Exsolved Augite Research Articles

HT P21/c–C2/c phase transition and kinetics of Fe2+–Mg order–disorder of an Fe-poor pigeonite: implications for the cooling history of ureilites

A natural Ca-poor pigeonite (Wo6En76Fs18) from the ureilite meteorite sample PCA82506-3, free of exsolved augite, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P21/c, was annealed up to 1,093°C to induce a phase transition from P21/c to C2/c symmetry. The variation with increasing temperature of the lattice parameters and of the intensity of the b-type reflections (h + k = 2n + 1, present only in the P21/c phase) showed a displacive phase transition P21/c to C2/c at a transition temperature T Tr = 944°C, first order in character. The Fe–Mg exchange kinetics was studied by ex situ single-crystal X-ray diffraction in a range of temperatures between the closure temperature of the Fe–Mg exchange reaction and the transition temperature. Isothermal disordering annealing experiments, using the IW buffer, were performed on three crystals at 790, 840 and 865°C. Linear regression of ln k D versus 1/T yielded the following equation: $$ \ln \,k_{\text{D}} = - 3717( \pm 416)/T(K) + 1.290( \pm 0.378);\quad (R^{2} = 0.988) $$ . The closure temperature (T c) calculated using this equation was ∼740(±30)°C. Analysis of the kinetic data carried out taking into account the e.s.d.'s of the atomic fractions used to define the Fe–Mg degree of order, performed according to Mueller’s model, allowed us to retrieve the disordering rate constants C 0 K dis + for all three temperatures yielding the following Arrhenius relation: $$ \ln \left( {C_{0} K_{\text{dis}}^{ + } } \right) = \ln \,K_{0} - Q/(RT) = 20.99( \pm 3.74) - 26406( \pm 4165)/T(K);\quad (R^{2} = 0.988) $$ . An activation energy of 52.5(±4) kcal/mol for the Fe–Mg exchange process was obtained. The above relation was used to calculate the following Arrhenius relation modified as a function of X Fe (in the range of X Fe = 0.20–0.50): $$ \ln \left( {C_{0} K_{\text{dis}}^{ + } } \right) = (21.185 - 1.47X_{\text{Fe}} ) - {\frac{{(27267 - 4170X_{\text{Fe}} )}}{T(K)}} $$ . The cooling time constant, η = 6 × 10−1 K−1 year−1 calculated on the PCA82506-3 sample, provided a cooling rate of the order of 1°C/min consistent with the extremely fast late cooling history of the ureilite parent body after impact excavation.

Kinetics of Fe2+-Mg order-disorder in P21/c pigeonite

The kinetics of the Fe-Mg intracrystalline exchange reaction in P21/c pigeonite (Wo10En47Fs43) free of exsolved augite, from the Paraná rhyodacite sample BTS308, was studied by single-crystal X-ray diffraction (XRD). Isothermal disordering annealing experiments, with oxygen fugacity controlled at the IW buffer, were performed on two crystals at 650, 700, 750, and 800 °C until the Fe-Mg exchange equilibrium was reached. The XRD data were collected from the two untreated crystals and after each annealing experiment. Structure refinements were carried out taking into account the recently discovered stronger preference of Mn for the M2 site compared to Fe2+. The linear regression of ln kD* vs. 1/T yielded the following equation:

Petrology of the Yamato nakhlites

Abstract— The Yamato nakhlites, Y-000593, Y-000749, and Y-000802, were recovered in 2000 from the bare icefield around the Yamato mountains in Antarctica, consisting of three independent specimens with black fusion crusts. They are paired cumulate clinopyroxenites. We obtained the intercumulus melt composition of the Yamato nakhlites and here call it the Yamato intercumulus melt (YIM). The YIM crystallized to form the augite rims, the olivine rims and the mesostasis phases in the cumulates. The augite rims consist of two layers: inner and outer. The crystallization of the inner rim drove the interstitial melt into the plagioclase liquidus field. Subsequently, the residual melt crystallized pigeonites and plagioclase to form the outer rims and the mesostasis. Three types of inclusions were identified in olivine phenocrysts: rounded vitrophyric, angular vitrophyric, and monomineralic augite inclusions. The monomineralic augite inclusions are common and may have been captured by growing olivine phenocrysts. The rounded vitrophyric inclusions are rare and may represent the composition of middle-stage melts, whereas the angular vitrophyric inclusions seem to have been derived from fractionated late-stage melts. Glass inclusions occur in close association with titanomagnetite and ferroan augite halo in phenocryst core augites and the assemblages may be magmatic inclusions in augites. We compared the YIM with compositions of magmatic inclusions in olivine and augite. The composition of magmatic inclusions in augite is similar to the YIM. Phenocrystic olivines contain exsolution lamellae, augite-magnetite aggregates, and symplectites in the cores. The symplectites often occur at the boundaries between olivine and augite grains. The aggregates, symplectite and lamellae formed by exsolution from the host olivine at magmatic temperatures. We present a formational scenario for nakhlites as follows: (1) accumulation of augite, olivine, and titanomagnetite phenocrysts took place on the floor of a magma chamber; (2) olivine exsolved augite and magnetite as augite-magnetite aggregates, symplectites and lamellae; (3) the overgrowth on olivine phenocrysts formed their rims, and the inner rims crystallized on augite phenocryst cores; and finally, (4) the outer rim formed surrounding the inner rims of augite phenocrysts, and plagioclase and minor minerals crystallized to form mesostasis under a rapid cooling condition, probably in a lava flow or a sill.

Exsolution and hydration of pyroxenes from partially serpentinized harzburgites

AbstractOrtho- and clinopyroxenes within partially-hydrated harzburgites from Elba and Val di Cecina (Italy) show lamellar exsolution textures and variable replacement by biopyriboles, talc-chlorite-serpentine mixed layers and serpentine. Chemical and geothermometric data suggest that the pyroxenes crystallized at 1240–1051°C, followed by subsolidus exsolution at slightly lower T (1145–1025°C for clinopyroxene lamellae + orthopyroxene matrix pairs and a 1033–982°C range for orthopyroxene lamellae + clinopyroxene matrix pairs).Investigation by transmission electron microscopy of exsolved enstatite and augite reveals a multistage hydration process. The first stage (highest T, probably in the amphibole stability field) leads to the formation of biopyribole lamellae within exsolved augite, leaving the enstatite matrix unaffected. The second stage (~500–300°C) corresponds to the topotactic replacement of enstatite by layer silicates (talc + chlorite + serpentine, with (001)layer silicates parallel to (100)enstatite). Enstatite is also replaced by randomly oriented, poorly crystalline serpentine. The last hydration stage (<300°C) corresponds to the disappearence of augite and recrystallization of serpentine, leading to completely hydrated bastites with random lizardite lamellae, polygonal serpentine and minor chrysotile.

Ca in orthopyroxene: structural variations and kinetics of the disordering process

The kinetics of the disordering process in Ca-rich orthopyroxenes was studied. The substitution of Ca for Mg and Fe in the orthopyroxene structure was investigated in order to evaluate the influence of this cation on the disordering process. Geometrical parameters obtained by X-ray single-crystal diffraction on different Ca-rich orthopyroxenes showed that Ca enters the octahedral M2 site of orthopyroxene, causing an enlargement of this polyhedron. As a consequence, both tetrahedral chains extend to allow matching between the tetrahedral and octahedral layers. The kinetic study was carried out at T = 730°C on an orthopyroxene from the volcanic rock sample L3. This orthopyroxene, with composition Wo 4 En 60 Fs 36 , contains thin exsolved augite lamellae but no Guinier-Preston zones. A series of isothermal annealing runs were performed on a single crystal in the presence of IW buffer followed by quenching. The ordering degree after each annealing run was determined by X-ray single-crystal diffraction. Analysis of the kinetic data using Mueller9s (1967) theory yielded a disordering rate constant of 2.69(8)·10 -2 min -1 . This value is in perfect agreement with that expected for a “normal” kinetic behaviour. The presence of Ca and augite lamellae in the orthopyroxene matrix do not seem to be responsible for the drastic change in the activation energy of the Fe-Mg exchange process observed in the Ca-rich orthopyroxene from the Johnstown meteorite.

Microtextures and crystal chemistry in P 2 1 / c pigeonites

¶A single-crystal X-ray investigation was performed on crystals of P21/c natural pigeonite with varying Ca and Fe* ( = Fe2+ + Mn2+) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae. The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated. The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples. In particular, it is shown that Ca and Fe* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and decrease the difference between the M2–O3 distances.

Orthopyroxene as a geospeedometer: Thermal history of Kapoeta, Old Homestead 001, and Hughes 004 howardites

Meteoritics & Planetary ScienceVolume 35, Issue 4 p. 887-887 Free Access Orthopyroxene as a geospeedometer: Thermal history of Kapoeta, Old Homestead 001, and Hughes 004 howardites M. C. DOMENEGHETTI, Corresponding Author M. C. DOMENEGHETTI Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy[email protected]Search for more papers by this authorG. M. MOLIN, G. M. MOLIN Dipartimento di Mineralogia e Petrologia, Università di Padova, C.so Garibaldi 37, 35122 Padova, ItalySearch for more papers by this authorM. TRISCARI, M. TRISCARI Dipartimento di Scienze della Terra, Università di Messina, Salita Sperone 31, 98166 Messina-S. Agata, ItalySearch for more papers by this authorM. ZEMA, M. ZEMA Centro Grandi Strumenti, Università di Pavia, Via Bassi 21, 27100 Pavia, ItalySearch for more papers by this author M. C. DOMENEGHETTI, Corresponding Author M. C. DOMENEGHETTI Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy[email protected]Search for more papers by this authorG. M. MOLIN, G. M. MOLIN Dipartimento di Mineralogia e Petrologia, Università di Padova, C.so Garibaldi 37, 35122 Padova, ItalySearch for more papers by this authorM. TRISCARI, M. TRISCARI Dipartimento di Scienze della Terra, Università di Messina, Salita Sperone 31, 98166 Messina-S. Agata, ItalySearch for more papers by this authorM. ZEMA, M. ZEMA Centro Grandi Strumenti, Università di Pavia, Via Bassi 21, 27100 Pavia, ItalySearch for more papers by this author First published: 04 February 2010 https://doi.org/10.1111/j.1945-5100.2000.tb01474.xCitations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. REFERENCES F. Wlotzka, Ed. (1992) The Meteoritical Bulletin 73. Meteoritics 27, 477– 483. Citing Literature Volume35, Issue4July 2000Pages 887-887 ReferencesRelatedInformation

Orthopyroxene as a geospeedometer: Thermal history of Kapoeta, Old Homestead 001, and Hughes 002 howardites

Abstract— Single crystals of orthopyroxene from small fragments of the Kapoeta, Old Homestead 001, and Hughes 002 howardites were studied by x‐ray diffraction and microprobe analyses. The Fe‐Mg equilibrium distribution coefficients kD of the crystals were used to calculate the closure temperatures (Tc) using the calibration by Stimpfl et al. (1999). The compositions, the presence of exsolved augite lamellae, and the Tc values (from 365 to 385 °C) obtained for Kapoeta orthopyroxene s suggest that our fragment comes from a diogenitic cumulate clast. The more Fe‐rich composition, the absence of exsolved lamellae, and the higher Tc values (from 583 to 605 °C) measured in the Old Homestead 001 orthopyroxenes suggest that this fragment comes from a cumulitic clast affected by fast cooling at high temperature. For the Hughes 002 orthopyroxenes, close in composition to Old Homestead 001, the different Tc values (339, 358, and 607 °C) recorded by the various crystals and the presence of augite lamellae in the crystals with the lowest Tc support the hypothesis that this howardite sample is an unheated breccia containing a mixture of cumulitic orthopyroxenes with different thermal histories.

Mineralogy and cooling history of the calcium‐aluminum‐chromium enriched ureilite, Lewis Cliff 88774

Abstract— The LEW 88774 ureilite is extraordinarily rich in Ca, Al, and Cr, and mineralogically quite different from other ureilites in that it consists mainly of exsolved pyroxene, olivine, Cr‐rich spinel, and C. The presence of coarse exsolved pyroxene in LEW 88774 is unique because pyroxene in most other ureilites is not exsolved. The pyroxene has bulk Wo contents of 15–20 mol% and has coarse exsolution lamellae of augite and low‐Ca pyroxene, 50 μm in width. The compositions of the exsolved augite (Ca33.7Mg52.8Fe13.5) and host low‐Ca pyroxene (Ca4.4Mg75Fe20.6) show that these exsolution lamellae were equilibrated at 1280 °C. A computer simulation of the cooling rate, obtained by solving the diffusion equation for reproducing the diffusion profile of CaO across the lamellae, suggests that the pyroxene was cooled at 0.01 °C/year until the temperature reached 1160 °C. This cooling rate corresponds to a depth of at least 1 km in the parent body, assuming it was covered by a rock‐like material. Therefore, LEW 88774 was held at this high temperature for 1.2 × 104years. The proposed cooling history is consistent with that of other ureilites with coarsegrained unexsolved pigeonites. Lewis Cliff 88774 includes abundant Cr‐rich spinel in comparison with other ureilites. The range of FeO content of spinels in LEW 88774 is from 1.3 wt% to 21 wt% [Fe/(Fe + Mg) = 0.04–0.6]. The Cr‐rich and Fe‐poor spinel in LEW 88774 has less Fe (FeO, 1.3 wt%) than spinels in other achondrites. We classify this spinel as an Fe, Al‐bearing picrochromite. Most ureilites are depleted in Ca and Al, but this meteorite has high‐Ca and Al concentrations. In this respect, as well as mineral assemblage and the presence of coarse exsolution lamellae in pyroxene, LEW 88774 is a unique ureilite. Most differentiated meteorites are poor in volatile elements such as Zn, but the LEW 88774 spinels contain abundant Zn (up to 0.6 wt%). We note that such a high Zn concentration in spinel has been observed in the carbonaceous chondrites and recrystallized chondrites. This unusual ureilite has more primitive characteristics than most other ureilites.

Dislocation-interface interactions in exsolved augite

Conventional and high resolution transmission electron microscopy has been used to study the effect of interphase boundaries on the deformation of a polyphase aggregate. The aggregate consists of augite which during cooling has exsolved enstatite and pargasite lamellae. It originates from a pyroxenite dyke of the Balmuccia peridotite massif located within the Ivrea Zone (NW-Italy). Deformation of the aggregate may have taken place during tilting of the Ivrea Zone at temperatures of about 650 °C. During deformation lattice dislocations interact with the interphase boundaries. Incorporation of lattice dislocations into the boundary leads to a change of the interface structure. The structural change may be interpreted as a superposition of a subgrain structure on the interface misfit structure. In general, tilt and twist components are produced, with tilts about the common chain axis being largest. Different mechanisms of dislocation-interface interactions are discussed.

Tholeiitic hypabyssal dykes: How many clinopyroxenes?

An augite-subcalcic augite trend is found in the coarse-grained tholeiitic dykes from the Ponta Grossa Arch (Brazil). Clinopyroxene is optically homogeneous but, for clinopyroxene approaching subcalcic augite, TEM/EDS shows heterogeneous exsolution products (lamellar exsolution and spinodal decomposition). The calcium content of this subcalcic augite estimated by single-crystal refinement [0.67 (6) atoms p.f.u.] agrees with the value obtained by TEM/EDS analysis of exsolved augite lamellae. Therefore, subcalcic augite does not exist as a single phase: it results from “contaminated” microprobe analyses arising from associated pigeonite and augite. Evolutive trends based on subcalcic augite compositions should therefore be considered with caution, unless detailed structural and microstructural data are available.

Geothermometry of exsolved augites from the Laramie Anorthosite Complex, Wyoming

Exsolved augite pyroxenes from the ferromonzonite border facies of the ferrosyenite in the Laramie Anorthosite Complex have been studied with the transmission electron microscope and the electron microprobe to determine their exsolution histories. The Lindsley and Andersen (1983) geothermometer gives initial crystallization temperatures of ≥1000° C for the bulk augite crystal (Wo32 En22 Fs46). Exsolved lamellae are predominantly pigeonites with very low calcium contents (Wo1–3 En23–24 Fs71–74) and have formation temperatures estimated to be in the range of 600 to 975° C. The uniform compositions of lamellae and hosts, despite the range in lamellar size and orientation, suggest that either 1) the ferromonzonite experienced an extended plateau in cooling or a reheating event at 600 to 650° C or 2) the pyroxenes recorded a blocking temperature. Two-feldspar geothermometry on exsolved feldspars also records 600° C and suggests that these low temperatures are not blocking temperatures.

Coexisting Ca-poor pyroxenes in the Main Zone of the Bushveld Complex

Electron microprobe analyses of Ca-poor pyroxenes in gabbroic rocks of the Main Zone of the Bushveld Complex reveal that inverted pigeonites have lower Mg/Fe ratios than coexisting hypersthenes. Textural relationships, however, indicate that the two Ca-poor pyroxenes did not crystallize simultaneously from the magma. Early pigeonite reacted with the magma to form hypersthene and the difference in the Mg/Fe ratio of these two pyroxenes reflects the difference of this ratio between early pigeonite and the magma at the time of reaction. Some of the grains of early pigeonite, now inverted to hypersthene, evidently escaped this reaction with the magma. Bulk compositions of pyroxenes intermediate between that of pigeonite and hypersthene are postulated on the grounds of varying amounts of exsolved augite in the hypersthene which has originated from pigeonite by reaction with magma.