A New Multi-Mineral Age Reference Material for 40Ar/39Ar, (U-Th)/He and Fission Track Dating Methods: The Limberg t3 Tuff

The phonolitic Limberg t3 tephra (Kaiserstuhl Volcanic Complex, Germany) was previously dated by the conventional K/Ar method yielding inconsistent results. We have re-dated this tephra layer with three independent methods. Fission Track (FT) external detector analyses on single apatite crystals (16.8 ± 1.3 Ma, 2s) and (U-Th)/He measurements on titanite and apatite (16.5 ± 1.0 Ma, 2s and 16.8 ± 1.0 Ma, 2s, respectively) are in close agreement with laser Ar/Ar dates on incrementally heated single crystals of sanidine (16.3 ± 0.4 Ma, 2s). Due to very rapid cooling, the He, FT and Ar thermochronometers provide one single age representing the eruption event. The different minerals are characterised by favourable properties with respect to their chemical composition, grain size and shape. In particular for the t3 sanidine, homogeneity has been demonstrated by electron microprobe analysis and on a grain-to-grain and grain-internal scale by single crystal incremental laser heating. Based on the agreement between independent methods and the mineral yield of this unit, the Limberg t3 tephra is proposed as multi-method age reference material for single grain laser Ar/Ar, FT and (U-Th)/He dating.     

Seleniferous minerals of palladium and platinum from ouro preto-bearing mineralisation in Brazil

Foram determinadas as composições químicas e as idades Rb–Sr de mica branca, feldspato potássico e de rochas totais das mineralizações de esmeraldas de Capoeirana e Belmont, de pegmatitos sem esmeraldas e dos gnaisses Borrachudos, Monlevade e Guanhães da região de Nova Era–Itabira–Ferros (Minas Gerais, Brazil). Os gnaisses graníticos Borrachudos, os gnaisses bandados Monlevade, seus respectivos pegmatitos e veios/schlieren pegmatóides, e os gnaisses Guanhães, adquiriram suas texturas e composições mineralógicas atuais há cerca de 1.9 Ga no contexto do evento Transamazônico.As rochas regionais encaixantes típicas das ocorrências de esmeraldas são os gnaisses Monlevade que pertencem a uma sequência metavulcano-sedimentar de tipo greenstone belt. O evento principal de formação de esmeraldas em Belmont e Capoeirana foi uma reação metassomática dos pegmatitos anatéticos ricos em Be com rochas ultrabásicas ricas em Cr durante o evento Transamazônico em torno de 1.9 Ga. Em Capoeirana nesse contexto os pegmatitos com feldspato potássico ricos em Be foram transformados em rochas de plagioclasio–quartzo. As idades Rb–Sr de cerca de 480 Ma de minerais das mineralizações de esmeralda resultaram da reequilibração de biotitas e feldspatos Transamazônicos durante o evento Brasiliano.Os pegmatitos não-metamórficos e sem esmeralda da região estudada foram formados no evento Brasiliano há 477±14 Ma. O grau de diferenciação dos pegmatitos, estudado em diagramas indicadores específicos como por exemplo Cs vs. K/Rb de micas brancas e feldspatos potássicos, varia desde pegmatitos cerámicos até muscovita-pegmatitos, à pegmatitos de metais raros e até berilíferos. Alguns dos pegmatitos apresentam marcante diferenciação interna.

Redox and nonredox reactions of magnetite and hematite in rocks

Redox and nonredox reactions causing pseudomorphic replacement of hematite by magnetite and magnetite by hematite are compared.Pseudomorphic replacements resulting from redox reactions are known as martitization [replacement of magnetite by hematite due to oxidation; reaction (1)] and mushketovitization [replacement of hematite by magnetite due to reduction; reaction (2)]. These two replacements cause characteristic ore textures and volume changes (reaction (1): increase of 1.66%; reaction (2): decrease of 1.64%). These small volume changes are the reason that martitization and mushketovitization are widespread in many rocks under condition, however, that oxidizing or reducing fluids (solutions) are present.The same initial and end products may also be involved in nonredox reactions. Reaction (3) is the replacement of hematite by magnetite due to simple addition of Fe²⁺ atoms under basic conditions. This reaction causes an increase of the volume of 47.6%. Reaction (4), causing a volume decrease of 32.2%, is the replacement of magnetite by hematite due to leaching of Fe²⁺ atoms under acidic conditions. From these volume changes it is concluded that reaction (4) may occur in many rock types, whereas reaction (3) is restricted to unlithified sediments only. However, ore textures caused by nonredox reactions are unknown and therefore their occurrence in rocks is hypothetical.     

Characterization and quantification of inorganic constituents of tropical peats and organic-rich deposits from Tasek Bera (Peninsular Malaysia): implications for coals

Since the Carboniferous, tropical latitudes have been the site of formation of many economic coal deposits, most of which have a restricted range of mineralogical composition as a result of their depositional environment, climatic conditions, and diagenesis. Mineralogical and microscopic investigations of tropical peats from Tasek Bera, Peninsular Malaysia, were performed in order to better understand some of these factors controlling the nature, distribution and association of inorganic matter in peat-forming environments. Distribution and nature of the inorganic fraction of peat deposits give insight into the weathering conditions and detrital input into the mire system. Because the inorganic composition of peat deposits is determined by plant communities, height of water table, and climate, the results of the quantitative and qualitative analysis can be used to reconstruct palaeoclimatic conditions.Tasek Bera is a peat-accumulating basin in humid tropical Malaysia with organic deposits of low- to high-ash yield and thus representative of many ancient peat-forming environments. Clay minerals dominate the mineralogical composition of the peat and organic-rich sediments, while quartz and clays dominate the underlying siliciclastic deposits. Kaolinite is the most abundant clay mineral in the organic deposits with minor amounts of illite and vermiculite. Particle size analyses indicate that >50% of the inorganic detrital fraction is <2 μm. Most detrital quartz grains range in size from fine silt to fine sand. The fine sand fraction accounts for a maximum of 5 wt.% of the inorganic constituents. In addition, abundant biogenic and non-biogenic, Al- and Si-rich amorphous matter occur. In the ombrotrophic (low-nutrient) environment, biogenic inorganic material contributes up to >75% of the ash constituents. As a consequence, the vegetational communities make an important contribution to the inorganic and overall ash composition of peats and coals. The ash content of the often inundated peat consists on average of 10% opaline silica from diatoms and sponge spicules, while the ash of the top deposits may have up to 50% biogenic silica. Hence, Al- and Si-hydroxides and the opaline silica from diatoms and sponges represent a large repository of Al and Si, which may form the basis of mineral transformation, neoformation and alteration processes during coalification of the peat deposits. As a result, most coal deposits from paleotropical environments are anticipated to have little to no biogenic inorganic material but high amounts of secondary clays, such as kaolinite (detrital kaolinite, resilisified kaolinite, or desilisified gibbsite) or illite, and various amounts of detrital and authigenetic quartz.