Detrital Monazite Research Articles

Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists

A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 μm scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525° C±25°C at 3.1±0.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists.

The diagenetic and economic significance of composite grains of monazite and hydrocarbon in Western Australian arenites

Composite grains consisting of detrital monazite and solid hydrocarbon, ranging up to 0.5 mm in diameter, have been found in Phanerozoic sedimentary rocks in three Western Australian basins. The most common form is a monazite core surrounded by a hydrocarbon envelope. The envelopes, which have conchoidal fracture and radial cracks, apparently congealed when hydrocarbons were polymerized and crosslinked by irradiation from the monazite. Some composite bodies contain fractured monazite grains within solid hydrocarbon and a few consist of a microbreccia of hydrocarbon and disseminated monazite fragments. The breakup of the monazite was probably a diagenetic chemical process that began when crosslinked hydrocarbons within cracks and open cleavages swelled as they were infiltrated by fluid hydrocarbons of low molecular weight. Five microprobe analyses of the monazite reveal the following ranges in percent composition: La 2 O 3 , 10.88–13.66; Ce 2 O 3 , 25.58–28.78; Pr 2 O 3 , 3.03–5.38; Nd 2 O 3 , 9.15–12.11; ThO 2 , 7.0–11.49; U 3 O 8 , 0–1.44. 2V γ extends from 12.5–15.5°. In the northern Perth Basin, the composite bodies, which contain cores of monazite derived from local gneissic basement rocks, formed after the growth of siderite cement but before extensive silicification. In the central Perth Basin, the composite bodies seem to have formed after the growth of kaolinite and before silicification. Textures associated with the bodies allow them to be slotted into the diagenetic sequence and they therefore show when the oil was present. Analysis of the bodies has indicated a contribution of oil from the Lower Triassic Kockatea Shale, but some may have grown from more than one generation of petroleum. Given the ubiquity of detrital monazite, similar bodies are likely to be useful diagenetic and economic indicators elsewhere.

Hydrocarbon rims on monazite in Permian-Triassic arenites, northern Perth Basin, Western Australia: Pointers to the former presence of oil

Black opaque hydrocarbon rims about 0.05 mm thick are preserved around detrital monazite grains in Permian-Triassic arenites of the norther Perth Basin, Western Australia. Chromatographic analysis indicates derivation from oil chemically like that elsewhere in the sequence, and the rims seem to be remnants of oil, which was largely flushed away. The hydrocarbon rims may have adhered to the monazite grains because of irradiation; they are not found on other grains. Monazite-rich heavy mineral bands are probably responsible for abnormally high radioactivity recorded locally by gamma-ray logs in wells penetrating the sandstones. A preliminary search has revealed similar rims around monazite in other Western Australian arenites ranging in age from Silurian to Cretaceous. Thin-section examination of heavy mineral concentrations may therefore be a useful, simple technique to indicate the former presence of hydrocarbons in dry arenites.

The composition and distribution of nodular monazite in the Lower Palaeozoic rocks of Great Britain

AbstractMillimetric, ellipsoidal monazite nodules found within Lower Palaeozoic sedimentary rocks in Wales, south-west England and Brittany are characterised by a pronounced zonation of light and heavy REE, an inclusion fabric of low-grade metamorphic minerals indistinguishable from the host rock and a low Th content. They are interpreted as the product of in situ recrystallization of detrital monazites derived from pegmatitic or granitic source rocks and are potentially useful as indicators of Lower Palaeozoic sedimentary rock provenance.

Postdepositional modification of uraninite-bearing quartz-pebble conglomerates from the Quirke ore zone, Elliot Lake, Ontario

Petrographic observations, backscattered electron imagery (spatial resolution = + or -0.1 mu m), and semiquantitative energy dispersive electron microprobe analysis indicate that postdepositional modification of the uraniferous conglomerates occurred in three stages. The first involved complete fluid-mediated leaching of Fe from detrital ilmeno-magnetite grains and mobility of U, Th, rare earth elements, Y, PO 4 (super -2) , and SiO 2 . Uraninite was replaced by coffinite ([U, Th]SiO 4 ) and quartz, detrital monazite was dissolved and slightly altered to urano-thorite ([Th, U]SiO 4 ), U reacted with TiO 2 derived from detrital grains to form composite grain aggregates composed of brannerite ( approximately UTi 2 O 6 ) and other alteration products, and coffinite together with a Y-rare earth elements-U phosphate precipitated in the matrix. The next stages involved the precipitation of the secondary pyrite and the occlusion of the conglomerate pore space by secondary quartz and sericite. The most probable Eh-pH fields for the ground waters involved in postdepositional modification are low to moderate Eh and slightly acid pH for ilmeno-magnetite leaching, and low Eh and near-neutral pH for pyrite precipitation. Both inferred Eh-pH fields fall within the thermodynamic stability fields of uraninite and coffinite in which uranium solubilities are low. Phosphate dissolved from monazite and apatite could have enhanced uranium solubility at the higher inferred Eh values. Conclusion that neither the deposition nor the preservation of detrital uraninite can logically be used to draw conclusions about the oxygen content of the Precambrian atmosphere at approximately 2,350 m.y. ago.--Modified journal abstract.

Evidence for the extreme age of certain minerals from the Dominion Reef conglomerates and the underlying granite in the Western Transvaal

Isotopic composition of galenas from the Witwatersrand indicated the existence of a hypothetical “parent” uraninite in this region; it was surmised to have originally crystallized ~3000 million years ago, and to have suffered alteration and expulsion of radiogenic lead later in its history. New age data are presented here for samples of “total rock”, and purified detrital monazite, from the Dominion Reef conglomerates which lie below the Witwatersrand System. These data give very strong evidence that radioactive minerals in the Dominion Reef have an age of crystallization close to 3100 million years. They are probably the oldest well-dated radioactive minerals. The analysis of “total rock” samples appears to be an important approach to the study of uranium minerals in ancient conglomerates. Clear-cut evidence concerning the time of deposition of the Dominion Reef conglomerate is still lacking.

Isotopic evidence on the origin and age of the Blind River uranium deposits

Isotopic analyses of lead extracted from a variety of minerals from Blind River. Ontario, are repeated. The detrital minerals monazite and zircon both give leadratio ages of 2500 million years. The uraainite ore gives a lead- ratio age of 1700 m a. Other isotopic evidence is quoted to suggest that the age of the sediment in which the uranium is found may also be approximately 1700 m y, or older. The lead found in pyrite, pyrrhotite, sericite, and feldspar has anomalous isotopic ratios which can be explained by the hypothesis that they received additions of radiogenic lead from the uraninites (presumed to be 1700 m y old) 1200 to 1300 m y ago. In any case the age of these minerals, in the sense of time of last chemical alteration, is not greater than 1450 plus or minus 150 m y. All our measurements can be interpreted without asauming a major period of mineralization more recent than 1000 m y ago, although we are unable to rule out such a possibility from our evidence. (auth)