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NWA 2656

Acapulcoite
Acapulcoite–Lodranite Clan
standby for nwa 2656 photo

Purchased 2004
no coordinates recorded A 386 g meteorite was found in the Sahara Desert in 2003. This was later determined to be part of an ~7.5 kg mass, which, along with other recovered fragments, compose a meteorite with a total weight of ~10 kg. The 386 g fragment was purchased by N. Oakes from a Moroccan dealer, and a sample was submitted for analysis and classification (T. Bunch and J. Wittke, NAU; A. Irving, UWS; D. Rumble III, CIW). Northwest Africa 2656 was determined to be an acapulcoite, exhibiting a recrystallized, polygonal, granular texture. Portions of this meteorite have been analyzed by NAU under several different names, including at least NWA 2699 (1,294 g), NWA 2714 (100 g), NWA 2866 (213 g), and NWA 2871 (3,467 g).

As a member of the lodranite/acapulcoite group, NWA 2656 has been distinguished from the members of the winonaite group, which exhibits similar recrystallized textures, through a plot of the Fa content of olivine vs. the Δ17O-isotopic value. From this diagram, it is apparent that these two groups plot in separate regions, and NWA 2656 is clearly resolved within the lodranite/acapulcoite group (D. Rumble, III et al., 2005). Northwest Africa 2656 consists of orthopyroxene, olivine, and plagioclase, along with minor FeNi-metal, troilite, schreibersite, Cr-diopside, and chromite, and exhibits a grain size of <1 mm. This meteorite has been shocked to stage S2 and has been weathered to grade W3.

A division of the acapulcoite–lodranite meteorite clan based on metamorphic stage was proposed by Floss (2000) and Patzer et al. (2003).

  1. primitive acapulcoites: near-chondritic (Se >12–13 ppm [degree of sulfide extraction])
  2. typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
  3. transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
  4. lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
  5. enriched acapulcoites (addition of feldspar-rich melt component)

Because both acapulcoites and lodranites are derived from the same parent body and have the same O-isotopic ratios, the plagioclase content in this meteorite is an important factor in making the distinction between acapulcoite and lodranite. Lodranites contain no plagioclase (or only trace amounts) since it was depleted from the restite during the partial melt phase. The two groups also have similar mineralogies, thermal histories, and cosmic ray exposure ages. Additionally, lodranites and acapulcoites have identical cosmogenic nuclide abundances and similar shielding conditions. Another factor which distinguishes acapulcoites from lodranites is their grain size. The grain size of NWA 2656 is more consistent with the finer-grained acapulcoites than with the coarser-grained lodranites—the division has been established by some at 500 µm, and the average grain size of NWA 2656 is 400 µm; however, further studies indicate an average grain size for portions of this meteorite of 0.6–0.7 mm, consistent with a lodranite classification. With many more samples to study, it is now evident that a continuum exists for the grainsizes of these two groups, and it has been proposed by Bunch et al. (2011) that an arbitrary group division is no longer justified; the term ‘acapulcoite–lodranite clan’ should therefore be applied to all members of the combined group.

For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The specimen shown above is a partial slice of NWA 2656 weighing 1.21 g.


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NWA 2775

Acapulcoite, FeO-rich
Acapulcoite–Lodranite Clan
standby for northwest africa 2775 photo
Purchased November 2005
no coordinates recorded A 222 g [231 g] partially fusion-crusted stone, which was found in Algeria, was eventually marketed in Erfoud, Morocco. The stone was purchased by American collectors and a portion was submitted for analysis to both the Northern Arizona University (J. Wittke and T. Bunch) and the University of Washington in Seattle (A. Irving). Northwest Africa 2775 was classified as an acapulcoite (see MetBull 91).

While NWA 2775 exhibits the typical recrystallized texture of acapulcoites, its olivine and pyroxene have FeO contents among the highest measured thus far in other acapulcoites. Notably, grain sizes have a range of 0.35–0.8 mm (ave. 0.55 mm), which is consistent with lodranites (>0.5 mm) according to one of the parameters that help distinguish lodranites from acapulcoites. However, with the many new samples available for study, it is now evident that a continuum exists for the grainsizes of these two groups, and it has been proposed by Bunch et al. (2011) that an arbitrary group division is no longer justified; the term ‘acapulcoite–lodranite clan’ should therefore be applied to all members of the combined group.

Oxygen isotopes were analyzed at the Carnegie Institute, Washington D.C. (D. Rumble III), and the Δ17O was found to be the lowest among all acapulcoites measured to date, but was determined to be consistent with an acapulcoite classification. A method to distinguish acapulcoites from winonaites was recently devised by Rumble III et al. (2005). Utilizing their diagram comparing the Fa mol% of olivine vs. the Δ17O‰, it is demonstrated that NWA 2775 (Fa14.5; ave. Δ17O –0.75‰) plots in a unique location at the edge of the acapulcoite field. The reason for the observed correlation between the Fa content in olivine and the Δ17O value for acapulcoites was considered by Irving et al. (2007). They suggested the possibility that a metal-poor impactor with a Δ17O value plotting close to the terrestrial fractionation line, similar to a brachinite, was mixed into the regolith of a body having olivine and an O-isotopic composition similar to a CH chondrite; subsequent to the collision, the mixture was thermally equilibrated.

The closure of the Hf–W chronometer on the ACA–LOD parent body occurred 4.5621 (±0.0014) b.y. ago, or 6.4 (±1.3) m.y. after CAI formation (Touboul et al., 2007). A slightly older Hf–W age of 3.84 (+3.6/–3.1) m.y. after CAI formation was calculated by Schulz et al. (2010). With other factors considered, they concluded that the metal melting point, or the cooling point at which redistribution of Hf and W between metal and silicate ended, occurred 4.1 (+1.2/–1.1) m.y. after CAIs. Although the ACA–LOD parent body reached higher temperatures than did the H chondrite parent body by assimilating a higher abundance of radiogenic nuclides during its earlier accretionary period, it also cooled more rapidly at high temperatures, possibly reflecting a smaller-sized planetesimal and/or a near-surface residence for the acapulcoites (Kleine et al., 2007). Another possibility for its early rapid cooling could be the fact that it experienced a collisional disruption early in its history forming sub-km- to multi-km-sized fragments, which eventually succumbed to gravitational reassembly.

For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The specimen of NWA 2775 shown above measures 35 mm × 19 mm and weighs 2.68 g. The photo below shows a magnified image of this acapulcoite. standby for northwest africa 2775 photo
Photos courtesy of S. Turecki


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NWA 2627

Acapulcoite, transitional subgroup
Acapulcoite–Lodranite Clan nwa 2627
click on photo for a magnified view Found 2004
no coordinates recorded A 68 g portion of a stone retaining considerable fusion crust was purchased in Erfoud, Morocco by M. Farmer in October of 2004. A sample was submitted for analysis to Northern Arizona University (T. Bunch and J. Wittke), and NWA 2627 was initially determined to be an anomalous winonaite. The stone has a recrystallized texture and is composed of a heterogeneous mixture of orthopyroxene (44 vol%), olivine (41 vol%), FeS (6 vol%), metallic phases (5 vol%), and plagioclase (3 vol%), along with minor abundances of merrillite, Ca-pyroxene, and chromite.

The mineralogy and composition of this meteorite are similar to those for the lodranites, and when plotted on a diagram comparing the Δ17O-isotopic value vs. Fa mol% in olivine (Rumble III et al, 2005), NWA 2627 clearly lies within the acapulcoite field (see Figure 2 plot in LPSC 38, #2254 [2007]). It has been suggested that this meteorite represents a transitional acapulcoite (Touboul et al., 2007) according to the following classification scheme by Floss (2000) and Patzer et al. (2003) based on metamorphic stage.

  1. primitive acapulcoites: near-chondritic (Se >12–13 ppm [degree of sulfide extraction])
  2. typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
  3. transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
  4. lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
  5. enriched acapulcoites (addition of feldspar-rich melt component)

The high temperatures experienced on the acapulcoite parent body, manifest in the recrystallized texture and occasional partial melt phase, suggest that it accreted very early in Solar System history when radiogenic 26Al was still extant. This was a time period spanning ~1–3 m.y. after CAI formation, after the accretion of differentiated parent bodies had occurred but before the accretion of chondritic parent bodies had begun (Touboul et al., 2007). However, this formation scenario is inconsistent with some acapulcoites having ages younger than that attributed to the complete extinction of radiogenic 26Al. An impact shock heating model has been proposed by Rubin (2007), the details of which can be found on the Monument Draw page.

Data for cooling rates indicate a more rapid cooling for the acapulcoite parent body than that which the bulk H chondrite parent body experienced (but similar to that of H4 chondrites; Kleine et al., 2007), consistent with a smaller acapulcoite parent body and/or a near-surface residence for the acapulcoites. The possibility also exists for a collisional disruption early in its history, forming sub-km- to multi-km-sized fragments, which eventually succumbed to gravitational reassembly. Compared to the lodranite lithological unit, both primitive and typical acapulcoite material is thought to have originated in the outermost layer of the asteroid where it cooled earlier and faster consistent with its older gas retention age, finer-grain size, and less intense metamorphism (<1–3% silicate partial melting). The lodranites experienced higher degrees of FeNi–FeS melting as well as silicate partial melting (~5–20%), with loss of an FeS and a basaltic component. The transitional acapulcoites exhibit features (e.g., HSE-rich metal) consistent with extensive melting of metal and sulfide phases, including melt migration and pooling, representing a continuum between the formation of acapulcoites and lodranites, or alternatively, representing formation at greater depths associated with core formation (Dhaliwal et al., 2017). For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Lodran page.

Northwest Africa 2627 has been weathered to grade W2/3 and has been shocked to stage S2. The 210 g acapulcoite NWA 4399 might be paired. The photo of NWA 2627 above is a 0.88 g partial slice, photographed by Jim Strope.


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NWA 1670

Angrite
Basaltic/Quenched, Picritic
standby for nwa 1670 photo
Purchased January 1, 2001
no coordinates recorded A single 30.6 g stone that was found in 2001 in Morocco, possibly in Attamina, was subsequently sold in Erfoud, Morocco to Bruno Fectay and Carine Bidaut. Northwest Africa 1670 was classified at two French institutions, the Université Pierre & Marie Curie (A. Jambon and O. Boudouma) and the Université d’Angers (J-A. Barrat), and has been described as a highly shocked angrite representative of an impact melt.

 

Northwest Africa 1670 has been described (Mikouchi et al., 2003; Jambon et al., 2008) as having a porphyritic texture, primarily consisting of a very fine-grained groundmass (82 vol%) composed of lath-shaped grains up to 1 mm in size consisting primarily of fassaite and plagioclase. Embedded within the groundmass are highly-magnesian (Fo9688) olivine xenocrysts (~18–20 vol%) measuring 0.5–3 mm in size, but it is considered likely they were all in the larger size range before sectioning (Mikouchi, 2014). Along the rims of the olivine xenocrysts in NWA 1670 are <1 mm-sized euhedral olivine phenocrysts that crystallized from the groundmass melt. As reported in D’Orbigny and A-881371, xenocrysts in NWA 1670 contain ~5–10 µm-sized inclusions consisting of FeNi-metal and sulfides, while some contain traces of fluid inclusions (Mikouchi et al., 2011; Mikouchi, 2014). These magnesian olivine xenocrysts were formed under reducing conditions before they were incorporated into the oxidizing parental melt of the groundmass (Mikouchi et al., 2015). The olivine xenocrysts are proposed to be zoned mantle material which was incorporated into an ascending magma and was subsequently quenched upon eruption onto the surface of a relatively large angrite protoplanet. However, an alternative formation scenario through a severe impact melting event is still under consideration (Mikouchi, 2014). Either way, angrites represent some of the earliest known differentiated material from a Solar System object, and with a U-corrected Pb–Pb age of 4.56437 (±0.00019) b.y., NWA 1670 is the oldest known angrite (Bizzarro et al., 2013).

 

As in other angrites, the plagioclase is nearly chemically pure anorthite (An99100), but is more Fe-enriched. Lesser amounts of calcic olivine are incorporated as patches within the fassaite. Accessory phases include spinel (both xenocrystic and groundmass types), FeS, kirschsteinite, Ti-magnetite, and Ca-silicophoshate. Ca-carbonate droplets (up to 5 µm) are trapped in pyroxene. Alkalies such as Na and K are lacking, possibly as a result of loss during impact events. Trace element and REE data for NWA 1670 are similar to that for the other quenched angrites, and along with the similar mineralogies, indicates a common magmatic origin (Sanborn and Wadhwa, 2010; Mikouchi et al., 2011).

 

Northwest Africa 1670 is typical in many respects to other angrites, being derived from a primary angritic source melt—the apparent differences among them can be attributed in large part to the accumulation of xenocrystic, highly magnesian olivine and to pyroxene accumulation. The groundmass texture and olivine zoning profiles in NWA 1670 are consistent with that of a more rapidly quenched melt located at very shallow depths. The low Si content and the overabundance of Ca in many mineral phases of NWA 1670 attests to melting in the presence of carbonate (Jambon et al., 2005). This is a process unique to angrites, which might illustrate one of the earliest stages of Solar System evolution.

 

It was proposed by Mikouchi et al (2001) that a rapidly cooling magma (~10–50°C/hour) entrained locally variable amounts of magnesian olivine xenocrysts derived from the mantle into the groundmass melt. Cooling rate data acquired with respect to chemical zoning of olivine xenocrysts gave consistent rates of 7–13°C/hour (Mikouchi et al., 2008). The lower Mn/Cr ratios obtained by Sugiura et al. (2003) are also consistent with rapid cooling within a thin lava flow at a depth of ~0.5–2 m. In further contrast to other angrites (with the exception of strongly shocked NWA 7203; Hayashi et al., 2018), NWA 1670 exhibits signs of a severe shock event, as evidenced by mosaicism and undulose extinction in olivine xenocrysts, and by the presence of cracks and impact-melt veins. In view of the shock deformation features present in NWA 1670 (and other quenched angrites) olivine xenocrysts, late metamorphism associated with an impact-shock event is considered a possibility (Jambon et al., 2008; Mikouchi et al., 2015, 2017).

 

The Mn–Cr ages of NWA 1670, Asuka 881371/12209, D’Orbigny, and Sah 99555 are identical and represent the oldest angrite crystallization ages. Despite the fact that D’Orbigny and Sah 99555 lack olivine xenocrysts, NWA 1670 likely originated from a common magma source, as did the two other olivine xenocryst-bearing (picritic) quenched angrites LEW 87051 and Asuka 881371/12209. By inferring the amount of dissolved olivine xenocrysts each of these angrites should contain, it was ascertained that they, along with NWA 1296, have similar bulk elemental compositions supporting a common magma source controlled by fractional crystallization with or without addition and resorption of Mg-rich olivine xenocrysts (Mikouchi and Bizzarro, 2012). Furthermore, the chemical composition of the NWA 2999 pairing group shows that it also derives from a picritic source magma, which thereafter experienced further fractional melting, metamorphism, and annealing, along with incorporation of an exogenous metal component (Baghdadi et al., 2015). NWA 1670 contains the most magnesian (Fo96) olivine xenocrysts of any angrite (or achondrite) known and also contains FeNi-metal grains, which suggests that it originated on a large, reduced angrite parent body having a significant metallic core; the xenocrysts subsequently experienced a period of oxidation prior to incorporation into the parental melt (Mikouchi et al., 2017).

 

In order to better constrain the properties of the differentiated angrite parent body core, van Westrenen et al. (2016) conducted a study modeling siderophile element depletions along with their metal–silicate partitioning behavior for the hypothesized angrite parental melt composition. A CV chondrite mantle composition was used for their calculations, along with a temperature and pressure (0.1 GPa) appropriate for a solidifying melt on a small planetesimal. Their results indicate that the observed siderophile element depletions of angrites are consistent with a core mass fraction of 0.12–0.29 composed of Fe and Ni in a ratio of ~80:20 (with a low S content), and that it was formed under redox conditions (oxygen fugacity) of ΔIW–1.5 (±0.45).

 

A CRE age of ~15–18 m.y. was calculated for the both NWA 1670 and LEW 86010 angrites, possibly representing a single ejection event (Eugster et al., 1991; Herzog and Caffee, 2014). This event might also include the angrite NWA 7812 with a CRE age of 20–21 m.y., since this age should be considered an upper limit based on the possibility that it contains a solar cosmic ray Ne component (Wieler et al., 2016). In addition, a similar CRE age of 20.3 (±2.2) m.y. was calculated by Takenouchi et al. (2019) for the quenched angrite NWA 7203. They recognized that NWA 1670 and NWA 7203 are the only angrites that exhibit shock features, which are manifest in the form of melt veins. An Ar–Ar age of 3.80 (±0.44) b.y. was ascertained, which Takenouchi et al. (2019) believe best represents the timing of this shock event.

 

Multiple episodes of impact, disruption, and dissemination of the crust can be inferred by the wide range of CRE ages determined for the angrites—<0.2–56 m.y. for thirteen angrites measured to date, possibly representing as many ejection events (Nakashima et al., 2008; Wieler et al., 2016; Nakashima et al., 2018). This range is consistent with a single large parent body enduring multiple impacts over a very long period of time, which would suggest that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 56 m.y. Alternatively, Nakashima et al. (2018) consider it plausible that there is currently at least two angrite (daughter) objects occupying distinct orbits: one representing the fine-grained (quenched) angrites with the shorter CRE age range of <0.2–22 m.y., and another representing the coarse-grained (plutonic) angrites with the longer CRE age range of 18–56 m.y. (see diagram below).
Cosmic-ray Exposure Ages of Angrites
standby for angrite cre age diagram
Diagram credit: Nakashima et al., MAPS, Early View, p. 14 (2018)
Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures’
(http://dx.doi.org/10.1111/maps.13039)

In a study of remanent magnetism in angrites, Weiss et al. (2008) discovered that a magnetic field with a strength of ~10 µT (microteslas), ~20% of that of present-day Earth, was imparted to the angrite PB during its earliest phase of crystallization. This magnetic field may be attributable to a number of possible causes; e.g., accretion to an orbit in close proximity to the early T-Tauri phase solar field, or perhaps more plausible, a magnetic field generated through an internal core-dynamo mechanism. Subsequent paleomagnetic intensity studies conducted for D’Orbigny, Sahara 99555, and Angra dos Reis by Wang et al. (2015) have established a natural remanent magnetization value for Angra dos Reis of ~15 µT (microteslas), demonstrating that this lithology formed under the influence of a significant core dynamo which existed ~11 m.y. after CAIs. By comparison, no natural remanent magnetization (paleointensity) > ~1 µT was detected for the earlier formed angrites D’Orbigny and Sahara 99555, which constrains the onset of the APB core dynamo to later than ~4 m.y. after CAI formation. It was also recognized that the strong solar nebula-generated magnetic field which had existed ~1.2–3 m.y. after CAIs (~50 µT, measured in Semarkona chondrules) had virtually disappeared by the time the earliest angrites were formed, indicating that the solar nebula had already been largely dissipated.
standby for angrite dynamo timeline diagram
Diagram credit: Wang et al., 46th LPSC, #2516 (2015)
A limited number of unique angrites are represented in our collections today, and they have been grouped as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample NWA 8535 (photo courtesy of Habib Naji). Another quenched angrite, NWA 7203 (photo courtesy of Labenne Meteorites), exhibits a striking variolitic texture. Interestingly, small fine-grained basalt clasts exhibiting textures and mineralogy generally consistent with a quenched angrite-like impactor are preserved in impact melt glass fragments recovered from the significant impact event that occurred ~5.28 m.y. ago near Bahía Blanca, Argentina (Schultz et al., 2006; Harris and Schultz, 2009, 2017; see photo below). The specimen of NWA 1670 pictured above is a 0.25 g partial slice.
standby for bahia blanca angritic fragments photo
Photo credit (left): Schultz et al., MAPS, vol. 41, #5, p. 755 (2006) (http://dx.doi.org/10.1111/j.1945-5100.2006.tb00990.x)
Diagram credit (right): Harris and Schultz, 40th LPSC, #2453 (2009)


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NWA 1617

Acapulcoite
Acapulcoite–Lodranite Clan
standby for northwest africa 1617 photo
Purchased June 2002
no coordinates recorded A small 23 g weathered stone was purchased in Agadir, Morocco by Moroccan dealer A. Habibi on behalf of American collector N. Oakes. A portion of the meteorite was submitted for classification to the University of Washington in Seattle (A. Irving and S. Kuehner), and it was initially classified as a winonaite (MetBull 88); however, this classification was eventually changed. Northwest Africa 1617 is composed primarily of magnesian bronzite and forsteritic olivine, along with minor FeNi-metal, troilite, chromite, and sodic plagioclase.

The classification as a winonaite was based on those components in NWA 1617 which represent a high degree of oxidation, such as is manifest in the lack of daubréelite and the occurrence of very Cr-rich chromite. More oxidizing conditions are consistent with the later-crystallizing, silicate-dominated inclusions of the high-Ni IAB irons, whereas more reducing conditions are expected to prevail in the graphite–troilite inclusions that predominate among the later crystallizing, low-Ni IAB irons. Also significant is the fact that the Ni content of metal in these irons was found to be positively correlated with the contents of fayalite and daubréelite (Benedix et al., 1995), consistent with what is found in NWA 1617. The plagioclase composition of NWA 1617 (Ab81) is almost identical to that found in the NWA 1457 winonaite. Likewise, the O-isotopic composition of NWA 1617 plots close to the winonaite group. Therefore, it was proposed that NWA 1617 was an unusual winonaite that extends the compositional limits of the winonaite/IAB iron group.

Notably, in the initial analysis it was found that the silicates in NWA 1617 are more FeO-rich (Fa11.6, Fs11.2) than those in typical winonaites (the most FeO-rich winonaite previously studied is Fa5.3, although silicate inclusions in the IAB-related iron Udei Station are compositionally close [Fa8, Fs9]). In fact, the values are more consistent with those of acapulcoites. In 2005, a method to distinguish acapulcoites from winonaites was devised by Rumble III et al. (2005). They utilized a diagram comparing the Fa mol% of olivine vs. the Δ17O‰, and on this diagram NWA 1617 (Fa11.6; Δ17O = –0.86 [±0.02] ‰) plots well within the acapulcoite field close to Monument Draw. Northwest Africa 1617 has now been reclassified as an acapulcoite (MetBull 90). The average grain size of NWA 1617 is 350 µm, which distinguishes it from the more slowly cooled lodranite component of this parent body which are distinguished by grain sizes larger than 500 µm. However, with many new members it is now evident that a continuum exists for the grainsizes of these two groups, and it has been proposed by Bunch et al. (2011) that an arbitrary group division is no longer justified; the term ‘acapulcoite–lodranite clan’ should therefore be applied to all members of the combined group.

For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The specimen of NWA 1617 shown above is a 0.53 g partial slice. The photo below shows a cut face of the main mass of this small stone. standby for northwest africa 1617 photo
Photo courtesy of N. Oakes—Meteorites–R–Us