TY - JOUR

T1 - Two types of impact melts with contrasting magnetic mineralogy from Janisjarvi impact structure, Russian Karelia

AU - Sergienko, Elena S.

AU - Kosterov, Andrei

AU - Kharitonskii, Petr V.

PY - 2017/5

Y1 - 2017/5

N2 - Palaeomagnetic and rock magnetic studies of impact-related rocks can provide important constraints for deciphering geophysical records from suspected impact structures, their geochronology, and, in the case of very large impacts, their effect on the Earth as a whole. However, the palaeomagnetic record in impact-related rocks may be ambiguous because of the uncertain origin of their natural remanent magnetization (NRM). Towards this end, we carried out a comprehensive rock magnetic and mineralogical study of tagamites (impact melts) from the Janisjarvi astrobleme, Russian Karelia. Chemical composition of magnetic minerals and non-magnetic matrix was evaluated by scanning electron microscopy (SEM) and X-ray analysis. Magnetic minerals were identified using thermomagnetic analysis at high and low temperatures, whereas their domain state was evaluated from hysteresis measurements and magnetic force microscopy. Janisjarvi tagamites appear to belong to two essentially different types arising from the differences in the impact melt crystallization conditions. Type I tagamites were likely formed by an extremely rapid cooling of a superhot melt with initial temperatures well above 2000 C-omicron. Type II tagamites originate from cooler and more iron-enriched melt. Common to the two types is that they both contain a substantial amount of fine inclusions in silicate matrix tens of nanometres to few micrometres in size, which appear to be a major, in some cases dominant, magnetic mineral carrying a significant part of rocks NRM. Structurally, these inclusions are heterogeneous objects consisting of two phases showing both chemical and magnetic contrast.

AB - Palaeomagnetic and rock magnetic studies of impact-related rocks can provide important constraints for deciphering geophysical records from suspected impact structures, their geochronology, and, in the case of very large impacts, their effect on the Earth as a whole. However, the palaeomagnetic record in impact-related rocks may be ambiguous because of the uncertain origin of their natural remanent magnetization (NRM). Towards this end, we carried out a comprehensive rock magnetic and mineralogical study of tagamites (impact melts) from the Janisjarvi astrobleme, Russian Karelia. Chemical composition of magnetic minerals and non-magnetic matrix was evaluated by scanning electron microscopy (SEM) and X-ray analysis. Magnetic minerals were identified using thermomagnetic analysis at high and low temperatures, whereas their domain state was evaluated from hysteresis measurements and magnetic force microscopy. Janisjarvi tagamites appear to belong to two essentially different types arising from the differences in the impact melt crystallization conditions. Type I tagamites were likely formed by an extremely rapid cooling of a superhot melt with initial temperatures well above 2000 C-omicron. Type II tagamites originate from cooler and more iron-enriched melt. Common to the two types is that they both contain a substantial amount of fine inclusions in silicate matrix tens of nanometres to few micrometres in size, which appear to be a major, in some cases dominant, magnetic mineral carrying a significant part of rocks NRM. Structurally, these inclusions are heterogeneous objects consisting of two phases showing both chemical and magnetic contrast.

KW - Magnetic properties

KW - Microstructure

KW - Rock and mineral magnetism

KW - Impact phenomena

KW - METEORITE CRATER

KW - PALEOMAGNETISM

KW - VREDEFORT

KW - CANADA

KW - LAKE

KW - DEFORMATION

KW - PETROLOGY

KW - BEARING

KW - AGE

U2 - 10.1093/gji/ggx077

DO - 10.1093/gji/ggx077

M3 - статья

VL - 209

SP - 1080

EP - 1094

JO - Geophysical Journal International

JF - Geophysical Journal International

SN - 0956-540X

IS - 2

ER -