Studying of phase relations in the system forsterite-diopside-jadeite (experiment at 7.0 GPa).

Butvina V.G., Litvin Yu.A.

Institute of Experimental Mineralogy RAS, Chernogolovka, Moscow district, butvina@iem.ac.ru, (496)5225876

Peridotites and eclogites, including diamond-bearing ones, are the basic ultra-basic and basic rocks of the upper mantle (Ringwood, 1969, 1975; Sobolev, 1974; Marakushev, 1985; Taylor & Anand, 2004). These rocks are presented in the assemblage of mantle xenolyths in kimberlites, but the basic minerals of peridotite paragenesis, olivine, orthopyroxene, garnet and clinopyroxene as well as of an eclogite paragenesis, garnet and omphacite are wide-spread synthetic inclusions in diamonds. The cases of finding minerals and peridotite and eclogite parageneses in diamond are described. It implies that these parageneses can have a single mantle source. However, the formation of peridotite and eclogite mineral parageneses at differentiation of the primary ultrabasite melt during physico-chemical single process is possible only at overcoming the “eclogite” thermal barrier (O’Hara, 1968; Litvin, 1991).

Eclogite genesis is one of the most difficult and discussional problems of modern petrology. Among investigators there is an opinion about eclogite heterogeneity not only on conditions of formation (crust, mantle), but also by composition of the initial rocks (para-, orthoeclogites) as well as by the way of their formation (magmatic, metamorphic, metasomatic). In literature diamond-bearing eclogite nodules of kimberlite pipes are often considered as metamorphic, which are formed at subduction of the Archean or of the Proterozoic oceanic crust (MacGregor & Manton, 1986; McCandless & Gurney, 1986, 1997 et al.). Only the presence of Na₂O in garnet and K₂O in clinopyroxene is a criterion of their participation in mantle magmatic processes.

Together with the hypotheses considered on eclogite origin there exists a version suggested in papers (Kushiro, 1972; Kushiro & Yoder, 1974), according to which mantle eclogites could be formed due to peridotite substance in the processes of fractional crystallization of ultrabasite magmas. The present paper is devoted to the experimental study of this problem.

Physico-chemical transition from peridotite assemblage to the eclogite one can be only ensured by the processes of fractional crystallization of mantle magmatic melts. The primary melting and magmatic evolution of mantle garnet lerzolite (or the Ringwood pyrolite) is controlled by a five-phase peritectics “p” Ol+Opx+Cpx+Grt+L and four cotectic curves conjugated to it (Litvin, 1991). In melting and evolution of melts of both olivine eclogites and coesite and corundum eclogites the corresponding five-phase eutectics are of a dominant importance. A general ridge for all elementary tetrahedrons (simplexes) is a line of compositions diopside-pyrope (clinopyroxene-garnet) which bimineral eclogite assemblages belong to. The internal section En-Di-Cor of the general tetrahendric diagram (symplex complex) separates olivine-saturated and silica-saturated compositions. “Eclogite” thermal barrier is “thermal barrier” on (O’Hara, 1968), on the cotectic line Opx+Cpx+Grt+L, connecting “peridotite” peritectic and “eclogite” eutectic points.

Meanwhile, at equilibrium (and fractional) crystallization of peridotite system in the peritectic point “p” orthopyroxene vanishes as a result of the peritectic reaction “orthopyroxene + melt ® clinopyroxene” (Davis, 1963; Litvin, 1991). With further temperature decrease the composition of the remnant melt is controlled by the nonorthopyroxene cotectics Ol+Cpx+Grt+L first, in the limits of the peridotite “simplex”, but then mechanism of fractional crystallization is also realized in the limits of the olivine-eclogite “simplex” up to the corresponding nonvariant eutectics.

The considered cotectics Ol+Cpx+Grt+L is of the greatest interest from the viewpoint of a possible change of compositions of remnant melts from olivine-normative to silica-normative ones. One can assume that under the conditions of fractional melt crystallization along the cotectic curve Ol+Cpx+Grt+L together with olivine jigging accumulation of incorehent elements, including Na, Fe etc. takes place. It leads to a gradual increase of jadeite component content in remnant melts what creates grounds for reactional interaction of jadeite and olivine components with olivine vanishing and garnet formation in accordance with the reaction found in (Litvin et al., 2004). A gradual decrease of olivine component content in remnant melts caused by that fact realizes a “turn” to the cotectic curve Ol+Cpx+Grt+L in the direction of the boundary section En – Di – Cor and, probably its exit to the line Di–Prp (clinopyroxene-garnet). Further under the conditions of fractional crystallization melt composition point can penetrate into the volumes of coesite-eclogite, kyanite-eclogite and corundum-eclogite “symplexes”. Thus, an overcoming of “eclogite” thermal barrier between olivine-normative peridotite-pyroxene and SiO₂ – normative eclogite compositions occurs. So, one can speak about the “destruction” of liquidus peridotite-eclogite thermal barrier in the limits of the peridotite “simplex” as a result of realization of two reaction mechanisms: (1) vanishing ofš orthopyroxene as a result of its peritectic reaction with the melt with clinopyroxene formation and (2) olivine vanishing as a result of its reactional interaction of jadeite with garnet formation. If with respect to the first mechanism definite experimental evidence exists (Litvin, 1991; Davis, 1963) then for the second mechanism it is absent.

Due to this fact the main purpose of this paper is an experimental study of phase relationships in the model system forsterite-dioside-jadeite at pressure of 7 GPa and foundation of possible physico-chemical correct transitions between peridotite and eclogite parageneses with overcoming liquidus “eclogite” thermal barrier. To construct a diagram of a ternary system forsterite-diopside-jadeite it is necessary to study its boundary binary sections forsterite-jadeite and fosterite-diopside as well as a number of internal polythermic sections. The section jadeite-diopside at 7 GPa has been studied earlier (Bobrov, Litvin, Kojitani, Akaogi, 2006; 2008) and it is characterized by the unlimited miscibility of jadeite and diopside components in solid and liquid states.

The first experimental results obtained at the initial stage of the investigation of this problem can be characterized as follows.

Forsterite-jadeite section.

The experiments in this section have been done in the temperature range of 1100-1800⁰C on which basis the construction of fusibility diagram of the system forsterite-jadeite at 7 GPa has been started. The obtained experimental data testify to the existence in the system of the liquidus fields Fo + L, Grt_SS + L and Cpx_SS (on the basis of jadeite phase) +L, as well as show indirectly a possible appearance of orthopyroxene as a liquidus phase. In subsolidus experiments olivine-bearing assemblages on the basis of the paragenesis Ol+Grt+Opx+Cpx are found. Garnet there is not a pure pyrope, but has the molecule Na₂MgSi₅O₁₂ (Na-majorite) what manifests itself in the direct correlation of Na and Si in the equations of this phase. Opx_SS is not a pure enstatite, but forms a complicated solid solution En+Jd+Mg-Ts. With jadeite content increase in the system olivine-bearing assemblages transfer into non-olivine ones, up to the assemblage Cpx + Grt (jadeite-clinopyroxene has also enstatite component) being indirect evidence of a peritectic character of solidus in this system) what has also been shown earlier in (Litvin et al., 2004). Due to this fact forsterite vanishes in subsolidus in the region of compositions rather enriched by jadeite component. The performed experimental investigations testify to complex topological relations of phases in this system at close solidus temperatures what needs further studies. The experimental investigations done earlier and referring to this system (Litvin et al., 2004; Gasparik & Litvin, 1997) testify to the appearance of a new phase of the composition Na₂Mg₂Si₂O₇, which role in the formation of subliquidus and subsolidus assemblages must be more studied.

Nevertheless, the obtained preliminary experimental data contain constructive data that make it possible to consider the basic problem of this work and start experimental investigations of liquidus phase relations of the system forsterite-diopside-jadeite.

The system forsterite-diopside.

The experiments in this section are given at pressure of 7 GPa in the range of temperatures 1600-1700⁰C. The system is pseudobinary due to the appearance of orthopyroxene component that forms an independent phase. According to the preliminary data liquidus assemblages of this system at 7 GPa are Fo + L É Di_SS + L, but the type of melting is eutectic. It agrees with the above investigations at pressure of 3 GP (Davis, 1963) where some pseudobinarity of the system forsterite-diopside caused by the appearance of orthopyroxene minal in clinopyroxene solid solution can be also seen.

The system forsterite-jadeite-diopside.

The experimental data and conclusions obtained for the boundary systems make it possible to start investigating liquidus surface for fusibility diagram of the ternary system forsterite-jadeite-diopside at P 7 GPa. For the experimental study polythermic sections of forsterite-(jadeite₅₀diopside₅₀) and forsterite-(jadeite₂₅diopside₇₅) have been chosen. The obtained data testify to the fact that olivineš vanishing and garnet formation are realized in both sections. The problem of further investigations is to search minimum concentrations of jadeite in the composition of this system where a total olivine vanishing takes place.

Thus, the performed experimental investigations of the model system forsterite-diospside-jadeite at pressure 7 GPa testify to the fact that forsterite (olivine) is a stable phase in the boundary system forsterite-diopside (olivine-clinopyroxene). While introducing rather low contents of jadeite component into the composition of this system the reaction of jadeite component with forsterite takes place in the melt. As a result, garnet appears as liquidus phase.

With the increase of the jadeite component concentration in the system the field of liquidus garnet expands, but a physico-chemical control of crystallization differentiation of the remnant melts transforms from the monovariant cotectics Fo + Di_SS + L through the invariant peritectic point Fo + Di_SS + Grt + L to the monovariant cotectics Grt + Cpx + L, which is responsible for crystallization of bimineral garnet-omphazite eclogite parageneses. The obtained experimental results testify unambiguously to the fact that in the system Fo-Di-Jd a physico-chemical mechanism of overcoming liquidus peridotite-eclogite barrier at mantle magma differentiation is realized. Thus, a gradual transition from olivine-bearing assemblages to those close by their characteristics to bimineral eclogites is provided.

Support RFBR: grants 07-05-00499, 08-05-00110, grant to the leading scientific school 5367.2008.5 (A.A. Marakushev), President grant MK-194.2008.5.

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