Late Precambrian marbles of unusual carbon-isotope composition, southeastern British Columbia
30
Citation
0
Reference
10
Related Paper
Citation Trend
Abstract:
Carbon- and oxygen-isotope analyses were made of both carbonates and graphites from several Precambrian metamorphic samples from British Columbia. The carbon-isotope data are unusual in that the δ 13 C values of many marbles are very high, up to 9.9, and the most positive values occur in the sillimanite-zone rocks. The δ 13 C values of graphite are also relatively high, and the 13 C fractionations between calcite and graphite suggest that (1) the rocks attained and retained carbon-isotope equilibrium during metamorphism, and (2) the temperature of graphite equilibration in marbles from the sillimanite zone is 650–690 °C, in agreement with temperatures estimated from mineralogical phase equilibria.Possible precursor carbonates for the 13 C-rich marbles in British Columbia include (1) Precambrian carbonate-secreting organisms such as algae that participate in reactions with large kinetic isotope effects, (2) carbonates that exchanged carbon isotopes with a large reservoir of organic material prior to metamorphism, and (3) travertines.Keywords:
Sillimanite
Carbon fibers
Cite
Abstract Detailed investigations of metamorphism and migmatization in the northern archipelago of Misterhult, south of Västervik, SE Sweden, show that two main events can be distinguished: 1. Period of prograde metamorphism: the maximum phase being a high-stage paragenesis with K-feldspar and sillimanite. Fleck and veined gneisses are formed, the former due to porphyroblastic growth (e. g. cordierite, andalusite) and/or metamorphic differentiation, the latter most likely due to metamorphic differentiation. A rise in fH2O and aK+ occurred, connected with the prograde metamorphism of argillaceous rocks (i.e. the breakdown of muscovite).2. Period of retrograde metamorphism: the most intense stage being equal to "greenschist facies". Raft migmatites and probably the major part of the agmatites form; the former by introduction of a K-rich volatile phase (from the younger granites) into a metasediment-metabasise-sequence, the latter due to the effect of non-hydrostatic pressure and the intrusion of a K-rich phase connected with diffusion of mainly plagioclase from the host rock (amphibolite). It can be stated that retrograde metamorphism affected the entire area, more or less intensively.
Sillimanite
Andalusite
Greenschist
Migmatite
Cite
Citations (14)
Carbon fibers
Stratotype
Cite
Citations (26)
Abstract The ratios of the stable isotopes of carbon ( 13 C/ 12 C) and oxygen ( 18 O/ 16 O) in calcite in clays that contain carbonate and that were fired at temperatures between 500 and 700 °C displayed a good correlation with the firing temperature for both isotopes. The isotopic composition indicated that the original carbonate was either completely decomposed or that it exchanged with environmental CO 2 to obliterate the original isotopic signature. The isotopic effects seem to be kinetically controlled. The isotope thermometer is used to compare refiring temperatures of pottery which was fired under controlled conditions. The differences between calculated and actual temperatures did not exceed 132 °C and in most cases was about 50 °C or less. Isotopic analysis of pottery from Tel Miqne‐Ekron and Deir el Balah, dated to the 13th–12th centuries BC, gave an average ancient firing temperature of about 600 °C. If shown to be of general validity, then perhaps this technique can be used also for estimating preparation temperatures of ancient mortar, especially when preserved in dry climates where later changes would be minimized.
Carbon fibers
Cite
Citations (4)
The study area is located about 110 kilometers southeast of Mashhad and approximately 40 kilometers southeast of Fariman. The area is considered to be a part of the central Iran zone. It includes metamorphic rocks with different protoliths consisting of pelitic, carbonate and quartz - feldspathic rocks. The metamorphism in the study area is considered to be a contact regional metamorphism with a low pressure - high temperature grade. According to petrographic studies, the metamorphism in the region reached the upper limit of amphibolite facies. On the basis of different mineral assemblages, three zones were recognized, including Andalusite-Cordierite, Sillimanite - Andalusite, and Sillimanite - K-Feldspar zones. Using Thermocalc software, the average temperature and pressure for the peak of metamorphism in the metapelitic rocks are 664 oC and 2.5 Kbar respectively. Based on calculated maximum pressure (2.5 Kbar), the depth of ~7.8 Km for the metamorphism is estimated. Keyword: amphibolite facies; contact regional metamorphism; Fariman; low P-high T; Thermocalc.
Geothermobarometry
Andalusite
Sillimanite
Isograd
Cite
Citations (0)
A large part of Central Sutherland is occupied by an injection complex in which rocks of the Moine Series are associated with much granitic and pegmatitic material. These migmatitic Moinian rocks have reached a higher grade of metamorphism than those which occur outside the injection complex. One feature characteristic of the high-grade migmatites is the presence of sillimanite in many of the pelitic and semi-pelitic rocks. Near the village of Kildonan, ten miles north-west of Helmsdale, sillimanite is not only abundant in the country rock, but occurs also in many pegmatitic and aplitic veins. The field and microscopic evidence shows that this mineral was formed as a result of metasomatic activity at a late stage in the history of the injection complex, when the general metamorphism was already on the wane. The sillimanite seems to have no direct connection with the conditions of regional metamorphism. It was formed under the influence of pegmatitic solutions. In view of the common use of this mineral as an index of the grade of regional metamorphism, it is of interest to describe the evidence on which the above conclusions are based.
Sillimanite
Pegmatite
Migmatite
Metasomatism
Pelite
Cite
Citations (27)
Similarity (geometry)
Cite
Citations (21)
Sillimanite
Orthoclase
Muscovite
Greisen
Cite
Citations (4)