Elevated 208, 207, 206Pb/204Pb by Volatile Degassing from Impact Melts

Abstract Two impact melt samples from the Lappajärvi crater (Scandinavia) are highly enriched in siderophile elements, such as Ir, Re, and Os. This indicates the presence of a meteoritic component. The simultaneous enrichments of Ni, Co, Cr, and Se suggest a chondritic projectile. Because of the relatively large indigenous contributions to Ni, Co, and Cr, it is not possible to distinguish between a normal and a carbonaceous chondrite. The high concentrations of relatively volatile elements could point towards a volatile-rich projectile.The two melt samples have high Re/Ir ratios compared to chondritic ratios. Enrichment of Re relative to Ir is very unusual in terrestrial impact melts. Loss of Re, because of volatilisation under oxidizing conditions or by weathering is frequently observed.The high Re/Ir ratios and the high abundances of relatively volatile elements either indicate the presence of a volatile rich phase or they characterize a type of meteorite, which has not been sampled. Some lunar highland rocks have a pattern of meteoritic elements rather similar to that observed for the Lappajärvi meteorite.The Lappajärvi crater is, after Rocheehouart, the second European crater where a significant amount of meteoritic component has been found.A melt sample from the Lake St. Martin crater (Manitoba), did not show any enrichment in meteoritic elements.

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Impact melts in the MAC88105 lunar meteorite: Inferences for the lunar magma ocean hypothesis and the diversity of basaltic impact melts

Geochimica et Cosmochimica Acta ◽

10.1016/0016-7037(91)90471-g ◽

1991 ◽

Vol 55 (11) ◽

pp. 3031-3036 ◽

Cited By ~ 13

Author(s):

G.Jeffrey Taylor

Keyword(s):

Magma Ocean ◽

Lunar Meteorite ◽

Impact Melts

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Reclassification of four aubrites as enstatite chondrite impact melts: Potential geochemical analogs for Mercury

Meteoritics and Planetary Science ◽

10.1111/maps.13252 ◽

2019 ◽

Vol 54 (4) ◽

pp. 785-810 ◽

Cited By ~ 5

Author(s):

Arya Udry ◽

Zoë E. Wilbur ◽

Rachel R. Rahib ◽

Francis M. McCubbin ◽

Kathleen E. Vander Kaaden ◽

...

Keyword(s):

Enstatite Chondrite ◽

Impact Melts

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Origin of Iron Meteorite Groups IAB and IIICD

Zeitschrift für Naturforschung A ◽

10.1515/zna-1980-0801 ◽

1980 ◽

Vol 35 (8) ◽

pp. 781-795 ◽

Cited By ~ 14

Author(s):

John T. Wasson ◽

John Willis ◽

Chien M. Wai ◽

Alfred Kracher

Keyword(s):

Parent Body ◽

Iron Meteorite ◽

Iron Meteorites ◽

Impact Melt ◽

Equilibrium Crystallization ◽

Ni Content ◽

Oxide Phases ◽

Host Metal ◽

Impact Melts ◽

The Impact

AbstractSeveral low-Ni iron meteorites previously assigned to group IAB are reclassified IIICD on the basis of lower Ge, Ga, W and Ir concentrations and higher As concentrations; the low-Ni extreme of IIICD is now 62 mg/g, that of IAB is 64 mg/g. The resulting fractionation patterns in the two groups are quite similar. It has long been established that, in contrast to the magmatic iron meteorite groups, IAB and IIICD did not form by fractional crystallization of a metallic magma. Other models have been proposed, but all have serious flaws. A new model is proposed involving the formation of each iron in small pools of impact melt on a parent body consisting of material similar to the chondritic inclusions found in some IAB and IIICD irons, but initially unequilibrated. These impact melts ranged in temperatures from ~ 1190 K to ~ 1350 K. The degree of equilibration between melt and unmelted solids ranged from minimal at the lowest temperature to moderate at the highest temperature. The lowest temperature melts were near the cotectic in the Fe-Ni-S system with Ni contents of ~ 12 atom %. Upon cooling, these precipitated metal having ~ 600 mg/g Ni by equilibrium crystallization. The Ni-rich melt resulted from the melting of Ni-rich sulfides and metal in the unequilibrated chondritic parent. Low-Ni irons formed in high temperature melts near the composition of the FeS-Fe eutectic or somewhat more metal rich. We suggest that the decreasing Ge, Ga and refractory abundances with increasing Ni concentration reflect the trapping of these elements in oxide phases in the unequilibrated chondritic material, and that very little entered the Ni-rich melt parental to the Oktibbeha County iron. The remaining elements tended to have element/Ni ratios in the melts that were more or less independent of temperature. The remarkable correlation between I-Xe age of the chondritic inclusions and Ni content of the host metal is explained by a detailed evolution of (mega)regolith in which these groups originated. The most Ni-rich melts could only be generated from an unequilibrated chondrite parent; as the continuing deposition of impact energy produced increasingly higher grades of metamorphism, the maximum Ni content of the impact melts (and their subsequently precipitated metal) gradually decreased.

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