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NWA 10019

Pyroxene-plagioclase pallasite, ungrouped
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Purchased January 2015
no coordinates recorded Within the Northwest Africa (NWA) dense accumulation area a single 580 g fusion-crusted meteorite was found along with 26 g of related fragments. The meteorite was subsequently sold to meteorite dealer Steve Arnold who submitted a type sample to the University of New Mexico (C. Agee and N. Muttik) for analysis and classification. Northwest Africa 10019 is an ungrouped pyroxene–plagioclase-bearing pallasite that exhibits significant compositional heterogeneity. It is composed of olivine (up to 6 mm), orthopyroxene (up to 5 mm), kamacite, and taenite, along with minor Ca-plagioclase, troilite, chromite, schreibersite, and Ca–Mg-phosphates (farringtonite, stanfieldite, and merrillite).

Northwest Africa 10019 is unique among all pallasites in that it contains a low abundance (<1 vol%) of Ca-plagioclase (An50–84); the pyroxene pallasite Choteau is the only other pallasite found to contain plagioclase, but in that pallasite it is highly albitic (Ab85.6). Boesenberg et al. (2016) observed that plagioclase in NWA 10019 is present as fine (10–50 µm) to coarse (2 mm) grains within olivine and orthopyroxene. The angular olivines in NWA 10019 have a Fa content (~Fa16.5) that is appreciably higher than typical main-group pallasites, but similar to the subset of main-group pallasites with anomalous silicates (Springwater, Rawlinna 001, Phillips County, and Zaisho). Boesenberg et al. (2016) identified a unique enclave in NWA 10019 that has more primitive phases than the rest of the pallasite, including Mg-rich chromite, slightly more magnesian olivine, and plagioclase that has a broader compositional range. In addition, they reported that the pallasite has an Fe/Mn value of 28–37.

To date, seven pyroxene-bearing meteorites having a pallasite-like composition have been characterized: the ‘Vermillion pallasite grouplet’ (Choteau, Vermillion, and Y-8451), Zinder, NWA 1911, NWA 10019, and LoV 263. Vermillion is composed of 86 vol% FeNi-metal and 14 vol% silicates, with the silicates consisting of 93% olivine and 5% pyroxene (4.9% opx and 0.1% cpx)—equivalent to a modal composition of ~0.7 vol% pyroxene. Wasson and Kallemeyn (2002) recognized that Vermillion might be related to the IAB complex iron meteorites. The 54.8 g Y-8451 pallasite contains 57 vol% silicates consisting of 97% olivine, 2% orthopyroxene, 0.4% clinopyroxene, and 0.4% augite. The silicates in Y-8451 are modally equivalent to ~1.6 vol% pyroxene (Boesenberg et al., 2000). The 46 g Zinder pallasite has a high modal abundance of pyroxene, similar to that in NWA 1911, estimated to be 28 vol% (Wittke and Bunch, 2003). The modal abundance of silicates in NWA 10019 is ~60%, comprised of olivine (~43–51 vol%) and orthopyroxene (~9–17 vol%) with pyroxene accounting for ~1–5 vol% of this pallasite (Boesenberg et al., 2016). The silicates in the 4.88 kg LoV 263 pallasite are comprised of approximately equal proportions of olivine and orthopyroxene.

In a study conducted by Gregory et al. (2016), it was ascertained that Choteau is compositionally and isotopically similar to both Vermillion and Y-8451, and it was concluded that these three pyroxene pallasites form a grouplet; they suggested that these meteorites should be termed ‘Vermillion pallasites’ (see the Vermillion page for additional details). The low-Ca pyroxene in Zinder, NWA 1911, NWA 10019, and LoV 263 is composed entirely of orthopyroxene (orthopyroxene in NWA 10019 contains ~100µm-sized clinopyroxene inclusions; Boesenberg et al., 2016), while that in the Vermillion pallasites comprises both orthopyroxene and clinopyroxene (Niekerk, 2005; Irving and Kuehner, 2013). Zinder contains a higher abundance of chromite compared to the Vermillion pallasites. The O-isotopic compositions of the Vermillion pallasites are distinct from the other four pyroxene-bearing pallasites, and many are associated with a number of established O-isotopic trends: the Vermillion pallasites plot near the field of acapulcoites and lodranites, and both NWA 1911 and NWA 10019 plot on the eucrite/mesosiderite fractionation line, which remains incompletely resolved from the bimodal fractionation trend of the main-group pallasites (Ziegler and Young, 2011; K. Ziegler, 2015). Although Zinder has been demonstrated to be associated with NWA 1911 (Boesenberg and Humayun, 2019), it plots on the terrestrial fractionation line due to a difference in δ17O values; however, terrestrial weathering may be the reason for this difference.

A separate O-isotopic analysis for NWA 10019 was conducted by Boesenberg et al. (2016), and it provided values which plot on an extension of the main-group near the pyroxene pallasite NWA 1911. However, many mineralogical features distinguish NWA 10019 from the main-group pallasites, including the presence of plagioclase, a significantly lower abundance of volatile elements (e.g., Ga, Ge, As and Au), high Al content in chromite, and metal that is more evolved than in any other pallasite. standby for nwa 10019 oxygen isotope diagram
Oxygen isotope composition of NWA 10019 compared to main-group pallasites and mesosiderites (left) and HEDs (right).
TFL = terrestrial fractionation line; EFL = eucrite fractionation line
Diagrams adapted from the Meteoritical Bulletin Oxygen Isotope Plots—The Meteoritical Society

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Diagram credit: Gregory et al., 47th LPSC, #2393 (2016)

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Diagram credit: Boesenberg et al., 47th LPSC, #2297 (2016) Based on the results of their study, Boesenberg et al. (2016) determined that NWA 10019 and the main-group pallasites formed from a similar O-isotopic reservoir but under very different petrologic conditions, and they concluded that NWA 10019 and the main-group pallasites derive from distinct parent bodies. Boesenberg et al. (2018) utilized a coupled Fe/(Fe+Mg) vs. Al/(Cr+Al) diagram in an analysis of chromite for various pallasites. They demonstrated that NWA 10019 chromite contains a relatively high Al content and plots in a unique compositional space (see diagram below). standby for chromite fe vs. al diagram
Diagram credit: Boesenberg et al., 49th LPSC, #1556 (2018) Based on all of the data gathered so far, it could be concluded that the pallasites in our collections represent at least seven separate parent bodies: 1) main-group; 2) Eagle Station group; 3) Milton; 4) Choteau + Vermillion + Y-8451; 5) Zinder + NWA 1911; 6) NWA 10019; 7) LoV 263. In addition, several pallasites with anomalous silicates (e.g., Springwater) and anomalous metal (e.g., Glorieta Mountain) could possibly increase the number of unique parent bodies. The specimen of NWA 10019 shown above is a 2.018 g partial slice. The photo shown below is a large slice from this unique pyroxene pallasite. standby for nwa 10019 slice photo
Photo courtesy of Steve Arnold


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NWA 1911

Pyroxene pallasite, ungrouped
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Purchased March 2003
no coordinates recorded A fresh (W1), complete stone, weighing 53.07 g, was retrieved by M. Farmer from a batch of meteorites shipped to him from Rissani, Morocco; this is the first pallasite recognized to be found in Northwest Africa. Northwest Africa 1911 was analyzed and classified at Northern Arizona University (Wittke and Bunch, 2003), and was found to have a modal composition of 24.3% FeNi-metal and 75% silicates, with the silicates consisting of 40.2% olivine and 34.5% orthopyroxene—the highest pyroxene content recorded for a pallasite. Minor troilite and chromite are also present, as well as trace merrillite.

In a study of NWA 1911 conducted by Boesenberg and Humayun (2019), they determined that the metal composition was extremely similar to that of the Zinder pyroxene pallasite. Utilizing a coupled Fe/(Fe+Mg) vs. Al/(Cr+Al) diagram in an analysis of chromite for various pallasites, they demonstrated that chromite in both NWA 1911 and Zinder contains a relatively high Al content and plots in a common unique compositional space (see diagram below). standby for chromite fe vs. al diagram
Diagram credit: Boesenberg and Humayun, 50th LPSC, #1438 (2019) Interestingly, the pyroxene pallasite Zinder was previously found to contain metal with a composition that is chemically identical to that of group IIIF irons (Boesenberg et al., 2017; Humayun et al., 2018). However, the IIIF irons formed in the carbonaceous region of the Solar System beyond Jupiter, whereas the negative ε54Cr and δ26Mg* values of Zinder indicate that it formed in the non-carbonaceous region of the inner Solar System (Wimpenny et al., 2019). See the Appendix Part III for further details about the two regions.

To date, seven pyroxene-bearing meteorites having a pallasite-like composition have been characterized: the ‘Vermillion pallasite grouplet’ (Choteau, Vermillion, and Y-8451), Zinder, NWA 1911, NWA 10019, and LoV 263. Vermillion is composed of 86 vol% FeNi-metal and 14 vol% silicates, with the silicates consisting of 93% olivine and 5% pyroxene (4.9% opx and 0.1% cpx)—equivalent to a modal composition of ~0.7 vol% pyroxene. Wasson and Kallemeyn (2002) recognized that Vermillion might be related to the IAB complex iron meteorites. The 54.8 g Y-8451 pallasite contains 57 vol% silicates consisting of 97% olivine, 2% orthopyroxene, 0.4% clinopyroxene, and 0.4% augite. The silicates in Y-8451 are modally equivalent to ~1.6 vol% pyroxene (Boesenberg et al., 2000). The 46 g Zinder pallasite has a high modal abundance of pyroxene, similar to that in NWA 1911, estimated to be 28 vol% (Wittke and Bunch, 2003). The modal abundance of silicates in NWA 10019 is ~60%, comprised of olivine (~43–51 vol%) and orthopyroxene (~9–17 vol%) with pyroxene accounting for ~1–5 vol% of this pallasite (Boesenberg et al., 2016). The silicates in the 4.88 kg LoV 263 pallasite are comprised of approximately equal proportions of olivine and orthopyroxene. standby for o-isotopic diagram
Diagram credit: Gregory et al., 47th LPSC, #2393 (2016) In a study conducted by Gregory et al. (2016), it was ascertained that Choteau is compositionally and isotopically similar to both Vermillion and Y-8451, and it was concluded that these three pyroxene pallasites form a grouplet; they suggested that these meteorites should be termed ‘Vermillion pallasites’ (see the Vermillion page for additional details). The low-Ca pyroxene in Zinder, NWA 1911, NWA 10019, and LoV 263 is composed entirely of orthopyroxene (orthopyroxene in NWA 10019 contains ~100µm-sized clinopyroxene inclusions; Boesenberg et al., 2016), while that in the Vermillion pallasites comprises both orthopyroxene and clinopyroxene (Niekerk, 2005; Irving and Kuehner, 2013). Zinder contains a higher abundance of chromite compared to the Vermillion pallasites. The O-isotopic compositions of the Vermillion pallasites are distinct from the other four pyroxene-bearing pallasites, and many are associated with a number of established O-isotopic trends: the Vermillion pallasites plot near the field of acapulcoites and lodranites, and both NWA 1911 and NWA 10019 plot on the eucrite/mesosiderite fractionation line, which remains incompletely resolved from the bimodal fractionation trend of the main-group pallasites (Ziegler and Young, 2011; K. Ziegler, 2015). Although Zinder has been demonstrated to be associated with NWA 1911 (Boesenberg and Humayun, 2019), it plots on the terrestrial fractionation line due to a difference in δ17O values; however, terrestrial weathering may be the reason for this difference.

Based on all of the data gathered so far, it could be concluded that the pallasites in our collections represent at least seven separate parent bodies: 1) main-group; 2) Eagle Station group; 3) Milton; 4) Choteau + Vermillion + Y-8451; 5) Zinder + NWA 1911; 6) NWA 10019; 7) LoV 263. In addition, several pallasites with anomalous silicates (e.g., Springwater) and anomalous metal (e.g., Glorieta Mountain) could possibly increase the number of unique parent bodies. Notably, the O-isotopic ratios for both Milton and the Eagle Station group pallasites plot on an extension of the trend line for the CV chondrites, and Choteau might be derived from the acapulcoite–lodranite parent body. Further information on the pyroxene pallasites can be found on the Vermillion page. The specimen of NWA 1911 shown above is a 6.47 g slice.


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Milton

Pallasite, ungrouped
‘CX’ trend
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Found October 2000
40° 17′ 15′ N., 95° 22′ 36′ W. While clearing rocks from his soybean field in Fairfax, Missouri, G. Wennihan found an unusually heavy one that had a rusty appearance. He tossed this 2,038 g rock into the back of his pickup truck to save. The large mass was eventually cut in half, and the unique appearance of the interior raised speculations that it originated in space. Eventually a friend of his who was a geology student, B. Rogers, took the strange rock to the geology department of Northwest Missouri State University where it was cleaned and examined. Although assistant geology professor Richard Felton and several faculty members examined the rock, it was Dr. Renee Rohs who recognized its resemblance to an Imilac specimen that she had seen years earlier while attending a class taught by Dr. Van Schmus (Horejsi and Cilz, 2002). Reasonably, the rock was taken to Dr. Van Schmus at the University of Kansas for his qualified opinion, and he immediately recognized that it was a pallasite. Samples of the pallasite were sent to the Institute of Meteoritics at University of New Mexico and to UCLA for thorough analyses.

Milton has a high abundance of small, angular olivines (73 vol%, Fo84.1) within an FeNi-metal matrix (Jones et al., 2003). The metal composition is relatively homogenous with respect to siderophile and highly siderophile elements. Chemical, mineral, and O-isotopic data indicate that Milton is not genetically related to other pallasites. The metal in Milton lacks taenite cloudy zones and shows no evidence of shock reheating, which attests to the fastest cooling rate among pallasites at >5000K/m.y. (Yang et al., 2010). The olivine in Milton is zoned in Ca and Cr, and has a higher molar Fe/Mn ratio than that of other pallasites. Likewise, the composition of the FeNi-metal is different from that of the main-group and Eagle Station pallasite groups. In addition, Milton has O-isotopic ratios that are distinct from all other pallasite groups, and as with the Eagle Station group, Milton demonstrates a relationship with the carbonaceous chondrite anhydrous mineral mixing line (slope = 0.94 ±0.01). Notably, the O-isotopic ratios for both Milton and the Eagle Station group pallasites plot proximate to an extension of the trend line for the CV–CK chondrites. standby for o-isotopic diagram
Diagram credit: Gregory et al., 47th LPSC, #2393 (2016) Some ungrouped irons have similar O-isotopic ratios to Milton but have significantly different iron chemistry, which excludes a genetic relationship. However, it was argued by Reynolds et al. (2006) that the high-Ni irons which comprise the South Byron trio (Babb’s Mill [Troost’s], South Byron, and Inland Forts [ILD] 83500) have similar metal compositions (siderophile element patterns) and similar structures (kamacite spindles and associated schreibersite) compared to metal in Milton, and therefore these meteorites might constitute a grouplet that originated on a common parent body. Moreover, all of these irons and the metal in Milton experienced a similar oxidation history during core formation, as evidenced by the presence of FeO-rich olivine, chromite, and phosphate, as well as the depletions in other easily oxidized elements (McCoy et al., 2008). Siderophile element abundances for these four meteorites were shown by McCoy et al. (2017) to have very similar values. Interestingly, isotopic compositions (Mo, Ru, and W) and HSE abundances of the IVB irons and the Milton–South Byron trio grouping fall within the range of the oxidized CV–CK chondrites (Hilton et al., 2018). Carbonaceous vs. Non-carbonaceous Irons
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Diagram credit: Hilton et al., 49th LPSC, #1186 (2018) McCoy et al. (2017) also recognized that the presence of volatile siderophile elements in these meteorites indicates they were not derived from a high-temperature condensation process contrary to other high-Ni iron groups such as IVA, but instead oxidation (nebular or parent body) and fractional crystallization were the dominant formation processes. The Milton pallasite is a product of an early stage of fractional crystallization compared to the main-group pallasites, as well as with regards to its fractional crystallization sequence among the South Byron trio irons. Based on HSE abundance patterns, Hilton et al. (2018) concluded that Babb’s Mill (Troost’s) was first in the sequence (representing the first 1% of crystallized melt) followed soon thereafter by South Byron (2%), with ILD 83500 having the highest content of incompatible P being last in the sequence (42%). If Milton is part of a core–mantle boundary then crystallization apparently proceeded inwards similar to the crystallization process some envision for the IVA and IIIAB irons following mantle removal on their respective parent bodies. In their analyses of O-isotopic composions in chromite for these four meteorites, McCoy et al. (2017) demonstrated that they plot along a similar trend line on an oxygen three-isotope diagram (see below). Together with previous petrographic and geochemical data, this new O-isotopic data provides strong evidence supporting a common source parent body. standby for o-isotopic diagram
Diagram credit: McCoy et al., 48th LPSC, #2241 (2017) In addition to the irons mentioned above, several other ungrouped ataxites could be members of this high-Ni iron group, including El Qoseir, Illinois Gulch, Morradal, Nordheim, and Tucson. However, because of the significant differences that exist in their content of refractory elements compared to that in the South Byron trio, further work is needed to establish a definitive connection (Kissin, 2010). Investigators have also explored the possibility of a genetic relationship between IVB irons and other meteorite groups. Based on O-isotopic analyses utilizing chromite grains from IVB irons Warburton Range and Hoba, Corrigan et al. (2017) found that IVB irons share close similarities to the Milton–South Byron trio grouping (see diagram below). The O-isotopic compositions of the IVB irons and the South Byron trio–Milton grouping also fall within the range of the oxidized CV–CK chondrites. Moreover, Corrigan and McCoy (2018) found that both IVB irons and the Milton–South Byron trio show evidence for early oxidation (e.g., both have a similar high Ni content of ~15.5–18 wt% and ~15–18 wt%, respectively), as well as evidence for late reduction (e.g., both contain reduced mineral phases such as troilite, daubréelite, and schreibersite). standby for o-isotopic relationship between groups diagram
Diagram credit: Corrigan et al., 48h LPSC, #2556 (2017)
Hoba: Δ17O = –3.4 [±0.2] ‰
Warburton Range: Δ17O = –3.4 [±0.4] ‰
Milton–South Byron trio: Δ17O = ~ –3.6 [±0.6] ‰ Silicate phases in Milton are enriched in the siderophile and highly siderophile elements which typically partition into metal phases (Hillebrand et al., 2004). Because of the unusual homogeneity of its metal and silicates, Milton has served as a good tool for J. Hillebrand (2004) to determine the in situ metal/silicate partition coefficients of a pallasite. Milton is a unique representative of its parent asteroid, and demonstrates that the petrogenesis of pallasites must have occurred in a similar way on multiple parent bodies. Interestingly, the ungrouped metachondrite NWA 10503 has been conjectured to have a possible affinity to the Milton pallasite (Irving et al., 2016). Not only does this unique meteorite share with Milton an association with carbonaceous chondrites as attested by their elevated silicate FeO/MnO ratios, but it also falls along an extension of the trend line established by Milton on an oxygen three-isotope diagram (see below). standby for nwa 10503 o-isotope diagram
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Diagram credit: Irving et al., 79th MetSoc, #6461 (2016) In an effort to better resolve potential genetic relationship that might exist between Milton and the CV chondrites, a Cr-isotopic analysis of olivine from the Milton pallasite was conducted by Sanborn et al. (2018). It is demonstrated on a coupled Δ17O vs. ε54Cr diagram (shown below) that Milton plots among the CV clan and plausibly shares a genetic relationship, but also that Eagle Station plots closer to the CK (or CO) chondrite group. It could be inferred that both the CV and CK planetesimals experienced a similar petrogenetic history in a similar isotopic reservoir of the nascent Solar System. Chromium vs. Oxygen Isotope Plot
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click on diagram for a magnified view

Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018) In an effort to further resolve differences between the CV and CK chondrite groups, Yin and Sanborn (2019) analyzed Cr isotopes in a significant number and broad range of meteorites. Their study included samples from each of the three CV subgroups (oxA, oxB, Red), anomalous CV3 chondrites, a C3-ungrouped, several CK members, and other potential CV-related meteorites including NWA 10503 and Milton (see top diagram below). It is demonstrated that the CV and CK meteorites are clearly resolved into two distinct isotopic reservoirs. In addition, it is shown by the ε54Cr values that NWA 10503 plots among the CV-related meteorites. A coupled Δ17O vs. ε54Cr diagram plotting all of the meteorites in their study is shown at the bottom below. Cr Isotope Weighted Average For CV and CK Chondrites
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17O vs. ε54Cr Diagram For CV and CK Chondrites
CK: orange shades; CV: green shades; Achondrites: open
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Diagrams credit: Yin and Sanborn et al., 50th LPSC, #3023 (2019)
In a study of two newly recovered ungrouped carbonaceous meteorites, the unequilibrated chondrite NWA 11961 and the dunitic breccia NWA 12264, Irving et al. (2019) further populated the O-isotopic trend line previously defined by NWA 10503 and the Milton pallasite; they termed this the ‘CX’ trend. However, Cr isotope data obtained for all of these meteorites have resolved both NWA 11961 and NWA 12264 as potential new carbonaceous parent bodies distinct from that of NWA 10503 and Milton, the latter previously considered possible members of the CV-clan (see diagrams below). ‘CX’ Oxygen Isotope Trend Line

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O–Cr Diagram for ‘CX’ Trend Meteorites
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Diagrams credit: Irving et al., 50th LPSC, #2542 (2019)
Based on all of the data gathered so far, it could be concluded that the pallasites in our collections represent at least seven separate parent bodies: 1) main-group; 2) Eagle Station group; 3) Milton; 4) Choteau + Vermillion + Y-8451; 5) Zinder + NWA 1911; 6) NWA 10019; 7) LoV 263. In addition, several pallasites with anomalous silicates (e.g., Springwater) and anomalous metal (e.g., Glorieta Mountain) could possibly increase the number of unique parent bodies. The specimen of Milton shown above is a 40.1 g partial slice sectioned from an 85 g slice that was acquired from the owner of the main mass, J. Piatek. The top two photos below show the cut face of a 500 g end section and the natural suface of a 677 g end section of Milton. The bottom photo shows a 2 cm-wide magnified image of an interior slice of Milton, courtesy of Dr. Laurence Garvey. standby for milton photo
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Photos courtesy of Dr. Jay Piatek

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Photo courtesy of Dr. Laurence Garvie—Arizona State University