Blue Jade in Iran

  • 1) Institute of Geosciences, Potsdam University, Karl Liebknecht Str. 27, 14476 Potsdam, Germany
  • 2) Department of Geology, Shahid Bahonar University, College of Arts and Sciences, Kerman, 76176, Iran
  • 3) Department of Geology, University of Tabriz, 51664 Tabriz-Iran
  • Citation of this article:

    Oberhänsli, R., Bousquet, R., Moinzadeh, H., Moazzen M., Arvin, M., 2007, Stability field and colour of blue jades: a case study from a new occurrence (Sorkhan, Iran), Canadian Mineralogist, 45, no. 6; p. 1501-1509



    Abstract

    A new occurrence of “blue jade” is described. This “blue jade” occurs as metasomatic veins in magnesite bodies within meta-ultramafic rocks of SE Iran. They are composed of almost pure jadeite, 90 to 99.5 mol. % Jd, contain minor amounts of Ba-bearing K- feldspar, lawsonite and katophoritic amphibole, but unlike other “blue” or “lavender jades” do not contain high amounts of Titanium. The jade veins formed at low- temperature high-pressure conditions, around 1.6 GPa and 420°C. Such P-T conditions are characteristic for cold subduction zones in which typically lawsonite blueschists to lawsonite eclogites form. Thermodynamic studies show that the mineral assemblage within the blue jade is strongly pressure- and temperature-dependent. Jadeitites containing two clinopyroxenes (jadeite and omphacite) are stable at high pressure (≥ 0.8 GPa) and low temperature (≤ 430°C) conditions, whereas blue jade with only one clinopyroxene (jadeite) forms at higher temperature or lower pressure. Based on these new calculations, PT conditions for all blue jade are re-examined.

    Résumé

    Une nouvelle occurrence de jades bleus est rapportée. Ces jades affleurent sous forme de veines métasomatiques dans des boudins de magnésite dans les roches ultra-mafiques du SE de l’Iran. Contrairement aux autres jades bleus et bleu-lavande décrites dans la littérature, la composition chimique des jades d’Iran n’indique pas de fortes teneurs en Ti. Ces jades se composent principalement de jadéite pure (entre 90% Jd et 95% Jd component). Les minéraux associés sont des Feldspaths riches en Ba, de la lawsonite et des amphiboles katophoritiques. De telles veines se forment dans des conditions de haute pression - basse température, aux alentours de 1,6 GPa -420°C. Ces conditions sont caractéristiques des zones de subduction à faible gradient géothermique dans lesquels se forment schistes bleus et éclogites à lawsonite. Une étude thermodynamique des jades bleus montre que la minéralogie de ces roches est fortement contrainte par les conditions de pression et de température. Ainsi les jades bleus contenant deux clinopyroxènes (jadéite et omphacite) sont stables à haute pression (≥ 0,8 GPa) et basse température (≤ 430°C) tandis que les jades bleus contenant seulement de la jadéite sont stables à plus haute température ou bien à plus basse pression. Sur la base de ces calculs, les conditions pression - température de l’ensemble des jades bleus décrites dans la littérature sont réévaluées.

    Introduction

    The beauty and wide-ranging expressiveness of jade have held a special attraction for mankind for thousands of years. Jade is, strictly speaking, a generic term for two different types of rocks, nephrites or jadeitites, dominated either by amphibole or jadeite respectively. Nephrites range mainly from medium to dark green or grey-green, but can also be white, yellowish or reddish. Rarer, and somewhat harder, therefore regarded as more precious, jadeitites display hues that include green, but also white or pink, and reds, black, brown, violet, lavender and blue. In both rock types, the way the colour is distributed varies considerably. Jadeitites, and especially blue jadeitites, are rather uncommon rocks. Generally, they are associated with subduction-related serpentinites along fault zones (Harlow 2001) and generally interpreted as crystallizing from hydrous fluids derived from dehydration of subducted slabs at high P/T (Manning 1998; Harlow 1994; Johnson & Harlow 1999). In an extensive review on the colour of jadeitite, Howard (2002) reported that most of the so-called blue jadeitite are in fact blue-green. Truly blue and lavender jadeitites are reported from only a few localities around the world. The most renowned occurrences are the Olmec blue jade (Quebrada Seca, Guatemala; Harlow et al. 2004) and blue jade from Ohmi-Kotaki in Japan (Chihara, 1971). Lavender jadeitite is known from Tavsanli in Western Anatolia, Turkey (Okay 1984), and green and bluish jadeites are well known in Burma (Chhibber 1934). Up to now, the blue colour is assigned to titanium rich omphacite (Harlow et al. 2004), which occurs in clots and veins in jadeitite. The Guatemalan “blue jade” contains assemblages of omphacite, phengite, titanite, zircon, allanite and rutile whereas the Japanese “blue jade” comprises omphacite, titanite, albite, and analcime assemblages. The Tavsanli “lavender jade” is rather homogeneous, but a patchy pattern allows the identification of an alkaline magmatic precursor that was metasomatized to jadeitite (Okay 1997). This jadeitite contains quartz, jadeite, K-feldspar, lawsonite, and aegirine. We report here a new locality of clear, sky-blue jadeitite occurring near Sorkhan in a blueschist belt of southeastern Iran. Jadeitite occur in a vein system along a serpentinite – magnesite contact. From this study, we discuss the composition and the stability of one- and two-pyroxene-bearing jadeitites.

    Geology of the Sorkhan sector

    In southeastern Iran, Mesozoic ophiolitic mélanges occur between the Zagros fold and thrust belt (representing the Arabian plate) and the metamorphic rocks and volcanic arc sequences of the Sanandaj-Sirjan Zone (northern continental margin of Tethys) (Fig. 1A). The coloured mélanges of the suture zone are complex and bear metamorphic as well as non-metamorphic sequences. Near Sorkhan, a belt containing high-pressure (HP)metamorphic rocks is juxtaposed against a sequence of non-metamorphic ultramafic to mafic rocks, considered to be of Precambrian age. Whole rock 40K-40Ar ages on gabbros and diabases near Sorkhan (Fig. 1B) range from 130 to 140 Ma and on amphibolites east of Sorkhan are 202 Ma old (Ghasemi et al. 2002).

    click on the picture to zoom or click here to download the picture
    Figure 1

    Figure 1: A: Geologic map of Iran showing the major ophiolite sutures, with inset of the studied area. B: Geological map of the Sorkhan area showing the ultramafic bodies containing magnesite lenses and the mélange formed by glaucophane-bearing serpentinite schists. The star indicates the sample locality.

    North of this area, at Ashin-e-Bala (Fig. 1B) serpentinized harzburgites contain talc, anthophyllite and enstatite (Sabzehei 1974), indicating a post-serpentinization metamorphic overprint. These meta-ultramafic rocks correlate with the Abdasht ultramafic suite, where chromian spinel associated with reddish chromian clinochlore is mined, and occur in a series of amphibolites, micaschists, marbles and greenschists that are mapped as glaucophane-schists (Hadjabad quadrangle). In this rock series blue amphibole can be recognized only locally and lower amphibolite facies to upper greenschist facies successions are prevalent. In mafic rocks, green amphibole commonly shows a thin rim of glaucophane indicating that all rocks underwent a HP-LT overprint. Micaschists near Ashin-e-Bala have been dated as around 80.7±1.5 Ma ( 40K-40Ar, phengite; Ghasemi et al. 2002) and around 93±5.9 Ma by 40Ar/39Ar (Agard et al. 2006). To the north of the meta-ultramafic suite, another rock association comprises a serpentinite mélange, composed of blueschists (sometimes pure glaucophanites), marbles and garnet micaschists in a serpentinitic matrix. These rocks, mapped as serpentinite schists, underwent metamorphism to the lawsonite-blueschist to lawsonite-eclogite facies (Sabzehei 1974). Along the northern contacts of these metamorphosed ultramafic bodies to the serpentinite mélange with blueschists, large lenses of magnesite occur. One of these lenses contains veins with mineralization of white and blue jadeitite. The jadeitite veins, completely enclosed in the magnesite lenses, are 5 to 15 cm wide, and show symmetrical zoning. The outer portions are snow white, while the inner portions (ca. 10 cm for the widest veins) show a clear, sky-blue colour.

    Petrography and chemistry

    In the blueschist mélange (serpentinite schists) north of the Sorkhan region, mafic rocks contain glaucophane, albite, phengite, and in some cases lawsonite and rutile. In rare cases, omphacite was found. Within this metamorphic belt large bodies of ultramafic rocks with chromite lenses are widespread. These mainly meta-dunitic rocks contain talc, enstatite, forsterite and antigorite. The associated chromites are rich in reddish chromian clinochlore. Along the contacts of the ultramafic rocks against the blueschist bearing serpentinte mélange, lenses (30 to 80 m by 5 to 20 m) of magnesite that are mined, developed metasomatically. In one of these magnesite lenses, veins of jadeitite (Fig. 2) show white and sky-blue colours.

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    here to download the picture
    Figure 2

    Figure 2: Photograph of a sample composed of blue and white jadeite

    The minerals forming the bluish veins were analyzed (Table 1) by means of electron microprobe (Camebax, SX100) at the GeoForschunsgZentrum (GFZ) in Potsdam (15 kV, 20 nA, ZAF correcton procedure) using natural and synthetic mineral standards: wollastonite (Si, Ca), orthoclase (Al, K), albite (Na), periclase (Mg), rhodonite (Mn), hematite (Fe), rutile (Ti), chromite (Cr). Raman spectra were obtained on a LabRam HR800 with green laser light (Nd: YAG laser, 532nm) at the Institute of Geosciences Potsdam. Major element composition of blue parts of the jadeitite vein was determined by XRF at GFZ (Phillips XRF-PW2400) using fused lithium-tetraborate disks. The estimate precision is better than 1-3 %. Trace elements were determined by ICP-MS using solution nebulisation after mixed acid digestion (HF-HClO4) under pressure (Dulsky, 2001).

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    here to download the picture
    Figure 3

    Figure 3: Photograph of a sample composed of blue and white jadeite

    The blue jadeitite contains the assemblage jadeite-lawsonite-amphibole plus minor Ba- beaing K-feldspar and white mica. The outer part of the jadeitite veins, generally white, are partly retrogressed and show white mica flakes and secondary albite. The inner part of the veins, with a blue hue, are extremely fresh and blue as well as white parts are composed of jadeite. The modal composition of the jadeitite ranges up to 99 vol.% jadeite. Jadeite has a dusty aspect and shows many 2 phase (liquid/gas) fluid inclusions. In the fine-grained (0.1-0.2 mm) jadeitite matrix additional minerals such as lawsonite, katophorite, Ba-bearing K-feldspar occur. Minute Ba-bearing K-feldspar is dispersed randomly: No textural relations could be deciphered. Lawsonite is found along white veinlets (0.1-0.2 mm wide) cutting the blue jadeite matrix as well as in the matrix (Fig. 3) and has been characterized both by Raman spectroscopy and electron microprobe.



    Table 1
    Table 1: Bulk rocks analyses of some representative rocks of the Entrelor area

    From the fact, that these veinlets are filled with jadeite, we conclude that these are closely related to the processes forming the jadeitite matrix. Later, significantly larger cracks (0.5 -1.2 mm wide) cutting white and blue jadeitite as well as the veinlets contain albite (Fig. 2). A third conspicious mineral forming small clots within the jadeitite matrix microscopically shows all textural features of amphibole. The observed amphibole is colourless in thin section, shows moderate birefringence (nγ –nα ~ 0.02), oblique extinction (Z ∧ c in plane // (010) ~ 15°). Rutile has not been found.
    In the Sorkhan jadeitites, clinopyroxene is close to the ideal jadeite composition (Table 1). All pyroxene analyses (>50) show jadeite contents close to the end member composition, ranging from 90 to 99.5 mol.% Jd. Both, FeOtot (0.04 to 0.5 wt. %) and TiO2 (0 to 0.1 wt. %) are lower than those reported from blue jadeitites elsewhere. Microprobe analyses of the amphiboles revealed the composition Na(Na1.1Ca0.9)Mg4.1Fe0.1(Al0.7)(Si7.5Al0.5)O22(OH)2 with s a very small amount of Fe and a slight excess of Mg. The composition fits on the join eckermannite [NaNa2(Mg4Al)Si8O22(OH)2] – magnesiokatophorite [Na(NaCa)(Mg4Al)Si7AlO22(OH)2]. The Sorkhan jadeitite is very pure.
    A bulk chemical analysis (Table 1) on a 1 cm3 sample of blue jadeitite yielded very low TiO2 (0.15 wt.%) whereas MgO (1.52 wt.%), Fe2O3 (0,27 wt.%) and CaO (2.90 wt.%) are relatively high. Except for TiO2 these minor amounts of major elements can be attributed to the accessory phases such as amphibole, feldspar and lawsonite, although Ca is also incorporated to some extent into jadeite ( X jd Ca = 0.05 -0.1). A Ti-phase was not observed in the high-pressure assemblage. The trace element analysis (Table 1) shows very low concentrations except for Ni, Sr and Li (62, 49, 57,7 ppm respectively), which may relate to the same accessory phases mentioned above.

    Stability field of blue jadeitite

    Using thermodynamic calculations (DOMINO/THERIAK, Gibbs free energy minimization, De Capitani & Brown 1987 and the database of Bermann, 1988) and the measured bulk-rock composition of the sample we can estimate the field of stability of blue jadeite. Our results (Fig. 4) show some remarkable features. The P-T space is divided into two areas. The low temperature side of the diagram (Fig. 4a-c, white) comprises assemblages with two clinopyroxenes - omphacite and jadeite – in assemblages with, depending on pressure, lawsonite-chlorite-glaucophane, lawsonite- amphibole, lawsonite-glaucophane-paragonite, quartz-lawsonite-paragonite, lawsonite- paragonite or garnet-feldspar-paragonite. The high-temperature assemblages (Fig. 4a-c, grey) contain only a single clinopyroxene phase, namely jadeite in similar assemblages containing garnet, feldspar, clinozoisite, paragonite, lawsonite, glaucophane or amphibole.
    Our calculation produces a relatively small, well-constrained field for the stability of the assemblage jadeite-lawsonite-magnesiokatophorite (Fig. 4a, dark grey; Jd-Lws-Am) at about 1.6 GPa and 420°C. The diagram shows the existence of a stability field for the assemblage two clinopyroxenes (Jd-Omph) - lawsonite – amphibole on the low- temperature side of the peak assemblage (Fig. 4a). By comparing the observed modal (Fig. 4b) and chemical (Fig. 4c) compositions of the Sorkhan blue jadeitite samples [Jd- Lws-Mkt] with our grid, an excellent fit is obvious. This assemblage contains very high modal amounts of pyroxene [94 % of rock] (Fig. 4b) and the calculated composition of the pyroxene [ X Jd 90 mol %] (Fig. 4c) fits the electron microprobe measurements [ X Jd>90 mol %].

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    Figure 4

    Figure 4: ressure – temperature diagrams calculated with the domIno software (de Capitani 1994) using the bulk-rock composi- tion of the blue jadeitite. a) Phase relations showing stability domains with one or two clinopyroxenes. b) Molar abundance of clinopyroxene in the rock. Note the increase of the modal abundance of clinopyroxene with temperature under high-pressure conditions. c) Content of the jadeite end-member in clinopyroxene, given for jadeite and omphacite. d) Comparison of P–T conditions from the literature (open symbols) with new P–T estimates (filled symbols) according to our calculations based on published data on mineral assemblages.

    Interestingly, the modal amount and composition of jadeite in the natural sample coincide very well with the petrogenetic grid (Tab. 2), whereas for amphibole a slight but significant misfit is obvious. This is due to the fact that the thermodynamic properties are not determined for all amphibole endmembers and solid solutions (e.g.: Ghiorso and Evans, 2002; Dale et al., 2005). Bearing in mind the problem of thermodynamic properties of amphiboles the fit produced in our calculation is astonishingly good. We believe that these calculations belong to the few cases that give proof of the excellent quality of the database (Bermann, 1988) used, including its mixing and activity models. In addition the P-T conditions deduced from the Sorkhan jadeitite fit well with the values deduced for the adjacent lawsonite blueschists (Agrad et al., 2006). We thus are quite confident about the significance of our PT grid although pressures and temperatures deduced for the Sorkhan area differ significantly from published values for similar metasomatic rocks with blue jadeitite, e.g.: Itoigawa-Ohmi in Japan (Morishita, 2005), Guatemala (Harlow 1994) or Myanmar (Goffé et al. 2000, Shi et al. 2003). We showed that observations similar to the ones recently published form lawsonite eclogites from Olmec, Guatemala (Tsujimori et al. 2005) can be reproduced accurately and therefore used the new thermodynamic calculations to re-estimate P-T conditions of the other occurrences of blue jadeitite based on their mineralogy (Fig. 4d).


    Table 2
    Table 2: Comparison of mineral compositions as calculated with DOMINO/THERIAK from thermodynamic data with caluclations from measured (EMPA) values from blue jade.

    All new P-T estimates for other blue jadeitite localities based on the published mineralogy (Japan: Morishita, 2005; Myanmar: Goffé et al. 2000, Shi et al. 2003; Turkey: Okay, 1997) range from 0.8 GPa, 300°C to 1.8, 500°C (Fig. 4d). For example the occurrence of zoisite in the Olmec blue jadeitite from Guatemala (circle; Harlow 1994) implies higher temperature conditions than originally estimated. Such results show that the Ca-phase (Zo, in the case of the Olmec blue jadeitite) is stable during HP conditions instead of being a breakdown product of jadeite (with albite) during retrogression. The Ca-phase (omphacite, zoisite or lawsonite) seems to be the determining criterion regarding the temperature conditions: occurrence of two clinopyroxenes (omphacite in equilibrium with jadeite) implies a stability field at low- temperature conditions (≤ 400-425°C) independently of pressure, whereas assemblages of jadeite in equilibrium with lawsonite or zoisite are stable under higher-temperature conditions (≥ 400-425°C).


    Acknowledgements

    We thank Bruno Goffé for stimulating discussions and a lively introduction on Myanmar and two-clinopyroxene jadeitites. Antje Müller and Onna Appel helped in the labs. We thank Dr. Rhede and the GFZ-Potsdam for the use of their EMPA facilities: The Iranian authorities and DAAD are thanked to make field work possible. The paper benefited from the reviews of D. Schwarz and R. Guistetto and comments by R. Martin. Paddy O’Brien’s scientific comments and his help as a native English speaker is appreciated. Last but not least, we thank Hakime who ran our field camp in Abdasht.

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