L. L. Perchuk

Crystal-melt equilibria involving potassium-bearing clinopyroxene as indicator of mantle-derived ultrahigh-potassic liquids: an analytical review

Abstract Crystal–liquid equilibria, including phase relationships of minerals with silicate and/or carbonate melts, are reviewed in order to understand the occurrence of clinopyroxene with up to 2 wt.% K 2 O ( KCpx ). This mineral occurs as inclusions in diamond from kimberlite pipes and in garnet from garnet–clinopyroxene potassium-poor silicate rocks intercalated with diamondiferous silicate–carbonate rocks of the Kokchetav Complex, northern Kazakhstan. The analysis of the available experimental data allowed estimation of the effect of P , T and compositional parameters on the equilibrium of KAlSi 2 O 6 (in Cpx )=1/4K 4 Si 2 O 6 +3/4Al 4/3 Si 2 O 6 (in melt ) in various silicate systems. A strong dependence of the K 2 O partition coefficient both on pressure and SiO 2 and Al 2 O 3 contents in the melt was identified and thermodynamically described. The resulting thermodynamic equation allows the calculation of pressure of the KCpx formation within interval 50–100 kbar for known melt compositions. A model for the formation of KCpx in deep-mantle potassium-rich carbonate–silicate magmas was derived using the available experimental and petrologic data. The formation of KCpx from any potassium-poor (K 2 O K p Cpx / L =K 2 O in Cpx /K 2 O of melt is much smaller then 1. The presence of Cpx with 1 wt.% of K 2 O as inclusions in Grt from potassium-poor rocks could be explained only by the crystallization of KCpx from potassium-rich silicate or silicate–carbonate magma in the presence of KCl brine at P >70 kbar. This chemical zoning of potassium-bearing clinopyroxene suggests its crystallization together with feldspar from silicate melt during rapid ascent of melt toward the Earths surface from depth about 200 km. Since Tschermak-type substitution has not been observed in either pyroxene, we suggested that feldspar resulted from the peritectic reaction KAlSi 2 O 6 +[SiO 2 ] L / fl =KAlSi 3 O 8 at relatively shallow mantle levels. A hypothetical phase diagram for the system Ca(Mg,Fe)Si 2 O 6 –KAlSi 2 O 6 is presented.

Two-dimensional numerical modeling of pressure–temperature-time paths for the exhumation of some granulite facies terrains in the Precambrian

Abstract Pressure ( P )–temperature ( T ) paths accurately record movement of metamorphic rocks within the Earth’s crust. 1D geodynamic modeling of P – T -time paths of regional metamorphism have explained many important features of the P – T evolution of metamorphic rocks. Further progress may be achieved using 2D numerical geodynamic modeling. Different types of P – T paths obtained for some granulites allow their 2D numerical modeling in terms of a theory of gravitational redistribution of material within the Earth’s crust. This modeling was done using the finite differences method for a Newtonian incompressible fluid. The P – T paths obtained geothermobarometrically for metapelites from the Limpopo granulite complex were compared with the P – T -time paths calculated numerically by monitoring the movement of different ‘samples’ in the thermal and gravitational field. The results of this monitoring support a hypothesis of isobaric cooling of granulites during the thermal interaction between rising hot granulites and sinking cool cratonic rocks. Effective viscosities of the rocks were gradually adjusted during the modeling by using the shape of the P – T paths. The calculated average viscosity of 10 19 Pa s for the granulites, supports an idea that the lower crust is a weak layer between upper crust (10 21 Pa s) and the Upper Mantle. The average rate of exhumation of granulites during the gravitational redistribution process is estimated at about 2.5 mm per year. This suggests exhumation of granulites from depths of about 30 km during approximately 10 Myr. The results of numerical modeling successfully explain the presence of two types of P – T paths observed in metapelites of the Limpopo granulite complex emplaced between the >3 Ga Kaapvaal and Zimbabwe cratons at ∼2.65 Ga.

Inherent gravitational instability of hot continental crust: Implications for doming and diapirism in granulite facies terrains

Modeling of in situ rock properties based on a Gibbs free energy minimization approach shows that regional metamorphism of granulite facies may critically enhance the decrease of crustal density with depth. This leads to a gravitational instability of hot continental crust, resulting in regional doming and diapirism. Two types of crustal models have been studied: (1) lithologically homogeneous crust and (2) heterogeneous, multilayered crust. Gravitational instability of relatively homogeneous continental crust sections is related to a vertical density contrast developed during prograde changes in mineral assemblages and the thermal expansion of minerals with increasing temperature. Gravitational instability of lithologically heterogeneous crust is related to an initial density contrast of dissimilar intercalated layers enhanced by high-temperature phase transformations. In addition, the thermal regime of heterogeneous crust strongly depends on the pattern of vertical interlayering: A strong positive correlation between temperature and the estimated degree of lithological gravitational instability is indicated. An interrelated combination of two-dimensional, numerical thermomechanical experiments and modeling of in situ physical properties of rocks is used to study the processes of gravitational redistribution within a doubly stacked, heterogeneously layered continental crust. It is shown that exponential lowering of viscosity with increasing temperature, in conjunction with prograde changes in metamorphic mineral assemblages during thermal relaxation after collisional thickening of the crust, provide positive feedback mechanisms leading to regional doming and diapirism that contribute to the exhumation of high-grade metamorphic rocks.

Thermal regime and gravitational instability of multi-layered continental crust : Implications for the buoyant exhumation of high-grade metamorphic rocks

Large-scale crustal thickening by tectonic and/or magmatic processes can lead to various complex patterns of multi-layered continental crust. It is well-known from one-dimensional thermal modeling that variations in total crustal thickness, mantle lithospheric thickness, thermal conductivities of the crust, and bulk radiogenic heat production of the crust will lead to variable geotherms in such heterogeneously stacked crust. By systematically changing the above parameters, we illustrate that variations on the order of 100–500°C will result at a depth of 30 km. Specifically, we show that geotherms are also strongly dependent on the pattern of vertical interlayering. Assuming a crustal structure composed of idealized granodioritic/gabbroic or granodioritic/dioritic compositional layer sequences, it can be shown that such gravitationally unstable, stacked, multi-layered continental crust can lead to temperature variations in geotherms of comparable magnitude as for the above parameters. Geotherms exhibiting the highest temperatures at a given depth are characteristic for gravitationally unstable structures in which the bulk of the granodioritic rocks underlie dioritic or gabbroic rocks. Thus a strong positive correlation between temperature and the estimated degree of gravitational instability of the multilayered crust is indicated. It is argued that the lowering of the viscosity of rocks with increasing temperature after tectonic or magmatic stacking will set the stage for processes of gravitational redistribution and buoyant exhumation of high-grade metamorphic rocks. Prograde changes in metamorphic mineral assemblages and partial melting during thermal relaxation after stacking provide positive feed-back mechanisms to enhance the possibility of gravitational redistribution. In keeping with the published results of Babeyko & Sobolev (2001) and Arnold et al. (2001), we find that gravitational overturn can be triggered only when external tectonic forces are active after stacking. Time-scales of 10 to 100 Myr are indicated for differential movement of rock masses on a kilometer-scale when the viscosity of crustal rocks is lowered to n × 10 21 Pa·s, but may be considerably less if zones of tectonic weakness in the crust lead to a further local decrease in effective rock viscosity.

Fluid control of charnockitization

Abstract The reactions Bt + Qtz = Opx + Kfs + H2O, Bt1 + Qtz + (K2O) = Or + Bt2 +(n H2O), Mg-Ts(Opx) + Otz + (K2O) = Or + En, Pl1 + (K2O) = Pl2 + Or + (Na2O) and Prg + En + Qtz + (K2O) = Or + Ed recorded in unique rock textures and mineral compositions from gneissic complexes of some granulite facies terrains have been studied. It is shown that chemical potentials of the perfectly mobile CO2, H2O and K2O in a metamorphic fluid govern the charnockitization process along the retrograde P-T paths typical of granulite facies rocks. The sensors indicating the contribution of thermodynamic parameters to the process are discussed. In many complexes charnockitization took place at a aflH2O similar to or even higher than that for initial gneisses. The origin of arrested charnockites in the Precambrian gneissic complexes is considered to be CO2/1bH2O fluid/gneiss interaction, which causes migration of the majority of the basic components (MgO, FeO, CaO, TiO2 etc.) from initial metamorphic rocks toward a transition zone; the scale of the migration depends strongly on the intensity of the interaction. The migration of the elements from a given rock volume propagates the shift of its bulk composition toward the charnockite (Kfs + Qtz + Fs eutectic and subsequent melting. This model may explain the existence of zoned metabasite-enderbite-charnockite complexes of Precambrian granulite facies terrains. The arrested charnockites are a small-scale model for the formation of these complexes.

Ultrapotassic clinopyroxene from the Kumdy-Kol microdiamond mine, Kokchetav Complex, Kazakhstan: Occurrence, composition and crystal-chemical characterization

Abstract We report data on the composition and crystal structure of the most K-rich (3.61 wt% K2O) natural clinopyroxene yet discovered. The studied crystal was found as a tiny inclusion in garnet from a garnet-clinopyroxene rock of the Kumdy-Kol microdiamond mine, Kokchetav complex, Northern Kazakhstan. Microprobe analysis yields the formula (Ca0.61Fe0.13Mg0.04Mn0.01K0.17Na0.05) (Al0.61Mg0.39)(Si1.61Al0.39)O6.00. Lattice parameters are: a = 9.773(1), b = 8.926(1), c = 5.269(1) Å, β = 105.75(1)°. The structure was refined up to Rall = 2.42% using 982 independent reflections. Substitution of K for Ca causes significant modification of the average structure. No evidence for an additional M2’ position was found. Crystal-chemical characteristics are compared with published data on both natural and synthetic K-bearing clinopyroxenes.

Ultrahigh potassium content in the clinopyroxene structure: an X-ray single-crystal study

Thecrystalstructuresandchemicalcompositionsoftwocrystalsofclinopyroxene(labelledK1andK2)synthesizedinthe model system CaMgSi2O6 - KAlSi2O6 at 7 GPa, have been investigated. Quantitative analysis leads to the following formulae: (Ca0.75K0.23Mg0.02)(Mg0.73Al0.27)(Si1.97Al0.03)O6.00and (Ca0.90K0.07Mg0.03)(Mg0.86Al0.14)(Si1.94Al0.06)O6.00, for K1 and K2 respectively. Latticeparametersare:a =9.803(2),b=8.985(2),c =5.263(1)A, =105.69(1)° forK1anda =9.744(1),b=8.904(2),c =5.273(1) A, b = 106.14(1)° for K2. The structures were refined to Rall = 1.16 and 2.02 %, respectively, using 982 independent reflections. Substitution of K for Ca causes strong modifications to the average structure, mainly a lengthening of the M2-O3C1 bond distance and a shortening of the T-O3A1 bond distance. The crystal K1 shows the largest volume for a M2 polyhedron ever reported for the clinopyroxene structure withdivalentcations inthissite. No evidence was found fora smallcation such as Mg coexistingwithCa in the M2 site that might act as a structural stabilizer for the accommodation of K in the clinopyroxene structure.

P-T paths and problems of high-temperature polymetamorphism

Many Precambrian granulite-facies metamorphic complexes contain so-called straight gneisses, which are massive rocks with a clearly pronounced blastomylonitic texture, lineation, and gneissosity. These rocks occur exclusively in high-temperature ductile shear zones, which can develop either during the primary exhumation of rock complexes or during the overprinting by high-temperature dynamometamorphism. The main criterion for distinguishing between these two types of straight gneisses is the configuration of their P-T trajectories, which are recorded in the mineral assemblages in these rocks and their host gneisses. Ductile shear zones developed in Archean granulite gneisses simultaneously with their exhumation, and, hence, their P-T trajectories are segments of decompression and/or isobaric cooling paths. Straight gneisses in Proterozoic polymetamorphic complexes commonly compose high-temperature ductile shear zones overprinted on Archean granulite complexes, and the P-T paths of these rocks are Z-shaped. This means that, at a constant pressure in the middle part of the continental crust, the Tmin of the older P-T trajectory corresponded to Tmax of the younger trajectory, and often Tmax–Tmin > 100°C. Such ductile shear zones commonly have a strike-slip morphology and can be easily seen in aerial photographs and discerned during structural geological surveying. These zones can overprint older gneisses without any notable thermal effect on the latter. Relations of this type were identified in the granulite complexes of Limpopo in South Africa, Sharyzhalgai in the southwestern Baikal area, and Lapland in the Kola Peninsula. The results of our research propose a solution for the well-known problem of the significant discrepancies between the isotopic ages in high-temperature-high-pressure complexes and the partial or complete distortion of radiogenic isotopic systems under the effect of a newly inflowing metamorphic fluid. The application of geochronologic techniques to these situations is senseless, and only P-T trajectories provide insight into the actual age relations between the discrete tectono-metamorphic stages. It is thus expedient to conduct not only structural studies of metamorphic complexes but also their detailed petrological examination and the calculation of their P-T paths before geochronologic dating.

Synthetic hypersilicic Cl-bearing mica in the phlogopite-celadonite join : A multimethodical characterization of the missing link between di-and tri-octahedral micas at high pressures

Abstract A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200-1250 °C, close to the solidus of the join diopside-jadeite-KCl, in association with diopside-jadeite pyroxene, K-rich aluminosilicate glass and/or sanidine and (K,Na)Cl. The mica shows a negative correlation between tetrahedral Si and octahedral (Al + Mg), suggesting an Al-celadonitic substitution (Si + VIAl + VI □ = IVAl + VIMg) and a chemical formula: K1.01(Mg2.45Al0.19□ 0.35)Σ=3(Si3.52Al0.48)Σ=4O10[(OH,O)1.66Cl0.34)]Σ=2. The presence of hydroxyl was confirmed by OH stretching modes at 3734 and 3606 cm-1 in the Raman spectra. Singlecrystal X-ray diffraction data provide the unit-cell parameters (space group C2/m, 1M polytype): a = 5.299(4), b = 9.167(3), c = 10.226(3) Å, β =100.06(4)°, V = 489.1(4) Å3. The structure refinement shows the presence of vacancies on the octahedral sites (15% for M1 and 6.5% for M2). Chlorine occupies a position about 0.5 Å from O4 with partial occupancy (0.39 apfu). Crystal-chemical mechanisms seem to govern chlorine incorporation in mica, since a large A site is necessary to locate the anion in the structure. A large A site results when the six-tetrahedra ring is hexagonal and the tetrahedral rotation angle α is 0°. Such a geometry is achieved either by increasing the annite component in biotite or by increasing the hypersilicic character of phlogopite through the Al-celadonite substitution. The present Si-rich mica shows a partial dioctahedral character due to the Al-celadonite substitution, which lowers the α angle and expands its stability field at high pressure. High aK₂O conditions, like in potassium-rich brine or potassic carbonatitic melts, increase the Alceladonite component in the phlogopite solid solution, explaining the association of Si-rich micas with inclusions of potassic liquids in kimberlitic diamonds.

Compressibility of synthetic potassium-rich clinopyroxene: In-situ high-pressure single-crystal X-ray study

Abstract The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca0.88K0.12)(Mg0.83Al0.17)(Si1.98 Al0.02)O6, was studied using high-pressure single-crystal X-ray diffraction methods. A four-pin diamond anvil cell with 4:1 methanol:ethanol pressure medium was used to achieve pressures to 9.72 GPa. Unit-cell data were measured at 17 pressures, and intensity data were collected at 6 pressures. Fitting the P-V data to the third-order Birch-Murnaghan equation of state yields V0 = 435.49(3) Å3, K0= 129(1) GPa, K’ = 2.7(3). Anisotropic compression was observed with unit strain axial ratios of 1:1.94:1.90. Unit-cell parameters decrease gradually as a function of pressure with axial compressibilities βb > βc ~ βa. They match those found for kosmochlor but are stiffer than those observed for synthetic diopside and hedenbergite. Compressibilities of the bond distances within the M2, M1, and T polyhedra show significant anisotropy. The incorporation of K into the clinopyroxene structure has little effect on its compressibility, although the concomitant substitution of Al in M1 from the K-Jd component reduces its compressibility. The K atom is softer than the M2 polyhedron and thus shrinks enough at high pressure to fit into the pyroxene structure.