Chemical analysis and X-ray diffraction revealed the biotite-alteration process to be vermiculitization, but gave no definite reason why biotites that retain more than 70% of their original Rb give usable Rb—Sr data. This may indicate that the lithosphere of the central part of the rift had a time integrated Th-depletion. When we produced the formula for , it was reasonable enough to think that there was little to no argon present in the original state of the rock, because argon is an inert gas, does not take part in chemical processes, and so in particular does not take part in formation. The Paritu Pluton is a sub-volcanic, epizonic, composite intrusion of hornblende-pyroxene granodiorite, pyroxene-hornblende quartz diorite, marginal pyroxene tonalite, hornblende-pyroxene quartz diorite, and biotite-pyroxene quartzdiorite porphyry as well as quartz-orthoclase aplites and pegmatitic quartzorthoclase- epidote-calcite druse cavities. Rb—Sr analysis of these fractions also showed a departure from a linear isochron when more than about 30% of the original Rb had been lost. After each period of time, the 87Rb in each rock decays to 87Sr producing a new line.
Attention has been focused on the feldspar phases of the three rock types because they are of unusual complexity and give insight into the development of the complex. When an is first formed, its will contain varying concentrations of rubidium and strontium, with some being high in rubidium and low in strontium, others being high in strontium and low in rubidium. To account for this, we first note that there is anisotope of Sr, 86Sr, that is: 1 non-radiogenic not produced by anotherradioactive decay process , 2 non-radioactive does not decay to anything else. The granitic core forms the bulk of the mass. Seawater isotope records, crustal evolution, tectonics and atmospheric evolution. Sr will contain both original and inherited alongwith radiogenic. Leucocratic xenoliths, including monzodiorites, diorites and syenogabbros, are distinctive among Hawaiian plutonic rocks in having alkali feldspar, apatite, zircon and biotite, and evolved mineral compositions e.
Data that possess scatter in excess of experimental error i. Microgranular enclaves and xenoliths are very rare. In addition, Rb is a highly that, during partial melting of the mantle, prefers to join the magmatic melt rather than remain in mantle. Permain and Triassic periods, and 205 © 5 Ma for the end of the Triassic. Indications are that the complex was intruded within a relatively short time interval at temperatures in the range 660°-900° C. The College of Earth Sciences Jilin University Changchun China About this article Cite this article as: Han, Y. This one additional piece of information about the initial state of the rock allows us to calculate its age.
This is improved to ±1. A survey of available Sr isotopic and traceelement data on Precambrian granites and granitic gneisses in south-east Norway Jacobsen and Heier, 1978; Killeen and Heier, 1974, 1975a-d; Smalley, 1983; Sundvoll, unpublished data shows that the older and younger Sveconorwegian granitic rocks in southern Norway were derived from compositionally different crustal sources. There is a wide age range with respect to the Paritu Pluton, which has produced porphyroblastic hornblende, epidote, and tourmaline in pre-pluton dikes but oligoclase, biotite, and hornblende in extrusive andesite. The application of Sr isotope stratigraphy is generally limited to carbonate samples for which the Sr seawater curve is well defined. Strontium, on the other hand, does take part in chemical reactions, and can substitute chemically for such elements as calcium, which is commonly found in.
In older sequences diagenetic alteration combined with greater uncertainties in estimating absolute ages due to lack of overlap between other geochronometers for example leads to greater uncertainties in the exact shape of the Sr isotope seawater curve. Constitution a betrayal of the American Revolution? The peraluminous Drammen batholith 650 km2 is the largest granite complex within the mainly alkaline province of the Permo-Carboniferous Oslo Rift, and peraluminous to metaluminous granites are also present in the southern part of the otherwise alkaline Finnemarka complex 125 km2. It seems highly unlikely that homogenization can be attained as the result of interaction between a viscous silicic magma and the wall-rocks during the final stages of crystallization. The accessories are titanite especially prominent , zircon, apatite, and opaques. However, this is worth it: it would, as I say, require a fluke for this to happen, so if we reject dates based on the mixing plot, then we will be throwing out a hundred bad dates for every good one.
Mafic minerals in samples from older and younger intrusions in the Permian Oslo rift were analysed to determine temperatures and oxygen fugacities of crystallization and possible genetic relations between different rock types. It is probable that these outcrops are all part of one continuous plug of granite. It is likely that provinces poor in lithophile elements existed in the crust during the Oslo rifting event. The one-feldspar granite contains a coarse perthite and approximates a residual albite-orthoclase-quartz liquid in composition. Isotopes in the Earth Sciences. This is a useful means of testing whether two data arrays may have the same age and. And of course this is not necessarily the case.
This is well known for the Cenozoic time-scale but, due to poorer preservation of carbonate sequences in the Mesozoic and earlier, it is not completely understood for older sequences. We can use this to tell us two important things 1. This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate. The Fe2+ biotite has a composition similar to that of biotite from calc-alkaline to sub-alkaline rock series. This granite, intruding Precambrian rocks just outside the southwestern corner of the Oslo Region, is interpreted as cogenetic with the main plutonic series of the Oslo Region. As a result, Rb is enriched in crustal rocks.
Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed. About half the time this will produce a straight line with negative slope: that is, it will slope down from left to right instead of up. So, composition of daughter must be homogenous when a new rock is formed. Crystal morphology and mineral inclusion studies reveal that Rb and Sr of pyrite mineral probably are preserved mainly in biotite, K-feldspar, and sericite mineral inclusions. The internal precision is ±7 m. This line is still linear but is steeper than the previous line. Applications of various geothermometers and oxygen barometers to plutonic rocks have been assessed in connection with this work.