25 research outputs found
Thermal history of nakhlites: A comparison between MIL 03346 and its terrestrial analogue Theo’s flow
No full textHigh resolution single-crystal X-ray diffraction (HR-SCXRD) and Mössbauer spectroscopy of the intracrystalline cation distribution have been performed on augitic core-crystals from a Miller Range nakhlite (sample MIL 03346,13) with approximate composition of En36Fs24Wo40. The Mössbauer data on the single-crystal yielded a very low Fe3+ content [Fe3+/Fetotal - 0.033(23)a.p.f.u.] that, together with the Electron microprobe analysis (EMPA) and the X-ray structural data allowed us to obtain the accurate cation site distribution and the Fe2+-Mg degree of order. This leads to a closure temperature (Tc) of 500 with a standard deviation of ±100°C that would correspond to a slow cooling rate, which is in disagreement with petrologic evidence that indicates that this sample originates from a fast cooled (~3-6°C/h) lava flow.In order to clarify this discrepancy we undertook (i) a SC-XRD study of an augite (~En49Fs9 Wo42) from a pyroxenite (TS7) of Theo's flow, a 120-m-thick lava flow regarded as a terrestrial analogue of MIL 03346; (ii) an annealing experiment at 600°C on a crystal from exactly the same fragment of MIL 03346. SC-XRD data from TS7 augite yields a Tc=600(20)°C, consistent with the cooling rate expected at 85m below the surface. This Tc is higher, although similar within error, to the Tc=500(100)°C obtained for MIL 03346; thus suggesting relatively slower cooling for MIL 03346 with respect to TS7. The annealing experiment on the MIL 03346 crystal clearly showed that the degree of order remained unchanged, further confirming that the actual Tc is close to 600°C.This result appears inconsistent with the shallow depth of origin (~<2m) assumed for MIL 03346, further supporting the discrepancy between MIL 03346 textural and petrologic evidence of fast cooling and the abovementioned Tc results obtained for augite. Therefore, a tentative scenario is that, soon after eruption and initial quench and while still at relatively high-T (~600°C), MIL 03346 was blanketed with subsequent lava flows that slowed down the cooling rate and allowed the augite Fe2+-Mg exchange reaction to proceed
Coupling between non-convergent ordering and transition temperature in the C2/c <-> P2(1)/c phase transition in pigeonite
No full textA Landau potential with linear-quadratic coupling has been developed to describe interactions between a non-convergent order parameter, Q(OD), for Fe/Mg ordering, and the order parameter, Q(D), for the C2/c-P2(1)/c phase transition in pigeonite. Spontaneous strain relationships, and expressions for the effect of ordering on the transition temperature derived from this expansion, have been tested by single crystal X-ray diffraction methods. Lattice parameters collected from a natural pigeonite crystal with composition En(47)Fs(44)Wo(9), in situ at temperatures up to 1050 degreesC, reveal that increasing Q(OD) could act to suppress Q(D) by a mechanism which includes overlapping and opposing strain fields. In a second experiment, the intensities of superlattice reflections (h + k = 2n + 1) were followed in situ at temperatures up to 500 degreesC. The crystal was heated ex situ successively at 700, 750, 800, and 850 degreesC between repeated in situ measurements in order to produce changes in the degree of cation order. The resulting data sets, giving the temperature dependence of Q(D)(2) for different fixed values of Q(OD), are consistent with the initial Landau model. In particular, they show a strong and linear dependence of transition temperature on Q(OD). The fourth order coefficient of the expansion describing the phase transition is perhaps also renormalized by changes in Q(OD). It is suggested that the influence of Q(OD) on the phase transition could be greater than the influence of the phase transition on the equilibrium variation of Q(OD)
The high-temperature P2(1)/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6) : structural and thermodynamic behavior
No full textA high-temperature in situ single-crystal X-ray diffraction study was performed from room T to 1150 °Con two crystals of Fe-free P2(1)/c clinopyroxenes of composition Ca0.15Mg1.85Si2O6 [cell parameters at room T: a = 9.651(2) Angstrom, b = 8.846(2) Angstrom, c = 5.202(1) Angstrom, beta = 108.38(2)degrees, V = 421.4 (2) Angstrom(3)] synthesized by isothermal annealing for 624 h at T = 1370 °C, P = 1 atm. A First order P2(1)/c-C2/c phase transition was found slightly below 1000 degreesC [T-c = 926(39) °C]. The transition was revealed by discontinuous changes in intensities and cell parameters. Prolonged heating at high temperature induced a non-reversible increase in the transition temperature up to more than 1150 °C, without apparent changes in the order of the phase transition. Coupling with strain due to incipient exsolution in a formerly almost defect-free sample is suggested to be responsible for increase in T-c. TEM observations of a sample from the same starting material after further annealing for 72 h at T = 1050 °C, P = 1 atm are consistent with the proposed incipient exsolution model. Annealing was found to induce the formation of a mottled texture oriented parallel to (101).
Results from structure refinement of data collected below the transition at T = 25, 500, 650, 800, and 1000 °C showed only minor changes in the chain configurations, which tire highly differentiated up to 1000 °C, confirming the strong First-order character of the transition
Intracrystalline “geothermometry” assessed on clino and orthopyroxene bearing synthetic rocks
No full textRecent discussion on the application of intracrystalline “geothermometers” based on the Fe–Mg order-disorder reaction in pyroxene in natural rocks, indicates that the available calibration equations for clino and orthopyroxenes (cpx and opx), which express the equilibrium intracrystalline Fe–Mg distribution coefficient kD(Fe–Mg) as a function of temperature, require independent validation. In this paper, we tested the available experimental calibrations for clino and orthopyroxenes by determining the site occupancies of these minerals in synthetic samples grown from a hydrous nepheline basanite in a piston-cylinder apparatus at 1050 °C at 2.0 GPa to 1170 °C at 3.0 GPa, and quenched very rapidly by shutting off the power. The site occupancies were determined by single crystal X-ray diffraction (SC-XRD) and used to calculate the closure temperature, TC, of cation ordering using available calibrations of kD(Fe–Mg) vs. T. The calculated TC values of both clino and orthopyroxenes were found to be close to the temperatures at which they were quenched, in line with expected kinetic behavior, when calibrations for cpx (Murri et al., 2016) and opx (Stimpfl et al., 2005) based on SC-XRD structure refinements were used. In particular, the smallest discrepancy between calculated and actual temperature is of the order of a few degrees (12 °C for cpx and 4 °C for opx), and the largest is of the order of tens of degrees (22 °C for cpx and 55 °C for opx). On the other hand, much lower TCs were obtained when calibrations based on Mössbauer determination of site occupancies were used. These results confirm that the two methods (i.e. SC-XRD and Mössbauer) give inherently different site occupancy data and that the same methodology should thus be used for both calibration and natural samples in the determination of cooling rate of host rocks
New augite geothermometer for nakhlites
No full textIntroduction: nakhlites, together with shergottites and chassinites constitute the “SNC” group of Martian meteorites. MIL 03346 is a nakhlite, that was found at Miller Range, in Antarctica and is mainly composed of clinopyroxene (79%), with minor olivine (1%), and 20% vitrophyric intercumulus material [1,2]. The clinopyroxene is augite with a homogeneous core (En36Fs24Wo40) and an iron enriched rim (En8Fs64Wo28). Since the first modern petrologic studies of Nakhla, the nakhlite group of meteorites has been interpreted as augite-rich cumulate igneous rocks, derived from basaltic magma, that erupted onto the surface of Mars [3,4]. The relative low closure temperature (Tc) of ca. 500 (±100)°C calculated for MIL 03346 by [5] with the available geothermometer [6] would correspond to a slow cooling rate that is in disagreement with the petrologic evidence for an origin from a fast cooled lava flow.
Geothermometer calibration: ex-situ annealing experiments combined with high-resolution single-crystal X-ray diffraction (HR-SC-XRD) on crystals from MIL 03346 clearly showed that the degree of order remained unchanged at 600°C thus suggesting that the actual Tc is close to this temperature. We then undertook an ex situ annealing experiments combined with HR-SC-XRD at 600, 700, 800 and 900 °C until the equilibrium in the Fe2+-Mg exchange on two crystals from exactly the same fragment of MIL 03346 sample in order to obtain a new geothermometer for augites from Martian nakhlites:
lnkD = -4421(±561)/T(K) + 1.46(±0.52) (R2=0.988)
where kD = [(Fe2+M1)(MgM2) / (Fe2+M2)(MgM1)]
Results: applying this new equation to MIL 03346 the closure temperature resulted to be 605(84)°C about 100°C higher than that calculated with previously available calibration (i.e. Brizi et al. 2000). However, this closure temperature of 605°C is still lower than expected based on petrologic evidences and suggests that MIL 03346 clinopyroxene bears record of a thermal event that postdates, or is superimposed on, the natural cooling path of the host lava flow
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
No full textA 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 TTr = 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 kD versus 1/T yielded the following equation: In KD = -3717(±416)/T(K)+1.290(±0.378); (R2 = 0.988). The closure temperature (Tc) 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 C0Kdis+ for all three temperatures yielding the following Arrhenius relation: In(C0K+dis) = In K0 - Q/(RT) = 20.99(±3.74) -26406 (±4165)/T(K); (R2 = 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 XFe (in the range of 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
Carbon polymorphs in Frontier Mountain ureilitic meteorites: A correlation with increasing the degree of shock?
No full textUreilites meteorite fragments present different levels of shock classified on the basis of optical observations of shock features in silicates. We have investigated by scanning electron microscopy (SEM), micro X-Ray Diffraction (XRD) and Micro-Raman Spectroscopy (MRS) five ureilitic fragments (FRO 95028, FRO 01089, FRO 97013, FRO 01088 and FRO 01012) with increasing degree of shock (from S2 to S6) with the purpose to observe if there is any correlation between the level of shock recorded by silicates and the presence of the different carbon polymorphs formed by shock. XRD patterns showed that all investigated samples contain nano-graphite. Besides this phase, sample FRO 95028, with S2 degree of shock, contains nanodiamond, while samples from level of shock S3 to S6 contain both nano-and micro -diamond. XRD results support the shock formation of micrometer-diamonds found in FRO 01089, FRO 97013, FRO 01088 and FRO 01012 with the assistance of (Fe, Ni)-alloys as catalysts at pressure >10 GPa (S3 shock level recorded by silicates). The formation of polycrystalline diamond is already allowed at pressure between 5-10 GPa. Temperature estimated by a graphite-thermometer based on MRS data, provided values in the range of 1291 to 1398 degrees C +/- 120 degrees C, revealing that there is not a considerable variation of the graphite temperature with the increasing degree of shock. (c) 2023 Elsevier B.V. All rights reserved
A TEM study of Ca-rich orthopyroxenes with exsolution products : implications for Mg-Fe ordering process
No full textOrthopyroxene from the Johnstown diogenite contains exsolution products. It shows anomalous kinetics for disordering Fe-Mg distribution, and the application of kinetic constants extracted from the disordering experiments carried out on Johnstwon orthopyroxene leads to aberrant cooling rates without physical meaning. Crystals of this orthopyroxene were studied by several transmission electron microscopy (TEM) techniques in order to elucidate the nature of the exsolution products, their topology, chemistry and probable relationship to kinetics. For comparison this study also included a terrestrial granulitic orthopyroxene with known "normal" disordering kinetics, and a Carich orthopyroxene of volcanic origin which was supposed to be free of exsolution products due to fast quenching. The TEM results show that all studied samples contain very fine clinopyroxene exsolution lamellae (several unit-cell wide). Additionally, only Johnstown orthopyroxenes contain single unit-cell wide Guinier-Preston zones. These represent a chemically distinct pyroxene and their distribution in Johnstown is pervasive. They are still present after annealing experiments at the temperatures of kinetic studies. This high density of defects is proposed to be responsible for the anomalous kinetic behaviour
A new calibration to determine the closure temperatures of Fe-Mg ordering in augite from nakhlites
No full textRecently it has been shown that the relatively low closure temperature (Tc) of 500 (100)°C calculated for augite from Miller Range nakhlite (MIL 03346,13) using the available geothermometers would correspond to a slow cooling rate inconsistent with the petrologic evidence for an origin from a fast-cooled lava flow. Moreover, previous annealing experiments combined with HR-SC-XRD on an augite crystal from MIL 03346 clearly showed that at 600 °C, the Fe2+-Mg degree of order remained unchanged, thus suggesting that the actual Tc is close to this temperature. In order to clarify this discrepancy, we undertook an ex situ annealing experimental study at 700, 800, and 900 °C, until the equilibrium in the intracrystalline Fe2+-Mg exchange is reached, using an augite crystal from Miller Range nakhlite (MIL 03346,13) with a composition of about En36Fs24Wo40. These data allowed us to calculate the following new geothermometer calibration for Martian nakhlites: lnkD=-4421(±561)/T(K)+1.46(±0.52)(R2=0.988),where KD=[(FeM12+)(MgM2)/(FeM22+)(MgM1)]. The application of this new equation to other Martian nakhlites (NWA 988 and Nakhla) suggests that for augite with composition close to that of MIL 03346, the Tc is up to 170 °C higher with respect to the one calculated using the previous available geothermometer equation, thus suggesting a significantly faster cooling in agreement with petrologic evidence
Volume thermal expansion along the jadeite–diopside join
No full textAn in situ single-crystal high-temperature X-ray diffraction study was performed on clinopyroxene crystals along the jadeite, (NaAlSi2O6 Jd)–diopside (CaMgSi2O6 Di) join. In particular, natural samples of jadeite, diopside, P2/n omphacite and three C2/c synthetic samples with intermediate composition (i.e., Jd80, Jd60, Jd40) were investigated. In order to determine the unit-cell volume thermal expansion coefficient (αV), the unit-cell parameters for all these compositions have been measured up to c.a. 1,073 K. The evolution of the unit-cell volume thermal expansion coefficient (αV) along the Jd–Di join at different temperatures has been calculated by using a modified version of the equation proposed by Holland and Powell (J Metamorph Geol 16(3):309–343, 1998). The equation aV(303K,1bar) = 2.68(3) × 10-5 + [1.1(1) × 10-8 × XJd]-[7.1(1.7) × 10-10 × X2 Jd] obtained from the αV at room-T (i.e., αV303K,1bar) allows us to predict the room-T volume thermal expansion for Fe-free C2/c clinopyroxenes with intermediate composition along the binary join Jd-Di. The observed αV value for P2/n omphacite αV(303K,1bar) = 2.58 (5) × 10-5 K-1 was compared with that recalculated for disordered C2/c omphacite published by Pandolfo et al. (Phys Chem Miner 1–10, 2012) [αV(303K,1bar) = 2.4(5) × 10-5 K-1]. Despite the large e.s.d.’s for the latter, the difference of both values at room-T is small, indicating that convergent ordering has practically no influence on the room-T thermal expansion. However, at high-T, the smaller thermal expansion coefficient for the C2/c sample with respect to the P2/n one with identical composition could provide further evidence for its reduced stability relative to the ordered one
