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Interaction of Mantle Rocks with Crustal Fluids: Sagvandites of the Scandinavian Caledonides

  • Sagvandite is an enstatite+magnesite rock formed from dunite or harzburgite bodies occurring as tectonically emplaced fragments from the upper mantle in many orogenic belts by interaction with CO2-bearing crustal fluids at upper amphibolite facies P-T conditions. Sagvandite bodies occur widespread in distinct nappes in the Scandinavian Caledonides in Norway. Common to all of the many sagvandite outcrops is their general structure of radial bundles of very coarse cm-sized enstatite crystals and interstitial magnesite. Often some strongly resorbed primary olivine is preserved, in addition to minor accessory Cr-spinel and chromite. The dunite to sagvandite conversion is governed by three metasomatic reactions:(1) carbonatization of peridotite by CO2-bearing fluids;(2) interaction with external fluids containing dissolved silica;(3) loss of Mg by dissolution of forsterite in NaCl-rich deep fluids. Simultaneous progress ξoverall of all three reactions in proportions that conserve the volume of the original dunite can explain the observed structure and mode of sagvandite. The relationship among the progress ξ of the three reactions suggests that loss of Mg by the ultramafic rock is the dominating process in the iso-volume conversion of dunite to sagvandite.
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Interaction of Mantle Rocks with Crustal Fluids: Sagvandites of the Scandinavian Caledonides

    Corresponding author: Kurt Bucher, bucher@minpet.uni-freiburg.de
  • 1. Mineralogy and Petrology, Institute of Earth and Environmental Sciences, University of Freiburg, 79104 Freiburg, Germany
  • 2. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266510, China

Abstract: Sagvandite is an enstatite+magnesite rock formed from dunite or harzburgite bodies occurring as tectonically emplaced fragments from the upper mantle in many orogenic belts by interaction with CO2-bearing crustal fluids at upper amphibolite facies P-T conditions. Sagvandite bodies occur widespread in distinct nappes in the Scandinavian Caledonides in Norway. Common to all of the many sagvandite outcrops is their general structure of radial bundles of very coarse cm-sized enstatite crystals and interstitial magnesite. Often some strongly resorbed primary olivine is preserved, in addition to minor accessory Cr-spinel and chromite. The dunite to sagvandite conversion is governed by three metasomatic reactions:(1) carbonatization of peridotite by CO2-bearing fluids;(2) interaction with external fluids containing dissolved silica;(3) loss of Mg by dissolution of forsterite in NaCl-rich deep fluids. Simultaneous progress ξoverall of all three reactions in proportions that conserve the volume of the original dunite can explain the observed structure and mode of sagvandite. The relationship among the progress ξ of the three reactions suggests that loss of Mg by the ultramafic rock is the dominating process in the iso-volume conversion of dunite to sagvandite.

0.   INTRODUCTION
  • Peridotites from the Earth mantle occur widespread as tectonically emplaced slivers and isolated bodies embedded in crustal rocks of orogenic belts. These occurrences vary in extent from cm-sized blocks to several km large masses or slabs. Many of these ultramafic rocks represent the mantle part of meta-ophiolites and are occasionally termed Alpine-type peridotites (Bodinier and Godard, 2004; Thayer, 1969; Wyllie, 1969; Ringwood et al., 1964). About 50% of the outcrop area of the Zermatt-Saas meta-ophiolite in the western Alps, for example, is made up of dozens of slabs and lenses of serpentinized harzburgite (Li et al., 2004a) associated with characteristic meta-rodingites (Li et al., 2008, 2004b).

    However, isolated peridotites from the subcontinental mantle unrelated to ophiolites occur widespread in mountain belts. In the Scandinavian Caledonides, for example, thousands of peridotite bodies occur at all levels of the nappe stack from the pre-Caledonian basement to the uppermost nappe in the stack (Bucher-Nurminen, 1991).

    Some of these mantle rocks in the crust of orogenic belts have been partly or fully carbonatized by reaction with CO2- bearing crustal fluids. Such carbonatized ultramafic rocks occur in three major varieties: Soapstone is a talc+magnesite rock typically derived from partial carbonatization of serpentinite at low temperature. Listvenite is a quartz+magnesite rock derived from serpentinite at low T by full conversion of all original Mg-silicates to carbonate and residual quartz (Kelemen et al., 2018; Menzel et al., 2018; Qiu and Zhu, 2018; Falk and Kelemen, 2015; Beinlich et al., 2012; Nasir et al., 2007). Sagvandite is an enstatite+magnesite rock derived from dunite or harzburgite by reaction with a CO2-rich fluid at high T (e.g., Ohnmacht, 1974). In this paper we present field, structure and chemical data from sagvandite occurrences in the Caledonian orogenic belt of Scandinavia and discuss the processes of fluid-rock interaction processes that produced them.

    Carbonated ultramafic rocks have received considerable attention in recent years because they represent natural examples of efficient CO2 sequestration by fixing free carbon dioxide from a fluid into a solid phase. The process has potential for being used in industrial scale sequestration of CO2 from fossil fuel burning (Eikeland et al., 2015; Gadikota et al., 2014; Kelemen et al., 2011; Lackner et al., 1995; Seifritz, 1990).

1.   THE SAGVANDITES
  • The Scandinavian Caledonides is a Paleozoic orogenic belt characterized by a stack of far traveled nappes resting on the Baltic foreland (Corfu et al., 2014; Gee et al., 2008, 1985; Roberts et al., 2007; Gee and Sturt, 1985). Lenses and knobs of ultramafic rocks varying in size from smaller than one meter to some km occur as tectonically emplaced fragments in the nappes of the Caledonides but also in the pre-Caledonian basement of the Baltic shield (Bucher-Nurminen, 1991, 1988; Moore and Qvale, 1977). Different nappes carry characteristic types of ultramafic rocks each. In the highest tectonic nappe of the stack serpentinite is prominent, tectonically lower nappes contain predominantly dunite and harzburgite. The carbonated dunites and harzburgites described in this paper are found in the Troms region of northern Norway in the Hei nappe of the Reisa nappe complex (Fig. 1) and in the Nordland region of northern Norway in the Rödingsfjell nappe complex (following the nappe terminology of the Norwegian Geological Survey, NGU). Carbonatized ultramafic rocks have also been described from the pre-Caledonian basement of the Western Gneiss Region of southern Norway (Moore, 1977; Moore and Qvale, 1977).

    Figure 1.  Simplified geologic map of the Sagelvvatnet area in Troms, northern Norway (modified after Zwaan et al., 1998). The original type-locality of sagvandite is indicated with a blue star (Pettersen, 1883). The locations of the sagvandites of the Sagelvvatnet are given on the inset together with the sagvandites from the Svartisen region (Fig. 6). Sagvandite locality: 19°03'54"E/69°09'35"N.

  • The original type sagvandite is located in Troms (Fig. 1), northern Norway, about 2 km south of Sagelvvatnet (literally for sawmill (sag) river (elv) lake (vatnet)). The first and name giving description of the rock near Sagelvvatnet has been given by Pettersen (1883).

    The sagvandite outcrop near Sagelvvatnet (Fig. 1) is an about 100 m long lens of enstatite-magnesite rock embedded in garnet-rich Bt-gneiss, locally containing also kyanite (abbreviations of mineral names from Whitney and Evans, 2010). The rocks belong together with dioritic and gabbroic rocks to the Hei nappe of the Reisa nappe complex. The Hei nappe from the upper allochthon is exposed directly under the main thrust to the overriding Lyngsfjell nappe of the uppermost allochthon (Zwaan et al., 1998). The sagvandites of the Troms region appear to occur exclusively in the Hei nappe. Ultramafic mounds in the Tromsø nappe consist predominantly of dunite. The hill at the locality Stabben (Fig. 2), for example, is a massive dunite with very little or no secondary rocks related to fluid-rock interaction present (Ravna et al., 2006).

    Figure 2.  (a) Stabben dunite knoll near Skutvikvatnet, Malangen area, Troms. The dunite hill belongs to the Tromsø nappe of the uppermost allochthon. (b) The rock consists predominantly of olivine with no serpentine, talc or carbonates.

    The Sagelvvatnet outcrop consists exclusively of sagvandite with no primary peridotite preserved (Fig. 3a). It is associated with a small amount of amphibolite. The sagvandite contains 70%–90% enstatite and 10%–30% magnesite. Very small amounts of phlogopite, tremolite, forsterite relics, Cr-spinel, pentlandite and chlorite can be present (Schreyer et al., 1972). The enstatite forms radial clusters up to 12 cm long (Fig. 3b). The weathering surface at the outcrop is porous because of carbonate dissolution (Fig. 3c). The distinct yellow color is related to the development of Fe3+-oxide-hydroxide in a 3–5 mm thick surface layer (Bucher et al., 2015). Sagvandite bodies further north in the Hei nappe display rare fracture-related reaction veins containing corundum (Schreyer et al., 1972). Detailed descriptions of the Sagelvvatnet body including rock composition data and petrologic discussion have been given by Schreyer et al. (1972) and Ohnmacht (1974).

    Figure 3.  (a) Original sagvandite outcrop south of Sagelvvatnet (blue star on Fig. 1). (b) Radial clusters of enstatite (typically 5–10 cm long En) and difficult to see interstitial weakly bluish and clear magnesite on unweathered surface at the outcrop shown on Fig. 3a. (c) Coarse porous weathering surface at the outcrop of Fig. 3a with magnesite dissolved and removed leaving only coarse enstatite at the outcrop wall.

    In the Svartisen area of the Nordland region south of the Troms region (Fig. 4) abundant sagvandite lenses and knobs have a similar size and are embedded in similar rocks as at the Sagelvvatnet outcrop. The Torsvik body north of Ørnes (10 km north of Fig. 4) is a particularly large and well-developed sagvandite occurrence. Field data, rock and mineral composition data can be found in Cribb (1988). The Torsvik sagvandite has a well-developed tremolite shell containing also minor phlogopite and occasional dolomite. Other studied occurrences in the Svartisen region (Sørensen, 1955a, b) are located at the Engabreen outcrops (Fig. 4).

    Figure 4.  Simplified geologic map of the Melfjorden area in Nordland, northern Norway (modified after Gustavson and Gjelle, 1991) showing 15 localities with characteristic sagvandite all located within the nappe of the upper allochthon below the Beiarn nappe. The Laupbakken locality of sagvandite presented in this paper is indicated with a blue star. The location of the sagvandites of the Svartisen region is given on the inset together with the sagvandites from the Sagelvvatnet area (Fig. 1). Laupbakken locality: 13°29'6"E/66°33'27.44"N.

    In the Melfjorden area (Fig. 4), southwest of Svartisen, similar bodies of sagvandite occur in upper amphibolite facies Grt-Hbl gneisses of the Rödingsfjell nappe below the Beiarn nappe of the uppermost allochthon (nappe terminology after Corfu et al., 2014). The sagvandite lenses occur at the same tectonostratigraphic level like the occurrences north of Svartisen and in the Troms region. The outcrop at Laupbakken is located directly at the fjord (blue star on Fig. 4);the Bergan lens crops out some 10 m above the fjord. On the extended southwest foreland of the Svartisen nappe (Fig. 4) a series of ultramafic bodies of similar size occur in the gneisses, the most prominent bodies are located at Kirkesteinen, Evendalen and Skavik. Near Oldervik some similar bodies of ultramafic rocks occur. At all localities typical coarse sagvandite dominates the ultramafic rocks (Fig. 5). In the contact zone to the enclosing Hbl-gneisses and high-grade meta-pelitic gneisses some thin shells of dark hornblende mantle the sagvandite bodies. Close to this contact zone tremolite is typically present, locally also green Cr-bearing actinolite. At some outcrops phlogopite is present in the contact shell to the surrounding gneisses. Dolomite can be present in the sagvandite close to the Hbl shell. Talc is rare in most sagvandites and clearly of late retrograde origin. No serpentine minerals are present. Below we present texture and composition data from the Laupbakken outcrop at Melfjorden. In the Beiarn nappe above the Melfjorden sagvandite bodies ultramafic outcrops display dunite or banded dunite-harzburgite rocks e.g., in the upper Beiarn and in the Saltfjellet area (Fig. 5).

    Figure 5.  Structures of the sagvandites of the Svartisen area (Fig. 6). (a) Kirkesteinen;(b) Oldervik;(c) Engabreen;and (d) layered dunite-harzburgite of the Beiarn nappe (Staupåtinden, Beiarndalen). Outcrop at (d) is about 2 m high.

    Common to all of the many sagvandite outcrops in Troms and Nordland is their general structure of radial bundles of very coarse cm-sized enstatite crystals and interstitial magnesite. Enstatite sheaf may reach 25 cm in length (Fig. 5a). The magnesite has often been removed from the surfaces at the outcrops giving the rocks a coarse porous appearance (Fig. 3c). The enstatite crystals in sagvandite are never parallel aligned and do not form a gneissic fabric in sharp contrast to the minerals of the enclosing Grt-Hbl gneisses.

  • The rocks from the Laupbakken outcrop at the shoreline of Melfjorden show particularly well-developed reaction textures that excellently characterize the processes of sagvandite formation. Typically there are the three major minerals present: olivine, orthopyroxene and magnesite. Olivine displays extreme resorption textures (Fig. 6). Dissolution of large indented Ol grains has left small isolated relic grains in optical continuity indicating that the original Ol has been much larger. The Opx is strongly euhedral and has overgrown relic Ol. The optical properties of Opx suggest that the pyroxene is enstatite. Magnesite is present as large crystals between the two silicates Ol and En (Fig. 6a). All three minerals are undeformed. However, a micro cleavage is well developed in the silicates, but in magnesite little micro fracturing occurs (Figs. 6b and 6c). The cleavage fractures do not show evidence for precipitation of late minerals from a fluid. The rocks are unfractured on a larger scale;veins and veinlets with late hydrate minerals such as anthophyllite, talc, serpentine or chlorite are extremely rare. Opaques are texturally not related to the olivine replacement by enstatite and magnesite. Small grains of Cr-spinel occur as inclusions in all three major minerals.

    Figure 6.  Photomicrographs of olivine resorption at the Laupbakken locality (Fig. 4). (a) Olivine to enstatite+magnesite reaction. Small olivine relics (Ol1 and Ol2) in optical continuity indicating the dissolution of originally much larger olivine grains. (b) Olivine replacement by enstatite. Note small olivine relics (Ol1) in optical continuity with the large central olivine. (c) Enlargement of the framed area in Fig. 6b (picture slightly rotated relative to Fig. 6b). Opaques are Cr-spinel overgrown by chromite.

2.   METHODS
  • The composition of minerals was determined using a CAMECA SX-100 electron microprobe at the University of Freiburg. All quantitative analyses were made using wavelength-dispersive spectrometers. Operating conditions were 15 kV acceleration voltage and 15 to 20 nA beam current with counting times of 10 s and a focused electron beam. The instrument was calibrated for each element analysed using well-characterized natural materials as standards. Data reduction was performed utilizing software provided by CAMECA.

    P-T-X conditions during fluid-rock interaction have been deduced from assemblage stability diagrams and assemblage models using Theriak/Domino software of de Capitani and Petrakakis (2010), and thermodynamic data from Berman (1988) (JUN92d.bs data-base and updates).

3.   COMPOSITION OF MINERALS IN SAGVANDITE
  • The composition of all minerals of the Laupbakken sagvandite is listed on the Tables 1 to 4. Olivine contains a relatively high amount of fayalite component, with an XMg of 0.80 (Table 1). Ca and Ni are low and variable. The orthopyroxene is rather pure enstatite (XMg 0.89 and 0.90). A small amount of Tschermak-component, both MgAl2SiO6 and MgCr2SiO6, is consistently present (Table 1). Magnesite contains 4 wt.% to 6 wt.% FeO and 0.2 wt.% to 0.8 wt.% CaO, corresponding to 90 mol%–94 mol% magnesite component.

    Mineral Ol
    #40
    Ol
    #41
    Opx
    #1
    Opx
    #4
    Opx
    #4
    Opx
    #9
    Opx
    #15
    Opx
    #16
    Opx
    #26
    SiO2 38.89 38.76 57.13 56.61 57.45 57.21 57.43 57.63 57.17
    Al2O3 0.01 0 0.75 0.77 0.76 1.01 0.7 0.33 0.79
    Cr2O3 0.06 0.01 0.19 0.12 0.12 0.17 0.07 0.1 0.07
    FeO 18.56 18.36 7.42 7.66 7.59 6.76 6.72 6.8 7.08
    MgO 42.42 42.51 34.44 34.36 33.81 34.97 34.6 34.5 34.32
    NiO 0.17 0.07 0.23 0.04 0.04 0.01 0.08 0.16 0.15
    CaO 0.01 0 0.11 0.08 0.08 0.07 0.09 0.14 0.09
    Total 1 100.12 99.83 100.3 99.69 99.9 100.21 99.72 99.73 99.68
    Formula 2
    Si 0.992 0.991 1.974 1.969 1.989 1.97 1.985 1.993 1.981
    Al 0 0 0.031 0.031 0.031 0.041 0.029 0.013 0.032
    Cr 0.001 0 0.005 0.003 0.003 0.005 0.002 0.003 0.002
    Mg 1.614 1.62 1.774 1.781 1.745 1.795 1.783 1.779 1.773
    Fe 0.396 0.393 0.214 0.223 0.22 0.195 0.194 0.197 0.205
    Ni 0.004 0.001 0.006 0.001 0.001 0 0.002 0.004 0.004
    Ca 0 0 0.004 0.003 0.003 0.003 0.003 0.005 0.003
    XMg 0.8 0.8 0.89 0.89 0.89 0.9 0.9 0.9 0.9
    1. Total includes traces of Ti, Mn, Na and K. 2. Formula normalized;Ol 4, Opx 6 oxygen.

    Table 1.  Composition of olivine and Opx (enstatite) at Laupbakken, Melfjorden

    Mineral Ath
    #27
    Ath
    #29
    Tlc
    #8
    Tlc
    #20
    Chl
    #5
    Chl
    #6
    Chl
    #7
    Chl
    #12
    Chl
    #13
    Chl
    #19
    SiO2 58.31 58.68 62.03 62.89 30.05 31.82 32.14 31.41 30.68 32.41
    Al2O3 0.08 0.04 0.17 0.16 20.74 17.05 17.64 18.63 18.85 14.98
    Cr2O3 0 0 0.09 0.08 0.92 2.32 2.27 0.93 2.19 2.87
    FeO 9.01 8.54 1.76 1.51 3.76 3.49 3.03 4.35 3.59 3.43
    MgO 28.97 29.31 30.67 29.91 32.25 33.11 33.32 31.71 32.72 34.01
    NiO 0.21 0.08 0.25 0.21 0.25 0.02 0.00 0.04 0.16 0.16
    CaO 0.42 0.44 0.05 0.05 0.00 0.00 0.00 0.01 0.02 0.02
    Total 1 97.1 97.12 95.07 94.87 88.1 87.83 88.47 87.21 88.36 87.93
    Formula 2
    Si 7.972 7.994 3.968 4.015 2.815 2.992 2.99 2.972 2.876 3.056
    Al 0.013 0.007 0.013 0.012 2.290 1.890 1.935 2.078 2.082 1.664
    Cr 0.000 0.000 0.005 0.004 0.068 0.173 0.167 0.070 0.163 0.214
    Mg 5.905 5.953 2.925 2.847 4.504 4.643 4.622 4.472 4.572 4.78
    Fe 1.030 0.973 0.094 0.081 0.295 0.274 0.236 0.344 0.282 0.271
    Ni 0.024 0.009 0.013 0.011 0.019 0.002 0.000 0.003 0.012 0.012
    Ca 0.061 0.064 0.003 0.003 0.000 0.000 0.000 0.001 0.002 0.002
    XMg 0.85 0.86 0.97 0.97 0.94 0.94 0.95 0.93 0.94 0.95
    1 Total includes traces of Ti, Mn, Na and K. 2 Normalized to # oxygen: Ath 23, Tlc, 11, Chl 14.

    Table 2.  Composition of hydrate minerals, anthophyllite, talc and chlorite

    Mineral Chr
    #14
    Chr
    #18
    Chr
    #33
    Cr-Spl
    #44
    Cr-Spl
    #45
    Cr-Spl
    #46
    Cr-Spl
    #47
    Mag
    #51
    TiO2 0.16 0.15 0.20 0.15 0.16 0.12 0.09 0.00
    Al2O3 12.08 12.31 13.47 22.92 24.81 22.96 23.94 1.14
    Cr2O3 53.25 52.20 49.95 43.52 41.17 42.89 42.25 4.24
    Fe2O3 1 62.36
    FeO 29.17 30.16 31.47 25.31 24.43 26.67 25.86 30.18
    MgO 4.04 3.94 3.58 7.85 8.64 7.03 7.79 0.33
    Total 2 98.71 99.07 98.67 99.82 99.31 99.73 100.34 98.35
    Al 0.492 0.499 0.546 0.858 0.92 0.864 0.882 0.052
    Ti 0.004 0.004 0.005 0.004 0.004 0.003 0.002 0
    Cr 1.454 1.418 1.359 1.093 1.025 1.083 1.054 0.13
    Fe3+ 3 0.046 0.078 0.084 0.044 0.048 0.05 0.072 1.818
    Fe2+ 0.796 0.788 0.821 0.629 0.595 0.662 0.61 0.978
    Mg 0.208 0.202 0.184 0.372 0.405 0.335 0.363 0.019
    Mg# 0.20 0.19 0.17 0.36 0.39 0.32 0.35 0.02
    Cr# 0.73 0.71 0.68 0.55 0.51 0.54 0.52 -
    1. calculated Fe2O3 and FeO given for magnetite only;2. Total includes traces of Mn, Ni, Ca, Na, and K;3. calculated Fe3+ and Fe2+ given in all formulas. Formula normalized to 4 oxygen.

    Table 3.  Composition of chromite, Cr-spinel and magnetite from ultramafic rocks at Laupbakken, Melfjorden

    Mineral CAM
    #21
    CAM
    #25
    CAM
    #36
    CAM
    #35
    CAM
    #23
    CAM
    #30
    SiO2 52.47 50.8 51.23 54.71 55.07 54.92
    TiO2 0.16 0.21 0.21 0.07 0.06 0.09
    Al2O3 7.37 8.76 7.32 3.71 3.76 3.48
    Cr2O3 0.21 0.62 0.63 0.49 0.02 0.01
    FeO 2.25 2.41 2.36 2.22 2.19 2.08
    MgO 21.51 20.82 20.88 22.58 23 23.22
    NiO 0.08 0.13 0.09 0.08 0.03 0.08
    CaO 13.4 12.93 13.17 13.28 12.87 13.08
    Na2O 1 1.17 1.05 0.57 0.51 0.48
    K2O 0.17 0.19 0.16 0.1 0.09 0.08
    Total 98.64 98.05 97.11 97.83 97.61 97.51
    Formula (normalized to 23 oxygen)
    Si 7.186 7.02 7.146 7.53 7.566 7.56
    AlIV 0.814 0.98 0.854 0.47 0.434 0.44
    AlVI 0.376 0.448 0.35 0.132 0.176 0.124
    Ti 0.017 0.022 0.022 0.007 0.006 0.01
    Cr 0.023 0.068 0.07 0.053 0.002 0.001
    Mg 4.392 4.289 4.342 4.633 4.711 4.764
    Fe 0.258 0.278 0.275 0.256 0.252 0.24
    Ni 0.009 0.015 0.01 0.009 0.003 0.009
    Ca 1.967 1.915 1.968 1.958 1.895 1.929
    Na 0.265 0.314 0.284 0.152 0.136 0.128
    K 0.03 0.034 0.029 0.018 0.016 0.014
    XMg 0.94 0.94 0.94 0.95 0.95 0.95

    Table 4.  Composition of Ca-amphibole from ultramafic rocks at Laupbakken, Melfjorden

    Rare local anthophyllite overgrowth on enstatite contains negligible amounts of Al and Cr;a small quantity of Ni and Ca is present (Table 2). The XMg of 0.86 is between XMg of olivine and enstatite. Talc contains a small amount of Ni. Talc has the highest XMg (0.97) of all analyzed minerals (Table 2). Chlorite has a significant but varying amount of Cr. Its Cr content reflects the source mineral that has been chloritized. Chl that formed from Cr-spinel contains about 1 wt.% Cr2O3, Chl associated with chromite has an average content of Cr2O3 of about 2.4 wt.% (Table 2).

    Three types of spinel are present in the analyzed sagvandite (Table 3): (a) Chromite-rich spinel (FeCr2O4). The Cr# of these spinels is about 0.71. They can be termed chromite. About 25% of the Cr-site is occupied by Al. (b) Mg-rich Cr-spinel. The Mg# of 0.35 is much higher than that of the chromites (Mg# ~0.19). These spinels are close to binary mixtures of Chr (FeCr2O4) and Spl (MgAl2O4). Both Chr and Cr-Spl contain very little Fe3+. (c) Rare magnetite contains about 4 wt.% Cr2O3. Magnetite is a late mineral overgrowing Cr-rich spinels. The composition of the spinels suggests that they are derived from dunite from fore-arc settings or alternatively from mafic cumulates (e.g., Pirard et al., 2013; Tamura and Arai, 2006).

    Two distinct types of Ca-amphibole are present. They are distinguished by different contents of edenite component that is different in Na and AlIV concentrations. The Cr and Ti contents are variable but generally higher in Na-rich Cam (Table 4). XMg is consistently lower in Na-rich Cam (0.94) compared to Na-poor tremolite/actinolite (0.95). Both types of Ca-amphibole are present in the contact zone to the surrounding gneiss and not in the main body of the sagvandite.

    Dolomite occurs primarily together with Ca-amphibole in the shells of the contact zone to the country rocks and contains between 1.5 wt.% and 5.5 wt.% FeO. Maximum MnO is 0.3 wt.%. Dolomite contains between 94 mol% and 98 mol% dolomite component.

4.   DISCUSSION AND INTERPRETATION OF DATA
  • The sagvandite near Sagelvvatnet (Fig. 1) occurs imbedded in Grt-rich Bt-gneiss containing kyanite and muscovite in addition to plagioclase and quartz. Staurolite is not present in the gneiss. This limits P-T conditions during formation of the metapelitic gneiss to about 8 kbar and 700℃. These conditions are constrained by the presence of kyanite and the absence of staurolite. The intersection of the Ky-Sil transition and the replacement of St by Grt+Ky is located at 7.7 kbar and 690℃ in Qz-bearing FASH rocks (Fig. 7.4 in Bucher and Grapes, 2011). In the KMFASH system the lower limit for the Ky-Grt-Bt assemblage is at about 720℃ at 8.5 kbar pressure given by a typical geotherm (Figs. 7.7 and 7.9 in Bucher and Grapes, 2011). At these P-T conditions melt can be present at high aH2O conditions. There is some field-evidence for minor partial melting at the E6 road cuts northwest of the sagvandite outcrop (blue star Fig. 1). The given P-T estimates must be regarded as minimum conditions. The upper T-limit is 750℃ at 9 kbar given by the absence of Sil. These conditions would be consistent with the Ky-Grt-Bt assemblage but not with the low degree of migmatization of the gneiss.

    At the sagvandite localities of the Svartisen area the peak P-T conditions of metamorphism indicated by the enveloping gneisses are 8 kbar and 730℃, nearly the same as in the Sagelvvatnet area. The conditions have been deduced quantitatively for the Engabreen locality (Fig. 4) from thermobarometry using Grt-amphibolite (Czirják, 1994) and Sil-Grt-Bt gneiss and dolomite marble containing tremolite and diopside (Hauser, 1994). The meta-pelitic gneisses at Engabreen contain sillimanite as a post-deformation mineral but also relics of earlier kyanite partly linked to deformation events. The assemblage of the metapelitic gneisses is Qz, Pl (An 10–20), Bt (XFe 0.52–0.56), Grt (XFe 0.83–0.87), Sil, Ms, Ky (early, partly resorbed), graphite and 7 different accessory phases including ilmenite (Hauser, 1994). The presence of dispersed fine-grained graphite in the gneisses implies that metamorphism occurred not in the presence of a pure H2O fluid phase. At P-T conditions in the range of 8–9 kbar and 700–750℃ the maximum aH2O was between 0.9 and 0.85 referenced to a standard state of pure H2O at P and T. The gneiss-marble association suggests the presence of a binary H2O-CO2 fluid. With aH2O buffered by the assemblage of graphite-bearing gneiss and ideal mixing of H2O-CO2 fluid the XCO2 of the fluid in gneiss was in the range of 0.1 to 0.2.

    These conditions are also relevant for the Torsvik locality where sagvandite has been described by Cribb (1988). The Torsvik body is enveloped by Ky-bearing Grt-Bt micaschists and more feldspathic gneiss. Staurolite-bearing Grt-Bt gneiss indicating somewhat lower peak T during metamorphism occurs about 10 km north of Torsvik.

  • The key reaction follows directly from the textural relationships (Fig. 6a) showing strongly resorbed olivine, euhedral enstatite and interstitial magnesite. The reaction

    replaces olivine from dunite by the sagvandite assemblage En+Mgs. It is written as a pure Mg reaction ignoring Fe of the fayalite component in olivine because olivine consists to more than 80 mol% of forsterite component (Table 1). The fayalite component (Fe) of olivine is mostly transferred to ferrosilite in the Opx and siderite in the breunnerite-magnesite carbonate. Note however, the data given in Table 1 are clearly inconsistent with an equilibrium partitioning of Mg-Fe between reactant olivine and product enstatite at upper amphibolite facies conditions. We suspect that metasomatism occurred at oxidizing conditions. The excess Fe-component in olivine has been transferred to secondary magnetite (Table 3) in a redox reaction not further discussed here. A passive Fe-enrichment of olivine caused by reaction (1) can be excluded because of the homogeneous Fe-distribution in relic olivine.

    Reaction (1) nearly went to completion using up all primary olivine in the majority of rocks. The reaction consumes CO2 from a fluid phase that had access to the olivine of the dunite. In some rocks strongly resorbed olivine survived (Fig. 6) and indicates that in these the reaction stalled because the CO2-bearing fluid on the reactant side has been completely consumed or could not be further supplied from the external source.

    Because the sagvandite bodies are embedded in vast masses of mica-rich gneiss the fluid contained also H2O in addition to CO2. At 8 kbar equilibrium of the principal sagvandite reaction (1) is at 800℃ for pure Mg-phases. Below this temperature forsterite is not stable together with a pure CO2-fluid and converts to enstatite and magnesite. In mixed volatile H2O-CO2 fluids the sagvandite assemblage may form under a wide range of P-T conditions. At the well-constrained T of 700℃ sagvandite forms at P ranging from 5 to ≫12 kbar (Fig. 7a). Below 5 kbar the harzburgite assemblage Fo+En is compatible with fluids ranging from pure H2O to pure CO2. At pressures above 8 kbar enstatite decomposes in very CO2-rich fluids forming the assemblage magnesite+quartz typical of listvenite (Fig. 7a). At high pressures harzburgite (forsterite+enstatite) is successively replaced by first sagvandite (enstatite+magnesite), then soapstone (talc+magnesite) and ultimately listvenite (quartz+magnesite) when infiltrated by fluids with increasing CO2 content. Since olivine relics are present in many of the sagvandite bodies reaction (1) effectively buffers the fluid to the harzburgite-sagvandite boundary (red dot in Fig. 7a). The XCO2 at the boundary is 0.375 at 8 kbar and 700℃. At 8 kbar sagvandite formation requires a minimum T of 680℃ (Fig. 7b). Below this T both harzburgite and sagvandite cannot stably coexist with a binary H2O-CO2 fluid. Soapstone and listvenite are predicted to form from Ol+Tlc by infiltration of a fluid with increasing CO2 content. At T below 580℃ stable serpentinite is present coexisting with pure H2O. With increasing CO2 in the fluid, first ophicarbonate forms, then soapstone and finally listvenite. Listvenite production requires fluids containing a decreasing amount of CO2 with decreasing T (Fig. 7b). At temperatures of 650℃, only 50℃ lower than at all Norwegian sagvandite occurrences, the harzburgite assemblage is limited to pressures below 6 kbar and CO2-rich fluid compositions (Bucher-Nurminen, 1990).

    Figure 7.  (a) P-XCO2 diagram at 700℃ for carbonated harzburgite showing the harzburgite-sagvandite conversion point (red dot). (b) T-XCO2 diagram at 8 kbar for carbonated harzburgite showing the harzburgite-sagvandite conversion point (red dot).

    After consumption of all olivine the sagvandite reaction (1) produced a rock containing 53 vol.% En and 47 vol.% Mgs. Since the observed amount of Mgs in the rocks ranges between 10 vol.% and 40 vol.% reaction (1) alone cannot explain the observed modal composition of the sagvandites. If sagvandites form from harzburgite rather than from dunite some of the enstatite could be inherited from the primary rock. The observed modal Mgs corresponds to 11 vol.%–46 vol.% En produced by reaction (1). Consequently ~50%–89% of the enstatite present would need to be primary enstatite. However, there is no evidence for the presence of two generations of enstatite, one primary En and one produced by reaction (1). Furthermore, there is no evidence for sagvandite formation from Ol-pyroxenites rather than dunite or Ol-rich harzburgite. All peridotite occurrences in all nappes of the stack are very Ol-rich rocks (Fig. 2). Also the serpentinite bodies are derived from Ol-rich primary peridotites (Bucher-Nurminen, 1991).

    A plausible alternative to an Opx-rich primary bulk rock composition is a metasomatic reaction adding SiO2aq dissolved in the fluid to a primary dunite (aq denotes SiO2 complex in an aqueous fluid). Reaction (2) can be written as

    Textural evidence for the significance of this reaction is the direct Ol-replacement by En (Figs. 6b and 6c) without associated magnesite. Since the harzburgite-sagvandite boundary is at about XCO2=0.37 (Fig. 7) it is suggested that sagvandite formed by infiltration of a CO2-bearing but nevertheless H2O-rich fluid. This is plausible because the sagvandite bodies are embedded in vast masses of micagneiss and micaschist (Figs. 1 and 4). The composition of the aqueous CO2-bearing fluid has been controlled by the assemblage of the enveloping gneiss containing predominantly quartz and feldspar. At 8 kbar, 700℃ and quartz-saturation, the fluid in equilibrium with the gneiss envelope contained 47 g L-1 SiO2aq corresponding to log mSiO2aq~ -0.25 (extrapolated from Walther and Helgeson, 1977). Reaction (2) buffers dissolved silica to aQz=0.4 corresponding to 18.7 g L-1 SiO2 at 8 kbar and 700℃. The reaction progresses until all olivine is used up and aQz then increases to quartz-saturation (aQz=1). However, relic olivine is present in many sagvandite occurrences and quartz has not been observed in the matrix of pure orthopyroxenites and Qz-veins are not present in any of these rocks indicating that the buffering capacity of reaction (2) has not been exhausted. The analysis suggests that both reactions run concurrently producing sagvandite from dunite.

    The problem with the two discussed reactions (1) and (2) is that both have a strongly positive ∆Vsolids resulting in a volume increase of 35% and 43% compared to olivine. There is no geological evidence that a massive volume increase is associated with the dunite-sagvandite conversion such as internal deformation of the ultramafic rocks (Figs. 3 and 5). Rather, the documented structure and texture of the rocks suggests that the volume of the rocks has been conserved during conversion. This suggests that a further reaction with a negative ∆Vsolids was important during sagvandite formation

    Reaction (3) converts forsterite to enstatite by removing Mg during interaction with the fluid. The reaction has been written with chloride as anion associated with Mg because deep fluids tend to have high chloride salinities (Bucher and Stober, 2010; Stober and Bucher, 2005a, b; Markl and Bucher, 1998; Trommsdorff and Skippen, 1987). Volume conservation requires that reaction (1) participates with 28% and reaction (3) with 72% in the dunite to sagvandite conversion. The volume conserving process produces a carbonate orthopyroxenite with 72 vol.% enstatite and 28 vol.% magnesite. This computed modal composition is close to the reported mean magnesite content of 24 vol.% from 17 recalculated rock analyses of the Troms region including the Sagelvvatnet outcrop (Ohnmacht, 1974). Yet, there is not obvious evidence for a high-Mg halo around the ultramafic bodies besides some phlogopite in the blackwalls. However, the cited textural evidence for the significance of reaction (2) above also supports reaction (3) that is the direct Fo-replacement by En (Figs. 6b and 6c) without associated magnesite.

    Reaction (2) produces 19 cm3 volume increase per mol forsterite converted, reaction (3) reduces the volume of the rock by 12 cm3 per mol forsterite reacted. Because the volume of the rock is preserved in the overall reaction, the volume increase caused by reaction (2) must be compensated by the concurrent extent of reaction (3): Rxn (2)=2·Rxn (3) if ∆Vsolids=0. Both reactions produce enstatite from olivine. Thus, if Rxn (2) progresses to some degree in the overall process the modal amount of magnesite present from Rxn (1) decreases. For sagvandites with a low magnesite content of only 10 vol.% the volume conserving process involves all three reactions simultaneously. Defining the extent of Rxn (1) as ξ=∆nFo/ν, where ∆nFo is the amount of Fo destroyed by Rxn (1) and ν the stoichiometric coefficient of Fo in the reaction. Rxn (1) ξ=1 converts 1 mole forsterite to 0.5 mol enstatite and 1 mol magnesite. If the extent of reaction ξ2=2ξ, and ξ3=6ξ, the volume of the rock is conserved in the overall process. The mode of the resulting rock is 89 vol.% enstatite and 11 vol.% magnesite, which is often observed at both the Troms and Svartisen sagvandite localities. The computed combination of extent of reaction ξ of the three individual reactions shows that Rxn (3) contributes the majority to the volume-conserved conversion of dunite to sagvandite.

5.   CONCLUSIONS
  • Sagvandites are carbonate-orthopyroxenites with the characteristic assemblage enstatite+magnesite formed by interaction of CO2-bearing saline crustal fluids with dunite or harzburgite fragments from the mantle tectonically emplaced in the lower crust during the Caledonian orogeny. The conversion of dunite to sagvandite involves three main reactions.

    (1) Reaction of forsterite (olivine) with CO2. It produces about equal volumes of enstatite and magnesite. It is an effective CO2 sequestration reaction that fixes CO2 from the fluid into a carbonate mineral. The reaction has a potential to be used as an industrial process for fixing man-made CO2 gas (Eikeland et al., 2015; Gadikota et al., 2014; Kelemen et al., 2011; Lackner et al., 1995; Seifritz, 1990). For such applications it would be reasonable to proceed with the carbonation to listvenite by converting also enstatite to quartz and magnesite (Hinsken et al., 2017). However natural listvenite formed from dunite and not from serpentinite is rare. (2) Reaction of forsterite (olivine) with dissolved silica in the fluid. It produces enstatite and is associated with an increasing volume of the rock. The structure of the sagvandite occurrences in the Scandinavian Caledonides of Norway suggests that the conversion was achieved by a volume conserving process. This requires an additional reaction that decreases the volume of the produced rock. (3) Reaction of forsterite (olivine) releasing MgCl2 to the fluid producing enstatite and decreases the volume of the product orthopyroxenite.

    Simultaneous progress of three reactions can explain the observed structure and mode of the sagvandites. The production of the carbonated ultrabasic rocks is a volume conserving process and the overall reaction ξoverall=ξ+ξ2+ξ3 matches closely the observed composition of magnesite-poor sagvandite bodies of the Caledonian orogenic belt in northern Norway if ξ=1, ξ2=2, and ξ3=6ξ. From these relationships its follows that loss of Mg by the ultramafic rock is responsible for 2/3 of the olivine destroyed and thus the dominating process in converting a certain volume of dunite to the same volume of sagvandite.

ACKNOWLEDGMENTS
  • We thank for the constructive suggestions and comments by two reviewers, which helped improving this contribution. We are grateful to Yanru Song for the thoughtful editorial handling of the manuscript. We are grateful to Kåre Kullerud and Fredrik Theisen for help with the fieldwork in Troms and Nordland. Attila Czirják and Markus Hauser are gratefully acknowledged for their company in the field and for contributing data to this study. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1257-2.

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