Precipitation of pyroxenes and Mg2SiO4 from majoritic garnet: simulation of peridotite exhumation from great depth

Our experimental simulations of the exhumation path of mantle peridotites show that high‐temperature (1400 °C) decompression of lherzolite from 14 to 13 and 12 GPa results in exsolution of interstitial blebs of diopside and Mg₂SiO₄ (wadsleyite) lamellae from majoritic garnet. At lower pressures (from 8 to 5 GPa, at T = 1400 °C) only enstatite exsolves as blebs at garnet boundaries. Continuous high‐temperature decompression from 14 to 7 GPa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims. No intracrystalline precipitation of pyroxene was observed in garnet, although such lamellae are found in some natural garnet peridotites. The explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens. Our data suggest that Mg₂SiO₄ and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites, whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths.     

Use of a simplified Mahalanobis distance approach to constrain the dispersion and provenance of Cr-pyrope populations at the Chidliak kimberlite province,Nunavut, Canada

Exploration for diamond-bearing kimberlites in the Chidliak project area by Peregrine Diamonds has generated a grid-like till sampling pattern across four discrete areas of interest totalling 402 km² that is densely populated with research-grade compositional data for 10,743 mantle-derived Cr-pyrope garnets. The available dataset is well suited to statistical analysis, in part due to the relatively unbiased spatial coverage. Previous workers showed empirically that the TiO₂ and Mn thermometry (Ti-TMn) attributes of Cr-pyrope populations at the Chidliak project may serve as source-specific “fingerprints”. In this work, we employ a simplified version of the multivariate Mahalanobis distance technique to formally examine the variability of, and differences between, Ti-TMn attributes of Cr-pyrope subpopulations recovered from a Laurentide-age glaciated terrain that also contains 30 known kimberlites within the four areas of interest. We show the simplified Mahalanobis distance approach enables accurate discrimination of Cr-pyrope subpopulations with subtly to distinctly different Ti-TMn attributes, and permits proper demarcation of their respective kimberlite source(s), specifically in areas with straightforward glacial histories. Redistribution and blending of Cr-pyrope subpopulations from known kimberlite sources is also observed, and typifies areas at Chidliak with complex late-glacial histories. Our results support <1 km horizontal scale subtle to obvious variability in the proportions of TiO₂-rich and high-temperature (> 1100 °C) Cr-pyropes between closely spaced kimberlite source(s) and also between physically adjacent magma batches within single kimberlite pipes. The local scale variability is attributed to protokimberlite fluid or melt interacting with, and metasomatizing discrete conduits within, the ambient diamond-facies peridotitic mantle at times closely preceding eruption of kimberlite magma batches at Chidliak.

     

Solidification depth and crystallization age of the Shiaidani Granodiorite: Constraints to the average denudation rate of the Hida Range,central Japan

Solidification pressure and crystallization age of the ~5 Ma Shiaidani Granodiorite (Hida Mountain Range, central Japan) are determined based on Al-in-hornblende geobarometry and U–Pb zircon dating. Al-poor patchy replacements developed in amphiboles are common in this granite and petrographic study revealed that the replacements include chloritized biotite and albitic plagioclase. These are probably the hydrothermally recrystallized domains, and should not be used for solidification pressure estimates. Magmatic rim of amphibole is characterized by Si < 7.3 a.p.f.u. (Al_IV > 0.7 a.p.f.u.), and utilized in solidification pressure estimate that yielded 0.17–0.29 GPa. The solidification age of the granite is estimated as ~5.6–5.2 Ma using U–Pb zircon dating. From these data, the lower limit of an average denudation rate after ~5.6–5.2 Ma for the area where Shiaidani Granodiorite is exposed is estimated as 0.93–2.5 mm/year.     

Paleogene-Neogene sediments of Bel’kov Island (New Siberian Islands): Characteristics of sedimentary cover in the eastern Laptev shelf

This work is aimed at characterizing the 40-m-thick Paleogene-Neogene sedimentary sequence of Bel’kov Island, which was accumulated in the marginal part of a fluvial plain. On the basis of the plant megafossils and spore-pollen data, the studied sediments were formed in the Late Eocene-Early Miocene with a probable sedimentation gap at the Eocene-Oligocene boundary. It was substantiated that the Paleogene-Neogene sediments were developed as a continuous cover over the entire area of Bel’kov Island. Later, they were eroded away from the island, but presumably preserved on the Bel’kov Horst and Zarya Strait. This suggests that the pre-Quaternary seismic complex, which continuously covers the Bel’kov Horst, also has Late Eocene-Early Miocene age. Selective dating of nonrounded detrital zircons from Lower Miocene conglomerates allowed us to substantiate the transportation of sediments from the Verkhoyansk-Kolyma Fold System along the Bel’kov-Svyatoi Nos Graben. The crystalline basement similar to the Ust’-Lena inlier was likely exposed on the East Laptev Horst and served as the source of metamorphic detritus, including zircons dated within 1850–2000 Ma.     

Aptian-Albian coaliferous sediments of Kotel’nyi Island (New Siberian Islands): New data on the section structure and ignimbrite volcanism

The Aptian-Albian sediments of Kotel’nyi Island are represented by a terrigenous coaliferous complex with the apparent thickness of approximately 700 m. The upper two thirds of their section enclose ignimbrites and rhyolitic ash tuffs. The integral thickness of volcanics is 170 m. A new sequence composed largely of acidic volcanics and sedimentary rocks is defined in the upper part of the Cretaceous section. The K-Ar age estimated for ignimbrite glasses is 110–107 ± 2.5 Ma, which corresponds to the first half of the Albian. The fossil flora list is added by several previously unknown forms. The macroflora of Kotel’nyi Island is similar to its Albian counterpart from the Kolyma-Indigirka region and allows Cretaceous sediments from the lower part the Kotel’nyi Island section to be dated back to the Aptian (?)-Albian (except for the terminal Albian). The palynological characteristic of rocks immediately contacting the dated volcanics appeared to be untypical of Albian sediments of Siberia and similar to that of the Late Neocomian palynocomplexes. This is partly explained by erosion and reworking processes. The examined continental sediments accumulated in post-orogenic extension settings. They constitute the lower strata of the Aptian(?)-Tertiary post-orogenic complex filling riftogenic depressions in the New Siberian Islands and Laptev Sea.     

Migration from the city to the countryside in Hungary and Poland

After the changes of 1989, urban-to-rural migration in Poland and Hungary remained stable compared to rural-to-urban migration. This was connected to a growing importance of suburbanisation around major Hungarian and Polish cities. Suburban areas around these cities are attracting increasing numbers of migrants from the major cities. Pollution and higher crime rates, as well as rising living costs and increasing uncertainties make many people to decide to leave the city for the surrounding countryside. Not only upper middle-class people move to the suburbs, but also lower status people, for whom life in the big city became too difficult. Long-distance migration from the cities to the rural areas is declining. Economic problems together with low employment opportunities prevent people from making such a move. Though the overall level of urban-to-rural migration did not change too much, this article shows the rapidly increasing importance of suburbanisation. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new microsplitter for unconsolidated sediments

Unrepresentative subsamples can be obtained from a sedimentary sample through repeated subdivision when using a mechanical microsplitter. Physical biases are often compounded by repetition and can result in wide variations among the estimates computed from the relative proportion of foraminifera of a given species. A new microsplitter, designed on a new principle, has been built to separate small representative subsamples. The device is based on the uniform distribution of unconsolidated sediments suspended in water. A subsample of the desired size is obtained by collecting sediment from an appropriate sedimentation area. Operation of this sampler is rapid and convenient and requires only one step, thus eliminating sampling biases introduced by repeated operations with other splitters. This new device was tested and found to be superior to the Otto Microsplitter according to some statistical goodness-of-fit tests conducted on the series.