Petrogenesis of lunar meteorite Northwest Africa 2977: Constraints from in situ microprobe results (original) (raw)
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Paragenesis and U-Pb systematics of baddeleyite (ZrO2)
Chemical Geology, 1993
The mineral baddeleyite (ZrO2) occurs in many terrestrial and lunar samples and some achondrite meteorites as a trace constituent that most often crystallizes in the late-stage, most chemically fractionated portions of mafic magmas together with apatite, ilmenite, + zircon, + zirconolite. In terrestrial mafic rocks these interstitial regions commonly contain Kfeldspar, Cl-rich mica, amphibole and a small amount of quartz. In addition, baddeleyite occurs in tektites as a dissociation product of zircon during meteorite impact and as rims on mantle zircon megacrysts recovered from kimberlites. Baddeleyite is a major carrier of Hf, Ti, Fe and U and strongly fractionates Hf from Zr as it can have very high Zr/Hf (> 50). The baddeleyite rims on mantle zircon xenocrysts have a unique chemical composition with signifcantly higher TiO2 and lower FeO contents. Baddeleyite is an ideal mineral for U-Pb dating because it has abundant U (up to 3000 ppm), negligible initial common Pb, rarely occurs as xenocrysts, and, in unmetamorphosed samples, experiences negligible Pb loss. As an example of the reproducibility of conventional U-Pb-baddeleyite dating, several individual fragments from a large crystal obtained from the Phalaborwa carbonatite, South Africa, yielded remarkable consistency with a mean 2°Tpb/2°rPb age of 2059.8 +0.8 Ma (2a), interpreted as the age of carbonatite emplacement. Perhaps the best example of the geological reproducibility of U-Pb-baddeleyite dating is the example of the Mackenzie dyke swarm in North America where eight dyke and sill samples, collected over distances of > 2000 km, all have consistent 2°Tpb/2°6pb ages of 1267 and 1268 Ma. The ability to obtain very precise (+ 1-2 Ma) and accurate U-Pb-baddeleyite ages for mafic magmatic outbursts on Earth, such as the Mackenzie event, is of paramount importance in attempting to make supercontinent reconstructions and in the global correlation of ancient mafic magmatic activity worldwide. In metamorphosed mafic rocks, baddeleyite can also survive very high grades of metamorphism hut often develops a rim of polycrystalline metamorphic zircon. The width of the zircon rims and the grain size of the zircon aggregate increase when baddeleyite is subjected to progressively higher grades of metamorphism. A particularly interesting example is the variably metamorphosed Hurwitz gabbro sill, Northwest Territories, Canada. In a relatively unmetamorphosed sample, the mean 2°Tpb/2°6pb age of 2111.2 +_ 0.6 Ma (2a) for four baddeleyite fractions with negligible zircon growth preserves the age of gabbro crystallization while baddeleyite from a middle to upper greenschist-facies sample collected along strike, and likely from within the same sill, contains significant metamorphic zircon rim development with corresponding discordance of between 12% and 31% depending on the size of the zircon rims. It is also noteworthy that both the primary age of crystallization and the age of metamorphism can be obtained from some metamorphosed gabbroic rocks. In an upper amphibolite-to granulite-facies coronitic gabbro from the Grenville Province, Ontario, Canada, magmatic baddeleyite and zircon retain a primary crystallization age of 1152 + 2 Ma while polycrystalline metamorphic zircon pseudomorphs after baddeleyite record the age of metamorphism at ~ 1050 Ma.
Meteoritics & Planetary Science, 2009
Abstract— 40Ar-39Ar data are presented for the unbrecciated lunar basaltic meteorites Asuka (A-) 881757, Yamato (Y-) 793169, Miller Range (MIL) 05035, LaPaz Icefield (LAP) 02205, Northwest Africa (NWA) 479 (paired with NWA 032), and basaltic fragmental breccia Elephant Moraine (EET) 96008. Stepped heating 40Ar-39Ar analyses of several bulk fragments of related meteorites A-881757, Y-793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm-Nd, U-Pb Th-Pb, Pb-Pb, and Rb-Sr methods. These three meteorites differ in the degree of secondary 40Ar loss with Y-793169 showing relatively high Ar loss probably during an impact event ˜200 Ma ago, lower Ar loss in MIL 05035 and no loss in A-881757. Bulk and impact melt glass-bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of 40Ar-39Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K-Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.
Geochimica et Cosmochimica Acta, 2010
We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an "An97 anorthosite" that has An contents similar to those of nearside FANs. Mg 0 (= molar Mg/(Mg + Fe) Â 100) values vary significantly from 45to45 to 45to80 covering the ranges of both nearside FANs and the Mg 0 gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up 2020% of the investigated slab (5.2 Â 3.6 cm 2) mainly consists of fragments of anorthosites ("An93 anorthosite") more sodic than nearside FANs. LG also contains an augite-plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg 0 variation indicates that the "An97 anorthosite" is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg 0 in mafic minerals. The highest Mg 0 values in mafic minerals for the "An97 anorthosite" and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at 203.8 Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts
Arabian Journal of Geosciences
Northwest Africa (NWA) 4255 is a meteorite found in the region of Tindouf (southwestern Algeria), classified as brecciated olivine diogenite. Based on textural observations and orthopyroxene compositions, two different lithologies were determined: harzburgitic and orthopyroxenitic. The orthopyroxenitic lithology contains orthopyroxene (Mg no. 73.99-75.68) and spinel (Cr no. 83.09-85.11, Mg no. 15.57-22.45). On the other hand, the harzburgitic lithology contains orthopyroxene (Mg no. 74.54-77.14) and olivine (Mg no. 70.94-72.57). The iron metal and the sulfides (Troilites) of this sample are present in both lithologies and are low in Ni (Ni < 0.1 wt%). The Fe/Mn ratio of orthopyroxenes ranges from 22.28 to 32.64 and show a large overlap between both lithologies. Lowest ratios are unusual; they are below the defined field for diogenites and olivine diogenites. Δ 17 O values are − 0.234 ± 0.003 (1σ) and confirm that the NWA 4255 originated from 4Vesta. The results of this study show that there is a genetic linkage between the two lithologies of NWA 4255 and correspond to in situ crystallization processes. This olivine diogenite reflects transition between two major magmatic processes in 4Vesta. The magma ocean of 4Vesta crystallized at equilibrium, allowing the formation of a dunitic and harzburgitic mantle. This late lithology is linked to the peritectic reaction between the olivines formed and the evolved liquid. Our sample then reflects this crucial step of separating this mantle from the residual liquid. This melt evolving on the peritectic allowed the formation of the observed harzburgitic assemblage and then evolves out from the peritectic reaction to proceed to a fractional crystallization process involving the formation of orthopyroxenite.
Earth and Planetary Science Letters, 2016
The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U-Pb isotopic systematics, but there is little understanding of shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate shock-effects and their linkage with U-Pb isotopic disturbance that has previously been documented by in-situ U-Pb isotopic analyses.
Meteoritics & Planetary Science, 2010
Northwest Africa (NWA) 1068 is one of the few olivine-phyric shergottites (e.g., NWA 1068, Larkman Nunatak [LAR] 06319, and Roberts Massif [RBT] 04262) that is not depleted in light rare earth elements (LREE). Its REE pattern is similar to that of the basaltic shergottite Shergotty, suggesting a possible connection between the olivine-phyric and the basaltic shergottites. To test this possible link, we have investigated the high-pressure nearliquidus phase equilibria for the NWA 1068 meteorite bulk composition. Our results show that the NWA 1068 bulk composition does not represent an unmodified mantle-derived melt; the olivine and pyroxene in our near-liquidus experiments are more magnesian than in the rock itself, which suggests that NWA 1068 contains cumulate minerals (extra olivine). We have then used these experimental results combined with the pyroxene compositions in NWA 1068 to constrain the possible high-pressure crystallization history of the parental magma. These results suggest that NWA 1068 had a complex polybaric history. Finally, we have calculated a model parental magma composition for the NWA 1068 meteorite. The calculated parental magma is an evolved basaltic composition which is too ferroan to be a primitive melt directly derived from the mantle. We suggest that it ponded and crystallized at approximately the base of the crust. This provided an opportunity for the magma to become contaminated by an ''enriched'' crustal component prior to crystallization. The results and modeling from these experiments are applicable not only to the NWA 1068 meteorite, but also to LAR 06319 and possibly any other enriched olivine-phyric shergottite.
Geochimica Et Cosmochimica Acta, 2009
Northeast Africa 003 (NEA 003) is a lunar meteorite found as a two paired stones (6 and 118 g) in Libya, 2000 and. The main portion ($75 vol%) of the 118 g meteorite, used for this study, (NEA 003-A) consists of mare-basalt and a smaller adjacent portion ($25 vol%) is a basaltic breccia (NEA 003-B). NEA 003-A has a coarse-grained magmatic texture consisting mainly of olivine, pyroxene and plagioclase. The late-stage mineral association is composed mainly of elongated plagioclase, ilmenite, troilite, fayalite, Si-K-rich glass, apatite, and a rare SiO 2 phase. Other accessory minerals include ulvö spinel, chromite, and trace Fe-Ni metal. Olivine and pyroxene contain shock-induced fractures, and plagioclase is completely converted into maskelynite.
Polar Science, 2013
The lunar meteorite Northwest Africa (NWA) 2200 is a regolith breccia with a ferroan feldspathic bulk composition (Al 2 O 3 ¼ 30.1 wt.%; Mg # ¼ molar 100 Â Mg/(Mg þ Fe) ¼ 59.2) and low Th content (0.42 mg/g). Lithologically, NWA 2200 is a diverse mixture of lithic and glassy clasts, mineral fragments, and impact glass spherules, all embedded in a dark glassy matrix. NWA 2200 contains some feldspathic brecciated rock components (ferroan anorthositic granulitic breccia, poikiloblastic granulitic breccia, and glassy melt breccia with an intersertal texture). The bulk compositions of these brecciated components indicate they are derived from ferroan troctolitic or noritic anorthosite lithologies (bulk Al 2 O 3 ¼ 26e30 wt.%; bulk FeO/MgO > 1.0). The bulk composition of NWA 2200 is more ferroan and feldspathic than the Apollo feldspathic regolith samples and feldspathic lunar regolith meteorites, and is also more depleted in incompatible elements (e.g., rare earth elements) than Apollo 16 feldspathic regolith samples. We conclude that NWA 2200 originated from a location different to the Apollo landing sites, and may have been sourced from the ferroan KREEP-poor highlands, "KREEP" materials are enriched in such elements as potassium (K), rare earth elements (REE), phosphorus (P).
Petrogenesis of lunar mare basalt meteorite Miller Range 05035
Meteoritics & Planetary Science, 2009
available online at Abstract-Miller Range (MIL) 05035 is a low-Ti mare basalt that consists predominantly of pyroxene (62.3 vol%) and plagioclase (26.4 vol%). Pyroxenes are strongly shocked and complexly zoned from augite (Wo 33 ) and pigeonite (Wo 17 ) cores with Mg# = 50-54 to hedenbergite rims. Coexisting pyroxene core compositions reflect crystallization temperatures of 1000 to 1100 °C. Plagioclase has been completely converted to maskelynite with signs of recrystallization. Maskelynite is relatively uniform in composition (An 94 Ab 6 -An 91 Ab 9 ), except at contacts with late-stage mesostasis areas (elevated K contents, An 82 Ab 15 Or 3 ). Symplectites (intergrowth of Fe-augite, fayalite, and silica) of different textures and bulk compositions in MIL 05035 suggest formation by decomposition of ferro-pyroxene during shock-induced heating, which is supported by the total maskelynitization of plagioclase, melt pockets, and the presence of a relict pyroxferroite grain. Petrography and mineral chemistry imply that crystallization of MIL 05035 occurred in the sequence of Fe-poor pyroxenes (Mg# = 50-54), followed by plagioclase and Fe-rich pyroxenes (Mg# = 20-50), and finally hedenbergite, Fe-Ti oxides, and minor late-stage phases. Petrography, bulk chemistry, mineral compositions, and the age of MIL 05035 suggest it is possibly source crater-paired with Asuka (A-) 881757 and Yamato (Y-) 793169, and may also be launch-paired with Meteorite Hills (MET) 01210. MIL 05035 represents an old (~3.8-3.9 Ga), incompatible element-depleted low-Ti basalt that was not sampled during the Apollo or Luna missions. The light-REE depleted nature and lack of Eu anomalies for this meteorite are consistent with an origin distant from the Procellarum KREEP Terrane, and genesis from an early cumulate mantle-source region generated by extensive differentiation of the Moon. . n.a. = not analyzed. a Opaque phases reported in include oxides, troilite, mesostasis, and symplectite. b Mesostasis in MIL 05035 by includes silica, fayalite, troilite, phosphate, and glass. c Average modal abundances were calculated using data for sections 6, 40, 31, and 34 of MIL 05035, corrected proportionally to their surface area. d Symplectites refer to intergrowth of pyroxene, fayalite, and SiO 2 .
Geochimica et Cosmochimica Acta, 2010
The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo 76-55 from core to rim), pyroxene (from core to rim En 70 Fs 25 Wo 5 , En 50 Fs 25 Wo 25 , and En 45 Fs 45 Wo 10 ), and Cr-rich spinel in a matrix of maskelynite (An 52 Ab 45 ), pyroxene (En 30-40 Fs 40-55 Wo 10-25 ,), olivine (Fo 50 ), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo >72 ) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ -2), followed by crystallization of the groundmass over a range of FMQ -1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.