Geochemical and mineralogical proxies for grain size in mudstones and siltstones from the Pleistocene and Holocene of the Po River alluvial plain, Italy
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Abstract:
Recent studies carried out on fine-grained sediments recovered from boreholes in the eastern plain of the Po River demonstrate that significant mineralogical and geochemical changes in the provenance of sediments occurred in coincidence with the Pleistocene-Holo-cene transition. An increase in ultramafic-sourced sediment, related to more important inputs from the Po River, is evident at the beginning of the Holocene. The effects of grain-size distribution and provenance variation were investigated on recent unconsolidated sediments, mainly silts and clays. Sediments were collected from ten boreholes in the area, and the geochemical and mineralogical data were compared to the grain-size data. Among the chemical indexes, Zr/V, Y/Rb, Y/V, SiO2/Al2O3, Fe2O3/SiO2, Na/Al increase from pure clay to fine sand together with some mineralogical ratios, including quartz/interstratified illite-smectite and feldspar/interstratified illite-smectite. Some provenance indexes, both mineralogical and geochemical (Ni/Al, Cr/Al, serpentine/sheet silicates), were found to be independent from grain-size and are therefore valid for a wide textural range of sediments. Several geochemical and mineralogical proxies for grain size were identified. In the present case, all these indexes are independent from provenance influence and can be used as direct proxies for the grain size of the sediment, as confirmed by the multiple regression analysis performed to evaluate median and sorting. The equations included the most significant ratios and work well for median values <30 μm.Keywords:
Illite
Unusual feldspar intergrowths were found in two metamorphosed basalt
dykes from Departure Rocks, Antarctica. They are composed of host calcic
plagioclase (Kf 0 _3 An7 5_9 1 Ab9_25) and potassium-rich alkali feldspar
(Kf90 _96 Ano_5 Ab0_8) lamellae. Their bulk composition is estimated to be
Kf An 21 65 Ab14. The complex and yet regular lamella textures of these
intergrowths suggest that they are exsolution products. This requires the
existence of a formerly homogeneous plagioclase, compositionally equivalent
to the estimated bulk composition of these intergrowths. Experimental
studies in the binary system anorthite K-feldspar and in the albite-poor
ternery feldspar system have proven the existence of such homogeneous
plagioclase, which has been synthesized at 1150°C and 10 [(bar under dry
conditions. The unusual feldspar intergrowths were homogenized under these
same conditions, further supporting the hypothesized exsolution origin of
these intergrowths. In an experiment of plagioclase crystallization in a
synthetic rock system, a K-rich plagioclase (Kf 19.6 An70 Ab10 . 4) was
obtained from a K-rich melt at 1140°C and 10 kbar, indicating that the
proposed parental plagioclase for the unusual feldspar intergrowths can
be formed in a complex (rock) system. Such K-rich melt could be generated
through interaction of the dyke magma and its surrounding charnockitic
magma.
It is concluded that limits of solid solutions in feldspar systems
are increased at high pressures due to the increase of feldspar melting
temperatures. Pressure may have an effect on the extend of feldspar solid
solutions, but it is considered to be very small.
Alkali feldspar
Anorthite
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Evidence from studies of K-feldspar and myrmekite the same terrane as that examined by Castle and Lindsley (1993) shows that when these investigators formulated their silica-pump for the origin of myrmekite, an inadequate analysis was made of the feldspars transitions to adjacent nonmyrmekite-bearing rocks. In their proposed model the myrmekite originates in response to kinetic effects associated with the exsolution of calcic alkalic feldspar into discrete potassium feldspar and plagioclase phases. This hypothesis was an outgrowth of the speculations of Tuttle (1952) and Tuttle and Bowen (1958) intended to explain many 'subsolvus' granites. Castle and Lindsley suggested that the diffusion rates of tetrahedral Al and Si through an exsolving ternary feldspar the presence of excess silica result quartz concentrating as vermicules myrmekite where Ca and Al are coupled plagioclase. Therefore, the silica-pump depends upon the prior existence of a K-rich, hightemperature, alkali feldspar whose composition is close to the crest of the solvus, where the potential for subsolvus exsolution is high. Such a composition would produce nearly equal percentages of K-feldspar and plagioclase.
Solvus
Alkali feldspar
Orthoclase
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Thermoluminescence dating
Optically stimulated luminescence
Alkali feldspar
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IRSL single-aliquot dating of three feldspar fractions from a Howieson’s Poort (SA) industry site has been investigated. In the case of the plagioclase fraction (r > 2.62 g cm �3 ) equivalent dose is similar to that of the other fractions (i.e. r < 2.58 g cm �3 , high-K-feldspar; and 2.58
Thermoluminescence dating
Alkali feldspar
Fraction (chemistry)
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Summary The feldspar phenocrysts of some British and Icelandic Tertiary acid glasses have been studied optically and chemically. Six new analyses of the feldspar phenocrysts are presented together with an anorthoclase from a glassy pantellerite. The analysed feldspars and all the other feldspar phenocrysts that have been examined show high-temperature optics, and confirm the results of previous investigations of plagioclase feldspar determinative curves. The relationship of the feldspar phenoerysts to the plagioclase components in the analysed residual glasses conforms to the synthetic plagioelase system, and the crystallization of feldspar in natural acid melts is considered.
Phenocryst
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