2016 year 61 volume 1:1 ~ 9

Wang Kechao , Xu Weibiao . Xinjiang are the world's longest shower meteors – Altai meteorites rain. Chinese Science Bulletin, 2016, 61:1-9 c, Wang k, Hsu w, b. 

The longest meteorite strewn field on Earth (in Chinese). 

Chin Sci Bull, 2016, 61:1-9, 2016 © doi: 10.1360/N972016-00536 China scientific journal www.scichina.com CSB.scichina.com Journal China Science magazine on the SCIENCE CHINA PRESS n972016-00536 yxm 

IN XINJIANG ARE THE WORLD’S LONGEST METEORITE STREWN FIELDS – THE ALTAY METEORITE FALL Wang Kechao ① ②, Xu Weibiao ① ③

① Planetary Sciences Chinese Academy of Sciences key laboratory , the Purple Mountain Observatory , Nanjing 210008; ② University Graduate School of Chinese Academy of Sciences , Beijing, 100039; ③ Space Science Institute , Macau University of science and technology , MacaoE-mail:wbxu@pmo.ac.cn

2016-04-26 received, 2016-06-12, 2016-06-13 accepted, 2016-09-07 online edition published National Natural Science Fund (41273079, 41573059), the Purple Mountain Observatory, asteroids and Macao's science and technology development fund of the Foundation (039/2013/A2) 

ABSTRACT

In 2011 year in the Xinjiang Autonomous Region a large iron meteorite was found, wuxilike (Wuxilike), 5 t.  The meteorite iron lines are the main minerals, taenite, plessite, accessory minerals phosphorus iron-nickel meteorite, meteorite-carbon iron (haxonite), troilite and daubreelite. Found in the same region of the Xinjiang iron meteorite (Armanty) and UL (Ulasitai) iron meteorite the minerals are consistent. Trace element geochemical analysis showed that Wuxilike and the Xinjiang iron meteorites and Ula Iron meteorites are in the IIIE Group. The three pieces of iron meteorites came from the same parent body. In its passage through the Earth's atmosphere, the explosion scattered stones in a range of up to 425 km, far more than the previous record, the Namibia Gibeon meteorite fall (long ellipse length of 275 km) and is the world's longest meteorite rain. The International society of meteorites has recently approved the pairing of iron meteorites including Xinjiang (Armanty), the wulasitai (Ulasitai) and wuxilike (Wuxilike) collectively to be known as the Altai meteorite rain (Aletai). Further study on the rain of meteorites to determine the evolution of the asteroid orbit and Earth history has important implications. 

KEYWORDS: Altay, iron meteorites, IIIE Group, meteorite rain 

Conventional meteorite rain distribution are in the thousands of meters to tens of meters, such as the 2013 year Chelyabinsk, Russian meteorite rain (20 km) and the 1976 Jilin meteorite rain (70 km). The biggest distribution was the Namibian Gibeon meteorite rain, the parent body disaggregating at a higher altitude after the blast, falling in an ellipse 275 km long and 120 km wide. In that region hundreds of pieces of iron meteorite individuals were found [1]. 

Research on the fall of large meteorites, the orbital evolution of near-Earth asteroids and the parent body’s interaction with Earth's atmosphere have important implications.

In 1898 in Qinghe County in Altay Prefecture in northern Xinjiang, agashi ao Bao Xiang Dong village South 20 km silver Bull ditch (45° 52ʹ 20ʺ n, 90° 30ʹ 24ʺ e) discovered a large iron meteorite, the Xinjiang iron meteorites (named Armanty) which weighed about 28 tons. It is the world's fourth-largest meteorite (Figure 1 (a)). The structure of the iron meteorite is octahedral, chemical type IIIE Group [2, 3]. 

In 2004 in Altay Prefecture there a 430 kg iron meteorite was found (Figure 1 (b)), Wulasitai (Ulasitai).  The petrology and mineral micro-elements analysis shows that the Wulasitai Xinjiang iron meteorite is chemical type IIIE [4]. 

In 2011 in Altay Prefecture in the Crane Valley (48° 3' 8' n, 88° 22' 19' e)  a piece of iron meteorite weighing up to 5 tons was found (Figure 1 (c)), Ukraine Xilike (Wuxilike). In (Figure 2) we can see that the 3 pieces of iron meteorite are distributed in a Southeast - Northwest direction on a line: Ulasitai southeast of the Xinjiang iron meteorite of by about 130 km, the Wuxilike iron meteorite northwest of Xinjiang by about 300 km, the distance from Wuxilike to Ulasitai being 425 km. Also in that area are reported finds of a massive number of large individual iron meteorites which correlate strongly with that line. In the Xinjiang Autonomous Region there is likely to be a scattering of meteorites of an unprecedented scale.

In 2011, the Wuxilike iron meteorite was subjected to petrologic and geochemical analysis of trace elements, with a view towards collecting more evidence to confirm that the Altay-Xinjiang meteorites were a mega-meteorite rain fall event. 

Figure 1 iron meteorite pictures. (A) In 1898 in Qinghe County in Altay Prefecture in northern Xinjiang, at Silver Bull Ditch, China's largest iron meteorite - the Xinjiang iron meteorites, weight 28 tons was

found; (B) In 2004, in the Ulasitai area, another iron meteorites weighing 430 kg was found. [from Xu and others [4]]; (C) In 2011 in the Aletai region of Xinjiang the Wuxilike iron meteorite was

found in Crane Canyon weighing 5 tons; (D) A slice of Wuxilike

Figure 2: a map showing the finding sites of three iron meteorites in Xinjiang province. The finding sites are along a southeast to northwest direction. The ellipse region is the possible strewn field of the Aletai

meteorite shower

1 ANALYTICAL APPARATUS AND METHODOLOGY

Mineral and petrologic experimental work at the Purple Mountain Observatory of celestial bodies was completed in the science and planetary science laboratory. The experimental apparatus used was a Nikon E400 POL polarized reflection microscope, a high precision Oxford INCA EDS Hitachi S-3400N II -low-vacuum scanning electron microscope, and a Japan Electronic JXA-8230 Electron microprobe. The first iron meteorite samples were prepared from surface polishing and final polishing with particle size of 0.3 um, and not encased with a carbon membrane to study main minerals and their morphology with an optical microscope. They were then coated with carbon film for scanning electron microscopy and electron microprobe analysis. The mineral form of iron meteorites were obtained from backscattered electron imaging with the Hitachi S-3400N II -low-vacuum scanning electron microscope and a working voltage of 15 kV. The chemical composition and analysis of minerals were obtained from the Japan Electronic JXA-8230 electron microprobe, operating at 15 kV, current 20 nA. Samples were obtained from the United States National Bureau of Standards for natural and synthetic metals and compounds, Cu, Co, Fe, Ni, and Cr use metal, S using pyrite, P using InP as standards. All data were validated using the ZAF (atomic number absorption fluorescence). 

Neutron activation analysis was completed in the laboratory at UCLA (University of California at Los Angeles) under the direction of Wasson and others [5]. For each sample for 2 dimensional analysis to reduce the error of sampling step. Filomena (North Chile) meteorites as standard for Fe, Co, Ni, w, Pt, Au, Ir element samples, NBS 809B nickel as standard for Cr, Cu, Au samples. Sample and standard sample thickness was 3.0 mm, to avoid visible troilite and iron-nickel meteorite phosphorus inclusions. Exposure to neutron flux at 1.8 x 1012 cm-2 s-1. In samples were exposed for 6, 15, 80 and 600 hours and counted four times.

2 ANALYSES 

2.1 Mineral petrological characteristics of major minerals in Wuxilike iron meteorite iron grain stones, taenite, plessite, accessory minerals phosphorus iron-nickel meteorite stone, Campbellite, troilite and meteorite Sulfur chromite, is a medium size octahedral iron meteorites. Davis structures (Figure 1 (d)) of iron in stone stripe width 0.28~1.62 mm between the average flat width of 0.89 ± 0.41 mm. ring of stone structures, Iron-rich Stone Center and edge of the nickel iron-rich . Usually stone structures from edge to core, change from one particle to another grain, stone found in the samples: spherical collection stone, comb-pattern stone, acicular plessite, mesh plessite (Figure 3 (a) and (b)). Meteorite-p Fe-NI Shi [(Ni,Fe) 3P] mineral particles there are many irregular parting (Figure 3 (c)), the slab of iron particles from the surrounding stone unconformity, biggest fossil of a width exceeding 1 mm, longer than 5 mm; A small meteorite Phosphorus and iron-nickel mineral wide only 2um, long only a few microns are rod-shaped inlays in the iron stone . Lath-phosphorus iron-nickel and iron meteorite stone contact Location, found 2 daubreelite troilite solution consisting of leaves, approximately 150 um (Figure 3 (d)). Alone was not found in the samples of meteorite- 3 theory of sulfur and iron. 

Campbellite appears in stone, mineral grains have many rules of parting, a special ”carbon rose” with stone and iron Nickel iron cross co (Figure 3 (e) and (f)), contains Campbellite plessite border presents a special serrated edge .

Taenite bands typical of the "M" shaped Ni distribution, center with low Ni, closer to the edge with higher Ni content. Center Ni content of nickel and grain was negatively related to the width of the stone, Co concentration and Ni content was negatively correlated, in the nickel iron Co average content of 0.40 ± 0.11 wt%, P pass the content of lower, less than 0.1 wt% (Table 1). 

Iron stone and iron in stone Ni content of 4.76 wt%~6.93 wt% (an average of 6.11 ± 0.71 wt%), Co content of 0.69 wt%~0.76 wt% (average 0.73 ± 0.02 wt%). Ni minimum (4.76 wt%) appear in from taenite in iron stone in molten particles, meteorite-carbon iron stone near the Co content than normal iron stone or stone Iron stone Co content is much higher , up to a maximum of 1.65 wt% (Table 1).

Lath-phosphorus iron-nickel meteorite stone [(Ni,Fe)3P] A more uniform (Table 2), the P content of 15.24 ± 0.07 wt%, Ni content of 19.64 wt%~ 21.00 wt%, at the junction of iron and stone and location of the crack, there is less Magnetite appears . Small, rod-shaped, phosphorus iron-nickel meteorite stone P contents of 15.14 ± 0.10 wt%, Ni content varies greatly (30.67%~ 47.87%), less phosphorus iron-nickel meteorite stone Ni content is higher . 

Campbellite Ni content is low, typically at 4.68 wt%~ 4.98 wt%, Co content is 0.38~0.43 wt%. Because C, meteorite-total carbon chemical analysis of iron is low, to 91.50 wt%~93.00 wt%. Iron meteorites Campbellite usually has two: cohenite [(Ni,Fe)3C] and haxonite [(Ni,Fe)23C6]. Cohenite 0.7 wt%~2.3 wt% Ni and 0.02 wt%~0.3 wt% Co, haxonite 3.5 wt%~ 5.6 wt% Ni and0.05 wt%~0.4 wt% Co; Ni and Co contents can haxonite cohenite distinguish, and Haxonite is found in Ni high-in-stone, is a special "carbon rose" structure of stone and iron and nickel iron cross co [4]. Ouchy Rick sample Campbellite Ni content of 4.68 wt%~4.98 wt%, Co content of 0.38 wt%~0.43 wt

%, and was typical of the ”carbon-Mary Rose” structure in stone , can determine the Campbellite haxonite. 

2.2 RESULTS OF NEUTRON ACTIVATION ANALYSIS

Wuxilike and Xinjiang iron meteorite sample table 1iron stone and nickel iron chemical composition (wt%) …trace element composition and Xu and others [4] ICP-MS measured new Xinjiang iron meteorites and Ula, the trace element composition of the samples included in table 3 were compared. UCLA 's neutron activation method Wuxilike and Xinjiang iron meteorite sample is consistent with the results of trace elements, in addition to As, Pt, W outside, other elements measured deviations are less than 3%. Xu and others [4] ICP-MS measured Xinjiang iron meteorites and Ulla trace element data are consistent, except for Cr (50%), W, Cu, Pt (30%) Outside, other elements measured deviations are less than 10%; Ulasitai Asthan the other 2 AeroLite, less 20% about. Cr enriched in sulfides, uneven distribution of sulphide particles may be caused by Cr elemental composition difference different great reasons. Worth noting is, UCLA and Xu and others [4], using the methods of analysis, may lead to system errors, and using the same method at the same time the analysis results can be better than sex . 

 

Figure 3 Mineral Petrology characteristics of iron meteorites. (a) Plessite, consisting of numerous spheroids; taenite (Tae) is in bright gray; Kamacite (Kam) is in gray and Haxonite (Hax) is in dark

gray. (b) Acicular kamacite in taenite matrix. (c) Schreibersite (Sch) distributed in kamacite of Wuxilike. (d) The dark gray lamellae of daubréelite (Dau) and the light gray lamellae of troilite (Tr)

constitute polysynthetic twin in the boundary of kamacite and schreibersite in Wuxilike; some magnetite (Mag) occurs near the cracks. (E) and (f) Haxonite in the Wuxilike iron meteorite, inlaying with kamacite

and taenite

3 DISCUSSION

3.1 Cooling rate of cooling rates recorded meteorites in its parent in the cold since the formation process of history, and its position in the matrix and matrix size close relationships. For the paired meteorites, the cooling rates should generally be the same. The cooling rate of iron meteorites is by simulating the structure of Vickers and cooling process of Ni in the iron stone and estimation of diffusion of nickel iron [6]. Measured using the nickel iron and a half wide and center Ni content, voted in Ulasitai iron meteorite with similar components simulation of cooling rate curve graph, as in Figure 4. Wuxilike cooling rates can be found in the 20~ 50℃/Ma, measured in the same way Ulasitai cooling rate for 10~40c/Ma [3], cooling rate of iron meteorites in Xinjiang is 20~40℃/ Ma[7]. Error within 3 blocks the cooling rate of iron meteorites. 

Figure 4 Wuxilike cooling rate estimates

3.2 Wuxilike chemical group and the paired early studies used bandwidth of iron stone in iron meteorites iron meteorite classified, 

The iron meteorites are divided into 3 categories : Hexahedral iron meteorite, the octahedral iron meteorites and nickel-rich iron meteorites without structure , octahedral iron meteorite subdivides into a coarse, coarse, medium size (width 0.5~1.3 mm), fine-grain , Extremely fine, transition and 6 subcategories . Wuxilike Iron stone bandwidth in the range of 0.28~1.62 mm, average of 0.89 ± 0.41 mm. and the Xinjiang iron meteorite 1.24 mm (1.08~1.43 mm) with width and Ulasitai 1.2 ± 0.2 mm (0.9~1.8 mm) of bandwidth, bandwidth is close to the average width, in the error range is consistent. Iron stone is to be noted that the bandwidth is not evenly distributed in iron meteorites, big change, statistics and sampling location has a lot to do with the result, randomness is quite strong. Belt width of the iron stone asteroids and meteorites Kernel of P, Ni content of cooling rate and the mother, different sets of iron meteorite can iron stone has the same width, it cannot reflect the relationship between the formation of iron meteorites, Iron stone bandwidth cannot be used as The main basis of iron meteorite group. Because of this , the international meteorite commonly used methods for chemical analysis of trace element iron meteorite classified , the iron meteorites into IAB, IC, IIAB, IIC, IID, IIE and IIF, IIG, IIIAB, IIICD, IIIE, IIIF, IVA, IVB and ungrouped class [8,9], IAB, IIE and IIICD causes for non-magmatic crystallization , Containing silicate inclusions, other types of magma Genesis . IIIAB IIIE iron meteorite group there is great similarity in chemical composition, it is difficult to differentiate. IIIE iron meteorite group iron meteorites are of a very small group, there are only 15 pieces of sample, is Scott and others [10] according to its smaller, Ga/Ni, Ge/Ni IIIAB iron meteorite group of small, isolated groups in the body.

Main distinguishing criteria are characteristics of accessory minerals haxonite, IIIAB Group iron meteorites without haxonite, IIIE iron meteorite groups contain characteristics under mine Haxonite. 

IIIAB Group IIIE iron meteorite group, Xinjiang, and Ulasitai and Wuxilike comparison of trace elements Ni element mapping (Figure 5). You can see IIIAB Group and IIIE trace element composition of the group is close , it is Ir-Ni IIIAB cluster IIIE Group coincide completely unable to distinguish , but Ni content of the IIIE Group IIIAB Group had lower rates of Ga, Au, Co, As. Xu and others [4] the measure of Xinjiang iron meteorites and Ulla sample IIIE Group correlation approach ; UCLA analysis of Xinjiang iron meteorites and Wuxilike of trace elements in Ga-Ni, As-Ni, Au-Ni, W-Ni map and IIIAB Group coincide , other trace elements and the IIIE Relevance of the Group closer . As mentioned earlier, only from trace element compositions indistinguishable IIIAB IIIE Group, key characteristics of accessory minerals haxonite, Because of Xinjiang iron meteorites and Wuxilike contain haxonite, an IIIE Group. Worthy of note is the UCLA analysis of Xinjiang iron meteorites and Wuxilike of trace elements in the diagram position coincide completely, says fully ming Wuxilike and Xinjiang are meteorites iron meteorite. 

Wuxilike of cooling rate is ~40℃/Ma, and Xinjiang iron meteorite stone, Ulasitai within close proximity , IIIAB 56~338℃ /Ma cold but rates quite different from [6]. And Wuxilike and iron meteorite in Xinjiang Stone, found in Ulasitai IIIE group characteristics of accessory minerals haxonite. 

In summary, can be considered Wuxilike and the Xinjiang iron meteorites, Ulasitai are meteorites. General Petrology and mineralogy and trace element Geochemical characteristics of, Wuxilike can be inferred and the Xinjiang iron meteorite stone,

Ulasitai belonging to the same group, IIIE Group. 

3.3 SIGNIFICANCE OF IRON METEORITES IN XINJIANG 

IIIE iron meteorite groups are magmatic differentiation causes iron meteorite, is determined by its small planet parent because of earlier radioactive decay and accretion of the solar and thermal mass and melting, under the action of gravity variation form Fe-Ni core and silicate mantle, slowly cools. Magma-shaped iron meteorite stone, the cooling process is slow, and is linear cooling. Cooling rate Relationship between RADIUS and the mother has the following functions:

R as ring RADIUS (km), CR for the cooling rate (C/Ma), g is constant and generally the 1. By measuring the cooling rate of iron meteorites, count calculated radius of matrix of iron meteorites. Cooling of individual iron meteorites rain in Xinjiang Rate of roughly 10~50c/Ma, take the average cooling rate 30℃/Ma, by (1) the computed matrix of RADIUS is 31 km. Metal nuclear radii of maximum of maternal radius of 0.5 times, then the parent metal of nuclear radii of not more than 15 km. 

Composition the composition of maternal iron meteorites in the solar nebula condensed Process material fractionation effects, magmatic differentiation effect of fractional

crystallization and magma. Fractional crystallization of magma fractionation element can be represented by Rayleigh fractional equation:

XS Is an element X in the solid phase concentration, and x is an element x in the initial concentration in the magma, f is a solid score of the original magma, and KX is the element in the solid phase and liquid phase concentration, partition coefficients. When the element X, y when separated from the magma crystallized when, simultaneous 2 equations you can eliminate others [4], Ni and Co with ICP-AES analysis method, other elements using ICP-MS method of measurement  others [4], Ni and Co with ICP-AES analysis method, other elements using ICP-MS method of measurement on the logarithm can be linear :

The slope of the curve is 2 element relationship between partition coefficients, log effect of -Ni on the map, and the distribution of different elements have different slopes : Because the KNi is 0.88, elements KX<1,-Ni log element has a positive slope of the curve on the graph; KX>1 elements , log element -Ni has negative curvature on slope [1]. 

Figure 5 shows, IIIAB Group in addition to Ga outside elements, IIIE Group in addition to Ga and Ir elements And other trace elements in log element -Ni graph is a straight line on it. UCLA analysis Wuxilike Xinjiang iron meteorites and trace elements have the same trends, show that these differences are the result of crystallization caused magmatic differentiation of chemical fractionation. 

Chen Yongheng, who [1] Iron meteorite parent body in the atmosphere found in Xinjiang Clean the heat caused erosion and surface under the action of thermal diffusion , element of redistribution occurs , in front of iron meteorites samples than other parts of the rich Ni, other trace elements have an enrichment . 

Wasson and others [11]iron meteorite discovered in Xinjiang's side out of ragged fracture traces , the edge of samples with other set of sample components are quite different . That the Office may be located in two Junction with a high-temperature phase -the capture of residual magma or 2 meter level contact surface of the initial nickel iron , accessory minerals are easy to gather here , different The crystals in the rock mineralogy characteristics and composition may be there is a big difference . 

This mechanism can result in iron meteorites and Wuxilike, Xinjiang Wu Russ rocks between mineralogy and composition differences, these differences may be the parent magma effects and atmospheric erosion ablation effect common with results. 

Iron meteorites rain in Xinjiang 3 individuals was IIIE in the Ni content of individual , than other IIIE iron meteorite group Ni The content of 14%, Comparing IIIE group changes of trace elements in different individual scope and trends , as well as differences on the study of rocks and minerals , conducive to the further inference IIIE little star mother experienced magmatic differentiation degree, differentiation and crystallization sequence of differentiation, such as history, study of trace elements in the matrix system Distribution law . 

4 CONCLUSION

Mineral petrology and trace element geochemical analysis shows that, in Altay Prefecture the recently discovered Wuxilike iron meteorite and early onset of Xinjiang iron meteorites and Ulla iron meteorite were paired meteorites, IIIE group. They come from the same parent body, passing through the Earth's atmosphere, and scattered after the explosion. Historically, Xinjiang Aletaidi area after a rare meteorite shower took place, land region of East -South - West direction, scattered range up to 425 km, far more than the Gibeon meteorite rain size (275 km), a distribution network is the world’s largest meteorite shower. Common meteorites rain distribution ranges from thousands of meters to tens of meters. Altai meteorites rain has a very long a meteorite landing distance, reflecting the size of its parent body, flight through the atmosphere and trajectory angle. It came apart in the air, the parent body especially high. Near-Earth asteroid orbit evolution and dynamics is an important study.  But due to finding so few meteorites for the current find too we are still not sure of the flight direction of its parent entering the atmosphere.  In this region we may also find more Altai individual iron meteorites. In order to identify meteorite rain fall ranges it will be very useful to collect meteorites in the region in the future. The International Meteorite Society in 2016 year formally approved the name Altai for the iron meteorite rain.

THANKS

UCLA Professor JT Wasson for his help with neutron activation analysis , Wang Ying, Deputy researcher, Li Shaolin and Wu Yunhua for experimental operations and Tan Jianyun who helped make the samples. The author gives his heartfelt thanks. 

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