Wang et al. Earth, Planets and Space (2021) 73:164 https://doi.org/10.1186/s40623-021-01492-3 EXPRESS LETTER Open Access New temperature and oxygen fugacity data of Martian nakhlite from Northwest Africa (NWA) 5790 and implications for shallow sulphur degassing Zilong Wang1, Wei Tian1* and Yankun Di2 Abstract Newly analysed titanomagnetite–ilmenite (Tim–Ilm) intergrowths from Martian nakhlite meteorite Northwest Africa (NWA) 5790 yielded crystallisation temperature up to 1032 °C and oxygen fugacity (fO ) up to ΔQFM 1.6, notably 2 + higher than previous estimates for nakhlite magmas (temperature < 950 °C, fO2 ΔQFM 0.5 to ΔQFM 1). To interpret how the magma was reduced from ΔQFM 0.5 to ΔQFM 1.6, we used= D-Compress− to model+ the sulphur − + degassing process within a single thick lava pile. For fO2 to signifcantly decrease in this extended range, a sulphur- rich (S content 4000–7000 ppm) Martian lava fow had to degas all the sulphur species at a certain fnal degassing pressure, which was 2–4 bar for NWA 988 and Lafayette and < 0.7 bar for Y-000593 and Nakhla. These fnal degassing pressure data are in good agreement with the Martian nakhlite burial depth estimated by other petrological and geochemical methods. These estimates are also comparable with the excavation depth of ~ 40 m based on the small (6.5 km in diameter) impact crater over the Elysium lava plain. The fO2-controlled sulphur degassing pressure may constitute a method for estimating the burial depth of sulphur-rich lava fows on Mars. Keywords: Martian meteorites, Nakhlite, Fe–Ti oxides, Oxygen fugacity, Temperature Introduction a petrographic texture similar to terrestrial basalts, Determining the oxygen fugacity (fO2) of Martian sher- believed to have been excavated from lava piles buried at gottite–nakhlite–chassignite meteorites is important the shallow surface of Mars (Mikouchi et al. 2006, 2012; to understand the volcanic eruption process in ancient Shuster and Weiss 2005; Righter et al. 2008; Cohen et al. Mars. Both shergottite and chassignite have relatively 2017). Terefore, the fO2 of nakhlite samples can directly reduced fO2, ranging from ΔQFM − 4 (4 logfO2 units provide constraints on lava fows erupted from ancient below quartz–fayalite–magnetite bufer) to ΔQFM − 0.5 Martian volcanoes. (Hewins et al. 2020; Udry et al. 2020). However, fO2 in Tere is a considerable variation of fO2 measured this range does not appear to represent magma erupted from Fe–Ti oxide phenocrysts in nakhlite samples, to the surface of Mars due to the high degree of crys- which ranges from ΔQFM − 0.5 to ΔQFM + 1 (Szyman- tallisation of shergottite and chassignite. Nakhlite has ski et al. 2010) for those ilmenite–titanomagnetite pairs that passing the Mg/Mn equilibrium test of Bacon and Hirschmann (1988) (for details see Additional fle 1: Fig- *Correspondence: [email protected] ure S1 and Table S1). However, the full range of fO2 in 1 The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry nakhlites has not yet been fully resolved, as Fe–Ti oxides of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China studied likely represent only the relatively low-tempera- Full list of author information is available at the end of the article ture history of these rocks. 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Earth, Planets and Space (2021) 73:164 Page 2 of 8 corresponding to the highest observed fO2 is < 950 °C, Na2O, 0.03 wt.% for MgO and CaO, 0.04 wt.% for TiO 2, much lower than the crystallisation temperature of clino- Cr2O3, MnO, FeO, and NiO, and 0.06 wt.% for SiO2. pyroxene phenocrysts in these melts (1100 °C, Righter et al. 2008). Most studies applied geothermometers and Ilmenite–titanomagnetite geothermometer oxybarometers to ilmenite–titanomagnetite pairs that 2+ 4+ 3+ Te exchange equilibrium of Fe + ­Ti for 2 Fe occurred as veinlets and marginal growth along the between ilmenite and titanomagnetite: cracks, which are considered to be late features (e.g., Szy- .Tim .Ilm ⇋ .Ilm .Tim manski et al. 2010; Treiman and Irving 2008), consistent Fe2TiO4 + Fe2O3 FeTiO3 + Fe3O4 with the low temperatures calculated. Ilmenite exsolu- tion lamellae intergrowths have been reported in Fe–Ti has been calibrated in several studies for use as a geo- oxide phenocrysts, which may record higher temperature thermometer (Andersen and Lindsley 1988; Ghiorso and growth (e.g., Balta et al. 2017; Righter et al. 2014; Imae Sack 1991; Powell and Powell 1977; Sauerzapf et al. 2008). et al. 2005), but none of these intergrowths has been Of these proposed geothermometer models, only the successfully analysed by electron probe microanalysis thermometer model of Sauerzapf et al. (2008) was experi- (EPMA) due to the inadequate thickness of the ilmenite mentally calibrated at a temperature range (800–1300 °C) lamellae. that includes the temperature range (900–1100 °C) Containing the highest mesostasis (glass phase) pro- recorded in Fe–Ti oxides in NWA 5790. Tus, the geo- portion, NWA 5790 is the nakhlite sample with the high- thermometer of Sauerzapf et al. (2008) was used in the est cooling rate (Corrigan and Velbel 2015; Mikouchi temperature estimations in this study. et al. 2012). Terefore, ilmenite–titanomagnetite pairs in NWA 5790 should record the highest closure tem- Results perature. We identifed titanomagnetite–ilmenite inter- Petrography and P–T estimations growths in Fe–Ti oxide phenocrysts in nakhlite NWA Te NWA 5790 sample in this study contains clinopyrox- 5790, which were wide enough (> 2 μm) to locate a single ene (55.7%), olivine (6.1%), Fe–Ti oxides (0.4%) and mes- EPMA spot. Ten, we determined the equilibrium tem- ostasis (37.8%) (Fig. 1a and Additional fle 1: Figure S2). perature and fO2 of these intergrowths. We conducted As shown in Fig. 1b, the Fe–Ti oxide grains are subhe- sulphur degassing modelling based on the newly obtained dral to anhedral, with a major axis of about 1 mm and a fO2 data and the extended variation range of fO2. Te minor axis of about 0.5–1 mm. Tere are two occurrence modelling results helped us constrain the degassing pres- domains in the titanomagnetite–ilmenite intergrowths: sure during the crystallisation of Fe–Ti oxide phenocrysts (1) pervasively distributed exsolution textures charac- and the emplacement depth (i.e. burial depth) of diferent terised by thin ilmenite lamellae occurring in titanomag- nakhlites. netite; and (2) ilmenite veinlets and rim regions along the cracks of titanomagnetite host crystals. EPMA was used Methods to analyse ilmenites from these two domains and their EPMA analysis host titanomagnetites. Ilmenite veinlets and rim regions Quantitative chemical analyses of minerals in the pol- are typically diferent in composition from lamella, with ished thin section NWA 5790 were conducted using a high TiO2, MgO, and MnO contents and low Al2O3, JEOL JXA-8230 electron microprobe analyzers (EMPA) Cr2O3, and FeO contents (Table 1). equipped with four wavelength dispersive spectrometers Titanomagnetite–ilmenite geothermometer and oxy- at Key Laboratory of Orogenic Belts and Crustal Evolu- barometer (Sauerzapf et al. 2008) were used to calculate tion of School of Earth and Space Sciences, Peking Uni- the equilibrium temperature and fO2 of Tim–Ilm lamel- versity. No feld emission gun was used. Te acceleration lae pairs, which are 1005–1032 °C and ΔQFM + 1.60 voltage and the beam current were 15 kV and 10 nA, to ΔQFM + 1.44, respectively (Table 1). As shown in respectively. Te beam diameter was set to “spot” mode Fig. 2a, the equilibrium temperature and fO2 obtained (minimum size of the instrument). Counting time of in this study are remarkably higher than that calcu- 20 s was used for both peck and background. Te SPI 53 lated for Tim–Ilm pairs from other nakhlites, includ- minerals standard (U.S.) was utilised for the quantitative ing Nakhla, Lafayette, Y-000593, and NWA 998 (Boctor analysis: sanidine was employed for K; diopside for Ca et al. 1976; Bunch and Reid 1975; Sautter et al. 2002; and Mg; rutile for Ti; jadeite for Na, Al and Si; chromium Szymanski et al. 2010; Treiman and Irving 2008). Te oxide for Cr; rhodonite for Mn; hematite for Fe; and temperature of 1032 °C narrowed the gap between the nickel silicide for Ni. At the fnal calibration stage, the crystallisation temperature of Fe–Ti oxide phenocrysts PRZ correction was performed. Detection limits of the and clinopyroxene phenocrysts, which is estimated to oxides were 0.01 wt.% for K 2O, 0.02 wt.% for Al2O3 and be 1100 °C (Righter et al. 2008). Tis suggests that the Wang et al. Earth, Planets and Space (2021) 73:164 Page 3 of 8 Fig. 1 a Hand-traced images of NWA 5790 sample in this study. Augite, Fe–Ti oxides, olivine, and intercumulus phase are shown in black, light grey, middle grey, and dark grey, respectively.