Cosmogenic Substances in the Zhamanshin Crater

Using high-resolution analytical electron microscopy, heterogeneous fragments of cosmogenic substances, such as nickelphosphide Ni₃P and the unnamed intermetallide ZnAl₂, are detected for the first time for a large meteorite crater, the Zhamanshin circular structure. Due to the impossibility of simultaneous finding these phases in the same meteorite, an assumption is made on the cometary nature of the impactor of the Zhamanshin crater.     

Tectonics and oil and gas potentials of the Kolyma and Primor'ye lowlands and adjacent shelves

The impact of gigantic meteorites momentarily creates extremely high pressures and temperatures, leading to the formation of distinctive objects — impact breccias and impactites — at the points where they fall. The Zhamanshin crater is one such place. This annular structure has a diameter of about 10 km, contoured by a low rim made up of a body of shattered Paleozoic rocks thrown out from the ring's center, from depths exceeding 100 - 200 m. Among these breccias have been found pieces and lenticular bodies of remelted rocks — impactites, called zhamanshinites. These thermally altered rocks from a series ranging from baked clays to cinder-like, partly recrystallized substances, glass, and black drops and spattered droplets (up to 1 - 5 cm) of isotropic glass that undoubtedly hardened as they fell through the air. The refused neogenic substances formed by selective evaporation are characterized by a considerable deficit of K and Na, excess Al₂O₃ and SiO₂, a lack of H₂O, and a sharp predominance of Fe⁺² over Fe⁺³. Their characteristic shape and distinctive structure and composition suggests that these remelted droplets are tektites, which were called irgizites when first found in the USSR. Their occurrence in the meteorite crater confirms the idea of a genetic unity between tektites and impactites. Selective evaporation of the more volatile elements is characteristic of the surfaces of small planets lacking an atmosphere, but is not characteristic of the earth's surface. The processes and substances occurring at the impact points of gigantic meteorites are therefore unique on the earth, but highly typical of the surfaces of the Moon, Mars, Mercury, and other relatively small celestial bodies.     

The natural high-pressure phase of cubic CdSe in impact glass from Zhamanshin crater

A CdSe high-pressure polymorph of the NaCl structural type of a₀ = 0.549 nm and Fm–3m space group was discovered in nature for the first time. Its composition is within range of CdSe–CdSe_1–х where x = 0.2 apfu. The phase was discovered as abundant nanosize inclusions in irgizite-type condensate glass separated from the sample of impact rock of the Zhamanshin crater (Central Kazakhstan). The treated mineral was presumably formed within a gas–plasma cloud at the moment of impact.     

中国大别山东南缘首次发现大坝陨坑构造

简介首次在大别山东南缘安徽省境内发现的大坝陨坑构造。经初步评价,知陨坑呈椭圆形,长轴呈北北东向,长约19km,短轴长约12km,最大坑深约2km,是一个有中央隆起区的复杂型陨坑。在卫片上陨坑显示环形影像,地貌形态为一洼地。陨坑基座保存尚好,可对它直观和追索陨坑构造边界。形成于230Ma左右的各类撞击变质岩石系列齐全,其中含有柯石英等典型的撞击变质矿物及撞击碎理等超微构造,特别是在陨坑基座内壁普遍发现有鉴别陨坑构造最可靠的标志——干裂自角砾岩,都证明大坝环形影像是一个典型的陨星撞击坑构造。它的发现,具有很大的科学及经济意义,对今后褶皱山区寻找和研究陨坑构造具有示范和指导作用,同时大大丰富了建设大别山世界地质公园的地质依据。无庸置疑此发现将促进大别山旅游业的发展及陨星撞击科普知识的传播。     

Condensate glasses from the Zhamanshin crater. I. Irghizites

The presence of condensate glass was established for the first time in natural terrestrial impact glasses. Its existence was previously predicted on the basis of shock evaporation experiments. Condensate glass was found in the irghizites of the Zhamanshin crater. Its characteristic features were described, including a combination of high silica content with the presence of nanoscale crystalline nuclei and typical globular morphology under TEM.     

Meteoritic material at five large impact craters

Sixteen crater samples were analyzed by radiochemical neutron activation analysis for Ge, Ir, Ni, Os, Pd and Re. Two impact melt rock samples from Clearwater East (22 km) showed strong, uniform enrichments in all elements except Ge, corresponding to 7.4% C1 chondrite material. Interelement ratios suggest that the meteorite was a C1 (or C2) chondrite, not an iron, stony iron, or chondrite of another type. An Ivory Coast tektite (related to the 10 km Bosumtwi crater) was enriched in Ir + Os and Ni to about 0.04 and 1.6% of C1 chondrite levels, but in the absence of data on country rocks, the meteorite cannot yet be characterized.Impact melt rock samples from Clearwater West (32km), Manicouagan (70km), and Mistastin (28 km) showed no detectable meteoritic component. Upper limits, as Cl chondrite equivalent, were Os ≤ 2 × 10^?3% (~0.01 ppb), Ni ≤ 2 × 10^?1% (~20ppm). Possible causes are high impact velocity and/or a chemically inconspicuous meteorite (achondrite, Ir,Os-poor iron or stony iron). However, a more likely reason is that some fraction of the impact melt remains meteorite-free, especially at craters with central peaks.Clearwater East is the first terrestrial impact crater found to be associated with a stony meteorite. Apparently the consistent absence of stony projectiles at small craters (< 1 km diameter) reflects their destruction in the atmosphere, as proposed by Öpik.     

世界陨石坑研究

本文综述了全球陨石坑研究的研究历史和最新成果、基本的概念、陨石坑的识别要点、世界著名的陨石坑、陨石撞击地球可能引起的岩浆活动、陨石撞击与生命演化等内容。确定一个陨石坑,要从有一定弧度的地貌开始,鉴别低平圆形地质体是陨石还是其它原因造成的,综合确定岩石的岩石学特征、岩石中是否有撞击变质矿物、残余陨石、重力异常。陨石撞击太阳系的所有行星。由于地球表面遭受严重的风化和侵蚀,地质学家很难发现陨石坑。截至2021年3月31日,全球陨石坑数据库中有190个经确认的陨石坑,但中国只有一个,中国地质学家在发现陨石坑方面应当积极努力。对一个陨石坑认识可能不很成熟,但往往能改变对一个地区的地质成因理论的认识,形成完整的陨石坑证据链可能需要几代科学家的不断努力。     

世界陨石坑研究

本文综述了全球陨石坑研究的研究历史和最新成果、基本的概念、陨石坑的识别要点、世界著名的陨石坑、陨石撞击地球可能引起的岩浆活动、陨石撞击与生命演化等内容。确定一个陨石坑,要从有一定弧度的地貌开始,鉴别低平圆形地质体是陨石还是其他原因造成的,综合确定岩石的岩石学特征、岩石中是否有撞击变质矿物、残余陨石、重力异常。陨石撞击太阳系的所有行星。由于地球表面遭受严重的风化和侵蚀,地质学家很难发现陨石坑。截至2021年3月31日,全球陨石坑数据库中有190个经确认的陨石坑,但中国只有一个,中国地质学家在发现陨石坑方面应当积极努力。对一个陨石坑认识可能不很成熟,但往往能改变对一个地区的地质成因理论的认识,形成完整的陨石坑证据链可能需要几代科学家的不断努力。     

Identification of the projectile at the Brent crater,and further considerations of projectile types at terrestrial craters

Impact melt samples from drill hole B1-59 at the 3.8 km diameter Brent crater (Ontario) have been analysed for siderophile trace elements indicative of meteoritic contamination. Samples from the basal melt zone at 823–857 m depth are enriched in Ir, Os, Pd, Ni, Co, Cr and Se over basement, with the abundance pattern suggesting a chondritic projectile for Brent. From a Ni-Cr correlation of 10 melt samples an L or LL chondrite is inferred. The contribution of an ultramafic country rock (alnoite) in the melt is too small to significantly influence its $\text{Ni}\text{Cr}$ ratio. Glass-rich breccias from the allochthonous breccias filling the crater also contain a meteoritic component. Interelement ratios (e.g. $\text{Ni}\text{Cr}$) are, however, fractionated relative to the melt zone samples. This, as well as the low Au content of all Brent samples, is probably a product of alteration.Additional data on impact melts from the 65 km diameter crater Manicouagan still did not reveal a meteoritic component, as also for the Mistastin crater (28 km diameter) where Cr analyses set an upper limit of 1% of an achondritic projectile component in the melt. Irghizites (tektite like glasses) from the Zhamanshin impact structure have been found to contain high Ni and Co concentrations, and our data show that Ir is also enriched. It is however not possible to define the projectile-type. Enrichment of an Ivory Coast tektite in Ir is confirmed. There are large differences in siderophile element concentrations among tektites, with otherwise similar chemical composition.There are now four known craters formed by chondrites (Clearwater East, Lapparjärvi, Wanapitei, and Brent), with Brent being the smallest of these. For smaller craters the projectiles appear to be limited to iron or stony-iron meteorites, because of atmospheric destruction of relatively small stony meteorites. It appears, however, that all major classes of meteorites are represented among the projectiles at terrestrial impact craters.     

Extracts of impact breccia samples from Sudbury, Gardnos, and Ries impact craters and the effects of aggregation on C60 detection

Fullerenes have been detected in carbonaceous chondrite meteorites and in breccia samples from meteorite impact craters, but questions have been raised about contradictory results from similar samples and the sensitivities and accuracies of different analytical methods. We analyzed samples from three impact craters and detected C₆₀ in samples from several locations; we also observed differences in the detection capabilities of various analytical techniques used in the search for fullerenes. The presence of C₆₀ in rocks from the Onaping Formation of the Sudbury impact crater was confirmed. Low levels of C₆₀ were also detected for the first time in samples from the Gardnos (Norway) and Ries (Germany) impact structures.We detected C₆₀ in these samples using surface-enhanced laser desorption/ ionization (SELDI), but the related technique of microprobe laser-desorption, laser-ionization mass spectrometry (μL²MS) did not observe C₆₀ above detection limits. We attribute the absence of μL²MS signal to aggregate formation caused by phthalic acid esters, which appear to easily contaminate samples either during storage or demineralization in plastic containers. The μL²MS technique is incapable of detecting aggregated C₆₀, but aggregation does not suppress detection in SELDI. Phthalate-induced aggregation did, however, enhance SELDI detection of C₆₀ in some cases, and we suggest that this enhancement may help explain previously reported differences in C₆₀ detection from natural samples between laser desorption mass spectrometry (LDMS, a technique analogous to SELDI that has detected fullerenes in meteorite and impact breccia samples) and high-pressure liquid chromatography (HPLC). This work highlights the effects of phthalates and other indigenous compounds or contaminants on certain mass spectrometric techniques and lends support to the idea that several complementary analytical methods should be employed to investigate complex natural samples.     

Hickman Crater,Ophthalmia Range,Western Australia: evidence supporting a meteorite impact origin ∗

A newly discovered, morphologically well-preserved crater with a mean diameter of 260 m is reported from the Ophthalmia Range, Western Australia. The crater is located in hilly terrain ～36 km north of Newman, and is situated in the Paleoproterozoic Woongarra Rhyolite and the overlying Boolgeeda Iron Formation. The morphometry of the crater is consistent with features characteristic of small meteorite impact craters. The rhyolite of the crater's rim exhibits widespread shatter features injected by veins of goethite bound by sharply defined zones of hydrous alteration. The alteration zones contain micro-fractures injected by goethite, which also fills cavities in the rhyolite. The goethite veins are interpreted in terms of forceful injection of aqueous iron-rich solutions, probably reflecting high-pressure hydrothermal activity by heated iron-rich ground water. None of these features are present in the Woongarra Rhyolite outside the immediate area of the crater. Petrography of the rhyolite indicates possible incipient intracrystalline dislocations in quartz. The Boolgeeda Iron Formation, which crops out only on the southern rim of the crater, displays brecciation and mega-brecciation superposed on fold structures typical of the banded iron-formations in the region. Geochemical analysis of two goethite veins discloses no siderophile element (Ni and PGE) anomalies, negating any contribution of material from an exploding meteorite. Instead, the strong iron-enrichment of the fractured rhyolite is attributed to a hydrothermal system affecting both the Boolgeeda Iron Formation and the Woongarra Rhyolite, and localised to the area of the crater. An absence of young fragmental volcanic material younger than the Woongarra Rhyolite is inconsistent with an explosive diatreme, leading us to a preferred interpretation in terms of an original impact crater about 80 m deep excavated by a ～10 m-diameter projectile and accompanied by hydrothermal activity. A minor north–south asymmetry of the crater, and an abundance of ejecta north, up to about 300 m northwest and northeast of the crater, suggest high-angle impact from the south. A youthful age of the structure, probably Late Pleistocene (10⁴–10⁵ years old), is indicated by damming of the drainage of a south-southeast-flowing creek by the southern crater rim.     

Reconstructing the Wolfe Creek meteorite impact: deep structure of the crater and effects on target rock

The Wolfe Creek Meteorite Crater is an impact structure 880 m in diameter, located in the Tanami Desert near Halls Creek, Western Australia. The crater formed?<?300 000 years ago, and is the second largest crater from which fragments of the impacting meteorite (a medium octahedrite) have been recovered. We present the results of new ground-based geophysical (magnetics and gravity) surveys conducted over the structure in July?–?August 2003. The results highlight the simple structure of the crater under the infilling sediments, and forward modelling is consistent with the true crater floor being 120 m beneath the present surface. The variations in the dip of the foliations around the crater rim confirm that the meteorite approached from the east-northeast, as is also deduced from the ejecta distribution. Crater scaling arguments suggest a projectile diameter of?>?12.0 m, a crater formation time of 3.34 s, and an energy of impact of ～0.235 Mt of TNT. We also use the distribution of shocked quartz in the target rock (Devonian sandstones) to reconstruct the shock loading conditions of the impact. The estimated maximum pressures at the crater rim were between 5.59 and 5.81 GPa. We also use a Simplified Arbitrary Langrangian–Eulerian hydrocode (SALE 2) to simulate the propagation of shock waves through a material described by a Tillotson equation of state. Using the deformational and PT constraints of the Wolfe Creek crater, we estimate the maximum pressures, and the shock-wave attenuation, of this medium-sized impact.     

Ion Microprobe Determination of Hydrogen Concentration and Isotopic ratio in Extraterrestrial Metallic Alloys

The isotopic ratio of hydrogen was measured in an iron meteorite and terrestrial native iron using an IMS 3f ion microprobe. The extraterrestrial D/H ratio (93 ± 9 × 10^?6) was close to the terrestrial value (105 ± 6 × 10^?6), and both samples had low H concentrations (7 ± 4 and 33 ± 11 ng g^?1 for the iron meteorite and the terrestrial sample, respectively). Experiments on artificially D‐enriched samples showed that the measured hydrogen signal is a combination of indigenous H and terrestrial atmospheric contamination. This contamination comes from the isotope exchange reaction between water adsorbed on the sample surface and atmospheric water, and would be continuously added to the indigenous H in the ion crater by the adsorbed water sinking into the crater during sputtering. Experiments showed that this contamination represents up to 20% of the signal but was within the uncertainty of the measured D/H ratio.     

Chemical analyses of Bosumtwi crater target rocks compared with the Ivory Coast tektites

The Bosumtwi crater, Ghana, was excavated in phyllites and greywackes with subordinate microgranite dykes and quartz veins of the 2000 Ma old Lower Birimian System with a granodiorite intrusion at Pepiakese on its northeastern side. New major and trace element analyses are presented for 7 phyllites, 5 greywackes, 2 microgranites, 3 Pepiakese intrusion rocks and 1 suevite using XRF and INNA.Means and standard deviations were calculated using all available modern analyses for each element in the Bosumtwi target rocks, Bosumtwi suevite glasses and Ivory Coast tektites. Good agreements between the means were found for the three groups with the suevite glasses and tektites having more limited compositional ranges than the target rocks. Least squares mixing between target rock types shows that the best fits to the tektite and suevite glass compositions require components of about a third or a quarter from the Pepiakese intrusion and some extra silica, derived from quartz veins, as well as the metasediments.The new data provide evidence for vapour phase fractionation of P₂O₅ and Na₂O in the tektites in addition to the previously reported Pb and Rb. Evidence for a meteoritic component in the tektites was found to be equivocal since the target rocks are probably a sufficient source of the meteorite indicator elements Ni and Ir.     

海南岛白沙陨击坑及其成坑陨石

海南岛白沙陨击坑是一个直径约３．５ｋｍ的环形镶边坳陷，组成陨击坑边缘的环形山连续性好，并具二元结构；下部是层理清晰的下白垩统紫红色砂岩，其中长石、石英等粒状矿物普遍受冲击破碎，发育有冲击微页理和击变玻璃，云母呈膝折状变形；上部是冲击角砾岩块杂乱堆垒成的溅射覆盖层，冲击角砾岩因冲击熔融结晶而貌似凝灰岩，但其中矿物成分十分复杂，含有镁橄榄石、镍纹石以及高密度石英等，岩石化学计算结果说明它是由砂岩变质而成的，与火成岩无关。坑内保留有回落角砾岩，常见到沿裂缝贯入的脉状角砾岩。在陨击坑内找到了重３．７５ｋｇ的石陨石碎块，其中含碱硅镁石、陨铁大隅石、四方镍纹石、陨硫钙石和陨硫铁等陨石标型矿物，但不具球粒结构，ＣａＯ含量为９．１９％，属富钙的无球粒陨石，认为是白沙陨石坑的成坑陨石。在陨击坑中找到富钙无球粒陨石，为陨击坑提供了最直接可靠的证据，也为石陨石撞击成坑提供了实例。     

内蒙古海拉尔发现陨石坑及科研意义

陨石撞击构造研究是天文地质学研究的热点,但是缺乏典型的研究基地,海拉尔陨石坑的发现,无疑是一个重要补充。海拉尔陨石坑坑区基础岩石为晚侏罗世火山岩,区域自然地理为平缓丘陵山地草原,陨石坑呈封闭圆形,中间筒状突起显示冲击锥地貌,直径320m,坑底到坑缘最大高差10m。为了保护性研究,没有进行进行破坏性取样分析撞击岩石矿物,根据排他比较分析法,认为该坑唯有陨石撞击成因可以解释,并且具有最关键的冲击锥地貌特征。这是一个我国唯一保存完整的可供直观参观的陨石坑,估计该陨石坑年龄应该在100万年以上。     

Fluid Inclusions from Anhydrite Related to the Chicxulub Crater Impact Breccias,Yucatan, Mexico: Preliminary Report

Abstract, Results of a study of fluid inclusions in anhydrite from drill hole Y-6 in the Chicxulub crater, of northwestern Yucatan, Mexico, are reported in this work. The Chicxulub crater was formed at the Cretaceous-Tertiary boundary by a meteorite impact. The resulting ejection breccias are composed mostly of hydrothermally altered crystalline basement material. The mineral assemblage pyroxene + anhydrite + quartz is associated with the hydrothermal alteration. The analyzed fluid inclusions in the anhydrite show highly heterogeneous phase assemblages within the same crystal plane. Fluid inclusion types include liquid plus vapor inclusions (L+V), vapor-rich inclusions (V), and inclusions containing daughter crystals (L+V+S). The eutectic temperatures indicate a brine composition dominated by CaCl₂-NaCl. Both the salinity and the homogenization temperatures show a wide range (from 3.6 to 23 wt% NaCl equivalent for the L+V inclusions, and 36 to 42 wt% NaCl equivalent for the L+V+S inclusions). The homogenization temperatures range from 100° to 500°C. These data represent cooling and boiling trends. We assume that the impact breccias were ejected at high temperature in an aqueous environment (above 500°C). This caused boiling of sea water and precipitation of anhydrite with its inclusions.     

岫岩陨石坑地质遗迹保护浅析

岫岩陨石坑是中国第一个被证实并被国际学术界认可的陨石坑,是极为珍贵的地质遗迹及自然遗产,具有重要的科学价值与开发利用价值。岫岩陨石坑范围应定为地质遗迹重点保护区,根据地貌单元、遗迹点分布特点及人文因素,将保护区划分为地质遗迹景观区、居民点保留区和自然生态区等3个分区;借鉴国外对陨石坑遗迹保护开发的经验和岫岩陨石坑的现状,提出了进行保护和开发的路线;针对地质遗迹景观区基础设施建设、居民点保留区和自然生态区功能性升级、场馆的建设等问题提出相应的措施与建议。     

荷叶塘陨石:一个L3型普通球粒陨石的岩石学和地球化学特征

荷叶塘为一块我国降落的原始3型普通球粒陨石,因此具有重要研究意义。本文对荷叶塘陨石光薄片及粉末样品的岩石学、矿物学和全岩组成地球化学特征进行研究,为这块陨石的深入研究提供重要基础数据。研究表明荷叶塘陨石具L3型陨石岩石学特征,具典型的球粒陨石结构,球粒清晰,球粒结构类型多,基质重结晶程度低,组成模式为:球粒80vol%,金属和硫化物含量为5vol%,基质15vol%。矿物化学成分表明,该陨石球粒以Ⅰ型(贫铁型)球粒为主,橄榄石Fa0.41-34.1(PMD=51),低钙辉石Fs1.82-27.2(PMD=88),Wo0.18-3.13(PMD=103),铁纹石中Co含量平均0.62%(PMD=20),矿物成份不均一程度高,橄榄石矿物结晶颗粒内部化学成分变化大,呈正环带分布,与岩浆型结晶顺序一致,球粒与基质及间隙物成分明显不同,表现为不同物质来源。化学成分全岩分析结果显示,荷叶塘陨石亲石、亲铁元素含量均为L型陨石特征。依据以上岩石矿物学和化学组成特征,依照陨石亚分类参数,将其类型划分为L3.4型普通球粒陨石。冲击变质程度S2,风化程度W1。研究结果表明荷叶塘陨石为一块受后期水、热蚀变和风化影响较少的原始类型陨石。组成矿物成分极不均一,在矿物晶体内部,球粒内部及球粒与基质间均有明显变化。     

The age of Dar al Gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics

Samarium-neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 ± 11 Ma and an initial ε_Nd¹⁴³ value of +36.6 ± 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.701249 ± 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace element-isotopic systematics observed in the martian meteorite suite.Combining the ε_Nd¹⁴²-ε_Nd¹⁴³ isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4.513_+0.033^−0.027 Ga. Using this age, the parent/daughter ratios of martian mantle sources are calculated assuming a two-stage evolutionary history. The calculated sources have very large ranges of parent/daughter ratios (⁸⁷Rb/⁸⁶Sr = 0.037-0.374; ¹⁴⁷Sm/¹⁴⁴Nd = 0.182-0.285; ¹⁷⁶Lu/¹⁷⁷Hf = 0.028-0.048). These ranges exceed the ranges estimated for terrestrial basalt source regions, but are very similar to those estimated for the sources of lunar mare basalts. In fact, the range of parent/daughter ratios calculated for the martian meteorite sources can be produced by mixing between end-members with compositions similar to lunar mare basalt sources. Two of the sources have compositions that are similar to olivine and pyroxene-rich mafic cumulates with variable proportions of a Rb-enriched phase, such as amphibole, whereas the third source has the composition of liquid trapped in the cumulate pile (i.e. similar to KREEP) after ∼99% crystallization. Correlation between the proportion of trapped liquid in the meteorite source regions and estimates of f_O2, suggest that the KREEP-like component may be hydrous. The success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean, and that assimilation of evolved crust by mantle derived magmas is not required. Furthermore, the decoupling of major element and trace element-isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions, whereas the major element abundances are limited by eutectic melting processes at the time of magma formation. Differences in major element abundances of parental magma, therefore, result primarily from fractional crystallization after leaving their source regions.