Thermal history of nakhlites: A comparison between MIL 03346 and its terrestrial analogue Theo’s flow

Abstract

High resolution single-crystal X-ray diffraction (HR-SCXRD) Analysis (EMPA) and the X-ray structural data allowed us to obtain the accurate cation site distribution and the Fe 2+ -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 MIL03346; 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 Fe 2+ -Mg exchange reaction to proceed.

In-situ high-temperature emissivity spectra and thermal expansion of C2/c pyroxenes: Implications for the surface of Mercury

“…The maximum found in the DiMDCV sample is known for Ca-rich C2/c pyroxenes. It has been observed in previous studies (Rossi et al 1987;Oberti and Caporuscio 1991;Boffa Ballaran et al 1998;Domeneghetti et al 2005Domeneghetti et al , 2013 and ascribed to the possible presence of Mg and Fe 2+ at the M2 site, giving rise to the so-called "M2′ split position". Therefore, a further set of refinements was performed using Mg 2+ and Fe 2+ scattering factors for the isotropic M2′ site.…”

Igneous and shock processes affecting chassignite amphibole evaluated using chlorine/water partitioning and hydrogen isotopes

“…; Domeneghetti et al. ), rapid late cooling of the chassignites is inferred to have taken place in a shallow intrusion (Monkawa et al. ; Mikouchi et al.…”

Volume thermal expansion along the jadeite–diopside join

“…Single-crystal X-ray diffraction (SCXRD) intensity data at room-T have been collected at the Dipartimento di Scienze della Terra e dell'Ambiente (Università degli Studi di Pavia, Italy) using a three-circle Bruker AXS SMART APEX diffractometer, equipped with a CCD detector (graphite-monochromatized MoKα radiation, 50 kV, 30 mA) and a 0.3-mm monocap collimator (see Domeneghetti et al 2013 for further details about data collection strategy). The Bruker SMART v. 5.625 software package was used for collecting a total of 3,360 frames (frame resolution 512 × 512 pixels) with four different goniometer settings using the ω-scan mode (scan width: 0.2° ω; exposure time: 10 s; detector sample distance: 4.02 cm) up to 2θ = 78°.…”