Pyroxene-plagioclase pallasiteOne of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, sometimes peridot (green gem quality clear olivine crystals), embedded in a matrix of Fe-Ni metal. The type specimen, weighing 680 kg, was found in the mountains near Click on Term to Read More, ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More Purchased January 2015 no coordinates recorded Within the Northwest Africa (NWA) dense accumulation area a single 580 g fusion-crusted meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More 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 olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More (up to 6 mm), orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More (up to 5 mm), kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More, and taeniteLess common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon." Click on Term to Read More, along with minor Ca-plagioclase, troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites., chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More, schreibersiteNi-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found in a variety of other meteorites including some acapulcoites, aubrites, enstatite chondrites and achondrites, lunars, ureilites, winonaites and a smattering of other meteorite types like CM, CO and CB. Schreibersite Click on Term to Read More, 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 pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification Click on Term to Read More pallasite Choteau is the only other pallasite found to contain plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More, 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% augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite Click on Term to Read More. 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 acapulcoitesPrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated Click on Term to Read More and lodranites, and both NWA 1911 and NWA 10019 plot on the eucriteMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after Click on Term to Read More/mesosiderite fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. Click on Term to Read More 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 elementsChemical elements that condense (or volatilize) at relatively low temperatures. The opposite of volatile is refractory. Volatile elements can be divided into moderately volatile (Tc = 1230–640 K) and highly volatile (Tc < 640 K). The moderately volatile lithophile elements are: Mn, P, Na, B ,Rb, K, F, Zn. The moderately Click on Term to Read More (e.g., Ga, Ge, As and AuThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The Click on Term to Read More), high Al content in chromite, and metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More that is more evolved than in any other pallasite. OxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read MoreisotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More composition of NWA 10019 compared to main-group pallasites and mesosiderites (left) and HEDs (right). TFL = terrestrial fractionation line; EFL = eucriteMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after Click on Term to Read More fractionation line Diagrams adapted from the Meteoritical Bulletin Oxygen Isotope Plots—The Meteoritical Society
Diagram credit: Gregory et al., 47th LPSC, #2393 (2016)
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). 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. Photo courtesy of Steve Arnold
PyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification Click on Term to Read MorepallasiteOne of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, sometimes peridot (green gem quality clear olivine crystals), embedded in a matrix of Fe-Ni metal. The type specimen, weighing 680 kg, was found in the mountains near Click on Term to Read More, ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More 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% olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More and 34.5% orthopyroxene—the highest pyroxene content recorded for a pallasite. Minor troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites. and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More are also present, as well as trace merrillite.
In a study of NWA 1911 conducted by Boesenberg and Humayun (2019), they determined that the metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More 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). 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 SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. beyond Jupiter, whereas the negative ε54Cr and δ26Mg* values of Zinder indicate that it formed in the non-carbonaceous region of the inner Solar SystemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with (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% orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More, 0.4% clinopyroxene, and 0.4% augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite Click on Term to Read More. 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. 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 acapulcoitesPrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated Click on Term to Read More and lodranites, and both NWA 1911 and NWA 10019 plot on the eucriteMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after Click on Term to Read More/mesosiderite fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. Click on Term to Read More 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 chondritesMeteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich Click on Term to Read More, and Choteau might be derived from the acapulcoite–lodraniteRare type of primitive achondrite named after the Lodran meteorite that fell in Pakistan in 1868. Initially, lodranites were grouped with the stony-iron meteorites because they contain silicates (olivine, orthopyroxene, and minor plagioclase) and Fe-Ni metal in nearly equal proportions. However, since discovery of the closely related acapulcoite group, lodranites Click on Term to Read Moreparent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More. 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.
PallasiteOne of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, sometimes peridot (green gem quality clear olivine crystals), embedded in a matrix of Fe-Ni metal. The type specimen, weighing 680 kg, was found in the mountains near Click on Term to Read More, ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More ‘CX’ trend 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 MeteoriticsScience involved in the study of meteorites and related materials. Meteoritics are closely connected to cosmochemistry, mineralogy and geochemistry. A scientist that specializes in meteoritics is called a meteoriticist. Click on Term to Read More 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 matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More (Jones et al., 2003). The metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More composition is relatively homogenous with respect to siderophile and highly siderophile elements. Chemical, mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More, and O-isotopic data indicate that Milton is not genetically related to other pallasites. The metal in Milton lacks taeniteLess common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon." Click on Term to Read More 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 olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More 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 chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than Click on Term to Read More 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 chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More. 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 elementLiterally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on patterns) and similar structures (kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More spindles and associated schreibersiteNi-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found in a variety of other meteorites including some acapulcoites, aubrites, enstatite chondrites and achondrites, lunars, ureilites, winonaites and a smattering of other meteorite types like CM, CO and CB. Schreibersite Click on Term to Read More) compared to metal in Milton, and therefore these meteorites might constitute a grouplet that originated on a common parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More. Moreover, all of these irons and the metal in Milton experienced a similar oxidationOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More history during coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More formation, as evidenced by the presence of FeO-rich olivine, chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More, and phosphate, as well as the depletions in other easily oxidizedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More elements (McCoy et al., 2008). Siderophile elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More 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 fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More within the range of the oxidized CV–CK chondrites (Hilton et al., 2018). Carbonaceous vs. Non-carbonaceous Irons Diagram credit: Hilton et al., 49th LPSC, #1186 (2018) McCoy et al. (2017) also recognized that the presence of volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). 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 crystallizationA crystallization process in which minerals crystallizing from a magma are isolated from contact with the liquid. It is a key process in the formation of igneous rocks during the process of magmatic differentiation. Also known as crystal fractionation. Click on Term to Read More were the dominant formation processes. The Milton pallasite is a product of an early stage of fractional crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More 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–mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in Click on Term to Read More 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 oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More 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. 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 elementsUsing research by Wood (2019), any of the elements with a relatively high condensation temperature of 1291 K < TC,50 < 1806 K in the solar nebula1. They are the first elements to condense out of a cooling gas. Refractory elements are the main building blocks of rocky planets, dwarf Click on Term to Read More 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 meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More 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 reductionOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More (e.g., both contain reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More mineral phases such as troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites., daubréelite, and schreibersite). 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] ‰ SilicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the 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 metachondriteTerm used to describe a metamorphosed chondrite. Also referred to as a type 7 chondrite. Metachondrites are texturally evolved rocks derived from chondritic precursors and some have been classified as primitive achondrites. Click on Term to Read More 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). click on image for a magnified view
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 chondritesMeteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich Click on Term to Read More, 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) chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More group. It could be inferred that both the CV and CK planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – Click on Term to Read More experienced a similar petrogenetic history in a similar isotopic reservoir of the nascent Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids.. Chromium vs. Oxygen IsotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More Plot 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 chondriteClass of carbonaceous chondrite named for the Karoonda meteorite that fell in Australia in 1930. They are more oxidized than all other carbonaceous chondrites and genetically distinct from CV chondrites. CK chondrites appear dark-gray or black due to a high percentage of Cr-rich magnetite dispersed in a matrix of dark Click on Term to Read More 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 click on photo for a magnified view
17O vs. ε54Cr Diagram For CV and CK Chondrites CK: orange shades; CV: green shades; Achondrites: open click on photo for a magnified view
Diagrams credit: Yin and Sanborn et al., 50th LPSC, #3023 (2019) In a study of two newly recovered ungrouped carbonaceous meteorites, the unequilibrated chondriteA chondrite with heterogeneous mineral compositions (e.g., olivine grains with differing FeO/(FeO+MgO) ratios. NWA 11961 and the dunitic brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More 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 click on photo for a magnified view
O–Cr Diagram for ‘CX’ Trend Meteorites click on photo for a magnified view
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 massLargest fragment of a meteorite, typically at the time of recovery. Meteorites are commonly cut, sliced or sometimes broken thus reducing the size of the main mass and the resulting largest specimen is called the "largest known mass". Click on Term to Read More, 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. Photos courtesy of Dr. Jay Piatek
Photo courtesy of Dr. Laurence Garvie—Arizona State University
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