Nivin, V.A., Vetrin, V.R., Balaganskaya, E.G., Ikorsky, S.V., Gannibal, M.A., Kirnarsky, Yu.M., Marty, B., Weiss, D., Verhulst, A., Demaiffe, D., 1999. Low mantle plume component in 370 Ma old Kola ultrabasic– alkaline–carbonatite complexes: evidences from rare gas isotopes and related trace elements. Rus. J. Earth Sci. 1, 179 – 222 (http://eos.wdcb.rssi.ru/rjes/). Tolstikhin, I.N., Kamensky, I.L., Marty, B., Nivin, V.A., Vetrin, V.R., Balaganskaya, E.G., Ikorsky, S.V., Gannibal, M.A., Weiss, D., Verhulst, A., Demaiffe, D., 2002. Rare gas isotopes and parent trace elements in ultrabasic–alkaline–carbonatite complexes, Kola Peninsula: identification of lower mantle plume component. Geochim. Cosmochim. Acta 66, 881 – 901. Tsarev, V.P., 1988. Features of Oil–Gas-Formation in Zones of Tectonic–Seismic Activation. Nauka, Novosibirsk. Ul’yanov, A.A., Ustinov, V.I., Turchkova, A.G., Pekov, I.V., 2001. Oxygen isotope cmposition of minerals from peralka-

112

V.A. Nivin et al. / Lithos 85 (2005) 93–112

line rocks of the Khibiny massif (Kola Peninsula, Russia). Vestnik Moskovskogo Universiteta, Series 4. Geology 4, 54 – 63 (in Russian). Voitov, G.I., 1975. Gas breath of the Earth. Priroda 3, 91 – 98 (in Russian). Voitov, G.I., 1977. Isotope characteristics of Khibiny spontaneous gases. Dokl. Akad. Nauk Ukr. SSR 236, 975 – 978 (in Russian). Voitov, G.I., Adushkin, V.V., Gokhberg, M.B., Nosik, L.P., Kucher, M.I., Nikulina, I.V., Pushkin, M.G., Zhogina, L.M., 1990. About chemical and isotope instabilities of gas jets in Khibiny. Dokl. Akad. Nauk Ukr. SSR 312, 567 – 571 (in Russian). Voytov, G.I., 1992. Chemical and carbon-isotope fluctuations in free gases (gas jets) in the Khibiny. Geochem. Int. 29, 14 – 24. Yakovenchuk, V.N., Ivanyuk, G.J., Pahomovsky, Ya.A., Men’shikov, Yu.P., 1999. Minerals of the Khibiny Massif. Moscow (in Russian). Zaitsev, A.N., 1996. Rhombohedral carbonates from carbonatites of the Khibiny massif, Kola Peninsula, Russia. Can. Miner 34, 453 – 468.

Zaitsev, A.N., Bell, K., Wall, F., Le Bas, M.J., 1997. Alkaline rareearth element carbonates from carbonatites of the Khibiny massif: mineralogy and genesis. Trans. (Dokl.) Russ. Acad. Sci. Earth Sci. Sect. 355, 786 – 790. Zaitsev, A.N., Wall, F., Le Bas, M.J., 1998. REE–Sr–Ba minerals from the Khibiny carbonatites, Kola Peninsula, Russia: their mineralogy, paragenesis and evolution. Mineral. Mag. 62, 225 – 250. Zaitsev, A.N., Demeny, A., Sindern, S., Wall, F., 2002. Burbankite group minerals and their alteration in rare earth carbonatites— source of elements and fluids (evidence from C–O and Sr–Nd isotopic data). Lithos 62, 15 – 33. Zak, S.I., Kamenev, E.A., Minakov, F.V., Armand, A.L., Mikheichev, A.S., Petersilie, I.A., 1972. The Khibiny Alkaline Massif. Nedra, Leningrad. Zezin, R.B., 1968. Carbonaceous species of the Khibiny alkaline massif (mineralogy, geochemistry, genesis). A Thesis for Degree of Candidate of Sci. Moscow (in Russian). Zubkov, V.S., 2001. Composition and speciation of fluid in the system C–H–N–O–S at P–T conditions of the upper mantle. Geochem. Int. 39, 109 – 122.