Serpentinization of the oceanic lithosphere and some geochemical consequences: Constraints from the Leka Ophiolite Complex, Norway

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Chemical Geology 249 (2008) 66 – 90 www.elsevier.com/locate/chemgeo

Serpentinization of the oceanic lithosphere and some geochemical consequences: Constraints from the Leka Ophiolite Complex, Norway K. Iyer ⁎, H. Austrheim, T. John, B. Jamtveit Physics of Geological Processes (PGP), University of Oslo, P.O. Box 1048, Blindern, N-0316, Oslo, Norway Received 15 August 2007; received in revised form 5 December 2007; accepted 8 December 2007 Editor: D. Rickard

Abstract Serpentinization of ultramafic rocks is an important process which modifies the petrophysical and geochemical properties of the affected rock. The Leka Ophiolite Complex (LOC) represents a part of the oceanic lithosphere which has been extensively serpentinized at the ocean floor. The ultramafic lithologies of the LOC preserve the history of hydration processes that took place during ocean-floor metamorphism over a wide range of decreasing temperatures. Serpentinization leads to the formation of less dense phases which results in a significant increase in bulk volume of the affected rocks. This change in density and volume of the ultramafic lithologies does not occur simultaneously in all lithologies and results in deformation of the surrounding rocks. The deformation is particularly evident in mm to dm thick, fractured and altered orthopyroxenite dykes in a dunite matrix. The fracturing process of the altered orthopyroxenite is driven by the reaction-assisted volume changes occurring in the dunites. Chemical and textural evidence show that major elements like Mg, Si and Al are essentially redistributed within the rock during serpentinization on an outcrop scale. However, there is abundant evidence for the mobility of Ca, Na, Fe and Mn at grain-size to regional scale. Alteration of primary clinopyroxenes to serpentine and clinochlore is a Ca-releasing reaction and contributes significantly to the mobilization of Ca in the fluids. This results in rodingitization of the crustal layer and also in the replacement of primary clinopyroxene, orthopyroxene and olivine by secondary diopside. Fe and Mn are mobilized simultaneously during the dissolution and subsequent precipitation of minerals throughout the serpentinization process. Olivine, in particular, is affected during the transport of iron and manganese. Metamorphic olivine has wide range of chemical compositions with Mg# as low as 0.68 and MnO contents of up to 1.5 wt.%. The fluid chemistry from high-temperature, ultramafic-hosted vent sites like Rainbow and Logatchev has elevated amounts of Fe and Mn suggesting that Fe and Mn could be transported out of the system as the degree of serpentinization increases. Therefore, serpentinization of the ultramafic part of the oceanic lithosphere may play an important role in constraining the global ocean chemical budget as well as ocean-floor mineral deposits. © 2007 Elsevier B.V. All rights reserved. Keywords: Ophiolite; Serpentinization; Petrography; Element mobilization; Volume change

⁎ Corresponding author. Tel.: +47 22859621; fax: +47 22855101. E-mail address: [email protected] (K. Iyer). 0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.12.005

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

1. Introduction Serpentinization is an important and ubiquitous process which strongly affects the petrophysical properties and geochemistry of the oceanic lithosphere. Serpentinization occurs in a variety of tectonic settings such as mid-ocean ridges and destructive margins (e.g., Mevel, 2003). It takes place during plate bending at the outer rise in a subduction zone setting (Ranero et al., 2003) and also occurs at the slab–mantle wedge interface where it may have implications for the mechanism of exhumation of high-pressure rocks (e.g., Gerya et al., 2002). Besides causing changes of rheology and density of mantle rocks (Escartin et al., 1997), serpentinization also influences other characteristic petrophysical properties such as magnetic susceptibility and seismic velocities (e.g., Bach et al., 2006, Ranero et al., 2003). Serpentinization may also lead to geochemical feedbacks between seawater and the oceanic lithosphere. Recent interest has been focused on the vent-fluid chemistry from ultramafic-hosted vent sites like Rainbow and Logatchev fields (Charlou et al., 2002) and the petrological and geochemical evolution of dredged or

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drilled samples from ultramafic outcrops at slow to ultraslow ridges (e.g. Andreani et al., 2007; Bach et al., 2004). The study of such samples however presents only a fragmented picture of the serpentinization process. Ophiolite complexes preserve records of progressive serpentinization in a complete suite of ultramafic rocks which may help us better understand the ocean-floor alteration of ultramafic rocks and its regional and maybe global geochemical effects. In this paper, we describe the petrological and chemical evolution of the ultramafic rocks of the Leka Ophiolite Complex (LOC) located in Nord-Trøndelag, Norway. The LOC offers a unique insight into hydration reactions in a variety of lithologies over a wide range of temperatures. 2. Geological setting and relevant lithologies The Leka Ophiolite Complex outcrops on the island of Leka, Nord-Trøndelag, Norway and is a part of the Upper Allochthon of the Scandinavian Caledonides (Furnes et al., 1988). U–Pb zircon dating of the quartz keratophyres of the LOC puts the age of formation of the LOC at 497 ± 2 Ma which is comparable to the

Fig. 1. Simplified geological map of the Leka Ophiolite Complex. Different ultramafic lithologies were sampled at Steinstind (filled orange circle) and Lauvhatten (filled brown circle). A. Field picture of interlayered wehrlite and dunites. Orthopyroxenite dykes (red) are also seen within the dunite. B. Locally crosscutting orthopyroxenite dykes within the dunites. The orthopyroxenite dykes are extensively fractured with the fractures oriented subperpendicular to the dyke–dunite contact. C. Field picture of harzburgites at Lauvhatten with a shear zone cutting through it (marked in red).

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Table 1 Analyses of whole-rock major-element compositions of harzburgites, dunites, wehrlites and orthopyroxenite dykes Rock type

Harzburgite

Sample no.

LE03

LE04

LE05

LE07

LE08

LE13

LE32

LE33

LE34

LE35

LE38

LE39

LE01_06

SiO2 (wt.%) Al2O3 Fe2O3 a MgO CaO Na2O K2O TiO2 P2O5 MnO LoI Total Mg# b MgO/SiO2 (molar proportion)

40.45 0.88 5.07 39.83 0.33 0.00 0.00 0.00 0.00 0.07 11.80 98.43 0.94 1.47

36.40 0.98 8.77 40.63 0.02 0.00 0.00 0.00 0.00 0.13 11.40 98.33 0.90 1.66

39.03 0.52 7.20 40.81 0.58 0.01 0.01 0.00 0.00 0.12 10.00 98.28 0.92 1.56

37.24 0.67 9.26 41.31 0.12 0.02 0.01 0.00 0.00 0.13 9.40 98.16 0.90 1.65

39.51 0.81 6.73 39.52 0.74 0.00 0.02 0.00 0.00 0.08 11.10 98.51 0.92 1.49

38.88 0.48 6.81 41.07 0.60 0.01 0.03 0.00 0.00 0.11 10.20 98.19 0.92 1.57

37.28 0.87 8.38 40.15 0.01 0.02 0.03 0.01 0.00 0.11 11.40 98.26 0.90 1.61

38.80 1.09 6.89 40.24 0.02 0.01 0.02 0.00 0.00 0.09 11.50 98.66 0.92 1.55

39.09 0.71 7.95 40.18 0.63 0.02 0.03 0.00 0.01 0.11 10.30 99.03 0.91 1.53

38.93 0.72 8.14 39.80 0.93 0.00 0.02 0.00 0.01 0.12 9.80 98.47 0.91 1.52

38.25 0.53 8.24 41.92 0.16 0.01 0.02 0.00 0.01 0.13 9.00 98.27 0.91 1.63

39.48 0.55 7.16 40.39 0.54 0.01 0.02 0.00 0.00 0.12 9.80 98.07 0.92 1.53

39.10 0.55 7.06 40.92 0.60 0.01 0.00 0.00 0.00 0.11 10.10 98.46 0.92 1.56

a

Analyzed total iron. Harzburgite Dunites Mg/(Mg + Fe2+), Fe2+ as total iron. LE02_06 LE03_06 LE04_06 LE05_06 LE08_06 LE09_06 LE10_06 LE11_06 LE12_06 LE13_06 LE01 LE15 LE20 LE26

b

magmatic age of the Karmøy Ophiolite Complex (Dunning and Pedersen, 1988). The LOC formed as a part of the oceanic lithosphere of the North Iapetus Ocean and was obducted during the Caledonian orogeny between the Ordovician and Silurian periods (Dunning and Pedersen, 1988; Titus et al., 2002) during which the mantle section was folded into an open fold (Maaløe, 2005). Two distinct sets of faults are observed in the LOC; the larger faults trend NE–SW and the smaller ones trend NW–SE (Titus et al., 2002). Shear zones are locally observed within the various rock units. Deformation zones consisting of breccia are also observed. The metamorphic grade in the LOC is upper greenschist to amphibolite facies (Prestvik, 1972). The Leka Ophiolite Complex is one of the most completely preserved ophiolites present in the Scandinavian Caledonides (Prestvik, 1972) and contains all the principle components of an ophiolite, including the mantle section, the layered crustal sequence and the overlying sediments (Fig. 1). The mantle harzburgites outcrop in the northern part of the island on either side of the ultramafic cumulates and are in tectonic contact with the metagabbros towards the east near Lauvhatten, which in turn are unconformably overlain by the metasediments of the Skei Group. The harzburgites are either in tectonic or transitional contact with the ultramafic cumulates and the contact is defined by an unconformity (Furnes et al., 1988). Harzburgites in the mantle section of the LOC display imperfect foliation defined by aligned crystals of pyroxene (subsequently altered to serpentine) attributed to high-temperature

plastic flow of the mantle (Furnes et al., 1988). Dunite bodies of various sizes occur locally within the harzburgite tectonite (Albrektsen et al., 1991). The layered cumulates consist of dunites interlayered with wehrlites and are well-exposed within the Skråa and Steinstind blocks. Dunites are the dominant rock type within the layered series and chromite layers are observed within them. The wehrlites outcrop as lensshaped pods which are several meters across within the dunites. The ultramafic cumulates and the metagabbros are separated by a sharp discordant boundary toward the southwest part of the island. Orthopyroxenite dykes, with a thickness ranging from less than 1 cm to several tens of cm, locally crosscut the dunites and wehrlites, and have variable orientations. They are usually aligned parallel to each other and to the layering, but it is not uncommon to observe them cut across each other as well, resulting in the formation of net-like patterns (Fig. 1B). They are invariably fractured and the spacing between the fractures is positively correlated with the thickness of the dykes (Iyer et al., in press). The least altered orthopyroxenite dykes are reddish-brown in color and change to a yellow-orange color when highly altered. The ultramafics of the LOC are all serpentinized to variable degrees. Previous studies were focused on the geochemistry and petrology of primary processes occurring in the mantle section and the layered ultramafic cumulates (e.g., Prestvik, 1972; Furnes et al., 1988), but very little work has been done on the serpentinization processes. The dunites, wehrlites and orthopyroxenite dykes were sampled along the northwestern part of the island

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

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Harzburgite

Dunites

LE02_06 LE03_06 LE04_06 LE05_06 LE08_06 LE09_06 LE10_06 LE11_06 LE12_06 LE13_06 LE01 LE15 LE20 LE26 38.04 0.56 7.45 41.52 0.06 0.00 0.00 0.00 0.00 0.10 10.00 97.74 0.92 1.63

37.24 0.33 8.04 42.05 0.05 0.00 0.00 0.00 0.00 0.12 9.90 97.73 0.91 1.68

38.39 0.50 8.32 40.84 0.06 0.00 0.00 0.01 0.00 0.13 9.90 98.15 0.91 1.59

39.22 0.45 6.98 41.71 0.37 0.00 0.00 0.00 0.00 0.12 9.50 98.34 0.92 1.59

38.32 0.56 7.57 41.19 0.04 0.00 0.00 0.00 0.00 0.12 10.80 98.60 0.92 1.60

37.17 0.35 9.13 41.41 0.10 0.00 0.00 0.00 0.00 0.16 9.90 98.22 0.90 1.66

39.61 0.61 7.50 40.06 0.73 0.00 0.00 0.00 0.00 0.14 9.90 98.55 0.91 1.51

38.67 0.68 7.84 41.38 0.10 0.00 0.00 0.00 0.00 0.11 10.10 98.88 0.91 1.60

37.10 0.44 8.34 40.74 0.71 0.00 0.00 0.00 0.00 0.13 11.20 98.66 0.91 1.64

37.35 0.45 7.87 40.54 0.25 0.00 0.00 0.00 0.00 0.10 11.60 98.16 0.91 1.62

35.77 0.02 8.27 45.12 0.03 0.00 0.00 0.00 0.01 0.13 9.00 98.35 0.92 1.88

36.76 0.08 9.58 41.06 0.00 0.01 0.02 0.00 0.00 0.15 10.80 98.46 0.89 1.67

34.76 0.16 8.57 43.52 0.01 0.02 0.03 0.00 0.00 0.12 10.70 97.89 0.91 1.87

34.98 0.22 8.04 44.13 0.01 0.02 0.03 0.00 0.00 0.11 10.00 97.54 0.92 1.88

(continued on next page )

in the layered ultramafic sequence (Steinstind) whereas the harzburgites were sampled in the northeastern part (Lauvhatten) in the mantle tectonites (Fig. 1). 3. Results 3.1. Analytical methods 57 powdered rock samples were analyzed by inductively coupled-atomic emission spectrometry (ICP-AES) method at Royal Holloway, University of London. All samples were dissolved twice, once after fusion with LiBO2 and once in HF and HClO4. This procedure is used to check the analysis and maximize the number of elements that can be determined. 0.2 g of powdered sample was weighed into a graphite crucible and 1.0 g of LiBO2 added. The powders were carefully mixed and fused at 900 °C for 20 min. The resulting mixture was dissolved in 200 ml of cold 5% nitric acid. Ga is added to the flux to act as an internal standard. This solution was then analyzed for Si and Al by ICP-AES using a Perkin Elmer Optima 3300R. The instrument was calibrated with natural and synthetic standards. Also, 0.2 g of powdered sample was dissolved in 6 ml of HF and HClO4 (2:1 mixture). This was then evaporated to dryness, cooled and dissolved in 20 ml of 10% HNO3. This solution was analyzed by ICP-AES for Fe, Mg, Ca, Na, K, Ti, P and Mn. The major elements are quoted as oxide-weight percent (wt.%). MgO/SiO2 ratios are given as molar proportions. Total Fe is reported as wt.% Fe2O3. The analytical precision is 1% for Si, Al, Fe, Mg and Ca and 2% for Na, K, Ti, P and

Mn. All elements are determined to 2 places of decimals (3 for Mn). Loss on ignition (LOI) is included in the reported totals. All analyses are listed in Table 1. Chemical compositions of minerals were analyzed using a Cameca SX-100 electron microprobe at the University of Oslo with an accelerating voltage of 15 kV and a current of 10 to 15 nA. Beam diameter used was 1 μm. Elements analyzed are Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, Cr and Ni calculated on the basis of a set of natural and synthetic standards. They are quoted as oxide-weight percent (wt.%) and in atom-per-formula unit (a.p.f.u). Representative analyses are listed in Tables 2–7. 3.2. Whole-rock chemistry The investigated lithologies of the Leka Ophiolite Complex are serpentinized to variable degrees (10–90%) as reflected by the high LOI values. There are no systematic correlations between the bulk MgO/SiO2, Fe2O3/SiO2 and MnO/SiO2 ratios of the analyzed dunites and harzburgites and the degree of serpentinization. MgO/ SiO2, Fe2O3/SiO2 and MnO/ SiO2 ratios scatter within the expected ratios of the original ‘dry’ mantle rocks (Maaløe, 2005; Neumann et al., 1995; Niu, 2004). MgO/SiO2 (0.98–1.17) vs. Al2O3/SiO2 (0.0–0.03) ratios of the harzburgites from Lauvhatten on the northern side of the island suggest that they belong to a depleted mantle suite (Niu, 2004). The harzburgites have Mgnumbers in the range 0.90–0.94. CaO contents vary from 0.01–0.93 wt.%. Harzburgite samples from the center of shear zones and from fracture/damage zones have higher

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Table 1 (continued ) Rock type

Dunites

Wehrlites

Sample no.

LE27

LE28

LE53

LE88

LE92

LE97

LE101

LE110

LE71

LE72

LE73

LE74

LE75

SiO2 (wt.%) Al2O3 Fe2O3 a MgO CaO Na2O K2O TiO2 P2O5 MnO LoI Total Mg# b MgO/SiO2 (molar proportion)

35.25 0.18 7.92 42.97 0.03 0.01 0.02 0.01 0.00 0.12 11.50 98.01 0.91 1.82

34.34 0.13 8.43 43.23 0.02 0.02 0.03 0.00 0.01 0.12 11.50 97.83 0.91 1.88

35.45 0.10 9.26 41.62 0.04 0.00 0.00 0.00 0.00 0.13 11.80 98.39 0.90 1.75

37.02 0.10 10.20 40.95 0.16 0.02 0.01 0.00 0.00 0.17 10.00 98.62 0.89 1.65

35.03 0.09 9.93 42.22 0.02 0.00 0.00 0.00 0.00 0.15 11.40 98.85 0.89 1.80

35.52 0.12 9.43 42.50 0.04 0.00 0.01 0.00 0.00 0.14 10.90 98.65 0.90 1.78

36.15 0.40 10.06 42.21 0.59 0.00 0.00 0.00 0.00 0.16 8.60 98.18 0.89 1.74

37.00 0.20 9.78 43.04 0.91 0.01 0.01 0.00 0.00 0.15 6.90 97.99 0.90 1.73

42.49 1.56 10.97 30.51 8.34 0.05 0.01 0.04 0.00 0.17 4.70 98.84 0.85 1.07

45.09 1.61 9.89 26.63 12.65 0.06 0.01 0.04 0.00 0.18 4.70 100.86 0.84 0.88

46.42 1.49 7.73 24.85 13.36 0.10 0.01 0.04 0.00 0.12 4.90 99.01 0.86 0.80

45.15 1.88 9.75 26.68 12.32 0.06 0.01 0.05 0.00 0.18 3.50 99.59 0.84 0.88

47.31 1.54 7.73 22.98 15.34 0.11 0.01 0.05 0.00 0.13 4.30 99.50 0.85 0.72

Wehrlites LE76

LE14

Orthopyroxenite dykes LE19

LE54

LE55

LE64

LE66

MgO/SiO2 ratios and lower CaO content than the wall rock harzburgite and grade towards dunite in composition. K2O, Na2O, TiO2 and P2O5 concentrations in the harzburgites are very low, usually below 0.03 wt.% and Al2O3 contents vary from 0.5–1.0 wt.%. MnO contents are slightly lower in the harzburgites than the other lithologies and fall in the range 0.07–0.13 wt.%. Dunites were sampled within the layered cumulate sequence of the LOC and have Mg-numbers in the range 0.89–0.92. CaO contents range between 0.01 and 1 wt.% but are usually below 0.5 wt.%. K2O, Na2O, TiO2 and P2O5 concentrations in the dunites are below 0.03 wt.% and Al2O3 contents vary from 0.02–0.50 wt.%. MnO contents of dunites range from 0.11–0.17 wt.%. Wehrlites are found in association with dunites in the layered cumulate sequence and have relatively lower Mgnumbers (0.84–0.86). The CaO content is high (12.3– 15.3 wt.%) except for one sample (8.3 wt.% CaO). K2O and P2O5 are extremely low (below 0.01 wt.%), while TiO2 contents are approximately 0.05 wt.%. The Al2O3 content is 1.5–1.9 wt.%, and Na2O varies from 0.05 to 0.1 wt.%. The MnO contents of wehrlites range from 0.12 to 0.18 wt.%. The Mg-number of the orthopyroxenite dykes vary from 0.88 to 0.91 and CaO contents are between 0.75 and 1.5 wt.%. P2O5 was not detected in the orthopyroxenite dykes (below detection limit), and K2O and TiO2 contents are between 0.01–0.03 and 0.01 wt.%, respectively. The Al2O3 and MnO contents are in the ranges 0.5–0.8 and 0.11–0.17 wt.% respectively. The Na2O contents of orthopyroxenite dykes are high when compared to orthopyroxenite dykes from the Bay of Islands Ophiolite (Varfalvy et al., 1997) and range between 0.2 and 0.8 wt.%.

LE90

LE93

LE99

LE100

LE103

LE104

The bulk-rock major-element compositions are related to the mineral assemblages present in the protolith before serpentinization (Fig. 2). 3.3. Petrography The mineral assemblages of the serpentinized ultramafites of the Leka Ophiolite Complex vary between the different lithologies. Amphiboles are classified after Leake et al. (1997) and serpentine textures after Wicks and Whittaker (1977) and O'Hanley (1996). 3.3.1. Harzburgites The mineral assemblage of the harzburgites sampled at Lauvhatten is composed of olivine, primary and secondary clinopyroxene, Cr-spinel, ferritchromite, magnetite and serpentine ± brucite/clinochlore. No orthopyroxene has been preserved in the harzburgites and its former existence is inferred from bulk-rock chemistry (see Fig. 2). The olivine in some of the harzburgites displays an apparent ‘cleavage’ and locally display deformation bands (Fig. 3A). The olivine is extensively fractured and adjacent grains generally show similar extinction angles. Relict olivine is also common in mesh-textured serpentine. Serpentine in mesh-textures occurs as oriented columnar lizardite with an apparent orientation perpendicular to the olivine grain boundary, i.e. the reaction front (Rumori et al., 2004). Mesh serpentine has been described as micro-crystalline lizardite (1T) with minor chrysotile (Viti and Mellini, 1998; Wicks and Whittaker, 1977). Primary clinopyroxene occurs as large grains that have been altered along grain boundaries and in fractures to serpentine. Secondary clinopyroxene is observed forming

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

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Table 1 (continued ) Wehrlites Orthopyroxenite dykes

Orthopyroxenite dykes

LE76

LE14

LE19

LE54

LE55

LE64

LE66

LE90

LE93

LE99

LE100

LE103

LE104

46.64 1.84 8.36 24.68 14.18 0.09 0.01 0.06 0.00 0.17 2.90 98.94 0.85 0.79

48.06 0.50 6.99 33.03 0.75 0.31 0.01 0.01 0.00 0.11 8.30 98.07 0.90 1.02

48.17 0.48 7.12 32.77 1.38 0.67 0.00 0.01 0.01 0.14 8.20 98.95 0.90 1.01

47.02 0.51 6.98 33.79 1.10 0.45 0.02 0.01 0.00 0.11 9.20 99.19 0.91 1.07

50.08 0.49 8.01 32.61 1.50 0.33 0.03 0.01 0.00 0.16 4.70 97.92 0.89 0.97

49.45 0.53 7.27 32.42 0.78 0.31 0.02 0.01 0.00 0.15 7.50 98.44 0.90 0.98

44.81 0.56 9.02 33.20 1.25 0.24 0.01 0.01 0.00 0.16 9.50 98.76 0.88 1.10

52.15 0.64 7.51 32.02 0.92 0.20 0.01 0.01 0.00 0.16 4.70 98.32 0.89 0.92

52.63 0.66 7.37 32.58 0.95 0.21 0.02 0.01 0.00 0.13 3.20 97.76 0.90 0.92

48.80 0.65 7.88 31.80 1.35 0.73 0.03 0.01 0.00 0.16 7.70 99.11 0.89 0.97

46.40 0.66 8.49 32.71 1.48 0.78 0.03 0.01 0.00 0.17 8.80 99.53 0.88 1.05

49.25 0.77 7.04 31.76 0.87 0.42 0.03 0.01 0.00 0.13 8.00 98.28 0.90 0.96

49.85 0.79 7.13 32.06 0.99 0.50 0.03 0.01 0.00 0.14 7.90 99.40 0.90 0.96

after olivine (Cpx2) and primary clinopyroxene (Cpx3; Fig. 3B). The alteration of primary clinopyroxene to serpentine and the formation of secondary diopside are observed within the same sample. Secondary clinopyroxene after olivine (Cpx2) is associated with the formation of fine-grained ferritchromite and magnetite (Fig. 3B). Texturally, they appear to have grown simultaneously. Serpentine occurs in association with brucite (intergrowth) in sub-domains formerly dominated by olivine. Blades of antigorite form after the serpentine and brucite intergrowth and commonly grow into the relict olivine crystals (Fig. 3C). Interpenetrating serpentine texture is also observed in domains consisting exclusively of recrystallized serpentine. Locally, serpentine and clinochlore intergrowths have been observed in association with the alteration of spinel. Primary Cr-spinel is rare in the harzburgites and is commonly completely altered to ferritchromite, rimmed by magnetite. 3.3.2. Orthopyroxenite dykes The mineral assemblage in the orthopyroxenite dykes is metamorphic in origin, with the exception of Cr-spinels. No relicts of original orthopyroxenes or clinopyroxenes have been preserved. Orthopyroxenite dykes consist of domains with relict boundaries of orthopyroxene grains that have been altered to bastites (Fig. 3D). The bastites, consisting of talc intimately mixed with serpentine, preserve the structure and deformation history of the orthopyroxene grain and, due to that, are sometimes kinked. The third-order interference colors displayed by the bastites indicate that enstatite was initially altered to talc which was then replaced by serpentine (O'Hanley, 1996). Metamorphic olivine forms small neoblasts and are confined to

domains within the relict cleavages of the bastites and around the bastites themselves. Amphiboles in the orthopyroxenite dykes can be texturally separated into two types. Relatively large (∼500 μm) idioblastic and pleochroic amphibole grains occur within the orthopyroxenite dykes. Amphibole of the second type occurs as small, disseminated grains throughout the orthopyroxenite dykes. They are generally smaller than 50 μm. Serpentine formed in the orthopyroxenite dykes can have various textures. The most common among them are interpenetrating blades of antigorite. The interpenetrating texture is observed in interstitial spaces where most or all of the primary mineral grains have been altered. Serrated serpentine is observed forming at the edges and within the cleavages of the bastites. Locally, the formation of serpentine and brucite intergrowth is restricted to sub-domains primarily consisting of olivine. Mesh cores are rare and occur when olivine grains have been completely serpentinized. Spinels are present as large idiomorphic grains throughout the rock and are slightly altered along fractures and grain rims. In samples that have been extensively hydrated, talc and olivine have been completely replaced by serpentine and amphibole with relict primary spinel. 3.3.3. Dunites Dunites are made up of olivine, serpentine ± brucite, Cr-spinel and magnetite. The dunites have been partially serpentinized and relict olivine grains are present in all the samples (Fig. 3E). Typically, serpentine and brucite form mesh-textures during the alteration of olivine. Relict olivine grains are present at the cores of the meshtextures although mesh cores consisting of serpentine are also sometimes observed. The contact between the

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K. Iyer et al. / Chemical Geology 249 (2008) 66–90

brucite and/or clinochlore. The formation of serpentine and clinochlore is ubiquitous in the wehrlites due to presence of large amounts of primary clinopyroxene. Alumina present in the clinopyroxene contributes to the formation of clinochlore within serpentine. Blades of antigorite commonly form interpenetrating textures in areas of intense serpentinization and recrystallization. Cr– Al spinels are extensively replaced by ferritchromite (Fig. 3F). Alteration of spinel begins at the rims and around fractures and progresses inwards. The alteration front consists of ‘fingers’ of porous ferritchromite extending into the spinel.

serpentine rim and olivine core is sharp. Adjacent olivine grains have similar optical orientations suggesting that they are fragments of a larger crystal. Magnetite grains associated with the serpentinization of olivine forms within the mesh rims. 3.3.4. Wehrlites Wehrlites are composed of primary and secondary clinopyroxene, olivine, serpentine ± brucite/clinochlore, Cr–Al spinel, ferritchromite and magnetite. Orthopyroxenes have not been found and have been completely altered to secondary diopside and serpentine which preserves the relict grain boundary. Primary clinopyroxene occurs as large grains, usually greater than 0.5 mm in size. Secondary clinopyroxene replaces olivine (Cpx2), primary clinopyroxene (Cpx3) and orthopyroxene (Cpx4) similar to the replacement textures seen in the harzburgites with the exception of Cpx4. Serpentine forms after the alteration of clinopyroxene and olivine. It is usually found intergrown with

3.3.5. Veins Three types of veins can be distinguished in the ultramafic rocks of the LOC based on textural characteristics and are described as V1–V3 (V1 being the earliest). V1-type veins consist of antigorite blades with interlocking textures, ribbon-textured serpentine or conical chrysotile, and cut across olivine and pyroxene

Table 2 Representative microprobe analyses of olivine Rock type Harzburgite

Dunite

Wehrlite

Orthopyroxenite dyke

Mineral

Olivine III Olivine II Olivine III Olivine I Olivine I Olivine I Olivine I Olivine I Olivine I Olivine II Olivine II Olivine II

SiO2 TiO2 Al2O3 FeO (tot) MnO MgO CaO Na2O K2O Cr2O3 NiO Total Oxygen p.f.u Si Ti Al Fe2+ (tot) Mn Mg Ca Na K Cr Ni Total cation Mg#

40.79 0.01 0.00 9.32 0.32 48.99 0.04 0.00 0.01 0.08 0.45 100.01 4

39.59 0.02 0.00 14.97 1.75 43.90 0.20 0.00 0.00 0.05 0.03 100.52 4

40.26 0.02 0.02 9.78 0.26 48.46 0.04 0.00 0.02 0.00 0.42 99.29 4

40.78 0.00 0.02 10.65 0.23 48.18 0.04 0.00 0.00 0.01 0.30 100.21 4

40.73 0.02 0.03 10.02 0.17 48.23 0.00 0.00 0.00 0.05 0.25 99.52 4

39.93 0.01 0.00 13.75 0.28 45.67 0.03 0.00 0.01 0.01 0.25 99.96 4

38.97 0.00 0.00 18.07 0.43 42.62 0.02 0.03 0.02 0.07 0.21 100.43 4

39.03 0.00 0.02 17.00 0.36 42.93 0.02 0.00 0.01 0.00 0.16 99.53 4

39.00 0.00 0.01 16.77 0.41 43.06 0.00 0.03 0.03 0.01 0.12 99.46 4

38.19 0.00 0.04 21.57 0.72 38.82 0.01 0.02 0.01 0.04 0.28 99.70 4

39.89 0.00 0.00 14.74 0.23 44.84 0.07 0.00 0.01 0.01 0.17 99.96 4

37.40 0.00 0.00 28.44 1.10 33.27 0.01 0.07 0.03 0.00 0.12 100.45 4

1.00 0.00 0.00 0.19 0.01 1.79 0.00 0.00 0.00 0.00 0.01 3.00

1.00 0.00 0.00 0.32 0.04 1.65 0.01 0.00 0.00 0.00 0.00 3.00

1.00 0.00 0.00 0.20 0.01 1.79 0.00 0.00 0.00 0.00 0.01 3.00

1.00 0.00 0.00 0.22 0.00 1.76 0.00 0.00 0.00 0.00 0.01 3.00

1.00 0.00 0.00 0.21 0.00 1.77 0.00 0.00 0.00 0.00 0.00 2.99

1.00 0.00 0.00 0.29 0.01 1.70 0.00 0.00 0.00 0.00 0.01 3.00

0.99 0.00 0.00 0.38 0.01 1.62 0.00 0.00 0.00 0.00 0.00 3.01

1.00 0.00 0.00 0.36 0.01 1.63 0.00 0.00 0.00 0.00 0.00 3.00

1.00 0.00 0.00 0.36 0.01 1.64 0.00 0.00 0.00 0.00 0.00 3.01

1.00 0.00 0.00 0.47 0.02 1.51 0.00 0.00 0.00 0.00 0.01 3.00

1.00 0.00 0.00 0.31 0.00 1.68 0.00 0.00 0.00 0.00 0.00 3.00

1.00 0.00 0.00 0.64 0.03 1.33 0.00 0.00 0.00 0.00 0.00 3.00

0.84

0.90

0.89

0.90

0.86

0.81

0.82

0.82

0.76

0.84

0.68

0.90 2+

2+

2+

Mg# = Mg /(Mg + Fe ).

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

73

Table 3 Representative microprobe analyses of clinopyroxene Rock type

Harzburgites

Wehrlites

Mineral

Cpx1

Cpx1

Cpx2

Cpx2

Cpx3

Cpx3

Cpx1

Cpx2

Cpx3

Cpx3

Cpx4

Cpx4

SiO2 TiO2 Al2O3 FeO (tot) MnO MgO CaO Na2O K2O Cr2O3 NiO Total Oxygen p.f.u Si Ti Al Fe2+ (tot) Mn Mg Ca Na K Cr Ni Total cation Mg#

52.96 0.06 2.97 1.96 0.14 16.56 24.08 0.25 0.01 1.17 0.05 100.19 6 1.92 0.00 0.13 0.06 0.00 0.90 0.94 0.02 0.00 0.03 0.00 4.00 0.94

53.14 0.00 2.35 1.90 0.10 17.03 24.37 0.18 0.01 1.09 0.11 100.28 6 1.93 0.00 0.10 0.06 0.00 0.92 0.95 0.01 0.00 0.03 0.00 4.01 0.94

55.26 0.03 0.01 0.70 0.00 18.06 26.31 0.00 0.00 0.09 0.05 100.52 6 1.99 0.00 0.00 0.02 0.00 0.97 1.02 0.00 0.00 0.00 0.00 4.01 0.98

55.43 0.00 0.03 1.08 0.00 17.91 25.97 0.03 0.01 0.10 0.09 100.65 6 2.00 0.00 0.00 0.03 0.00 0.96 1.00 0.00 0.00 0.00 0.00 4.00 0.97

52.72 0.02 0.08 5.56 0.18 14.58 25.95 0.00 0.01 0.18 0.00 99.30 6 1.97 0.00 0.00 0.17 0.01 0.81 1.04 0.00 0.00 0.01 0.00 4.02 0.82

52.91 0.01 0.25 5.60 0.30 14.48 25.51 0.12 0.01 0.07 0.05 99.31 6 1.98 0.00 0.01 0.18 0.01 0.81 1.02 0.01 0.00 0.00 0.00 4.02 0.82

53.92 0.06 1.44 3.08 0.14 16.68 24.57 0.19 0.01 0.36 0.03 100.46 6 1.96 0.00 0.06 0.09 0.00 0.90 0.96 0.01 0.00 0.01 0.00 4.01 0.91

55.17 0.00 0.05 1.54 0.03 17.81 26.23 0.06 0.02 0.00 0.00 100.92 6 1.99 0.00 0.00 0.05 0.00 0.96 1.01 0.00 0.00 0.00 0.00 4.01 0.95

53.22 0.04 0.03 7.71 0.50 12.99 25.45 0.04 0.02 0.01 0.00 100.01 6 1.99 0.00 0.00 0.24 0.02 0.73 1.02 0.00 0.00 0.00 0.00 4.01 0.75

52.55 0.04 0.03 9.79 0.42 12.08 25.21 0.01 0.00 0.00 0.00 100.13 6 1.99 0.00 0.00 0.31 0.01 0.68 1.02 0.00 0.00 0.00 0.00 4.01 0.69

54.21 0.00 0.23 1.65 0.09 17.75 25.26 0.13 0.00 0.31 0.00 99.64 6 1.98 0.00 0.01 0.05 0.00 0.97 0.99 0.01 0.00 0.01 0.00 4.02 0.95

54.62 0.01 0.24 1.52 0.01 17.49 25.67 0.19 0.04 0.31 0.00 100.11 6 1.98 0.00 0.01 0.05 0.00 0.95 1.00 0.01 0.00 0.01 0.00 4.01 0.95

Mg# = Mg2+/(Mg2+ + Fe2+); Cpx1 = primary clinopyroxene; Cpx2 = secondary clinopyroxene after olivine; Cpx3 = secondary clinopyroxene after primary clinopyroxene; Cpx4 = secondary clinopyroxene after orthopyroxene.

grains. The width of V1-type veins can be up to 500 μm. V1-type veins with conical chrysotile have been described by Andreani et al. (2007) and have high modal amounts of oxides. The vein-infill is almost isotropic under crossed-polarized light. V2-type veins are up to 250 μm wide; consist of banded serpentine with the bands oriented parallel to the vein margin and crosscut V1-type veins. The bands in the V2-type veins are interpreted as a mixture of protoserpentine + chrysotile + polygonal serpentine (Andreani et al., 2004) and always occur as infill in fractures present in the orthopyroxenite dykes. V3-type veins are composed of calcium carbonates with a width up to 200 μm and are interpreted as late-stage features. 3.4. Mineral chemistry 3.4.1. Olivine Three different kinds of olivine are determined based on chemical and textural evidence (Table 2; Fig. 4). Relict primary olivine is found in dunites and wehrlites

and is termed as Olivine I. The second type of olivine is found throughout the orthopyroxenite dykes and in certain domains of the harzburgites. Olivine II in the orthopyroxenite dykes is interpreted to have formed during breakdown of orthopyroxene to olivine and talc. They have relatively higher MnO and FeO (total) contents than primary olivine and are designated as Olivine II. The third type of olivine has a high forsterite component and is designated as Olivine III. Olivine III has been detected in the harzburgites and forms ‘rims’ around Olivine II (Fig. 5). Olivine II and Olivine III in harzburgites show a continuous range in composition with respect to manganese and iron content. Olivine I in the dunites has Mg-numbers between 0.86 and 0.90. Olivine I in wehrlites has a higher fayalitic component with Mg-numbers around 0.81– 0.82. MnO contents for Olivine I in dunites and wehrlites are less than 0.4 wt.%. Mn and Fe contents are positively correlated for all groups. MnO contents in Olivine II are exceptionally high and can reach 1.5 wt.% in the harzburgites and Mg-numbers can be as low as

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Table 4 Representative microprobe analyses of amphibole Rock type

Orthopyroxenite dyke

Mineral

Tremolite

SiO2 TiO2 Al2O3 FeO (tot) MnO MgO CaO Na2O K2O Cr2O3 NiO Total Oxygen p.f.u. Si Ti Al Fe2+ Fe3+ Mn Mg Ca Na K Cr Ni Total cations Mg#

54.50 0.03 3.49 2.49 0.06 21.62 12.68 1.54 0.01 1.10 0.14 97.66 23.00 7.54 0.00 0.57 0.26 0.03 0.01 4.46 1.88 0.41 0.00 0.12 0.02 15.30 0.94

Winchite 57.58 0.00 0.16 4.18 0.25 21.98 11.02 2.32 0.09 0.01 0.03 97.61 23.00 7.97 0.00 0.03 0.48 0.00 0.03 4.54 1.64 0.62 0.02 0.00 0.00 15.33 0.90

57.95 0.02 0.27 3.26 0.28 22.90 9.09 3.41 0.13 0.10 0.11 97.52 23.00 7.97 0.00 0.04 0.28 0.09 0.03 4.70 1.34 0.91 0.02 0.01 0.01 15.42 0.94

Richterite 57.49 0.00 0.25 4.76 0.36 22.29 8.04 3.84 0.10 0.41 0.01 97.55 23.00 7.97 0.00 0.04 0.51 0.04 0.04 4.60 1.19 1.03 0.02 0.04 0.00 15.50 0.90

57.74 0.04 0.16 5.16 0.46 21.76 8.16 4.27 0.17 0.10 0.02 98.03 23.00 7.99 0.00 0.03 0.54 0.06 0.05 4.49 1.21 1.14 0.03 0.01 0.00 15.55 0.89

57.81 0.01 0.19 5.15 0.49 22.03 7.81 4.68 0.14 0.12 0.07 98.50 23.00 7.97 0.00 0.03 0.58 0.01 0.06 4.53 1.15 1.25 0.02 0.01 0.01 15.64 0.89

Mg-Hornblende

Edenite

53.34 0.07 4.11 2.75 0.06 21.65 12.66 1.28 0.01 0.87 0.11 96.92 23.00 7.43 0.01 0.67 0.17 0.15 0.01 4.49 1.89 0.35 0.00 0.10 0.01 15.28 0.96

52.56 0.05 4.29 3.90 0.12 20.76 11.72 2.87 0.05 1.07 0.03 97.42 23.00 7.37 0.01 0.71 0.39 0.07 0.01 4.34 1.76 0.78 0.01 0.12 0.00 15.57 0.92

52.02 0.08 5.55 2.93 0.00 21.14 12.74 1.85 0.01 1.31 0.07 97.70 23.00 7.24 0.01 0.91 0.24 0.10 0.00 4.38 1.90 0.50 0.00 0.14 0.01 15.43 0.95

52.24 0.07 5.16 3.00 0.06 20.85 12.67 2.30 0.02 1.25 0.08 97.70 23.00 7.29 0.01 0.85 0.35 0.00 0.01 4.34 1.90 0.62 0.00 0.14 0.01 15.52 0.93

Mg# = Mg2+/(Mg2+ + Fe2+).

0.68 for Olivine II in the orthopyroxenite dykes. All olivines investigated are poor in CaO and Cr2O3 (below 0.05 and 0.1 wt.%, respectively) and have NiO contents lower than 0.45 wt.%. 3.4.2. Clinopyroxene Clinopyroxenes have been observed only in the wehrlites and harzburgites. Two types of clinopyroxene have been distinguished; primary (Cpx1) and secondary (Cpx2–4; Table 3). Primary clinopyroxenes (Cpx1) are augitic in composition and contain 1.0–3.0 wt.% Al2O3 and 0.05–0.30 wt.% Na2O. FeO contents of primary clinopyroxene are between 1.5 and 4.0 wt.%. Secondary clinopyroxenes (Cpx2–4) tend toward the diopsidic end-member and usually contain lower amounts of Na2O and Al2O3, below 0.1 wt.% and 0.8 wt.% respectively. However, secondary clinopyroxene formed after primary clinopyroxene (Cpx3) has a higher hedenbergite component than that formed after olivine (Cpx2) or orthopyroxene (Cpx4) with FeO (total) contents ranging from 5.5–7.0 wt.% for the former

and less than 4 wt.% for the latter. Na2O contents of secondary clinopyroxene after orthopyroxene (Cpx4) are higher than other secondary clinopyroxenes (Cpx2 and 3) and are similar to that measured in primary clinopyroxenes (Cpx1). Cr2O3 contents in clinopyroxenes are positively correlated to the Al2O3 and can reach up to 1.2 wt.% in the primary clinopyroxenes but are usually below 0.4 wt.% in secondary clinopyroxenes. 3.4.3. Amphiboles Amphiboles have been detected only in the orthopyroxenite dykes. Five different kinds of amphibole are classified based on chemical compositions and using nomenclature from Leake et al. (1997) (Table 4); magnesiohornblende, edenite, tremolite, winchite and richterite (Fig. 6). Estimation of ferric iron in amphiboles was carried out using the format laid out in Appendix 2 of Leake et al. (1997). Large, pseudomorphic amphiboles are magnesiohornblende or tremolite. Disseminated, smaller grains of amphiboles are tremolite, winchite or richterite.

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

75

Table 5 Representative microprobe analyses of talc Rock type

Orthopyroxenite dyke

Mineral

Talc

Talc

Talc

Talc

Talc

Talc

Talc

Talc

Talc

Talc

Talc

Talc

SiO2 TiO2 Al2O3 FeO (tot) MnO MgO CaO Na2O K2O Cr2O3 NiO Total Oxygen p.f.u Si Ti Al Fe2+ (tot) Mn Mg Ca Na K Cr Ni Total cation Mg#

60.65 0.00 0.47 3.21 0.00 28.72 0.05 0.11 0.07 0.07 0.07 93.42 11 3.97 0.00 0.04 0.18 0.00 2.80 0.00 0.01 0.01 0.00 0.00 7.02 0.94

62.60 0.01 0.37 2.08 0.00 29.65 0.00 0.11 0.04 0.01 0.02 94.91 11 4.00 0.00 0.03 0.11 0.00 2.83 0.00 0.01 0.00 0.00 0.00 6.99 0.96

62.49 0.00 0.30 2.62 0.00 29.78 0.01 0.13 0.02 0.04 0.10 95.51 11 3.99 0.00 0.02 0.14 0.00 2.83 0.00 0.02 0.00 0.00 0.01 7.01 0.95

62.64 0.03 0.21 2.35 0.02 29.54 0.04 0.16 0.06 0.03 0.08 95.16 11 4.01 0.00 0.02 0.13 0.00 2.82 0.00 0.02 0.00 0.00 0.00 7.00 0.96

63.00 0.00 0.27 2.28 0.01 29.70 0.02 0.11 0.02 0.01 0.09 95.55 11 4.01 0.00 0.02 0.12 0.00 2.82 0.00 0.01 0.00 0.00 0.00 6.99 0.96

61.77 0.01 0.42 2.39 0.04 29.49 0.04 0.23 0.08 0.11 0.06 94.67 11 3.98 0.00 0.03 0.13 0.00 2.83 0.00 0.03 0.01 0.01 0.00 7.02 0.96

62.60 0.02 0.78 1.71 0.00 29.84 0.01 0.35 0.02 0.10 0.14 95.61 11 3.98 0.00 0.06 0.09 0.00 2.83 0.00 0.04 0.00 0.00 0.01 7.01 0.97

63.08 0.00 0.63 1.75 0.00 30.67 0.03 0.34 0.04 0.05 0.03 96.66 11 3.97 0.00 0.05 0.09 0.00 2.88 0.00 0.04 0.00 0.00 0.00 7.03 0.97

63.29 0.01 0.27 1.75 0.04 31.15 0.01 0.16 0.04 0.02 0.09 96.82 11 3.97 0.00 0.02 0.09 0.00 2.91 0.00 0.02 0.00 0.00 0.00 7.03 0.97

62.66 0.00 0.49 1.79 0.00 29.69 0.02 0.25 0.02 0.03 0.01 95.03 11 4.00 0.00 0.04 0.10 0.00 2.83 0.00 0.03 0.00 0.00 0.00 7.00 0.97

62.72 0.00 0.37 2.01 0.03 30.30 0.00 0.16 0.02 0.01 0.06 95.69 11 3.98 0.00 0.03 0.11 0.00 2.87 0.00 0.02 0.00 0.00 0.00 7.01 0.96

63.06 0.00 0.47 1.76 0.08 30.77 0.00 0.26 0.06 0.00 0.13 96.59 11 3.97 0.00 0.03 0.09 0.00 2.89 0.00 0.03 0.00 0.00 0.01 7.03 0.97

Mg# = Mg2+/(Mg2+ + Fe2+).

Magnesiohornblende contains between 1.83 and 1.89 a.p.f.u. Ca and 0.34 and 0.63 a.p.f.u. Na. Si contents are between 7.03 and 7.44 a.p.f.u. and Al contents are between 0.67 and 1.13 a.p.f.u. Mg# of magnesiohornblende ranges from 0.92 to 0.96. Edenite occurs as rims around magnesiohornblende and has higher Na contents between 0.62 and 0.78 a.p.f.u. Mg# of edenite is between 0.91 and 0.92. Ca and Na contents of tremolite range from 1.63– 1.95 a.p.f.u. and 0.14–0.62 a.p.f.u. respectively. Al contents range between 0.02 and 0.60 a.p.f.u. while the Si contents range from 7.54–7.99 a.p.f.u. Mn and Cr concentrations in tremolites are between 0.0 and 0.03 a.p.f.u. and 0.0 and 0.12 a.p.f.u., respectively. Mg# for tremolites is between 0.90 and 0.97. Winchites and richterites contain higher amounts of Na and Mn and lower amounts of Al and Ca than the tremolites. Si concentrations for winchite and richterite are between 7.96 and 7.98 a.p.f.u. while Na and Mn contents range from 0.87–1.25 a.p.f.u. and 0.03– 0.06 a.p.f.u., respectively. Ca and Al concentrations are between 1.15 and 1.35 a.p.f.u. and 0.02 and 0.04 a.p.f.u.,

respectively. Mg# is between 0.90 and 0.95 for winchites and between 0.88 and 0.91 for richterites. 3.4.4. Talc Talc has been detected only in the orthopyroxenite dykes (Table 5). It occurs as flakes within the bastites formed after primary orthopyroxene and has low amounts of Al2O3 and minor substitution of Fe2+ for Mg2+. MnO contents in talc are low (b 0.1 wt.%). The concentration of Al2O3 and Cr2O3 in talc is low, below 1 wt.% and 0.5 wt.% respectively. The Mg# for talc is high with an average of 0.96 (Mg# = Mg / Mg + Fe) corresponding to FeO contents of up to 3.5 wt.%. 3.4.5. Serpentine Serpentine is ubiquitous within the ultramafic lithologies of the LOC and is classified into three groups based on textural and chemical analysis (Table 6; Fig. 7). Serpentine and brucite intergrowths associated with mesh-textured olivine are distinguished by their lower SiO2 contents (26–40 wt.%). Al2O3 and Cr2O3 contents are usually below 0.5 wt.% and 0.2 wt.% respectively.

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K. Iyer et al. / Chemical Geology 249 (2008) 66–90

Table 6 Representative microprobe analyses of serpentine Rock type

Harzburgite

Mineral

Atg

Dunite

Srp + br Srp + fbr (mc) Srp + cli Srp

SiO2 44.22 37.71 TiO2 0.02 0.02 Al2O3 1.07 0.01 FeO (tot) 2.58 4.95 MnO 0.09 0.16 MgO 39.52 40.62 CaO 0.00 0.12 Na2O 0.01 0.01 K2O 0.01 0.00 Cr2O3 0.10 0.00 NiO 0.12 0.28 Total 87.73 83.88 Oxygen p.f.u 7 – Si 2.04 – Ti 0.00 – Al 0.06 – Fe2+ (tot) 0.10 – Mn 0.00 – Mg 2.72 – Ca 0.00 – Na 0.00 – K 0.00 – Cr 0.00 – Ni 0.00 – Total cation 4.93 – Mg# 0.96 –

21.92 0.00 0.06 14.85 0.65 42.47 0.10 0.00 0.01 0.01 0.44 80.50 – – – – – – – – – – – – – –

34.32 0.00 12.14 4.73 0.00 33.60 0.02 0.01 0.00 0.81 0.09 85.73 – – – – – – – – – – – – – –

40.78 0.00 0.00 4.07 0.07 39.41 0.07 0.00 0.00 0.04 0.00 84.45 7 1.98 0.00 0.00 0.17 0.00 2.86 0.00 0.00 0.00 0.00 0.00 5.02 0.95

Orthopyroxenite dyke

Wehrlite

Srp + br Srp + fbr (mc) Atg

Srp + fbr (mc) Atg

Srp + fbr (mc) Srp + cli

36.42 0.00 0.01 5.21 0.17 42.81 0.03 0.00 0.00 0.00 0.23 84.89 – – – – – – – – – – – – – –

37.15 0.02 0.18 11.48 0.23 28.83 0.18 0.00 0.06 0.07 0.11 78.30 – – – – – – – – – – – – – –

36.63 0.01 0.30 10.38 0.43 34.46 0.25 0.03 0.00 0.00 0.24 82.73 – – – – – – – – – – – – – –

24.29 0.00 0.02 16.58 0.42 39.66 0.02 0.01 0.01 0.00 0.09 81.11 – – – – – – – – – – – – – –

44.48 0.01 0.25 4.86 0.02 37.94 0.00 0.00 0.00 0.01 0.04 87.61 7 2.07 0.00 0.01 0.19 0.00 2.64 0.00 0.00 0.00 0.00 0.00 4.92 0.93

42.56 0.01 2.06 4.39 0.08 37.87 0.06 0.02 0.01 0.19 0.13 87.37 7 1.99 0.00 0.11 0.17 0.00 2.65 0.00 0.00 0.00 0.01 0.00 4.95 0.94

34.93 0.00 11.98 4.70 0.00 34.39 0.03 0.01 0.02 0.08 0.12 86.28 – – – – – – – – – – – – – –

Mg# = Mg2+/(Mg2+ + Fe2+); Atg = antigorite blades; Srp + br = serpentine and brucite intergrowth; Srp + fbr (mc) = serpentine and ferroan brucite intergrowth in mesh cores after olivine; Srp + cli = serpentine and clinochlore intergrowths. Note: A.p.f.u. for serpentine mixtures has not been calculated as the stoichiometry is not known.

Mesh cores after olivine consisting of serpentine and ferroan brucite have higher FeO (total) and MnO contents up to 20 and 1.2 wt.%, respectively. The formation of serpentine and ferroan brucite has been attributed to serpentinization reactions at low fluid fluxes (Bach et al., 2006). Serpentine and clinochlore forms intergrowths associated with clinopyroxene and spinel and have Al2O3 contents ranging between 6 and 18 wt.%. Cr2O3 contents can be up to 2 wt.%. Blades of antigorite have SiO2 values between 42 and 45 wt.% and can contain 4 wt.% Al2O3.

Al2O3 contents are negligible. Magnetite has the highest amount of Fe3+ and Cr2O3 contents are generally lower than 10 wt.%. Al2O3 and MnO contents in magnetite are also lower than for the other minerals in the spinel group. Ferric iron shows negative correlation with Cr and Al contents in the spinels (Fig. 8).

3.4.6. Spinel Spinel is classified as magmatic Cr-spinel, ferritchromite and magnetite (Table 7). Primary Cr-spinel contains between 40 and 55 wt.% Cr2O3 and 8 and 30 wt.% Al2O3 (Cr# = 0.70–0.80). Cr and Al are negatively correlated. MnO contents in Cr-spinel are between 0.2 and 1.0 wt.%. Ferritchromite is characterized by higher Fe3+ and a slight increase in MnO contents (0.4–1.2 wt.%). Cr2O3 contents in ferritchromite are much lower than Cr-spinel (10–38 wt.%) and

The ultramafic rocks of the Leka Ophiolite Complex have undergone serpentinization and display a wide range of mineral reactions and textures indicative of fluid–rock interaction. Austrheim and Prestvik (in press) have studied the rodingites associated with the LOC and conclude that although some hydration could have taken place during obduction and the Caledonian metamorphic event, the majority of the serpentinization process occurred during ocean-floor metamorphism. Dredged ocean-floor samples also show

4. Discussion 4.1. Evidence for ocean-floor metamorphism

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

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Table 7 Representative microprobe analyses of spinel Rock type

Harzburgite

Dunite

Wehrlite

Orthopyroxenite dyke

Mineral

P-Sp

Ftc

Mgt

P-Sp

Mgt

P-Sp

Ftc

Mgt

P-Sp

P-Sp (alt)

Mgt

SiO2 TiO2 Al2O3 FeO (tot) MnO MgO CaO Na2O K2O Cr2O3 NiO Total Oxygen p.f.u Si Ti Al Fe2+ Fe3+ Mn Mg Ca Na K Cr Ni Total cation Cr#

0.00 0.01 26.53 19.71 0.40 11.63 0.04 0.07 0.00 40.66 0.01 99.05 4 0.00 0.00 0.96 0.45 0.06 0.01 0.53 0.00 0.00 0.00 0.99 0.00 3.00 0.51

0.06 0.07 0.19 66.92 1.48 0.84 0.01 0.01 0.00 22.04 0.14 91.75 4 0.00 0.00 0.01 0.90 1.30 0.05 0.05 0.00 0.00 0.00 0.68 0.00 3.00 0.99

0.03 0.05 0.03 86.10 0.05 0.47 0.02 0.04 0.00 3.57 0.72 91.08 4 0.00 0.00 0.00 0.94 1.89 0.00 0.03 0.00 0.00 0.00 0.11 0.02 3.00 0.99

0.06 0.12 9.37 28.67 0.37 6.61 0.00 0.04 0.00 53.36 0.00 98.59 4 0.00 0.00 0.38 0.65 0.17 0.01 0.34 0.00 0.00 0.00 1.44 0.00 3.00 0.79

0.12 0.04 0.01 88.12 0.17 0.23 0.00 0.02 0.00 0.90 0.21 89.81 4 0.00 0.00 0.00 0.98 1.96 0.01 0.01 0.00 0.00 0.00 0.03 0.01 3.00 0.99

0.01 0.24 31.18 31.29 0.36 8.80 0.00 0.00 0.01 26.67 0.00 98.56 4 0.00 0.01 1.13 0.59 0.21 0.01 0.40 0.00 0.00 0.00 0.65 0.00 3.00 0.36

0.02 0.59 0.47 69.64 0.65 0.57 0.00 0.03 0.00 21.59 0.09 93.65 4 0.00 0.02 0.02 0.96 1.29 0.02 0.03 0.00 0.00 0.00 0.66 0.00 3.00 0.97

0.05 0.20 0.06 87.66 0.06 0.24 0.04 0.02 0.01 3.65 0.14 92.12 4 0.00 0.01 0.00 0.98 1.87 0.00 0.01 0.00 0.00 0.00 0.11 0.00 3.00 0.98

0.06 0.13 10.94 26.05 0.23 7.40 0.00 0.00 0.00 54.56 0.03 99.40 4 0.00 0.00 0.43 0.63 0.10 0.01 0.37 0.00 0.00 0.00 1.45 0.00 3.00 0.77

0.00 0.08 10.94 30.31 0.52 4.16 0.01 0.04 0.00 51.59 0.06 97.72 4 0.00 0.00 0.45 0.76 0.12 0.02 0.22 0.00 0.00 0.00 1.42 0.00 3.00 0.76

0.48 0.00 0.00 91.32 0.04 0.05 0.01 0.02 0.00 0.10 0.00 92.01 4 0.02 0.00 0.00 1.01 1.96 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.00 1.00

Cr# = Cr3+/(Cr3+ + Al3+); P-Sp = primary spinel; P-Sp (alt) = slightly altered primary spinel; Ftc = ferritchromite; Mgt = magnetite.

evidence of rodingitization (Aumento and Loubat, 1971; Honnerez and Kirst, 1975). The alteration reactions occurring in the LOC, furthermore, require a large amount of water to produce the observed, hydrous mineral assemblages. The average H2O content measured is about 10 wt.% for the orthopyroxenite dykes, harzburgites and dunites. This translates to around 300 kg H2O per m3 rock. Although the process of

serpentinization does not occur instantaneously but in sequence over a range of temperatures (see Section 4.2), this huge amount of water could be available during ocean-floor metamorphism of the ultramafic rocks. Pseudomorphic replacement textures, such as mesh cells and bastites, observed in the ultramafic lithologies of the LOC are typical of retrogressive hydration reactions occurring under static conditions (Type 3

Fig. 2. A. Relationship between serpentinized ultramafics and end-member minerals in MgO–SiO2–CaO (molar proportion) ternary diagram. All lithologies tend towards the dominant minerals that composed the rock before alteration. B. Ternary diagram of minerals and serpentinized ultramafics in the MgO–SiO2–H2O system.

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Fig. 3. A. Photomicrograph of olivine with apparent cleavage (Ol–Cl) next to ‘normal’ olivine grain (Ol) in harzburgite. B. BSE image of secondary olivine (Ol) with serpentine and brucite (Srp+br) forming within fractures in harzburgite (sample LE06). Secondary clinopyroxene (Cpx2, Cpx3) forms after olivine and primary clinopyroxene (Cpx1), respectively. Fine-rained ferritchromite and magnetite (bright grains) are common. Green arrows indicate veins of secondary clinopyroxene. C. BSE image of antigorite blades (Atg) forming after serpentine+brucite (Srp+br) intergrowth. Antigorite is often observed to penetrate through the olivine (Ol). D. Photomicrograph of talc/serpentine bastites after orthopyroxene with high birefringence colors in orthopyroxenite dyke. Areas dominated by olivine occur between the bastite grains. E. BSE image of mesh-texture in dunite. Relict olivine grains (Ol) are surrounded by a fine-grained intergrowth of serpentine and brucite (Srp+br). Mesh cores (m-core) are also observed and consist of a mixture of serpentine and ferroan brucite. F. BSE image of altered primary Cr-spinel in wehrlite. The grain is rimmed by ferritchromite (Ftc) and serpentine and clinochlore (Srp+cli) are formed in fractures.

Fig. 4. Mn (a.p.f.u.) vs. Mg# binary plot for olivines in various lithologies shows an inverse relationship. Fe contents are positively correlated to manganese contents.

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

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Fig. 5. A. BSE image of secondary olivine grain in harzburgite (sample LE05). Olivine II appears as ‘patches’ within recrystallized Olivine III. Numerous grains of ferritchromite (Ftc) and magnetite (Mgt) form over Olivine III while Olivine II is largely free of such inclusions. Serpentine and brucite intergrowths (Srp + br) are present within the fractures in the olivine and later recrystallized to antigorite (Atg). B. Profile of manganese concentration (a.p.f.u.) across the olivine grain (dashed line in Fig. 5A) describes a bell-shaped curve. Distance between two points is 2 μm.

serpentinization after Wicks and Whittaker, 1977). The successive formation and replacement of amphibole, talc and serpentine in the orthopyroxenite dykes also suggests a continuous decrease in temperature during hydration. Our findings support the conclusions that most of the serpentinization within the LOC occurred at the ocean-floor. However, late antigorite formation postdating mesh-texture formation in the olivine present in the various lithologies of the LOC may have been related to increasing temperature during Caledonian regional metamorphism after obduction or due to excess silica present in the fluids (Wicks and Whittaker, 1977; O'Hanley, 1996; Li et al., 2004). 4.2. Dominant phase transformations occurring in the ultramafic rocks of the Leka Ophiolite Complex The forward progress of hydration reactions depends on the presence of a fluid phase. In case of serpentinization of the oceanic lithosphere, questions regarding the timing of hydration and the mechanism by which the

fluid enters the rock are central. In the following, we compare the reactions expected assuming continuous fluid access to the rock and compare this with the observed mineral reactions. Equilibrium phase relations for representative bulk-rock compositions for each lithology were calculated using Perple_X (Connolly and Petrini, 2002) and thermodynamic data from Holland and Powell (1998) in the CMSH system (Fig. 9). The measured FeO content in the samples was recalculated to an equivalent (molar) amount of MgO. Adjustment of CMSH-endmember activities to account for the presence of Fe does not result in major shifts of the reactions. Fluids were treated as pure water and H2O values were obtained from the measured LOI. Addition of CO2 and CH4 into the fluid phase would shift the reaction curves to lower temperatures. The model is treated as a ‘batch’ experiment where the total amount of water was allowed to react freely with the other components throughout the given temperature and pressure range. Phase relations were also modeled to allow for episodic fluid infiltration events, i.e. a fixed amount of water (a fraction of the LOI

Fig. 6. (Na + K)A vs. Si plot for measured amphiboles shows a complete solid-solution series between calcic and sodic-calcic amphiboles.

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Fig. 7. Plot of Al2O3 vs. (MgO + FeO)/SiO2 shows a clear discrimination between the 3 types of serpentine and serpentine containing intergrowths observed in the ultramafic rocks of the LOC.

value) is introduced at regular intervals of temperature. This does not result in any shift in the reaction curves predicted by the presented model but the amount of hydrous phases will depend on the amount of fluid present in each infiltration event. Modal volumes of mineral phases were calculated for decreasing temperatures along continental (30 °C/km) and oceanic (70 °C/km) geotherms. The mineral modes for the continental and oceanic geotherms are very similar as the phase boundaries are only slightly dependent on pressure. This results in a slight shift of the mineral reactions across the phase boundaries towards higher temperatures (∼ 25–40 °C) for the continental geotherm. The phase changes, however, remain identical and the mineral phase assemblage for the oceanic geotherm will be used here (Fig. 9). Hydration of the ultramafic lithologies of the LOC is dominated by the following four reactions in the model: 4CaMgSi2 O6 þ 5Mg2 Si2 O6 þ2H2 OY Cpx

Opx

ðR1Þ

2Ca2 Mg5 Si8 O22 ðOHÞ2 þ 2Mg2 SiO4 Tremolite

Olivine

5Mg2 Si2 O6 þ2H2 OY 2Mg3 Si4 O10 ðOHÞ2 þ 2Mg2 SiO4 Opx

Talc

Olivine

ðR2Þ

these reactions occurring in representative samples of the various ultramafic lithologies and compare them with the actual reactions derived from textures. 4.2.1. Harzburgites (sample LE05) The harzburgite present in the mantle section of the LOC contains neither primary orthopyroxene nor talc. Relict primary clinopyroxenes have been altered around grain rims and along cleavages to serpentine indicating that no fluids were available for (R1) to proceed. The reaction of primary orthopyroxene to talc and olivine (R2) and subsequently to serpentine (R3) possibly occurred in the early stages of hydration (Fig. 9A), but no textural support for this reaction is found. Mesh-textures with relict olivines are common in the harzburgites. Mesh serpentine consists of fine-grained intergrowths of serpentine and brucite (R4). Approximately 30% of the harzburgite is composed of relict olivine with minor amounts of clinopyroxene while the rest is serpentine and brucite. This is consistent with a LOI of ca. 10.0 wt.%. 4.2.2. Orthopyroxenite dykes (sample LE55) Primary clinopyroxene in the orthopyroxenite dykes has been completely altered to amphiboles as described by (R1). Textural observations indicate that olivine is not

Mg3 Si4 O10 ðOHÞ2 þ 6Mg2 SiO4 þ9H2 OY Talc

Olivine

ðR3Þ

5Mg3 Si2 O5 ðOHÞ4 Serpentine

2Mg2 SiO4 þ3H2 OY Mg3 Si2 O5 ðOHÞ4 þ MgðOHÞ2: Olivine

Serpentine

Brucite

ðR4Þ These modeled reactions take place below 800 °C, 650 °C, 530 °C and 400 °C, respectively at a pressure of 200 MPa. In the following, we discuss the progress of

Fig. 8. Binary plot of Cr + Al vs. Fe3+ in spinels. Cr3+ and Al3+ are replaced by Fe3+ as alteration progresses towards ferritchromite and magnetite.

K. Iyer et al. / Chemical Geology 249 (2008) 66–90 Fig. 9. Phase diagrams for various lithologies using Perple_X with temperature ranging from 100–900 °C and pressure from 10–1000 MPa. Phases associated with temperatures and pressure fields are indicated next to red dots. Red and blue dotted lines represent the continental and oceanic geotherms, respectively. The modal volume percent of the solid phases (normalized to 100% after removal of water) for each lithology is also plotted with respect to the oceanic geotherm. Note: water was treated as a thermodynamic component and values were obtained from the LOI. ol = Olivine, en = enstatite, di = diopside, sp = spinel, tr = tremolite, clin = clinochlore, ta = talc, atg = antigorite, br = brucite, phA = phase A, H2O = water.

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K. Iyer et al. / Chemical Geology 249 (2008) 66–90

associated with (R1) which suggests that the formation of amphibole after primary clinopyroxene could have occurred in an open system. Primary orthopyroxene has been completely altered to bastites which preserve the grain boundaries of the altered orthopyroxene and forms talc and metamorphic olivine (R2). Secondary olivine and talc are subsequently altered to antigorite (R3). Serpentine and brucite intergrowths have been detected in domains dominated by the presence of secondary olivine (R4), although this is quite rare. Thin orthopyroxenite dykes are often completely altered to hydrous phases with only serpentine and amphibole present. 4.2.3. Dunites (sample LE15) There are no textural relicts of primary orthopyroxene left in the dunites of the layered series. The high susceptibility of orthopyroxene to alteration may have wiped out all traces of it. The phase calculations based on bulk-rock chemistry show that orthopyroxene was a minor phase (Fig. 9C) which may have altered to talc and olivine (R2) and subsequently serpentine depending on the availability of H2O (R3). The hydration of the dunites of the LOC is dominated by the alteration of olivine to serpentine and brucite (R4) which results in meshtextures consisting of relict primary olivine as cores surrounded by fine-grained intergrowths of serpentine and brucite. Approximately 60% of the olivine present in the dunite has been serpentinized, consistent with ca. 11 wt.% LOI when corrected for the presence of brucite. 4.2.4. Wehrlites (sample LE73) Primary clinopyroxene in the wehrlites have undergone alteration to intergrowths of serpentine and clinochlore and no traces of amphiboles have been detected. (R1) seems not to have occurred in the wehrlites. Olivine displays mesh-textures surrounded by a fine-grained mixture of serpentine and brucite (R4). Relict grain boundaries of primary orthopyroxene are preserved and this mineral has been completely altered to secondary clinopyroxene and serpentine. The wehrlite was originally composed of about 40% olivine and the rest was clinopyroxene with minor orthopyroxene. Approximately 40% of the olivine has been serpentinized while the clinopyroxenes have suffered little alteration (∼20%). Therefore, serpentine makes up around 30% of the wehrlite accounting for ca. 5 wt.% LOI. 4.3. P–T relations and reaction history in the Leka Ophiolite Complex Comparison of the calculated phase diagrams with the assemblages present in the rocks suggests that the

various lithologies were hydrated at different stages. As discussed above, a variety of reactions took place simultaneously within the different lithologies but a few are restricted to specific lithologies. In the following, we discuss the timing and temperatures of the various reactions based on textural and chemical relations. 4.3.1. Stage 1 (High-T alteration) Stage 1 is characterized by the complete replacement of primary clinopyroxene by Ca-amphibole in the orthopyroxenite dykes (R1) below 800 °C. (R1) has not occurred in the harzburgites and wehrlites suggesting that fluids were available only to the orthopyroxenite dykes at this stage. Since the orthopyroxenite dykes are embedded in a dunite matrix, it is likely that the dunites were also exposed to water, but did not react due to the lack of original pyroxenes. Tremolite is the most common amphibole formed with lower modal amounts of magnesiohornblende. However, these temperatures are the upper limit for the reaction of primary clinopyroxene to amphibole. It is also likely that the alteration of clinopyroxene and some of the orthopyroxene occurred simultaneously at lower temperatures (b650 °C) resulting in the formation of amphibole. The formation of calcicamphibole after clinopyroxene may alternatively occur in an open system through a reaction such as (R5). The excess Ca and Si in the fluid, in conjunction with the breakdown of orthopyroxene, could then result in the formation of the disseminated grains of amphibole observed in the orthopyroxenite dykes (Allen and Seyfried, 2003; Bach et al., 2004; Paulick et al., 2006; Austrheim and Prestvik, in press). 5CaMgSi2 O6 þ6HClY Ca2 Mg5 Si8 O22 ðOHÞ2 Cpx

Tremolite

ðR5Þ

þ3CaCl2ðaqÞ þ 2SiO2ðaqÞ þ 2H2 O: 4.3.2. Stage 2 Stage 2 is defined by the breakdown of orthopyroxene. The reaction of orthopyroxene (R2), in the presence of fluids, to talc and secondary olivine (Olivine II) is welldocumented in the orthopyroxenite dykes (Fig. 9B). Experimental calibrations suggest that the breakdown of orthopyroxene to talc and olivine begins at approximately 650 °C at 100 MPa (Pawley, 1998; Melekhova et al., 2006) in the presence of fluids. Allen and Seyfried (2003) showed that orthopyroxene alters at temperatures higher than 400 °C at 50 MPa when hydration of olivine is too sluggish to proceed. The formation of bastites after orthopyroxene indicates that hydration took place under static conditions (see Fig. 3D). The resulting Olivine II is

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

high in iron and manganese contents. Previous studies have also found that secondary olivine formed due the alteration of orthopyroxene is richer in iron and manganese (Kimball et al., 1985). The chemical composition of Olivine II in the orthopyroxenite dykes is uniform for a given sample but can vary among different samples implying that the chemistry was dependent on the original chemical composition and amount of orthopyroxene present in the rock (Fig. 10). Olivine II has also been observed in some areas of the harzburgites but no talc has been detected (see Fig. 5A). Olivine II in the harzburgites is interpreted to have formed due to a different process than that described above (see Section 4.6).

83

by olivine. The formation of brucite is possible only when the activity of silica in the fluid is low else the formation of serpentine is favored indicating that nearby orthopyroxene and talc have been completely altered (in Stages 2 and 3) and the system had locally evolved to lower silica activity before the formation of serpentine and brucite mixtures. Mesh cores are formed locally when olivine replacement is complete. The mesh-core intergrowths contain higher amounts of ferroan brucite (see Fig. 3E). Magnetite is formed due to the oxidation of fayalitic component of olivine (R6) and/or iron in ferroan brucite and serpentine as the system evolved to more oxidizing conditions. Fe2 SiO4 þ2H2 OY 2Fe3 O4 þ3SiO2 þ 2H2 :

4.3.3. Stage 3 Talc and olivine react to serpentine at temperatures lower than 550 °C at 200 MPa as long as fluids are available (R3). Serpentine is formed directly from this reaction due to high silica activity in the presence of talc (Frost and Beard, 2007). Blades of antigorite are commonly seen penetrating grains of Olivine II. 4.3.4. Stage 4 The hydration of olivine to serpentine and brucite (R4), in the presence of fluids, occurs during Stage 4 at temperatures lower than 400 °C for all rock types. Static conditions during serpentinization are indicated by the formation of mesh-textures after olivine (see Fig. 3E) (Type 3 serpentinization, Wicks and Whittaker, 1977). The mesh-textures formed during the breakdown of olivine consist of a fine-grained intergrowth of serpentine and brucite (R4). The formation of brucite indicates that the temperatures were at least below 400 °C at 100 MPa (O'Hanley, 1996) and is also predicted by the phase diagrams (Fig. 9A, C and D). The formation of serpentine and brucite after olivine is also observed in the orthopyroxenite dykes but is locally restricted to domains dominated

Fig. 10. Manganese concentrations in secondary olivine (Olivine II) in orthopyroxenite dykes are negatively correlated to the forsterite content. Olivine II compositions in a given sample of orthopyroxenite do not vary much but can be significantly different in different samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fayalite

Magnetite

ðR6Þ

Texturally, we observe that veins of secondary diopside crosscut fractures in olivine filled with serpentine ±brucite, indicating that secondary diopside formation occurred either during or after mesh-texture formation (see Fig. 3B). The stability field of tremolite + olivine ranges between 450 and 825 °C at Ptotal =P(H2O) =500 MPa, while diopside is stable at higher and lower temperatures (Peacock, 1987). Phase calculations also indicate that tremolite, formed during high-temperature alteration in the harzburgites and wehrlites, would be unstable below 500 °C at about 200 MPa and breaks down to secondary diopside. However, textural relations show that clinopyroxene was previously unreactive (in Stage 1) in the harzburgites and wehrlites and has been directly altered to serpentine at grain rims and along cleavages. The alteration of primary clinopyroxene (Cpx1) to serpentine results in excess Ca and Si in the fluids (R7; Frost and Beard, 2007). Secondary clinopyroxene forms after olivine (Cpx2) (R8), primary clinopyroxene (Cpx3) and orthopyroxene (Cpx4) (see Fig. 3B). Secondary diopside after orthopyroxene has been observed in the wehrlites suggesting that orthopyroxene in the wehrlites could have survived alteration occurring in Stage 2. Formation of secondary diopside is encouraged due to the excess Si4+, Mg2+ and Ca2+ in the fluid (Stripp et al., 2006). The coexistence of diopside, olivine and an aqueous fluid suggests that the αCa++/α2H+ ratio and αSiO2(aq) are relatively high and low, respectively. The pH value of the fluid at this stage would be high (Allen and Seyfried, 2003). The resulting secondary clinopyroxene is lower in Al2O3 and Na2O contents (except Cpx4). Al2O3 lost during formation of secondary clinopyroxene also promotes the formation of associated intergrowths of serpentine and clinochlore which acts as a sink for the mobilized Al2O3. Magnetite grains are observed in association with the formation of secondary diopside after olivine, which could be the result of

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K. Iyer et al. / Chemical Geology 249 (2008) 66–90

oxidation of the fayalitic component in olivine. Iron present in the primary spinels also undergoes oxidation resulting in the formation of ferritchromite (see Fig. 3F). The formation of ferritchromite in turn leads to an increase in dissolved chromium which, in conjunction with ferric iron, results in the precipitation of numerous fine-grained ferritchromite and magnetite (R9). The formation of magnetite and ferritchromite in association with Cpx2 suggests that Fe present in olivine was oxidized during the formation of secondary clinopyroxene (see (R8)). Aluminium released during alteration of Cr-spinel is associated with intergrowths of clinochlore and serpentine (R10; Mellini et al., 2005). 3CaMgSi2 O6 þ6Hþ ðaqÞ Y Mg3 Si2 O5 ðOHÞ4 Pri: Cpx

Serpentine

þ 4SiO2ðaqÞ þ 3Ca2þ þ H2 O Mg2 SiO4 þ3SiO2ðaqÞ þ 2Ca2þ ðaqÞ þ 2H2 OY Olivine

2CaMgSi2 O6 þ4Hþ ðaqÞ

ðR7Þ

ðR8Þ

Sec: Cpx

CrQspinel þ Fe3þ þ H2 OYFerritchromite þ Magnetite þ Al3þ þ H2 Al3þ þ Serpentine þ H2 OYClinochlore:

ðR9Þ ðR10Þ

Rodingite formation in the Leka Ophiolite Complex most likely occurred during clinopyroxene alteration to

serpentine and formation of mesh-textures. The lower alkali contents in the resulting rodingites could have been one of the sources which influenced the formation of Na-rich amphiboles in the orthopyroxenite dykes although sodium present in the reacting seawater could have also played an important role. 4.3.5. Stage 5 Progressive serpentinization leads to formation of interpenetrating blades of antigorite from existing serpentine and brucite (see Figs. 3C and 5A). This is characteristic of Type 7 serpentinization (Wicks and Whittaker, 1977) which is related to a temperature increase. There could have been an increase in temperature associated with nonpseudomorphic antigorite formation during the obduction of the oceanic lithosphere and Caledonian regional metamorphic event (O'Hanley, 1996). However, it is also possible that silica was present in the fluid which promoted the formation of antigorite. The reactions described above show that chemical mobility during serpentinization is extensive. The serpentinization of the ultramafic units of the LOC is related to episodic fluid infiltration events, changes of the oxidation state and temperature of the system and sequences of rather closed and open systems at variable scales (Fig. 11). Major chemical components, such as Mg, Si and Al seem to be redistributed within a given lithology, at the outcrop scale. However, the described reactions and textural sequence show that reactions are restricted to fluid availability. Fluids are needed to dissolve the precursor

Fig. 11. Schematic drawing of alteration sequences of minerals in various lithologies of the Leka Ophiolite Complex inferred from textural and thermodynamic observations. Abbr: Hzb = harzburgite, Wehr = wehrlite, Opx dykes = orthopyroxenite dykes, Cpx = Clinopyroxene, Opx = Orthopyroxene, Ol = olivine, Amph = amphibole, Tlc = talc, Ftc = ferritchromite, Mgt = magnetite, Cli = clinochlore, Srp = serpentine, Bru = brucite, Atg = antigorite.

K. Iyer et al. / Chemical Geology 249 (2008) 66–90

minerals, rearrange the ions in solution and precipitate new phases (Putnis, 2002; John and Schenk, 2003). At high fluid–rock ratios and in an open system, the fluid flow is able to cause the mobilization of minor and trace elements. 4.4. Reaction-assisted fracturing during serpentinization One of the important petrophysical effects of serpentinization is the density/volume changes occurring in the serpentinized rocks. There are 3 major phase transformations (R2–R4) which have an enormous effect on the density of the ultramafites during serpentinization. The reactions have not gone to completion in any of the lithologies. This is an interesting observation as it raises the question of why the rocks ran out of fluids. The orthopyroxenite dykes and harzburgites of the LOC are the only lithologies that previously contained appreciable amounts of primary orthopyroxene. The reaction of orthopyroxene to talc and olivine (R2) in the presence of fluids, and subsequently to serpentine (R3), controls a major part of the density changes in the orthopyroxenite dykes which takes place between 500 and 650 °C at 100 MPa (Stages 2 and 3; Fig. 12). The mineral assemblage in the dunites consists mainly of olivine and the density changes during serpentinization are dominated by the reaction of olivine to serpentine and brucite (R4) at temperatures lower than 400 °C at 100 MPa (Stage 4; Fig. 12). This reaction also takes place in the harzburgites and wehrlites. The changes occurring in the ultramafic rocks, especially the orthopyroxenite dykes and surrounding dunites, occur at different temperatures and are thus temporally separated which plays an important role in the deformation of these lithologies. The density changes

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predicted by the episodic fluid infiltration model occur at similar temperatures as described above but the change in density is less drastic due to limited amount of fluid available to form hydrous phases. One example of reaction-induced deformation is observed in the orthopyroxenite dykes of the Leka Ophiolite Complex which are extensively fractured with the fractures being sub-perpendicular to the contact between the orthopyroxenite dykes and the dunite matrix (see Fig. 1B). The 2-D fracture pattern of the dyke is made up of polygons (see Fig. 2D from Iyer et al., in press). The fracture pattern in the orthopyroxenite dykes closely resembles those seen in slowly fracturing quasi-2D layers undergoing contraction (e.g. Bohn et al., 2005a,b,c). Although fractures generated by volume-changing reactions occur frequently in geological systems (Engvik et al., 2001; Jamtveit et al., 2000, in press) and the physical process is also understood (Malthe-Sørenssen et al., 2006), fracture patterns in expanding systems, such as serpentinization, are less well understood. As discussed above, hydration of the orthopyroxenite dykes occurs at higher temperatures than serpentinization of the surrounding dunite. The volume changes occurring in the system, at lower temperatures, will therefore be dominated by the serpentinization of dunite during which the orthopyroxenite dykes are mostly unreactive. Approximately 60% serpentinization of dunite causes a volume increase of 25% and is also reflected in the bulk volume change calculated from the phase transformations. The expansion occurring in the dunite during serpentinization will effectively ‘squeeze’ the orthopyroxenite dykes and cause them to fracture. The geometrical and statistical characteristics of the 2-D fracture networks in the LOC are typical of patterns generated during hierarchical fracturing where the layer

Fig. 12. Plots of bulk density and volume changes during serpentinization occurring in the ultramafic lithologies of the LOC. The plots are calculated with respect to the oceanic geotherm for all lithologies.

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is progressively broken up in to smaller domains. This process is described by Iyer et al. (in press). Fracturing of the dyke initiated with a few fractures which break the layer into discrete domains. The formation of later fractures during progressive serpentinization is controlled by older generations of fractures. The domains thus formed are usually four-sided and angles at fracture junctions are typically 90° and 180° due to formation of T-junctions. T-junctions occur as late fractures meet older fractures at right angles. 4.5. Evidence for calcium and sodium mobility in the Leka Ophiolite Complex We have shown that calcium is highly mobile at the mm to cm scale during the process of serpentinization leading to the formation of various calcium-bearing phases in different lithologies. Clinopyroxene is the major, primary calcium-bearing phase present in the ultramafic rocks of the LOC. Alteration of clinopyroxene to amphibole, serpentine and/or chlorite releases significant amounts of calcium (R5, R7) while formation of secondary diopside is a calcium consuming reaction (R8). These reactions are particularly evident in the harzburgites present in the LOC where the serpentinization of primary clinopyroxene leads to the formation of secondary clinopyroxene replacing olivine and primary clinopyroxene. Evidence for the mobility of calcium is also observed on a larger scale in the rodingites present in the plagioclase-rich layers of the crustal sequence (Austrheim and Prestvik, in press). The rodingites are associated with fracture and shear zones suggesting that fluid and element transport occurred along them. The sequence of calcium releasing and consuming reactions occurring in the ultramafics, which have often not run to completion, could enrich the fluid in Ca which causes rodingitization in the crustal layers. The rodingites have low concentrations of Na2O and K2O suggesting removal of alkalis during this process. The mobilization of alkalis could be one of the factors that caused the formation of Na-rich amphiboles like edenite, winchite and richterite observed in the orthopyroxenite dykes.

positively correlated. The alteration of orthopyroxene results in an increase in the iron and manganese concentrations in the resulting secondary Olivine II (R2) in the orthopyroxenite dykes. The formation of Olivine II is also observed at the contact between the orthopyroxenite dyke and the surrounding rock. Olivine II (Mg# = 0.70–0.76; Mn = 0.011– 0.024 a.p.f.u.) is observed forming at the rims of and along preferential fluid pathways in primary olivine (Olivine I) as shown in Fig. 13. In a closed system, the dissolution of Olivine I (Mg# = 0.85–0.88; Mn = 0.005–0.010 a.p.f.u.) would result in the precipitation of antigorite with a higher Mg# (0.90–0.93) and lower manganese contents (b0.005 a.p.f.u.). The excess iron and manganese is taken up by the precipitating Olivine II. The higher iron contents would also increase the stability field of Olivine II. The iron to manganese ratios in Olivine II (Fe/Mn = 30–50) are much higher than the values expected in a closed system (Fe/Mn = ∼31). However, in an open system, the fluid composition may have been influenced by the breakdown of orthopyroxene occurring in the adjacent orthopyroxenite dykes which could have supplied the higher iron and manganese concentrations. Indeed, the iron and manganese contents of Olivine II formed at the contact between the orthopyroxenite dyke and the surrounding rock falls within the field described by Olivine II present in the orthopyroxenite dykes (see Fig. 4). Two types of secondary olivine are observed within the harzburgites present in the mantle section of the LOC (see Fig. 5A). Olivine II (Mg# = 0.84–0.86, Mn = 0.020– 0.035 a.p.f.u.) occurs as ‘patches’ and is surrounded by another secondary olivine with a high forsterite

4.6. Effect of iron and manganese mobility on olivine chemistry during serpentinization The described serpentinization process occurred sequentially and affected discrete domains at different times and to variable degrees, indicating pulses of low and high fluid fluxes. Iron and manganese are highly mobile during this process and the variations of both are

Fig. 13. BSE image showing the formation of Olivine II (Ol II) after primary olivine (Ol I) at the contact between orthopyroxenite dyke and surrounding matrix rock. Olivine II forms around the rims and along fluid channels within primary olivine. Antigorite (Atg) forms directly during the hydration of primary olivine. Note: dark, round patches are dust particles.

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component (Olivine III; Mg# = 0.88–0.90; Mn = 0.004– 0.010 a.p.f.u.). This also occurs in the cumulate section as described by Austrheim and Prestvik (in press). Olivine III contains numerous fluid and solid inclusions, some of which are serpentine, and is related to the formation of serpentine, secondary clinopyroxene and oxides. Most manganese profiles across Olivine II and III describe a bell-shaped curve (see Fig. 5B) with the highest manganese and iron concentrations in the core, whereas some profiles display plateaus of constant manganese and iron concentrations in their centers. The Fe/Mn ratios of Olivine II in the harzburgites (Fe/ Mn = 9–20) are much lower than those of Olivine II associated with the orthopyroxenite dykes. The formation of Olivine II in the harzburgites can be described by the reaction of primary olivine in the harzburgite with fluids to form serpentine with lower iron and manganese contents than the primary olivine. The process is a dissolution–precipitation process (Putnis, 2002) and is similar to that described above for Olivine II in the contact zone. This results in lower modal amounts of relict primary olivine present in the harzburgite. Manganese and some iron present in the fluid are locally incorporated into Olivine II. However, the fluid oxidizes some of the iron present in the primary olivine leading to the formation of the ferritchromite and magnetite. This results in lower Fe/Mn ratios of Olivine II in the harzburgites compared to those within the orthopyroxenite dykes. There are two possible scenarios which may explain the formation of Olivine III within the harzburgites. In the first scenario, later infiltration of an oxidizing fluid would lead to oxidation of the fayalitic component in Olivine II which results in the formation of Olivine III and increasing Fe3+ content observed in the oxides. The resulting manganese profile described by the aforesaid scenario would be ‘step-like’ at the reaction front between Olivine II and Olivine III. Chemical relaxation (diffusion) could later result in bell-shaped profiles. However, diffusion is unlikely to occur at the low temperatures during serpentinization. Olivine II observed at the contact between the orthopyroxenite dykes and the surrounding rock also does not show any evidence of Fe–Mg–Mn diffusion within olivine (small-scale variations in the chemistry of olivine observed in Fig. 13 would not have survived the diffusion process). In the second scenario, the formation of Olivine III occurs at grain boundaries of and along fluid pathways present in the ‘relict’ Olivine II due to the oxidation of iron present in the latter. The formation of Olivine III and serpentine occurs contemporaneously which is evident from the serpentine inclusions within Olivine III. Ferric iron and some manganese liberated during this process are taken up by

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the oxides. The oxidizing properties of the fluid decreases as the reaction front moves into the relict olivine grain resulting in lower amounts of iron oxidized towards the core of Olivine II. It is also likely that magnesium transport occurred simultaneously within the fluid as the reaction front progresses inwards. This dissolution– transport–precipitation process would lead to a shallow gradient observed in the bell-shaped manganese profiles instead of the expected step-like gradient described in the first scenario. The plateaus present in the center of some manganese profiles suggest that the oxidation/reaction front was not able to propagate towards the core of the Olivine II grain. The process described above indicates that the chemical gradient is a result of changes in fluid properties during the progress of the reaction front into the mineral grain (cf. John and Schenk, 2003). The mobility of iron and manganese is also observed during the formation of ferroan brucite and serpentine after olivine. Initial hydration of olivine results in the formation of serpentine and brucite (R4) and mesh-textures (see Fig. 3E). As alteration of the relict olivine progresses, silica dissolved in the fluid is locally transferred towards the outer region of the reacting zone eliminating brucite and forming only serpentine (Mg# = 0.96; Mn = b.d.; Fig. 14). There is also a simultaneous transfer of iron and manganese to the inner region of the reacting zone and leads to the formation of ferroan brucite halos (MgO = 43–45 wt.%; FeO = 19–20 wt.%; MnO = 1.0–1.2 wt.%; SiO2 = 11– 15 wt.%) with high manganese contents around the olivine grains (Mg#= 0.90; Mn = 0.005 a.p.f.u.). Ferroan brucite is

Fig. 14. BSE image of alteration of olivine (Ol) to serpentine and ferroan brucite. Transfer of silica occurs towards the outer region forming lizardite (Lzd) and iron and manganese is simultaneously transferred towards the inner region forming halos of manganese-rich ferroan brucite (F-br) around the reacting olivine. Subsequent oxidation of ferroan brucite results in the formation of magnetite (Ox) around olivine grains.

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also formed in areas where the reaction has gone to completion retaining the shape of the relict olivine core. Magnetite is formed due to the subsequent oxidation of ferroan brucite and also forms rims around olivine. Manganese would thus be lost to the fluid phase and could escape out of the system. 4.7. Implications of iron and manganese mobility on vent-fluid and global ocean-floor chemical budgets The Rainbow (36°14′N) and Logatchev (14°45′N) hydrothermal fields are hosted in ultramafic rocks along the Mid-Atlantic Ridge and fluids in these systems vent at 365 °C and 350 °C, respectively (Charlou et al., 2002; Douville et al., 2002). The high mobility of iron and manganese during the breakdown of orthopyroxene and olivine, as described above, could account for the high amounts of iron and manganese present in mantle-hosted vent fluids. The vent fluids also contain high amounts of H2 and CH4 which is a product of the oxidation of Fe2+ to Fe 3+ (R6, R9). Therefore, the serpentinization of ultramafic rocks could be an important factor in constraining the iron and manganese budget of the oceans and the formation of mineral deposits on its floor due to seawater and vent-fluid interaction (Tivey, 2007). 5. Conclusions 1. The ultramafic rocks of the Leka Ophiolite Complex allow us unique insight into the alteration of minerals over a wide range of temperatures. The resulting products are not only a function of temperature variations but are also dependent on the precursor assemblage and preserve the reaction history of the peridotite body as it underwent ocean-floor serpentinization. 2. Serpentinization of ultramafic rocks is associated with density and volume changes which contribute to deformation and fracturing of the associated lithologies. The fracture patterns observed in the serpentinized rocks are formed by the coupling of fluid migration and stress-generating reactions and provide important controls on fluid migration rates and subsequent alteration of rocks. 3. Serpentinization of ultramafic rocks of the LOC is associated with the mobilization of the elements Ca, Na, Fe and Mn. The removal and subsequent transport of calcium in the fluids results in the formation of rodingites within the lower crustal section of the Leka ophiolite complex and facilitates the formation of secondary clinopyroxene after olivine, primary clinopyroxene and orthopyroxene elsewhere in the LOC. The simultaneous mobilization of Fe and Mn is

an important feature which controls the mineral chemistry of the metamorphic olivine formed during serpentinization. Alteration of the olivine to hydrous phases and oxides could contribute to the global ocean chemical budget and the formation of oceanfloor mineral deposits. Acknowledgements The authors would like to thank Muriel Erambert, University of Oslo for her help in carrying out EMP mineral analysis. The authors are also grateful to David Rickard, Cin-Ty Lee and Horst Marschall for their helpful comments and reviews. This study was funded by a Centre of Excellence grant from the Norwegian Research Council to PGP. References Albrektsen, B.A., Furnes, H., Pedersen, R.B., 1991. Formation of dunites in mantle tectonites, Leka Ophiolite Complex, Norway. Journal of Geodynamics 13, 205–220. Allen, D.E., Seyfried, W.E., 2003. Compositional controls on vent fluids from ultramafic-hosted hydrothermal systems at mid-ocean ridges: an experimental study at 400 °C, 500 bars. Geochimica Et Cosmochimica Acta 64, 1531–1542. Andreani, M., Baronnet, A., Boullier, A.M., Gratier, J.P., 2004. A microstructural study of “crack-seal” type serpentine vein using SEM and TEM techniques. European Journal of Mineralogy 16, 585–595. Andreani, M., Mevel, C., Boullier, A.M., Escartin, J., 2007. Dynamic control on serpentine crystallization in veins: constraints on hydration processes in oceanic peridotites. Geochemistry Geophysics Geosystems 8, Q02012. doi:10.1029/2006GC001373. Aumento, F., Loubat, H., 1971. The Mid-Atlantic Ridge near 45° N. 16. Serpentinized ultramafic intrusions. Canadian Journal of Earth Sciences 8, 631–663. Austrheim, H., Prestvik, T., (in press). Rodingitization and hydration of the oceanic lithosphere as developed in the Leka ophiolite, north-central Norway. Lithos. doi:10.1016/j.lithos.2007.12.006. Bach, W., Garrido, C.J., Paulick, H., Harvey, J., Rosner, M., 2004. Seawater–peridotite interactions: first insights from ODP Leg 209, MAR 15 degrees N. Geochemistry Geophysics Geosystems 5, Q09F26. doi:10.1029/2004GC000744. Bach, W., Paulick, H., Garrido, C.J., Ildefonse, B., Meurer, W.P., Humphris, S.E., 2006. Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274). Geophysical Research Letters 33, L13306. doi:10.1029/2006GL025681. Bohn, S., Douady, S., Couder, Y., 2005a. Four sided domains in hierarchical space diving patterns. Physical Review Letters 94, 054503. Bohn, S., Pauchard, L., Couder, Y., 2005b. Hierarchical crack pattern as formed by successive domain division. I. Temporal and geometrical hierarchy. Physical Review E 71, 046214. Bohn, S., Platkiewicz, J., Andreotti, B., Adda-Bedia, M., Couder, Y., 2005c. Hierarchical crack pattern as formed by successive domain division. II. From disordered to deterministic behavior. Physical Review E 71, 046215.

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