Oxygen Isotope Thermometry of DaG 476 and SaU 008 Martian Meteorites: Implications for Their Origin

Abstract

We report the equilibration temperatures derived from the oxygen isotope thermometry of pyroxene-olivine pair from the Dar al Ghani (DaG) 476 (1200 +105/−90 °C) and Sayh al Uhaymir (SaU) 008 (1430 +220/−155 °C) meteorites showing a difference of over 200 °C at the face values. Regardless of the large associated uncertainties, contrasting geochemical and isotopic characteristics such as oxygen fugacities, hydrogen isotopic compositions (referred to as the D/H ratios), olivine abundances, presence of merrillite and/or apatite, and their chlorine contents between the two meteorites are observed in the literature. These opposing features lend support to the idea that the relative difference observed in the estimated temperatures is probably real and significant, thus providing insights into the Martian mantle magmatism. Based on our temperature estimation and previous magmatic models, we propose that SaU 008 could have been originated from a deeper depleted mantle source. However, DaG 476 may have been produced by the partial melting of the entrained pockets of the depleted mantle similar to that of the SaU 008’s source at a relatively shallower depth. Both meteorites erupted as a relatively thick lava flow or a shallow intrusion at approximately the same time followed by a launch initiated by a single meteoritic impact 1.1 million years (Ma) ago.

1. Introduction

The SNC (Shergotty-Nakhla-Chassigny) meteorites are the main source of information about the interior of Mars. They represent rocks from Martian volcanoes and magmatic intrusions into the crust of Mars [1,2,3,4]. These mafic to ultramafic rocks provide unique samples that can be used to decipher and understand the magmatic processes and the crystallization history of Mars [5,6]. Based on the crystallization ages of SNCs (150–4500 Ma) obtained from various geochronometers such as Sm–Nd, Rb–Sr, Pb–Pb, and Ar–Ar [7,8,9,10,11,12,13,14,15,16,17], it appears that the Martian magmatism extended over most of the solar system history that agrees well with the time span of crater retention ages [18]. It is believed that shergottites—the most abundant of the Martian meteorites found to date—have been generated by the partial melting of the Martian mantle [8,19]; therefore, their geochemical signatures can provide insights into the Martian mantle chemistry. Furthermore, the depleted shergottites—for example, having low chondrite-normalized La/Y (~0.1), low initial ⁸⁷Sr/⁸⁶Sr (~0.7013), low magmatic oxygen fugacity (i.e., quartz-fayalite-magnetite, QFM −3.5), and large positive ε¹⁴³Nd values [5,7,8,20,21]—originated from a depleted mantle source region that has not seen significant alteration or melting since planetary differentiation at 4504 ± 6 Ma [9,17]. Compared to the nakhlites and chassignites, shergottites, with the exception of Northwest Africa (NWA) 7635 (2405 Ma) [17], represent more than three-quarters of the known Martian meteorites (i.e., SNCs) with relatively young crystallization ages (150–574 Ma) [7,13,22]. However, according to several independent measurements made by the spacecraft, shergottite-like rocks are found in older terrains [23]. The depleted shergottites are relatively older than the intermediate and enriched types that include Yamato 980459 (Y980459), Dar al Gani (DaG) 476, Sayh al Uhaymir (SaU) 005/008, Dhofar 019, and NWA 7635. Most of the depleted shergottites were launched from the Mars surface by a single impact [17]. These meteorites are related via identical ejection ages (i.e., mean ejection age = 1.1 ± 0.2 Ma; N = 11) and compositionally identical mantle source [17]. There is no physical evidence that could suggest that these rocks are derived from the same terrain on Mars [15]. However, the majority of the depleted shergottites share similar ejection ages (1.1 ± 0.2 Ma; [17], except NWA 5990, NWA 7032, QUE 94201 (3 Ma) and Dhofar 019 (18 Ma), suggesting that most of them might have launched in a single impact probably from a similar terrain. Scientific community is working on possible craters on Mars to find out potential candidate locations where SNCs could have originated on the red planet and so far, several such craters have been found carrying characteristics identical to SNCs. Recently, it is found that young volcanic sequences (i.e., Tharsis plateau) on Mars may possibly be the source of shergottites [24,25]. Large-scale impacts on Martian surface have produced mechanical deformation ([26,27,28,29] and refs. therein) of major rock-forming minerals (i.e., px-ol), transformation of plagioclase to maskelynite ([30] and refs. therein), and formation of melt pockets, veins, and bubbles in the glass [31,32]. Shock features in olivine and pyroxene include planar deformation features, twinning, and mosaicism and these are abundant in both DaG 476 and SaU 005 [33]. On the other hand, shock effects in plagioclase are commonly observed in Martian and Lunar meteorites of basaltic origin [34,35]. Plagioclase can resist complete loss of crystallinity (i.e., changing to glass) up to ~45 GPa [36]. Furthermore, solid-state transformation of plagioclase to maskelynite needs not necessarily reset isotope systems [30].

In order to understand the magma generation in the Martian interior and its eruption mechanism, several models based on numerical and geochemical data were proposed [15,22,37,38,39]. Geochemical models regarding the source regions of the shergottites suggest that these regions have formed either as a result of solidification of a Martian magma ocean [7,8,40,41] or by the interaction between mantle and crustal reservoirs [20,42].

The volatile budget in Martian magmas is also important to understand the eruption history on Mars [37] and the chemistry of the SNCs [43,44,45,46]. Water content can constrain the extent of oxidation [47] that may possibly affect the oxygen isotope compositions of the mantle minerals through isotopic exchange. Martian meteorites have shown the presence of water within Mars’ interior [48,49,50,51,52,53,54,55]. However, Martian magmas may have suffered a partial loss of the volatiles by degassing upon eruption [56]. To counter this uncertainty, several studies have been carried out to estimate the pre-eruptive water contents of Martian magmas on the basis of H-isotopes [53,54,57]. Furthermore, an experimental study [58] has suggested that at least 2 wt. % of water is required to reproduce shergottite-like mineralogy and their possible temperatures of crystallization.

Over the last several decades, oxygen isotope compositions in coexisting rock-forming minerals have been used as a robust thermometric tool [59] and for confirming the isotopic equilibrium. For example, pyroxene-olivine pair is particularly important to interpret the oxygen isotope data of mantle samples and was successfully applied to Lunar basalts to estimate their crystallization temperatures [60,61,62]. However, terrestrial basalts show higher uncertainty in temperatures based on pyroxene-olivine thermometry due to their crystallization at higher temperatures leading to small isotopic fractionation. A part of oxygen isotope data of minerals in SNCs are compiled in our previous study (see Table 2 in [63]). Application of these data to estimate temperatures in SNCs is limited; for example, oxygen isotope compositions of various mineral pairs in Shergotty were utilized for this purpose [64]. To our knowledge, there is no other reporting of oxygen isotope thermometry on SNCs. Olivine-phyric shergottites [65] are the most promising primitive meteorites given their relatively high bulk-rock Mg# values $\left[=100\times \mathrm{molar}\mathrm{Mg}/\left(\mathrm{Mg}+\mathrm{Fe}\right)\right].$ Our previous work was mainly dealing with the oxygen isotope compositions of bulk materials and mineral separates from SNCs [63]. The data were used to discuss the homogeneity of Martian mantle as a whole because we included samples from all representative members of the SNCs, thus representing different mantle sources. However, this contribution deals with a subset of oxygen isotope data of two meteorites (DaG 476 and SaU 008) from our previous paper [63] that mainly focuses on the oxygen isotope thermometry based on major-rock forming minerals (i.e., px-ol). It is fortuitous to have data of mineral fractions of these Martian meteorites from our previous work [63] because they are commonly studied depleted shergottites (olivine-phyric) having several characteristics in common (as mentioned above), such as being highly shocked, having identical ejection ages, relatively similar crystallization ages, and hot desert finds. The objective of the present study is to apply oxygen isotope thermometry to the olivine-phyric shergottites given their parent melt compositions closest to equilibrium with the Martian mantle. We used oxygen isotope compositions of the pyroxene-olivine mineral pair to estimate the equilibration temperatures of DaG 476 and SaU 005/008 to investigate their igneous petrogenesis to better understand the Martian mantle and generation of shergottite magmas.

2. Methodology

2.1. Samples

Shergottites are commonly divided into basaltic, olivine-phyric, and lherzolitic groups based on their mineralogy [65]. SaU 008 was found in close proximity of the SaU 005 find location on the same day (i.e., 26 November 1999) from the deserts of Oman and they are recognized as paired along with several other meteorites [66]. Furthermore, SaU 008 shows a porphyritic texture with large olivine phenocrysts in a fine-grained groundmass of pigeonite and shocked-plagioclase, that is, maskelynite. On the other hand, DaG 476 is the first Martian meteorite found from hot desert in Dar al Gani, Sahara Desert, Libya (1 May 1998), having porphyritic texture; it is composed of olivine megacrysts in a fine-grained groundmass of pigeonite and maskelynite. Average olivine and pyroxene modal abundances are, respectively, 17.2 vol % and 58.5 vol % (i.e., ranging from 10.4 to 24 vol % and 54 to 64.6 vol %, respectively) [33,67,68]. Both SaU 008 and DaG 476 meteorites are grouped as olivine-phyric shergottites given their highly depleted light rare earth elements (LREE) abundances.

Both DaG 476, SaU 005 and their pairings are similar in their texture, mineralogy, chemistry and exposure age [69]. However, the former is extensively weathered in terrestrial conditions [70] compared to the latter possibly due to their different residence periods on earth (i.e., DaG 476 = 85 ka, SaU 005 = 13 ka; [71]). Additionally, significant weathering for DaG 476 may also led to higher chlorine content (840 ppm; [33]) and K/La ratio (2630–4610; [7]) compared to those of SaU 005 (Cl = 143 ppm; [72], K/La = 1465–1770; [8]). Furthermore, olivine in DaG 476 and its pairings contains Sr (e.g., 47–87 ppm) [73] and Ba (e.g., 36–84 ppm) [33,73] concentrations that are higher by a factor of 10–100 and 6–10, respectively, compared to those in their counterpart shergottites found in Antarctica [74]. The elevated Sr and Ba contents are due to terrestrial weathering resulting in the formation of secondary sulfate and carbonate [72,74].

2.2. Oxygen Isotope Measurements

Precise triple oxygen isotope measurements have recently been done on a suite of SNC meteorites including DaG 476 and SaU 008 by adopting a laser-assisted fluorination mass spectrometry at Laboratory for Stable Isotope Science (LSIS), Western University, Canada [63]. The distinctively colored minerals (i.e., pyroxene relatively darker than the lighter olivine) were separated by handpicking from a crushed specimen of the bulk meteorite under the optical microscopy. The separated fractions of pyroxene and olivine minerals were treated with 6 M HCl at 70 °C for 2–3 min in order to remove any terrestrial weathering products to make sure that our data represent actual isotope values for the minerals. Pre-dried samples (1–2 mg) were loaded as grains on the sample holder and placed in the sample chamber. After complete evacuation of the vacuum line, a reaction was performed by heating the sample with a 25 W CO₂ laser (10.6 µm wavelength; Merchantek, Bozeman, MT, USA; Model MIR10-25) in a BrF₅ atmosphere to extract oxygen gas. Later, oxygen gas was purified using cryogenic metal traps and a heated KCl salt trap, giving yields of better than 95%. The triple oxygen isotope ratios were determined using a Delta V Plus mass spectrometer in a dual inlet mode integrated with Isodat3.0 software for system controls and data acquisition. Data were reported as delta notation (δ) with respect to the Vienna Standard Mean Ocean Water (VSMOW; [75]). Details of the analytical procedures are reported elsewhere [63].

3. Results and Discussion

3.1. Oxygen Isotope Thermometry

Oxygen isotope thermometry is based on two fundamental assumptions: (i) isotopic equilibrium is maintained; (ii) and the exchange occurs by diffusion in a closed system [76]. Mineral data of both meteorites fall on a Martian fractionation line (i.e., MFL; [63]) with a slope of 0.532 ± 0.009 (SE) (Figure 1) which shows an isotopic equilibrium has been achieved and thereby satisfying the first assumption. On the other hand, for second assumption the fast grain boundary (FGB) diffusion model [77,78] provides a flexible way to deal with stable isotope diffusion which suggests that the mass balance and diffusion for any number of minerals in a rock are linked. This is in contrast to the Dodson model, which proposes diffusion between a single mineral and an infinite reservoir [79]. Although no distinct closure temperature for any mineral is assumed in the FGB model, calculations are made during a transition towards a progressively slower diffusion during which a mineral ultimately no longer achieves measurable exchange and is effectively closed [76].

Oxygen isotope fractionation among common rock-forming minerals such as olivine and pyroxene (i.e., cpx), can be calculated by using a polynomial expression for individual minerals for all temperatures above 400 K (i.e., 127 °C) [80]. Three oxygen isotope plot of the pyroxene-olivine pair in both shergottites, along with their respective bulk composition, shows that the minerals are in isotopic equilibrium (i.e., slope = 0.532 ± 0.009 (SE); Figure 1), as reported earlier [63] for the Martian fractionation line that is based on mineral separates. Furthermore, lower δ¹⁸O values in olivine compared to that of pyroxene (

Table 1. Oxygen isotopic compositions of pyroxene and olivine in shergottites. Estimated temperatures are also listed.

Sample	Mineral	δ18O (‰) ± SE	N	∆Di-Fo	T (K) †	1σ	T (°C) †	T (K) ‡	Data Source
DaG 476	px (Di)	4.708 ± 0.045	2			105			[63]
DaG 476	ol (Fo)	4.301 ± 0.024	3	0.407 ± 0.05	1470	−90	1200	1480	[63]
SaU 008	px (Di)	4.533 ± 0.045	2			220			[63]
SaU 008	ol (Fo)	4.223 ± 0.045	2	0.310 ± 0.06	1700	−155	1430	1720	[63]
NWA 2046	px (Di)	4.62 ± 0.09	2			330			[84]
NWA 2046	ol (Fo)	4.27 ± 0.17	2	0.35 ± 0.19	1595	−890	1320	1620	[84]
ALHA 77005 *	px (Di)	4.72 ± 0.07	1			130			[84]
ALHA 77005 *	ol (Fo)	4.28 ± 0.05	6	0.44 ± 0.09	1410	−175	1140	1445	[84]
NWA 1950 *	px (Di)	4.62 ± 0.05	4			165			[84]
NWA 1950 *	ol (Fo)	4.33 ± 0.03	6	0.29 ± 0.06	1765	−230	1490	1780	[84]

DaG = Dar al Ghani. SaU = Sayh al Uhaymir. NWA = Northwest Africa. ALHA = Allan Hills, Antarctica. SE = Standard Error. px = pyroxene. ol = olivine. Di = diopside. Fo = forsterite. N = number of individual runs. Oxygen isotope data taken from [63,84]. Errors in δ¹⁸O data of DaG 476 and SaU 008 represent the external reproducibility of several individual runs of San Carlos olivine (5.256 ± 0.045, N = 5), except DaG 476 ol [63]. However, errors in δ¹⁸O data of other shergottites are taken from [81]. Additive propagated errors on ∆_Di-Fo are calculated by taking the square root of the sum of the squares of uncertainty in δ¹⁸O values. ^† Calculated using Equation (5) derived from [80]. ^‡ Calculated using Equation (6) derived from [81]. Samples marked with asterisk (*) are lherzolitic shergottites.

) are also indicative of the fact that the isotopic equilibrium has been achieved between the mineral pair [80]. The expressions for the oxygen isotope fractionation of diopside and forsterite minerals are shown in Equations (1) and (2), respectively [80]. We calculated the f_Di and f_Fo values at temperatures between 600 °C and 1700 °C with a stepwise increment of 100 degrees. We selected higher temperatures for our calculations given the fact that oxygen isotope fractionation between these minerals have been studied experimentally at 600–1300 °C [81], and extrapolated to the typical temperature range of mantle melting zone (i.e., ~1400–1700 °C; [82]). The squared and cubed terms in the expressions (Equations (1) and (2)) are found to be negligibly low and has been ignored to get a simplified expression (Equation (3)). By plotting 10⁶T⁻² against ∆_Di-Fo, we get a straight line (Equation (4)) with a slope of 0.8298 and intercept of 0.0233 (Figure 2). The parameters of the regression line (i.e., slope and intercept; Figure 2) are used to calculate temperatures (Equation (5)) for the mineral pair (pyroxene-olivine) from DaG 476 and SaU 008 meteorites assuming that their oxygen isotope fractionation during fractional crystallization are identical to that of the diopside and forsterite, respectively [83]. Temperatures of the mineral pair, calculated using the isotope fractionation relation given in Equation (6) derived from 1000lnα = A × 10⁶T⁻² [81], show an excellent agreement.

$$f_{Di}=9.237\times \left(\frac{{10}^6}{T^2}\right)-0.199\times {\left(\frac{{10}^6}{T^2}\right)}^2+0.0053\times {\left(\frac{{10}^6}{T^2}\right)}^3$$

(1)

$$f_{Fo}=8.326\times \left(\frac{{10}^6}{T^2}\right)-0.142\times {\left(\frac{{10}^6}{T^2}\right)}^2+0.0032\times {\left(\frac{{10}^6}{T^2}\right)}^3$$

(2)

where

$${\Delta }_{Di-Fo}=f_{Di}-f_{Fo}$$

$${\Delta }_{Di-Fo}\mathrm{vs}.\left(\frac{{10}^6}{T^2}\right)$$

(3)

$${\Delta }_{Di-Fo}=0.8298\times \left(\frac{{10}^6}{T^2}\right)+0.0233$$

(4)

Rearranging Equation (4),

$$\left(\frac{{10}^6}{T^2}\right)=\frac{\left({\Delta }_{Di-Fo}-0.0233\right)}{0.8298}$$

$$T\left(K\right)=\sqrt{\frac{0.8298\times {10}^6}{\left({\Delta }_{Di-Fo}-0.0233\right)}}$$

(5)

$$T\left(K\right)=\sqrt{\frac{A\times {10}^6}{1000ln\alpha }}$$

(6)

where α and A represent the fractionation between the minerals and the coefficient factor, respectively. For diopside-forsterite pair, A = 0.92 ± 0.11 [81]. A comparison has been made between the temperatures estimated for both meteorites using Equations (5) and (6) (Figure 3). Errors associated with the estimated temperature of SaU 008 are relatively larger than those of the DaG 476. There is a 230 degrees difference between the face values of the two meteorites despite the large uncertainties in temperature estimation and errors overlap by 30 degrees (Figure 3).

3.2. Petrogenetic Relationship

The mantle melting zone shows temperature stratification (i.e., increase with increasing depth) that range from ~1400 °C to ~1700 °C [82]. Melt trapping in the olivine of DaG 476 has been reported at ~1275 °C followed by ascent and cooling that assisted crystallization of abiotic macromolecular carbon, spinel, and pyroxene in the melt inclusions of DaG 476 [82]. Our temperature estimation (~1200 °C) using pyroxene-olivine oxygen isotope thermometer for DaG 476 show a close resemblance. The oxygen fugacities of DaG 476 and SaU 005, estimated by olivine-pyroxene-spinel equilibria [47], are QFM −3.4 and QFM −2.5, respectively, and correspond to relatively lower mean temperature for the former (i.e., 980 °C; [47]) than that of the latter (i.e., 1086 °C; [47]. Higher temperature (1430 °C) estimated for SaU 008 using diopside-forsterite oxygen isotope thermometry implies that the magma could have originated at higher pressure, potentially due to its deeper residence in the mantle (Figure 4). The systematically higher temperatures from px-ol oxygen isotope thermometry than those from the ol-px-sp equilibria may be due to the disequilibrium that existed between all 3 phases in the latter [47]. However, oxygen isotopic compositions of bulk materials and minerals of both meteorites are well in isotopic equilibrium (Figure 1), suggesting that estimated temperatures are robust. Higher olivine modal abundance in SaU 005 (22–31 vol %; [6], Ni = 310 ppm; [85]) may support the idea that SaU 005 in comparison to DaG 476 (10–24 vol %; [6], Ni = 240 ppm; [85]) had crystallized at deeper levels in the Martian interior. Furthermore, the range in SaU 005 olivine Mg# (68 at core) [86] compared to that of other shergottites is restricted (est. 60–72; see Figure 3c in [87]). It is evident that olivine Mg# is directly proportional to its MgO content that, in turn, is correlated with olivine crystallization temperatures (1050–1525 °C) observed in basalts (see Figure 1 in [88]). In addition, forsterite (Fo) contents and Mg# values of olivines are often used interchangeably to assess the MgO content and are inversely related to their δ¹⁸O compositions as observed in olivine from CI chondrite, Orgueil [89]. The results of the fractional crystallization modeling by MELTS algorithm (i.e., a software for thermodynamic modeling of phase equilibria in magmatic systems) [15] using Y980459 as parent composition reveal that the change in temperature (1170–1270 °C) during sequential crystallization of minerals (e.g., opx to pigeonite) from a liquid is associated with a proportional change in mean En content (e.g., pigeonite = 56, opx = 78; [15]). Mean En content in the pigeonite of DaG 489 (paired with DaG 476) is 64 (En_57–72; [90]), which is within the range shown by the mean En content obtained from fractional crystallization model for the crystallization sequence of opx to pigeonite (i.e., 56–78; [15]). The temperature estimated for DaG 476 from oxygen isotope thermometry (1200 °C) is consistent with the temperature range (1170–1270 °C) obtained from MELTS fractional crystallization modelling [15]. The olivine-phyric shergottites represent a significant and important subgroup of Martian shergottites [65]. They contain cumulate olivines with relatively high core Mg# values, which suggest that they could represent unfractionated liquids formed by the direct melting of the Martian mantle, namely, primitive melts [55]. Application of the oxygen isotope data of minerals formed from the primitive melts (i.e., px-ol) provides useful information regarding the temperature conditions of Mars’ interior. However, the sensitivity of this thermometry to small analytical uncertainties in oxygen isotopic compositions of these minerals could produce large errors. For example, the temperatures calculated for other shergottites using their px-ol data [84] are associated with large errors (Table 1). Regardless of the errors, we will discuss our estimated temperatures at their face values that may likely be different given other contrasting chemical and isotopic signatures observed between the two meteorites.

The partial melting processes in the source regions of magmas are greatly influenced by the volatiles (e.g., water, halogens) that are partly lost upon eruption [56], making the estimation of water contents indigenous to the magma difficult. However, experimental studies suggest that ~2 wt. % water may be sought to reproduce the mineralogy and temperatures of crystallization of shergottites [57,91,92,93]. Furthermore, D/H isotope compositions of DaG 476 show negative correlation between δD (i.e., 352 ± 18 to 2347 ± 85‰) and water abundance (relatively higher) in their feldspathic glasses (shocked plagioclase; e.g., maskelynite). The reverse is the case for SaU 005 (e.g., show positive correlation; δD = −105 ± 17 to 3257 ± 41‰, low water) [94]. The contrasting trends in both meteorites suggest a mixing of a fractionated Martian water component having high δD (3000–4000‰) with possibly a terrestrial component having low δD (~0‰) [94]. The difference in D/H ratios between the two meteorites may possibly be related to the difference in the source regions of the two meteorites.

Recently, it has been observed that halogens (i.e., Cl, F) have identical influence on crystallization temperatures of magmas similar to the effect of water [95,96,97]. Therefore, water and/or halogens may be required to reproduce the mineralogy of the shergottites. Given the uncertainty associated with estimation of indigenous water and halogen contents in the Martian magmas, it is rather useful to apply oxygen isotope thermometry using nominally anhydrous minerals. Olivine liquidus temperature (i.e., 1430 °C for Y980459) depends on melt composition and pressure [15]. The composition of the liquidus olivine is a function of melt composition, pressure, and temperature [98]. The fractional crystallization of mid-ocean ridge basalt (MORB) on terrestrial conditions is mainly affected by the water content in the magma. This is supposed to decrease the melt liquidus temperature and to suppress the plagioclase crystallization relative to olivine and clinopyroxene [99]. Experiments suggest that the parental magma of Y980459 could have been hydrous [100]. Furthermore, it is envisaged that water in the Martian mantle is heterogeneously distributed based on the estimates from the SNCs (i.e., 14–250 ppm; [46]. However, depleted shergottite mantle source is relatively water-poor (i.e., 14–23 ppm; [46]) compared to the enriched ones (i.e., 36–73 ppm; [46]). Mantle derived water that exhumed on the Martian surface in large fluxes during the earlier history of Mars (Noachian to early Hesperian periods; [23]), has been followed by further water contributions from shergottite-like magmatism during the Amazonian era [23].

Recently, pressure (i.e., 2.7 GPa) and temperature (i.e., 1600 °C) of formation of NWA 6234, an olivine-phyric shergottite, were estimated from olivine-melt Mg-exchange thermometry [55]. They suggested that high pressure and temperature (i.e., PT) conditions indicate that NWA 6234 may have derived from a unique source region deeper in the Martian interior. Furthermore, NWA 6234 recorded that merrillite reacted with an OH-Cl-F-rich melt to form apatite [55]. Shergottites contain trace quantities of apatite that is relatively more abundant in the enriched ones compared to their depleted counterparts [46]. It has been observed that all phosphate grains present in SaU 094 and SaU 150 meteorites, paired with SaU 005/008, represent merrillite, and no apatite has been found [69,101]. However, petrographic observations revealed that both merrillite and apatite are present in DaG 476 [101]. Apatite observed in DaG 476 have patchy texture that is consistent with anhedral growth on the rims of other late-stage minerals, such as merrillite and pyroxene [46]. However, whether it is the product of igneous crystallization or formed later by fluid-rock interactions is not well known. The chlorine content in DaG 476 (840 ppm; [33]) is approximately six times larger than that of the SaU 005 (143 ppm; [72]). Most of the rock’s chlorine budget comes from the mineral apatite. This suggests that the magmas in equilibrium with the apatite could have been enriched in chlorine. Moreover, high Na-contents (1.0–1.2 wt %; [55]) in merrillite of DaG 476 demonstrate that merrillite crystallized earlier than plagioclase. These observations suggest that PT conditions could have been different for the formation of these meteorites and probably confirm that they could have been originated from source regions of the Martian mantle at different depths. Furthermore, DaG 476 may have encountered with OH-Cl-F-rich melt during its ascent, resulting in the reaction with merrillite to form apatite, something that SaU 008 did not experience.

4. Conclusions

Oxygen isotope fractionation between pyroxene and olivine minerals during crystallization in the magma chamber provide useful means to estimate the equilibration temperature. Oxygen isotope thermometry using pyroxene-olivine (proxy for diopside-forsterite) pair is applied to calculate the temperatures in two olivine-phyric shergottites (i.e., DaG 476 and SaU 008), showing that the two meteorites have 230 °C difference in their equilibration temperatures at the face values (i.e., SaU 008 = 1430 +220/−155 °C; DaG 476 = 1200 +105/−90 °C), even though the values are associated with large errors. To support the difference in our estimated temperatures, we investigated that both meteorites show contrasting geochemical and isotopic characteristics, such as (i) oxygen fugacities, (ii) olivine abundances, (iii) D/H ratios, (iv) occurrence of merrillite/apatite minerals, and (v) chlorine contents. Earlier magmatic models have proposed that olivine-phyric shergottites were originated from a depleted and wet magma sources. Equilibration temperatures suggest that SaU 008 could have sourced the deeper depleted mantle reservoir whereas DaG 476 was potentially produced by the partial melting in the entrained upwelling of the similar mantle at relatively shallower depth. We also propose that both meteorites erupted simultaneously as a relatively thick lava flow or a shallow intrusion followed by a launch initiated by a single meteoritic impact circa 1.1 Ma.