ALBERT

Notes: [Auszug] The volcanic layer may be only 200 m thick at 45° N and oxidation of titanomagnetite may be responsible for the decrease in amplitude of the magnetic anomalies away from the ridge ...

Notes: A good correlation between opaque mineralogy and rock magnetic properties has been established in a study of a small number of oriented samples from four kimberlite dykes and one pipe in the Koidu complex of Sierra Leone. the pipe sample is mineralogically distinctive, in that it alone contains deuteric titanomagha-emite after titanomagnetite, and haematite after chlorite. All of the kimberlites contain xenocrystic, upper-mantle derived ilmenite that has undergone sub-solidus reduction to form Mg-Ti magnetite. Three of the dykes contain serpentinized olivine with microscopic to sub-microscopic magnetite and identifiable metal Ni-Fe alloys, and finely textured Mg titanomagnetite and magnetite as atolls surrounding complexly zoned Cr-Mg-Al spinels. Consistent with these observations, Curie and blocking temperatures range from 520 to 590deg;C and high alternating field (AF) coercivities imply single domain or pseudo-single domain particles as the bulk carrier of natural remanent magnetization (NRM). In the pipe sample, however, the remanence persisting above 600deg;C is comparatively strong. Preliminary remanent vectors, isolated by either thermal or AF demagnetization, from four of the five samples are moderately consistent, and yield a mean (n= 4) of D= 323deg;, I= -17deg;, k= 18deg;, and s̀95= 22deg;.

Notes: An intensive study has been made of 30 samples from within, and two bakes samples from below a single Tertiary basalt in Eastern Iceland. The samples are spaced along a measured vertical traverse from bottom to top. The lava is 16·8 m thick, and is reversely magnetized.The quantities measured for each core were:〈list xml:id="l1" style="custom"〉1Oxidation state as revealed by microscope studies in polished sections of the iron-titanium oxide.2Oxidation state of the whole rock as determined by FeO and Fe2O3 analyses.3Titanomagnetite and separate ilmenite grain sizes.4Magnetic susceptibility.5Saturation magnetization.6Curie points and total heating curve in a strong magnetic field.7Natural magnetization after demagnetization in several alternating fields.8Koenigsberger's Q factor, slightly modified.9A new measure of the resistance of the specimen to alternating field demagnetization, the ‘stability factor’.10The position of the core above the base of the lava.The above quantities vary greatly with position in the lava. The results of these measurements show the relationships of these variables to each other; in particular how the palaeomagnetic stability depends on oxidation state of the iron-titanium oxides. Stability is high in the lower two-thirds of this lava, and not just at the chilled margins.Two anomalous results are (1) that the Curie point is virtually constant regardless of oxidation state (unlike some other lavas), and (2) that the whole rock oxidation state measured by Fe2O3/(2FeO + Fe2O3) does not correlate simply with the oxidation state of the iron-titanium oxides as seen in the microscope.

Notes: Abstract Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ13 CPDB = − 12.3 to − −3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO2) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.

Notes: [Auszug] ALTHOUGH a very large volume of palaeomagnetic data is now available1 from igneous materials, relatively few examinations have been made of the petrological details of the particular rocks involved. Rarer still are investigations of the variation of magnetic and associated petrological properties ...

Notes: [Auszug] Analysis of cores collected from a sequence of lavas in Eastern Iceland has made possible an accurate calculation of the average rate of reversal of the Earth's magnetic ...

Notes: Abstract Oxidation variation between the cooling faces of a single lava has been studied by polished section examination, and Fe2O3: FeO chemical analysis. The oxidation state, which is at a maximum toward the center of the lava, developed during the initial cooling, and has very marked magnetic expression. The Th: U ratio varies in harmony with the oxidation state emphasizing the problem of “resentative” sampling of the body. Examination of thirteen additional bodies suggests that in lavas there exists at least a partial dependence of the oxidation variation development on the lava thickness or interior cooling rate.