Northwest Africa 5790: Revisiting nakhlite petrogenesis (original) (raw)
Related papers
Meteoritics & Planetary Science, 2012
The nakhlites, a subgroup of eight clinopyroxenites thought to come from a single geological unit at the Martian surface, show melt inclusions in augite and olivine. In contrast to olivine-hosted melt inclusions, augite-hosted melt inclusions are not surrounded by fractures, and are thus considered preferential candidates for reconstructing parent liquid compositions. Furthermore, two types of augite-hosted melt inclusion have been defined and characterized in four different nakhlites (Northwest Africa [NWA] 817, Nakhla, Governador Valadares, and NWA 998): Type-I isolated inclusions in augite cores that contain euhedral to subhedral augite, Ti-magnetite, and pigeonite plus silica-rich glass and a gas bubble; Type-II microinclusions that form trails crosscutting host augite crystals. Fast-heating experiments were performed on selected pristine primary augite-hosted melt inclusions from these four samples. Of these, only data from Nakhla were considered robust for reconstruction of a nakhlite parental magma composition (NPM). Based upon careful petrographic selection and consideration of iron-magnesium ratios, our data are used to propose an NPM, which is basaltic (49.1 wt% SiO 2), of high Ca ⁄ Al (1.95), and K 2 O-poor (0.32 wt%). Thermodynamic modeling at an oxygen fugacity one log unit below the QFM buffer using the MELTS and PETROLOG programs implies that Mg-rich olivine was not a liquidus phase for this composition. Our analysis is used to suggest that olivine megacrysts found in the nakhlites are unlikely to have coprecipitated with augite, and thus may have been introduced during or subsequent to accumulation in the magma chamber, possibly from more evolved portions of the same chamber.
Minerals, 2019
Apatites from Martian nakhlites NWA 10153 and NWA 10645 were used to obtain insight into their crystallization environment and the subsequent postcrystallization evolution path. The research results acquired using multi-tool analyses show distinctive transformation processes that were not fully completed. The crystallization history of three apatite generations (OH-bearing, Cl-rich fluorapatite as well as OH-poor, F-rich chlorapatite and fluorapatite) were reconstructed using transmission electron microscopy and geochemical analyses. Magmatic OH-bearing, Cl-rich fluorapatite changed its primary composition and evolved toward OH-poor, F-rich chlorapatite because of its interaction with fluids. Degassing of restitic magma causes fluorapatite crystallization, which shows a strong structural affinity for the last episode of system evolution. In addition to the three apatite generations, a fourth amorphous phase of calcium phosphate has been identified with Raman spectroscopy. This amorp...
Meteoritics & Planetary Science, 2011
The nakhlites contain small proportions of Cu-Fe-Ni sulfide minerals; we have studied these sulfides in Northwest Africa (NWA) 998, Nakhla, Governador Valadares, and NWA 817 with optical microscopy, scanning electron microscope, and electron microprobe. Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are uniformly low (0.02 to 0.05 ± 0.03 vol%), i.e., a factor 5 lower than in shergottites. Sulfides occur within the glassy mesostasis, as composite two-phase Fe-Ti oxide-sulfide grains, intimately associated with interstitial grains or locally enclosed in postcumulus melt inclusions (e.g., Governador Valadares) in olivine. They exhibit a uniform low-Ni monoclinic pyrrhotite composition ± chalcopyrite. There is a gradation of sulfide grain sizes and textures across the nakhlites flow(s): droplets in NWA 817; resorbed blebs in Governador Valadares; more massive, true intercumulus blebs in Nakhla and NWA 998. These nakhlites also show evidence for sulfide weathering. Hot desert finds (e.g., NWA 998 and NWA 817) show a few percent fracture-filling iron (oxy) hydroxides of likely terrestrial origin. Original sulfides are 50% altered in our NWA 998 section, with iron (oxy) hydroxides at grain boundaries and as complete pseudomorphs. The compositions of unaltered pyrrhotites are homogeneous, close to that of the monoclinic endmember Fe 7 S 8 , and are too sulfur-rich to have been in chemical equilibrium with the late magmatic redox state fixed by the fayalite-magnetite-quartz buffer. Therefore, the compositions of the pyrrhotites must have been altered during the later stages of magmatic crystallization, by assimilation of S-rich regolith and hydrothermal circulation. 770 V. Chevrier et al.
Geochimica et Cosmochimica Acta, 2013
Nakhlite Miller Range (MIL) 03346 contains many secondary phases resulting from aqueous processes, including formation of poorly crystalline iddingsite-like veins in olivine, the precipitation of Ca-sulfates and Fe,K-sulfates from evaporating fluids, alteration of titanomagnetite to secondary Fe-oxides, and the dissolution of magmatic Ca-phosphates and residual glass in the mesostasis. A surprising variety of alteration products occur in association with olivine in MIL 03346, including: patches of incipiently-altered olivine, large Si-enriched olivine-hosted veins (up to 10 lm across) some of which are complex in morphology and are composed of several phases, small Fe,S(±K)-rich veinlets that crosscut the Si-enriched veins, Ca-sulfates filling cracks in olivine, and secondary Ca-phosphates. Elemental abundances and distributions in these alteration products are consistent with the mobilization of elements from readily dissolved phases in the mesostasis such as phosphates and residual glass. Under favorable weathering conditions, these phases dissolve more readily than pyroxenes, plagioclase, and even olivine at low pH. The occurrence (crosscut and devolatilized by the fusion crust) and composition of Si-enriched alteration veins in olivine are consistent with their formation on Mars. Si-enriched, poorly crystalline alteration products and secondary Ca-sulfates commonly occur in nakhlites, but the habit and composition of these alteration products differ between meteorites. Elemental distributions in these secondary phases suggest at least two episodes of alteration have affected MIL 03346, and subtle differences in secondary minerals and chemistry indicate that each nakhlite experienced its own unique alteration history either on Mars, Earth, or both. The variable Al content and range of morphologies of the olivine-hosted Si-enriched veins suggest variable alteration conditions consistent with a water-limited regime. If the secondary phases in MIL 03346 can be shown to have formed on Mars, their chemistry will provide important clues to the aqueous environments and processes at the time of their formation. However, elevated S and REEs, Ce anomalies, and association of secondary minerals with postimpact cracks and voids indicate that terrestrial weathering has significantly affected MIL 03346. This work highlights the difficulty in distinguishing pre-terrestrial aqueous alteration from later chemical weathering of susceptible mineral phases even in meteorites with limited terrestrial modification.
The lavas and pyroclastic rocks of Nyiragongo volcano (East African Rift) range in composition from olivine melilitite to nephelinite and minor alkali olivine basalt, and include rare examples of strongly peralkaline combeite nephelinite. In peralkaline nephelinites at Nyiragongo, titanium is hosted in mineral assemblages with Ti-bearing magnetite ± perovskite ± Ti-rich clinopyroxene ± götzenite. Combeite and götzenite occur as groundmass minerals in holocrystalline melilite nephelinite, which also carries kirschsteinite (replacing melilite phenocrysts), recrystallized nepheline + kalsilite phenocryst aggregates and a range of late accessory minerals including delhayelite. The compositions of coexisting nepheline and kalsilite in phenocryst aggregates and groundmass suggest a crystallization temperature of ca. 600 °C for the götzenite- and combeite bearing mineral assemblages. The textural features of the rock agree with an origin of holocrystalline nephelinite (with or without götzenite and combeite) by recrystallization of glass-bearing, nepheline-kalsilite and melilite porphyritic peralkaline nephelinite due to thermal metamorphism and metasomatism within the volcanic edifice. A chemographic analysis of the Ti-bearing mineral assemblages of götzenite-bearing and götzenite-free peralkaline nephelinite suggests that götzenite is stabilized by elevated fluorine activity combined with moderately high (for nephelinite) silica activity. At increasing peralkalinity, götzenite is likely to break down to perovskite-bearing mineral assemblages coexisting with combeite.► Unique occurrence of rock-forming götzenite and combeite in peralkaline nephelinite. ► The HFSE mineralogy is controlled by the activities of silica, alkalis and fluorine. ► New insights into magmatic and sub-solidus processes in peralkaline nephelinite. ► Contact metamorphism and metasomatism within an active volcano.
Magmatic origin of low-Ca olivine in subduction-related magmas: Co-existence of contrasting magmas
Chemical Geology, 2006
Unravelling the origin of different components contributing to subduction-related magmas is a prerequisite to understanding the sources and processes involved in their origins. Mafic, high-Ca subduction-related magmas from geographically-diverse areas (Indonesia, Solomon Islands, Kamchatka, Valu Fa Ridge) contain two populations of olivine crystals, of which only the high-Ca population (CaO = 0.3–0.5 wt.%) crystallized from the melt that dominantly contributed to the whole rock composition. Forsterite-rich (Fo90–94), low-Ca (CaO < 0.15 wt.%), high-Ni (NiO > 0.3 wt.%) olivine crystals, which constitute 16–37 vol.% of total olivine population, are generally interpreted as mantle or lithospheric xenocrysts. However, in these samples, the olivine shape and chemical zoning, the composition of included minerals (orthopyroxene, clinoenstatite and Cr-spinel) and presence of melt inclusions, are indications that these crystals are phenocrysts from a mafic magma with high silica and low calcium contents. The coexistence of contrasting magmas (mafic high-Ca silica-poor versus low-Ca silica-rich) within a number of arc systems and their mixing may not be a rare event, and should be taken into account when developing models of arc petrogenesis.
Differentiation of natrocarbonatite magma at Oldoinyo Lengai volcano, Tanzania
Mineralogical Magazine, 1998
Natrocarbonatite magma, erupted as lava flows in the Tanzanian volcano Oldoinyo Lengai in June and November of 1988, has evolved chemically since its formation. The June and November flows of 1988 display increasing Cl, F, Ba, K, Mg and Mn, concomitantly with Na, Ca and P depletion. Furthermore, the June magma, at the time of eruption, had higher Cl, F, Ba and K contents and lower Ca than the November magma and evolved to higher levels of Cl, F, Ba and K content and lower Ca, Na and P. The mineralogy of the lavas reflects these trends. Crystallization of fluorite and halite–sylvite solid solution, usually as a symplectic intergrowth, occurs when Cl and F concentrations reach the critical value necessary to stabilize both minerals and explains why neither occurs as a phenocryst phase. Natrocarbonatite magma has undergone considerable and rapid magmatic evolution, probably in small and separate magma chambers. Two minerals, nyerereite and gregoryite, have dominated the crystallization...
Geochimica et Cosmochimica Acta, 2020
Northwest Africa (NWA) 7042 is an intermediate, permafic shergottite consisting of two generations of olivine (early zoned olivine Fo 41-76 , and late-stage fayalitic olivine Fo 46-56), complexly zoned pyroxene (En 35-64 Fs 22-46 Wo 5-34), shock-melted or maskelynitized feldspar (An 5-30 Ab 16-61 Or 1-47), and accessory merrillite, apatite, ilmenite, titanomagnetite, Fe-Cr-Ti spinels, pyrrhotite, and baddeleyite. The zoned olivine grains have been pervasively modified, containing conspicuous brown Mgrich cores surrounded by colorless, unaltered Fe-rich overgrowth rims. This textural relationship suggests that the cores were altered at magmatic temperatures prior to crystallization of the rims on Mars. Launch-generated shock veins in NWA 7042 also crosscut and displace several of the altered olivine grains indicating that alteration occurred before ejection of the meteorite. While this type of olivine alteration is rare in shergottites, it is similar to deuterically altered olivine in basalts and gabbros on Earth, caused by residual water-rich magmatic fluids. Transmission electron microscopy analysis of the olivine alteration did not reveal the high-temperature phases expected from this process; however, NWA 7042 has also been subjected to extensive terrestrial weathering, which may explain their absence. The potential presence of deuterically altered olivine in NWA 7042 has significant implications, as it is the third martian meteorite where deuteric alteration of olivine has been observed (the others being NWA 10416, and Allan Hills 77005). The different mantle sources for the parental melts of these three meteorites would suggest many, if not all martian mantle reservoirs have the potential to produce water-rich magmas.
Open Journal of Geology
Nyiragongo volcanic eruptions of 1977 and 2002 emitted silica-undersaturated lavas named melilite-nephelinites with microlithic to sub-porphyritic textures, and consisted of olivine, clinopyroxene (augite), phlogopite, melilite, magnetite, and rare plagioclases. This melilite-nephelinite as an evolved rock, shows low SiO 2 (38.40-39.52 wt%) and MgO (3.10-4.01 wt%), and relatively high FeOt (13.76-14.10 wt%), Al 2 O 3 (15.01-16.48 wt%), CaO (11.00-12.29 wt%) and Na 2 O + K 2 O (10.34-11.85 wt%). Unlike LA-ICP-MS on silicate melt inclusions (SMIs) hosted in augite show a pristine melt of picrobasaltic (low Ti-picrite) rock poor in SiO 2 (31.14-32.26 wt%), FeOt (2.19-2.79 wt%), Al 2 O 3 (8.01-9.57 wt%), and Na 2 O + K 2 O (2.34-3.05 wt%), while enriched in MgO (20.27-28.63 wt%), and CaO (24.95-33.17 wt%). The sums (∑REEs) for lavas and SMIs are ranging 712-799 and 43-119 ppm respectively. REE contracted multi-element patterns showed a W-feature for most lavas except for SMIs. High Rb/Sr, and low Ba/Rb, Zr/Nb, and Sm/Hf ratios of lavas suggest a phlogopite-rich source of materials.