tters 255 (2007) 289305www.elsevier.com/locate/epslEarth and Planetary Science LeOman diopsidites: a new lithology diagnostic of very hightemperature hydrothermal circulation in mantleperidotite below oceanic spreading centres

Marie Python a,b,, Georges Ceuleneer a, Yoshito Ishida b, Jean-Alix Barrat c, Shoji Arai b

a Centre National de la Recherche Scientifique, Universit Paul Sabatier U.M.R. 5562 Observatoire Midi-Pyrnes,14, av. . Belin 31400 Toulouse, France

b Faculty of Science, Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japanc Centre National de la Recherche Scientifique, Universit de Bretagne Occidentale U.M.R. 6538 I.U.E.M.,

Place Nicolas Copernic 29280 Plouzan, France

Received 19 August 2006; received in revised form 14 December 2006; accepted 22 December 2006Available onliEditor: C.P. Jaupart

ne 9 January 2007Abstract

Maficultramafic dykes scattered in the mantle section of ophiolites are generally crystallisation products from common silicatemelts.In the frame of a global survey of these melt migration relics in the Oman mantle harzburgites, we discovered a peculiar lithology madeessentially of pure diopside (0:95b MgMgFetotal b1) whose characteristics do not match a magmatic or mantle origin. Arguments against amagmatic ormantle origin for these diopsidites combine compositional and textural evidence. In spite of their refractory composition, theyare strongly depleted in Cr (Cr2O3b0.2 wt.%). By the same way other minor elements (Al, Ti, Na) and rare earth elements havepeculiarly low abundances and plot away frommagmatic differentiation trends. In a few samples, the paragenetic association includes pureanorthite (An% up to 0.99), minor amounts of forsterite (FoN0.95) or traces of andradite. When not deformed, the diopsides areautomorphic and their texture points tometamorphic growth in amatrix composed of antigorite and/or carbonate.Diopsidites have texturesreminiscent to that of skarns developing in contact metamorphic halos or that of rodingite frequently present in the serpentine bodies of theophiolitic crust. They frequently appear as dykes (former cracks), with a fewmm to few tens of cmwide transitional zones, which containhigh amounts of hydrous minerals, between the diopsidite facies and its host rock. The diopsidites are not randomly distributed in theOman ophiolite, being more abundant near former asthenospheric diapirs emplaced at shallow depth in the lithosphere. We interpret thediopsidites as the footprint of very high temperature circulation of seawater and carbonated fluids (N800 C), which may have leachedplagioclase rich lithologies before penetrating the mantle (as shown by a well developed positive Eu anomaly). Our data confirm theprediction of McCollom and Shock [T.M. McCollom, E.L. Shock, Fluid-rock interactions in the lower oceanic crust: Thermodynamicmodels of hydrothermal alteration, J. Geophys. Res. 103 (B1) (1998) 547575.] who proposed that common anhydrous minerals likepyroxene, plagioclase and olivinemay crystallise from high temperature fluids intermediate between silicatemelts and supercritical water.This confirms that there is no clear-cut thermal and chemical boundary between the fields of magmatic and hydrothermal crystallisations. 2007 Elsevier B.V. All rights reserved.Keywords: Oman; hydrothermalism; serpentine metamorphism; diopside; dyke Corresponding author. Tel.: +81 76 264 6539; fax: +81 76 264 6545.E-mail addresses: marie@earth.s.kanazawa-u.ac.jp (M. Python), Georges.Ceuleneer@dtp.obs-mip.fr (G. Ceuleneer),

y_ishida@earth.s.kanazawa-u.ac.jp (Y. Ishida), ultrasa@kenroku.kanazawa-u.ac.jp (S. Arai).

0012-821X/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2006.12.030

mailto:marie@earth.s.kanazawa-u.ac.jpmailto:Georges.Ceuleneer@dtp.obs-mip.frmailto:y_ishida@earth.s.kanazawa-u.ac.jpmailto:ultrasa@kenroku.kanazawa-u.ac.jphttp://dx.doi.org/10.1016/j.epsl.2006.12.030

290 M. Python et al. / Earth and Planetary Science Letters 255 (2007) 2893051. Introduction

Since the discovery of black smokers at oceanicspreading centres in the late seventies [2,3] hydrother-mal circulation of seawater in the oceanic crust is rec-ognised as a major mechanism of heat and chemicaltransfer process within the earth (e.g. [46]). It is wellestablished that hydrothermal circulation pervasivelyaffects the mineralogical and chemical composition ofthe upper oceanic crust down to the base of the sheeteddyke complex (e.g. [7,8]). At deeper levels, in thegabbroic crust, the footprint of hydrothermal alterationis also ubiquitous, although the hydrothermal veinnetwork becomes less and less dense as the crust/mantleboundary is approached [911].

The model of large, permanent, magma chamberspopular in the late seventies and early eighties [12] actedas a conceptual boundary to the likelihood of seawaterpenetration in the mantle itself. It is now recognised thatthe magma bodies are much smaller and short-lived thanpreviously expected, especially at slow spreading centreswhere a significant proportion of the seismic crust is madeof altered peridotite intruded by gabbroic plutons (e.g.[1315]). Moreover, hydrothermal vents have been dis-covered directly on outcrops of peridotites (e.g. [1618]).

The maximum extent of hydrothermal cells at depthand the related question of maximum temperaturereached by hydrothermal fluids in these cells remain,however, poorly constrained by direct observation. Inthe oceanic crust, the continuity in chemistry andcrystallisation temperatures between magmatic andhydrothermal amphiboles makes the interpretation ofamphibole bearing rock uncertain. Moreover, at veryhigh temperature, hydrous phases do not crystallise inspite of the presence of water in their parent melts.Accordingly, if interaction with seawater occurs at thosetemperatures, its detection remains difficult. In themantle, to identify the footprint of high-temperaturehydrothermal circulation is even more tricky for twomain reasons: (1) mineralogical transformations relatedto waterrock interaction are less spectacular and ofmore ambiguous interpretation in peridotites than in thecase of gabbroic lithologies [10,19], and (2) the peri-dotites are very sensitive to lower temperature alteration(essentially serpentinisation at temperature b600 C,theoretically down to 0 C) that can obscure the signatureof oceanic spreading-related hydrothermal processes.

In the frame of a systematic survey of pyroxeniticand gabbroic dykes, veins and porous flow featuresexposed in the Oman mantle peridotites, we have dis-covered peculiar veins, made essentially of purely mag-nesian diopsides whose chemical composition, textureand relation with the host serpentinous peridotite cannotbe accounted for by crystallisation from a pure silicatemelt. The present paper is devoted to a field, petrog-raphic and geochemical study of these lithologies.

2. Geological setting and previous work

The ophiolite of Oman belongs to the ophiolite beltenclosed in the Alpine mountains chain; it is one of thebest exposed section of oceanic lithosphere in the world.The precise tectonic setting of the Oman ophiolite is stilldebated (ocean ridge, (e.g. [2022]) versus arc-relatedbasin, e.g. [23,24]) but, whatever the answer, it repre-sents a former spreading centre where basaltic MORB-like magmas are emplaced on a deep seafloor in anextensional environment.

The upper crustal section, from the basalts to the baseof the sheeted dyke complex, was pervasively altered byhydrothermal fluids and the primary lithologies weremetamorphosed in the greenschist facies, resultingcommonly in the crystallisation of an albite+actinolite+chlorite+epidote+quartztitanite paragenesis, whereasthe gabbro/dyke transition zone is characterised byamphibole veins bearing essentially actinolitic hornblendeand more rarely common hornblende [2527,15]. Micro-thermometric analyses on fluid inclusions in hydrothermalparageneses have shown that the reaction temperatureswere comprised between 300 C and 360 C [28,9] but ahigher temperature alteration stage was recorded in theODP-Hole 504B sheeted dykes with temperatures up to425 C [8]. It was early known that hydrothermal fluidscan penetrate the gabbroic crust underlying the sheeteddyke complex [29,30]. Seawater probably circulates up tothe higher level of gabbro in large cracks and leads mainlyto the crystallisation of amphibole [25,15,31]. Thepossibility that hydrothermal fluids can penetrate downto the lower gabbroic section at the ridge axis was alsoearly suspected (e.g. [29,30]) and recent studies in thedeeper gabbroic section of the Oman ophiolite and ofmid-ocean ridges lead to the conclusion that interactions at veryhigh temperature between seawater and gabbros at thecrust/mantle boundary may occur [32,33,11].

In the layered gabbros, a complete transition from lowtemperature hydrothermal parageneses (greenschists at thetop of the section) to the high temperature magmaticcumulates (mostly gabbros at the lower part of the crust) isobserved and no clear boundary can be defined betweengabbros transformed by hydrothermal processes andpurely magmatic gabbros. The high orthopyroxenecontent of some gabbros found in the oceanic or ophioliticcrustal section, aswell as their chemical composition couldbe easily explained if interactions with seawater occurs at

291M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305some stage during the meltingcrystallisation processleading to their generation, data as Sr and O isotopesanalyses suggesting a seawater contribution [3436].

In the mantle section, mafic and ultramafic dykes arevery common; they are interpreted as channels used bymantle partial melts to reach the surface. The systematicpetrographic and microprobe study of about 1000 of thesedykes distributed all along the Oman ophiolite [37,38],and trace element and isotopic studies [39,34] have shownthat most of them belong to two main magma suitescontrasted in terms of petrographical and chemicalcharacteristics; a first one issued from a MORB-typemagma (cumulates from olivine tholeiites) and the secondone issued from parent melts richer in silica andultradepleted in incompatible elements (likely cumulatesfrom boniniticandesitic magmas). At the scale of theOman ophiolite (i.e. about 350 km80 km), these twofamilies crop out in different areas which hardly overlapeach other (Fig. 1). Dykes from both families areFig. 1. Global map of the Oman ophiolite showing the general distribution odots show the location for each sampled diopsidite dyke.frequently hydrothermally altered, the secondary miner-alogical assemblages (dominated by chlorite, actinotetremolite, serpentine, etc.) do not show any significantdifferences with the ones observed in crustal gabbros,pointing to similar temperatures ofwaterrock interaction.

At the Mid-Atlantic Ridge, the chemical compositionof hydrothermal vents reveals the signature of serpentini-sation reactions resulting from the circulation of seawaterin the mantle [4042]. Some authors proposed that theheat generated by this reaction alone may contribute tomaintain the hydrothermal circulation. However, the foot-print of high temperature hydrothermal processes in abys-sal peridotites were obscured low temperature alteration.Near the Maqsad area in the Oman ophiolite, the mantlecrust transition zone is crosscut by normal faults roughlyparallel to the paleo-ridge axis. In the neighborhood ofthese faults, the mantle can be totally altered in a chloritetalccarbonate assemblage bearing more or less abundantsulphides or magnetite. The analysis of sulphide-bearingf each family of dyke as defined by Python and Ceuleneer [38]. Black

292 M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305serpentines suggests a hydrothermal origin for the altera-tion of peridotites [43]. Rare epidoteprehnitechloriteveins in serpentinised peridotites suggest temperatures ofabout 250350 C but in absence of other mineralogicaltransformations, the alteration temperature remains poorlyconstrained.

3. High magnesian diopsides dykes in Omanperidotites

3.1. Field and petrographical characteristics

From place to place, an exotic kind of dyke, that doesnot belong to any of the main suites reported above nor totheir alteration products, was found. In the field, this kindof dyke appears as whitish to pale greenish in colour withhighly variable width (from a few centimetres to severalmetres). The mineralogical assemblage (dominated bypure diopside) and mineral chemistry (see Section 3.2)contrast with those of the majority of the Omanese dykes.They are not very common in the mantle section butconsidering their similarity, at first sight, with highlyTable 1Main field and microscopic characteristics for each sample

Label Sample Locality Orientation %Pl

92OG8 Gabbro Maqsad 170E90 6097M5 Gabbro Maqsad 165E70 2097M38a Diopsidite Maqsad 175W80 097RU6b Gabbro Rustaq 60S80 4097SA15c Diopsidite Sarami 155E65 099HI56b Diopsidite Sudum 13090 099HI59a Diopsidite+host Sudum 099HI69 Diopsidite Hilti 170W85 099HI70a Diopsidite Hilti 170W85 099HI72a Diopsidite Hilti 170W85 099SD16 Diopsidite Samad 135N80 000HI82a1 Diopsidite+host Hilti 10E70 000HI82a2 Diopsidite Hilti 10E70 000HI82b Diopsidite Hilti 10E70 000HI82c Diopsidite Hilti 10E70 000HI82d Host Hilti 000HI82e Diopsidite+host Hilti 10E70 000NA58 Gabbro Nakhl 165E30 4500RU21 Diopsidite Rustaq 100S80 004OM22b Diopsides in serp. Wuqbah 004OM53 Diopsides in serp. Batin 005SDM2 Diopsidite Sudum 130W70 005SDM8 Diopsidite+host Sudum 175E80 005SDM9 Diopsidite Sudum 17090 0

Locality is the name of the massif where the sample was taken; see Fig. 1 foproportions (Pl=plagioclase, Ol=olivine, Cpx=clinopyroxene, serp.=serpentmillimetres. The nature of ore mineral is precised in the last column, for the thinthe W.R. in brackets indicates that chromite was found only in the wall harzbualtered gabbros or with plagiogranites we may haveunderestimated their abundance during our initial survey.

The large majority of these diopsidites are virtuallymonomineralic, being made of almost 100% purediopside. This monomineralic composition has beenconfirmed by X-ray diffraction on whole rock powders.Diopside is associated in a few samples to anorthite,forsterite or to andradite and/or to more or less abundantamount of fibrous hydrous phases (tremolite, antigorite).Spinels are very uncommon and are present only closeto the contact with the host harzburgite. When presentthey are in most cases chromium spinels, sometimessurrounded by magnetite. Sulphides were not observed.One sample contains up to 10% of titanite (00NA58, seeTable 1 and Fig. 4(d)).

The samples which are exclusively composed ofdiopside show various textures that always differ from thecumulate textures of the magmatic dykes. They rangefrom fibrous, with minerals presenting a well-developedpreferred orientation (Fig. 4(a)), to cataclastic with largebroken grains embeddedwithin a very fine grainedmatrix(Fig. 4(c)). These two kinds of textures may be observed%Ol %Cpx %om GS(mm)

Ore mineral

0 40 0 1 0 80 0 b3 0 100 0 b3 0 60 0 3 0 100 0 15 0 100 0 N15 10 85 5 2 Chromite0 99 b1 1 Chromite0 100 0 b1 0 100 0 2 0 99 b1 35 Magnetite30 69 1 1 Chromite (in W.R.)0 100 0 15 1 99 0 3 0 100 0 15 90 0 0 15 0 100 0 15 0 45 10 15 Titanite0 100 0 1 0 b3 1 1 Chromite0 5 1 1 Chromite0 100 0 12 Chromite0 100 1 b1 Chromite (in W.R.)0 100 1 15 Chromite (in W.R.)

r more precise location. Columns 5, 6, 7 and 8 give estimated mineraline and om=ore mineral). Column GS gives the average grain size insection made in the sample labelled 00HI82a (00HI82a11 to 00HI82a15),rgite very close (at the scale of a thin section) to the diopsidite veins.

Fig. 2. Field view of hydrothermal dykes. (a) A diopsidite dyke seen inthe landscape. (b) Whitish to greenish dyke and surroundingharzburgites. The thickness is variable and relationships withsurrounding peridotite may be sharp or gradual, these two modes ofoutcropping being observed for one single dyke.

293M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305in one dyke with intermediate textures showing elongatedgrains alternating with fine grained zones (Fig. 4(b)).

Contacts between diopsidite veins and their hostperidotite vary from rather sharp to progressive asshown in Figs. 2(b) and 3. All kinds of relationshipsFig. 3. Sawn surface of a sample showing the gradational contactbetween surrounding peridotite and diopsidites may beobserved in a single outcrop (see Fig. 2(b)) and the natureof that surrounding peridotite may also range fromstandard harzburgites (containing 1020% of Opx). Theperidotite wall rock is more or less serpentinous and theproportion of serpentine increases when approaching thediopsidite dyke, reaching 100% at the contact. Directcontact between the olivine of the wall rock and diopsidewas not observed, these twominerals being separated by amore or less developed (few mm to few tens of cm) zoneof antigorite or tremolite. The antigorite itself is replacedby needles of amphibole, mainly tremolite but occur-rences of anthophyllite were noticed at the contact witholivine. In some thin diopsidite veins, ghosts of mantleolivine grains can still be identified. The boundariesbetween former grains are marked by an abrupt change inthe grain size of diopside needles (Fig. 5(a)). In the centreof these thin veins, euhedral diopsides embedded withininterstitial antigorite are commonly observed (see Fig. 5(b)); exceptionally, micro-metric grains of andraditepresenting the same characteristics are also present.

Four samples have themodal composition of gabbros,being made of about 50% diopside and 50% anorthite.One of them, which was taken in a brecciated zone (anormal fault parallel to the strike of the sheeted dykecomplex in the Maqsad area) has a mylonitic texture andthe three other samples a granular texture with anaverage grain size of about 0.1mm (Table 1). The samplecontaining euhedral titanite is also rich in anorthite(Table 1 and Fig. 4(d)) while the others do not includeany accessory mineral. In these samples, more or lesseuhedral diopsides appear as millimetre sized grains in afiner matrix of anhedral plagioclase.

In some samples, veins of carbonate cross centi-metre-sized grains of diopside (Fig. 5(c)). These veinshave a maximum thickness of 0.5 mm. Interestingly,minute grains of perfectly euhedral diopside havebetween a diopsidite vein and the surrounding harzburgite.

Fig. 4. Thin sections (crossed Nicol prisms) showing various textures of the diopsidite. The most commonly observed texture is fibrous withelongated grains (a); the deformation of the diopside grains is always low but dislocations at the grain boundaries are common and lead to a cataclastictexture (b) and (c). The compositions of the three samples in (a), (b) and (c) are monomineralic and are constituted by diopside. The microphotographof sample 00NA58 (d) show the microgranular aspect of samples containing plagioclase. Diop.: diopside; Anort.: anorthite; Tit.: titanite.

294 M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305crystallised within the carbonate and subhedral crystalsof diopside grew from the rim of the vein giving to thewhole the aspect of a geode (see Fig. 5(c)).

Finally, we have found diopside grains scattered inserpentinised dunites away from any recognised diopsi-dite dykes in the Batin area of theWadiTayin massif andin the Wuqbah massif (samples 04OM53 and 04OM22b,see Table 1). In this kind of samples, within a totallyserpentinous dunite, the cores of the serpentine meshnetwork are filled byminute grains (b100m) of diopsidethat can be easily confused with relics of fresh olivine.

3.2. Chemistry

We analysed about 25 samples of diopsidites with theToulouse University microprobe (Cameca SX50) andthe Kanazawa University microprobe (JEOL 8800) witha standard program, using an acceleration voltage of20 kV, a probe current of 20 nA and a probe diameter of3 m. The trace element compositions of minerals fromtwo samples taken in the core and at the rim of the samedyke (respectively samples 05SDM8a and 05SDM8cfrom the wadi Sudum) were analysed using laser abla-tion (193 nm ArF excimer: MicroLas GeoLas Q-plus)-inductively coupled plasma mass spectrometry (LA-ICP-MS, Agilent 7500S) at the Incubation BusinessLaboratory Center of Kanazawa University [44].Analyses were performed by ablating a 132 mdiameter spot on clinopyroxene at 6 Hz with an energydensity of 6 J/cm2 per pulse. The primary calibrationstandard (NIST SRM 612 glass) was analysed at thebeginning of each batch of 3 or 4 sample analysis, with alinear drift correction applied between each calibration(see [45] for analytical details and data quality, and [46]for element concentration of NIST SRM 612 used forcalibration). Data reduction was facilitated using Si asinternal standards based on the SiO2 contents of Cpxobtained by microprobe analysis, following a protocolessentially identical to that outlined by Longerich et al.[47]. In this paper, we will present the chemical data forthe three main phases constituting the diopsidite:diopside, forsterite and anorthite. Other phases likeandradite, antigorite, spinel, tremolite, etc. are scarce inthe main body of the dykes and their chemistry will notbe detailed here, we will just mention that, whenpresent, tremolite is rather poor in Al (Al2O3 b5 wt.%,

Fig. 5. Microphotographs and simplified sketches showing specific textural characteristics of diopsidites. (a) Needles of diopside are growing in amicrograined matrix composed of a mix of serpentine, carbonate and diopside. The fine grained matrix may appear as a vein, or as olivine-likesubrounded pocket; it is surrounded by fine (few tens of micrometers) diopside grains. (b) Euhedral grains of diopside appear frequently withinfibrous serpentine (antigorite). (c) Relationships between the diopsides and a vein of carbonates. Carbonate vein is causing hydrolic fracturation of thecoarser grains of diopside and small euhedral grains of diopside are crystallising within this vein.

295M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305with an average value at 1.5 wt.%) and in Fe (Fe totalmeasured as FeO b3 wt.%, with an average value at1.1 wt.%).

Microprobe data show that the chemistry of diopsideand related forsterite and anorthite presents moderate butsignificant variations from one sample to the other and,within a single thin section, from one grain to the other.The TiO2 content ranges from 0% to 0.49%; Al2O3 from0% to 1.74%; Cr2O3 varies from 0% to 0.81% in allsamples but one in which it reaches 1.2% (Table 2 andFig. 6). The Xmg of clinopyroxene varies from 95% to100% and the forsterite content of olivine from 94% to98% (Table 2 and Fig. 7). These ranges of valuescharacterise the variability from one sample to the other as

Table 2Major element chemical characteristics of each constitutive mineral

Label Nature Olivine Pyroxene

An% Pl MnO NiO Fo% TiO2 Al2O3 Cr2O3 Na2O Xmg

92OG8 Diop.+An. 99.53 0.06 0.35 0.01 0.03 98.6097M5 Diop.+An. 97.78 0.03 0.36 0.02 0.02 97.6197M38a Diop. 0.00 1.18 0.00 0.18 91.9697M38a Diop. 0.00 0.81 0.00 0.00 99.3797RU6b Diop.+An. 95.78 0.00 0.24 0.00 0.07 97.0697SA15c Diop. 0.00 0.09 0.00 0.02 98.9099HI56b Diop. 0.00 0.63 0.00 0.02 97.8199HI59a Diop.+For. 0.02 0.12 95.72 0.00 0.12 0.03 0.02 98.8599HI59a Diop.+For 0.17 0.34 94.23 0.05 1.39 0.61 0.04 97.4999HI69 Diop. 0.00 0.38 0.04 0.01 98.9399H170a Diop. 0.05 1.08 0.05 0.12 97.8799H172a Diop. 0.25 1.74 0.00 0.02 94.4499H172a Diop. 0.00 0.07 0.01 0.00 98.9999SD116 Diop. 0.03 0.08 0.04 0.03 96.3400H182a11 Diop.+Trem. 0.02 0.11 94.19 0.05 0.07 0.02 0.01 97.6700H182a12 Peridotite 0.05 0.13 89.9000H182a13 Peridotite 0.17 0.46 90.9000H182a13 Diop.+Trem. 0.13 0.40 93.40 0.00 1.81 0.86 0.09 93.1300H182a14 peridotite 0.19 0.50 90.30 0.00 1.69 0.72 0.00 90.8400H182a14 Diop.+Trem. 0.14 0.37 93.7000H182a15 Peridotite 0.11 0.38 91.0000H182a15 Diop.+Trem. 0.12 0.56 93.9700H182b Peridotite 0.02 0.10 93.6400H182c Diop. 0.00 0.08 0.01 0.00 95.1300H182d Peridotite 0.20 0.38 90.60 0.00 1.66 0.68 0.00 91.0400HI82e Peridotite 0.04 0.11 90.95 0.00 1.64 0.69 0.00 90.9700NA58 Diop.+An.+Tit. 99.13 0.00 0.11 0.01 0.01 98.6700RU21 Diop. 0.03 0.30 0.02 0.01 95.3300RU21 Diop. 0.01 1.22 0.05 0.16 98.6905SDM02a Diop. 0.02 0.17 0.02 0.00 97.9405SDM02b1 Diop.+And. 0.00 0.59 0.11 0.13 95.8305SDM02b1 Diop.+And. 0.00 0.00 0.00 0.00 98.7505SDM02b2 Diop. 0.01 0.33 0.00 0.08 96.6605SDM02b2 Diop. 0.00 0.03 0.00 0.02 97.8005SDM2b2 Diop. 0.01 0.57 0.00 0.05 92.6605SDM2b2 Diop. 0.01 0.21 0.20 0.05 97.3705SDM2b2 Diop. 0.00 0.22 0.00 0.02 97.6505SDM08a Diop. 0.00 0.05 0.01 0.00 97.3205SDM08a Diop. 0.00 0.12 0.05 0.03 97.4205SDM08a Diop. 0.00 0.18 0.00 0.00 99.6905SDM08a Diop. 0.00 0.00 0.04 0.01 99.7905SDM08b Diop. 0.00 0.24 0.12 0.03 97.9505SDM08b Diop. 0.02 0.11 0.00 0.02 99.1505SDM08b Diop. 0.00 0.05 0.02 0.00 99.6905SDM08c Diop.+For. 0.03 0.53 97.76 0.00 0.03 0.03 0.00 99.4105SDM09a Diop. 0.04 0.06 0.02 0.00 97.9205SDM09a Diop. 0.01 0.02 0.06 0.03 99.2205SDM09b Diop.+Trem. 0.02 0.10 0.02 0.01 96.8405SDM09b Diop.+Trem. 0.00 0.01 0.04 0.01 99.17

For each sample, few representative analyses are given. The column Nature gives the main phases included in the sample: Diop.: diopside; An.:anorthite; For.: forsterite; Tit.: titanite; Trem.: tremolite. In the column Pyroxene is given the composition of Opx for the samples of peridotite and Cpxfor the others.

296 M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305well as the internal variability within one single thinsection. In spite of this heterogeneity, it remains that allsamples share common and geochemical characteristicsspecific to diopsidites, which are depleted in all minorelements, both compatible and incompatible (Ti, Al, Cr,Na, etc., see Table 2 and Fig. 6).

Fig. 6. Variation of TiO2, Al2O3 and Cr2O3 content versus Xmg inclinopyroxene. The light grey surface and dashed line show thecompositional domain for most of Omanese dykes (N900 samples,[38]) and the dark grey surface shows the compositional domain ofOmanese peridotites [48,49]. Sample 00HI82 is shown in Fig. 3; this isa sample of peridotite containing thin (b1 cm) veins of diopsidite. Thesolid triangles represent the composition of the peridotite of this sample(out of any vein); grey circles: composition of olivine (Fig. 7) anddiopside (this figure) included in the veins of this sample; black crosses:composition of other diopsidite dykes, solid squares: composition ofdiopsides from the sample 04OM53 from the Batin dunite (the legend isshown on the chromium graph and is the same for all other graphics).

Fig. 7. Evolution of NiO versus forsterite content in olivine. Thesymbols used are the same as those of Fig. 6. Two samples onlycontain olivine; both of them were taken in the same area in the wadiSudum. The core of large dykes does not contain any olivine and thismineral was analysed in the host rock (black triangles), near the rim ofdiopsidite veins (grey circles) and included in the dyke rim wheninterpenetration of host rock within the dyke is observed (blackcrosses). The chemical fields are the same as those of Fig. 6.

297M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305For all the samples, the content in Na, Ti, Cr of thepyroxenes is almost always below the detection limit orclose to it (Fig. 6). In average three samples showrelatively high Ti content (TiO20.250.3%), but theTi content of the diopside from the titanite rich gabbro issystematically below the detection limit. Na content ofCpx is always below 0.15% and Cr content is rarely over0.1%, Al content is exceptionally low, below 1.3%except for some individual analyses.Fig. 8. Evolution of the anorthite content of plagioclase versus theXmg of diopside. Only four samples contain plagioclase and none ofthem olivine. As Omanese peridotites usually do not contain anyplagioclase, they have not been represented on this graphic. Thechemical field for the troctolite and olivine gabbro families (in lightgrey) and for the gabbronorite family (dashed line) are given; the blackdots represent the plagioclase bearing diopsidite dykes.

Fig. 9. Chondrite normalised [69] pattern for diopsides from 2 samples ofone dyke of the wadi Sudum. Open square: sample 05SDM8c (rim of thedyke); black circles: sample 05SDM8a (core of the dyke), in situ laserablation onminerals; thick line: sample 05SDM8a,whole rock chemistry.

298 M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305Olivine grains enclosed in the dyke rim (see Fig. 3and Table 2) present very high magnesian compositionbut, at a very short scale, the forsterite content of olivinebecomes normalwith compositions similar to the onesof other Oman harzburgites [48,49]. Ni content ofolivines ranges from about 0.25% to 0.6% (Fig. 7),which is rather high compared to mantle harzburgite. Atthe scale of one sample, the Ni content is highly variable(1.25% NiO). By the same way, the forsterite contenttends to increase in the host rock when approaching thedykes, reaching the highest values in the olivineembedded in the dyke rims (see Fig. 7).

Plagioclase is present in four samples, its chemicalcomposition is close to pure anorthite (An% varyingFig. 10. Sketch of hydrothermal circulation below the Omanese ancient spreadbut may reach the upper mantle. We suggest that the diopsidites form wherethe surface. Accordingly, conditions for diopsidites formation are expectsegmentation of the spreading centre (not shown).from 93% to N99%, see Table 2 and Fig. 8). This isconsistent with the very low content of Na in thediopsides, which is the highest in the samples having theless anorthitic plagioclase (sample 97RU6b).

Finally, preliminary trace element data on twosamples show REE patterns typical of Cpx except fora pronounced positive Eu anomaly (see Fig. 9). Thesedata being acquired with an in situ method (LA-ICP-MS), contamination with plagioclase can be excluded.As for other minor elements, the concentrations in REEare very low and show large heterogeneity. The globalREE concentration ranges from 0.1 to 1 time thechondrite concentration (Fig. 9); the whole rock patternof sample 05SDM8a appears as a thick line in Fig. 9 andappears to be a good average of the individual lasermeasurements.

3.3. Distribution in the Oman ophiolite

All the occurrences of these diopsiditewere found inthe uppermost part of the mantle section within a fewhundred metres below the mantle crust transition (seeFig. 1). Except in the Rustaq and Nakhl areas where theyare sub-perpendicular to the mantle/crust boundary, theirorientations (northsouth in the Hilti area, N130150 inthe Maqsad vicinities) are sub-parallel to the paleo-Mohowith variable dip. When present, evidence for crackpropagation (dyke tips) are lateral or downward ratherthan upward (by reference to the paleo-Moho).

In the field, the greenish fibrous aspect of these dykesis similar to that of totally altered gabbros and theing centre. Hydrothermal fluids circulate mainly within the upper crustdownwelling hydrothermal fluids meet percolating magmas en route toed to be highly transient and controlled by tectonic and magmatic

299M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305particularly high degree of serpentinisation of thesurrounding peridotite contributed to this misleadingimpression. As a consequence, we did not systemati-cally sample diopsidite before we realised they wereactually not altered gabbros, so that the preliminary mapin Fig. 1 may be hedged. However, it suggests that thedistribution of diopsidite dykes and veins is not randomat the scale of the ophiolite, the majority of them beinglocated in the MORB zones as defined by Python andCeuleneer on the basis of the distribution of the maficdykes [38]. This is coherent with the observation that, inthe field, these features are spatially related tooccurrences of troctolitic impregnations in the hostharzburgites.

A large majority of the diopsidite dykes were foundin the MORB zone of the Hilti area. A more carefulprospection showed no diopsidite outcrop off theMORB zone in this area. Five dykes are located inthe NakhlSumailSamad troctolite/olivine gabbrozone, four within the troctolite zone and one in theborder of the olivine gabbro zone (Fig. 1). In the Saramiarea, only one dyke was sampled. It is located in anarrow zone containing olivine gabbro dykes close tothe mantle/crust transition. Only the three samplingpoints of the Rustaq/Wuqbah area seem to be away ofany troctolite or olivine gabbro dyke, one of them (point04OM22b, see Fig. 1) is one of the above describedserpentinous dunite containing small grains of diopside.

4. Summary and discussion

As far aswe know, the lithologywe report on has neverbeen described in ophiolites nor among rocks sampled inthe present-day oceanswhatever the tectonic setting (mid-ocean ridge versus subduction zone). We call it diop-sidite because, in most samples, it is made of nearly100% very high Mg# diopside but some of thesediopsidites may contain a significant proportion of avery anorthitic plagioclase and/or of a very magnesianolivine. We summarise the salient field, petrographic andgeochemical features of these rocks and propose a pos-sible scenario for their genesis.

4.1. Diopsidites have not crystallised from commonmagma types

The composition of clinopyroxenes in diopsidites isquite distinct from the one of common pyroxenites andgabbros dykes cross cutting mantle peridotites and fromthe one of clinopyroxenes in mantle peridotites them-selves (Fig. 6). The combination of very high Xmg(from 0.95 to the pure magnesian end member) withvery low Cr and Al contents is one of the main geo-chemical evidence for a non magmatic origin of thediopsidites. As a matter of fact, mantle and cumulatediopsides rarely exceed 0.94 in Xmg and their Crcontent increases exponentially as their Xmg increases,a behaviour related to the compatible nature of Cr inCpx. Typical diopsides from mantle peridotites and fromprimitive (high Xmg) cumulates have concentrations inCr2O3 ranging from 0.5% to 2.5% and in Al2O3 higherthan 1.5% [50,38, and references from PetDB [51]].These values contrast markedly with the very low Cr2O3 and Al2O3 concentrations in our diopsides (with,respectively, average values of 0.05% and 0.3%), inspite of their high Xmg. The atypical character of Cpxfrom our diopsidites, compared to those in mantleperidotites and in cumulate pyroxenites and gabbros, iscorroborated by the behaviour of other minor elements(Ti, Na) that also have low concentrations frequentlybelow the detection limit of electron microprobe andplot clearly away from fractional crystallisation andpartial melting trends (Fig. 6). When present in theparagenetic association, olivine and plagioclase showsimilar characteristics as diopsides, i.e. they are tooclose in composition to pure end members (too for-steritic and anorthitic) to fit with common magmatic ormantle compositions (Figs. 7 and 8).

Diopsidites are generally devoid of ore minerals(oxides and sulphides) and other accessory mineralsapart from one sample that contains a significantamount of titanite. In this sample, titanite grains areeuhedral (see Fig. 4(d)) suggesting that they are notrelics from a previous oxide gabbro but that theycrystallised together with the diopside grains. Inmagmatic differentiation processes, titanium is incom-patible and is incorporated in oxides (ilmenite, rutile)or titanite generally as the magma becomes relativelyevolved in terms of Xmg. The evolution of the Ticoncentration in the magmatic clinopyroxenes islargely conditioned by the behaviour of these Ti-richoxides [52,38,36, and references from PetDB [51]].This scenario cannot be transposed to the case of ourdiopsidites: the occurrence of titanite in one sample isnot correlated to different Xmg and Ti concentrationsin its clinopyroxene relative to those from otherdiopsidites. Although we do not understand preciselythe reason for titanite crystallisation in this peculiarsample, it clearly does not result from standardfractional crystallisation processes from silicate melts.

Finally, the occurrence of andraditic garnet in somediopsidites can, here also, hardly be attributed to standardmagmatic processes and calls for a combination ofphysico-chemical conditions (bulk chemical composition

300 M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305of the system, temperature, Pfluid, etc.) rarely encounteredin magmatic systems.

4.2. Diopsidites crystallised in very thin cracks andreplaced former serpentinous peridotites

Diopsidites occur frequently in dyke-like structures,i.e. planar structures with more or less sharp contacts withthe country rock. It is tempting to interpret these as formercracks eventually filled with crystallisation products ofthe melt or fluid that circulated in these cracks. However,the contact between the pure diopsidite and its hostperidotite is always progressive and characterised byreplacement textures andmineralogies. In the centre of thelarge dykes, these replacement textures disappear, leavingroom for pure fibrous or cataclastic textures. Within somethin veins, replacement textures are observed up to thevery centre of the vein. It is clear that the thickness of theveins or dykes measured in the field by using the colourcontrast between the diopsidite and its host does notstrictly correspond to the former width of the channelswhere the melt or fluid circulated. Although thisparameter cannot be determined precisely, it is clearlymuch thinner than what could be deduced from theuncritical interpretation of structural measurements.

Where replacement textures are observed, the protolithis always a mantle derived peridotite (harzburgite ordunite), not a former gabbroic cumulate. Replacementoccurred at the expense of peridotites that sufferedvariable degrees of serpentinisation. Ghosts of the meshtextures characteristic of partially serpentinised olivinecan be beautifully preserved even in rocks made presentlyof 100% diopsides. Single diopside grains can bepseudomorphic on relics of olivine grains preservedduring former transformation of the peridotite intoassemblages of high temperature serpentines (antigorite).In that respect, diopsidites are different from rodingitesdespite both rocks share some field characteristics.

4.3. Where did diopsidites come from?

Diopsidite dykes are preferentially observed in man-tle harzburgites immediately underlying the mantle crusttransition zone (paleo-Moho). Evidence for downward(i.e. away from the Moho) crack propagation is fre-quently observed in the field. In many occurrences, theformation of diopsidites dykes was contemporaneouswith intense shearing: as a matter of fact, a perfectcontinuum exists between sheared (mylonitic and por-phyroclastic textures) and undeformed (crystal growth)textures. Less commonly, diopsidites occur as aconstituent of the matrix within brecciated fault zones.These observations point to emplacement of diopsiditedykes in areas of intense tectonic activity and high fluidpressure, although the crystallisation of the diopsidite isnot restricted to sheared areas and thus not triggered bythe solid state deformation itself.

When the global distribution of diopsidites in theOman ophiolite is considered, it appears that they crop outpreferentially in those areas of the mantle section thatwere intensely percolated by hot primitive magmas ofMORB geochemical affinity (now dykes and porous flowchannels of troctolites and olivine gabbros [39,38]). Theseareas have been interpreted as former asthenosphericwindows where hot mantle diapirs made their waythrough the lithosphere up to the base of the mantle/crusttransition [37,38]. A relationship certainly exists betweenthe genesis of diopsidites and the thermal and structuralsegmentation of the Oman paleo-spreading centre.

4.4. Diopsidites as products of prograde metamorph-ism of serpentines induced by the circulation of veryhigh temperature hydrothermal fluids

On the basis of the observations and petrological datapresented above, we propose that the deep penetration ofhydrothermal fluids in the mantle can be traced by thediopsidite veins. The use of an anhydrousmineral to followthe path of fluids may look strange at first sight but issupported by experiments and models. McCollom andSchock [1] showed that all common anhydrous igneousphases, including pyroxenes, olivine and plagioclase, cancrystallise from hydrothermal fluids provided the temper-ature is high enough (typically in the range of 750900 C)and that it could be hard to decipher between igneous andhydrothermal mineral assemblages. Our observations fitwith their prediction although in complex natural systemsthe hydrothermal processes may have a very specificgeochemical signature (here, an abnormally high Xmg andabnormally low in Cr, Al, etc. content in Cpx) super-imposed on the mineralogical composition. Accordingly,we propose that the distinction between igneous and hy-drothermal minerals is not so ambiguous in the real world.

A possible scenario accounting for diopsidite forma-tion is illustrated in Fig. 10. Diopsidites have never beenobserved in the crustal section of Oman nor in gabbrossampled along present day ridges in spite of extensivesurvey of hydrothermal alteration mineralogy [7,53,26,54,8,33,11, etc.]. Accordingly, we can reasonably sup-pose that conditions (pressure, temperature, chemistry ofthe protolith) for their formation are not realized in thecrustal environment. In Oman, it appears that diopsiditesform preferentially just beneath the mantle crust transi-tion, and that they are peculiarly abundant close to former

301M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305impregnations of troctolites, diagnostic of segregation andcrystallisation of very hot magmas at shallow depth(probably above 1200 C [5557, etc.]). We propose thatdiopsidites form in a narrow transition zone sandwichedby the downward propagating front of hydrothermalfluids and by the top of a partially molten asthenosphericdiapir that made its way through the lithosphere.

The conditions for diopsidites formation are likelyvery transient, in agreement with their somewhatchaotic mode of occurrence (cataclasites) and bythe great textural heterogeneity pointing to quitevariable rates of nucleation and growth, diagnostic ofnon equilibrium conditions. Such transient conditionsmade possible the preservation of steep gradients inmetamorphic mineralogy in the peridotites hosting thediopsidite veins.

When reaching the mantle crust transition, hydro-thermal fluids had already lost most of their charge inalkalic elements by precipitation of minerals likeamphiboles, epidotes, albite, etc. in crustal hydrothermalveins, but they were selectively enriched in elementslike Ca and Eu due to their interaction with the gabbrosfrom the lower crust. In the mantle, fluids circulatedlikely in a network of narrow channels or veins, leadingto the serpentinisation along these veins, the crystal-lisation of diopsides being the result of the transforma-tion of this serpentine. The so called diopsidite dykesare actually reaction zones, up to several dm inthickness, around a much narrower central part(typically less than 1cm) that can be interpreted as themain fluid channel. The lithostatic pressure at the timeof fluid circulation can be estimated to about 0.2 MPa,given the overlying crust thickness of about 6 km,making unlikely the opening of widely open cracks,which is in agreement with our observations.

A typical sequence of prograde metamorphism isrecorded when moving from the far field peridotite tothe centre of diopsidite veins. This is consistent withprogressive increase of the fluids temperature as theycirculate in the vein network. As a dike is approached,antigorite and tremolite (chlorite, talc and anthophyl-lite) replace both primary mantle minerals (olivine andpyroxenes) and low temperature serpentine. Inside thedyke, the temperature has risen above the stability fieldof these hydrous minerals and diopsides (andradite andanorthite) crystallise. The temperature rise may be acombined effect of the heat released by the exothermicserpentinisation reaction in the wall rock and of heatadvection by convection of fluids, the magmas crystal-lising troctolites in the nearby diapir acting as aninfinite heat source for maintaining the convection ofhydrothermal fluids.The important (15%) volume increase induced by theserpentinisation reaction in the peridotite immediatelyadjacent to the diopsidites will contribute to close thecracks and limit the fluid propagation on long distances inthe mantle. This is likely the reason why the diopsiditesare relatively scarce at the scale of the Oman ophiolite.

4.5. Conditions of diopsidite crystallisation

The diopsidite textural characteristics (fibrous elongat-edminerals, euhedral grains of diopsideswithin pockets ofserpentine, heterogeneous grain sizes, etc.) point to rapidcrystal growth and out of equilibrium conditions. Thesetextural evidence together with non magmatic chemicalsignatures lead us to interpret these diopsidite lithologiesas the product of metamorphism induced by the reheatingof host rock in the presence of fluids. One of the bestanalog is likely the skarns and other features observed incontact metamorphism context [5860].

The residual textures observed in some diopsidites(ghosts of olivine, pseudomorphic diopsides on olivineor orthopyroxene grains) suggest that diopsides form byCa enrichment of a mesh textured serpentinite. In thatrespect, diopsidites are quite similar to rodingites deve-loping at the boundary of serpentinite and mafic bodiesin some ophiolitic complexes and continental greenrocks belts [6164]. So that we may imagine similarprocesses for the formation of rodingites and diopsi-dites. In the formation process of rodingites, the serpen-tinisation of an ultramafic body leads to the breakdownof clinopyroxene into serpentine and the release of a Ca-enriched fluid. This fluid reacts with the nearby maficrocks (gabbros, diorites, basalts, etc.) leading to thereplacement of the mafic minerals (plagioclases, pyro-xenes, amphiboles) by the rodingitic assemblages[61,65,62,63, and references therein].

Contrary to the common rodingites which derivefrom a mafic protolith, the diopsidites are the transfor-mation product of an ultramafic rock. This could explainthe differences in textures and chemical compositions asthe lack of aluminium and the high magnesiumconcentration in the diopsidites compared to therodingites (forsterite are usually absent from rodingites)but cannot justify the extreme deficit of iron and thesilicon enrichment that lead to the genesis of thediopsidites. During the rodingitisation process, the totalamount of iron slightly decreases but not at the level thatis recorded in the diopsidites. In the Oman ophiolite, acommon olivine contains between 8 and 10 wt.% ofFeO while the total iron content in diopsides is usuallylower than 1 wt.%, about 0.7 wt.% in average for a dykeconstituted of 100% of diopsides. Moreover, the

302 M. Python et al. / Earth and Planetary Science Letters 255 (2007) 289305rodingitisation of mafic rocks causes a slight decrease inSiO2 content while the replacement of a rock constitutedpredominantly by olivine with the 100% diopsideassemblage implies a large enrichment in silica. Thus,rodingites and diopsidites share some common char-acteristics and in both cases, the genetic processinvolves the metasomatism of a protolith with a Ca-rich fluid and the replacement of the primary lithologyby a Ca-rich one. Nevertheless, the genesis of thediopsidites as large dykes included within harzburgitecannot be totally explained by a classical rodingitisationprocess and parameters such as the nature of theprotolith and the temperature or the chemistry of theinvolved fluid differ in the formation of the one or theother of these lithologies.

A natural candidate for the protolith is a serpentinousharzburgite or dunite. As a matter of fact, the chemicalcomposition of serpentine is poor in Fe, Cr, Al and solubleelements such as Na or K. When embedded in a matrix ofserpentine both diopside and andradite can presentperfectly euhedral crystal shapes. This suggests that diop-side and andradite grew at the expense of serpentine ratherthan being relics of former, partially altered, high tempera-ture assemblages. Tiny, irregular, carbonate veins areobserved in some thin sections. Here also, textural rela-tionships suggest that diopside can grow into the carbonatematrix as in the case of serpentine. It also happens thatcoarse grains of diopside are broken by the carbonateveins, suggesting that the formation of the carbonate veinswas contemporaneous with diopside crystallisation.

The nature of the fluids needs to be constrainedprecisely by specific studies but, given the predomi-nance of diopside in the paragenesis, we can infer that itwas a hydrated fluid rich in Ca, carbonates (CO2) andprobably in silica. The well-developed positive Euanomaly (Fig. 9) suggests that these fluids have leachedplagioclase rich lithologies (gabbros from the overlyingcrustal section?) before penetrating the mantle.

The temperature of metamorphism can be con-strained roughly by the mineralogical assemblage.The stability field of diopside is quite large; it maycrystallise at temperatures as low as 300 C but is alsostable near the solidus. Nevertheless, the assemblagediopside+forsterite+antigorite at the rim of the dyke isnot stable, as shown by the systematic presence ofantigorite between diopside and forsterite; this suggestsa temperature over 550 C. On the other hand, still at therim of the dyke, the presence of anthophyllite inassociation with olivine suggests that this temperaturewas at least 620 C in the wall rock [59].

The assemblage antigorite+diopside is stable up to500 C. Over this temperature, the antigorite+diopsideassociation will break down to tremolite and forsterite,and diopside will reappear over 750 C. Antigorite andforsterite are absent from the centre of the diopsidite dyke;in the CaOSiO2MgOH2O system, anorthite cancrystallise only in extreme conditions, over 900 C inassociation with diopside [6668, and references therein].The various mineralogical assemblages suggest large var-iation in the conditions of the diopsidite formation, fromthe serpentinisation field near 500 C for the diopside+antigorite assemblage near the rim of some dykes, toextremely high temperatures over 900 C for anorthite+diopside equilibrium in the core of some other dykes.

Acknowledgement

We are grateful to Fabienne de Parseval, Anne-MarieRoquet, Raphal Peyron at the thin section manufactur-ing and to Philippe de Parseval at the microprobe servicefor their help at Toulouse University. Marc Monnereau,Kenji Suzuki, Yoshitoshi Takemoto and Hilal al Azriwere of great help for our field work. Franois Fontan(Toulouse) kindly performed XRD analyses. We alsowould like to thank Satoko Ishimaru, Akihiro Tamuraand Christophe Monnin for everyday support anddiscussion, and two anonymous reviewers who contrib-uted to improve the manuscript.

Financial support for this work was provided by theCentre National de la Recherche Scientifique in Francein the frame of the Dorsale and Dynamique et bilande la Terre programs, and by the Japan Society forPromotion of Science in Japan.

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Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation .....IntroductionGeological setting and previous workHigh magnesian diopsides dykes in Oman peridotitesField and petrographical characteristicsChemistryDistribution in the Oman ophiolite

Summary and discussionDiopsidites have not crystallised from common magma typesDiopsidites crystallised in very thin cracks and replaced former serpentinous peridotitesWhere did diopsidites come from?Diopsidites as products of prograde metamorphism of serpentines induced by the circulation of.....Conditions of diopsidite crystallisation

AcknowledgementReferences