H. W Fairbairn

KAr age values in pelagic sediments of the North Atlantic

Abstract K-bearing mineral components in pelagic sediments from several locations in the North Atlantic show K-Ar age values generally in the range 200–400 m.y., indicating that most of the K is held in stable allogenic mineral structures. The high content of K in the non-car-bonate fraction indicates that it is part of a major component of the sediment. Different size fractions show a minor change in age value. X-ray analysis and studies of age values in eolian material and the components of marine shales suggest that the K-bearing phase is dominantly illite that is in part transported to the oceanic region by wind from continental source regions of unmetamorphosed ancient (mostly Paleozoic) marine shales. Ocean currents may contribute to the transportation and mixing of the material. Differences in age values at depth in a dated bottom core on the equator suggest that there were changes in direction of the transporting medium, or change of source, during glacial stages in the Pleistocene, which may be a basis of stratigraphic correlation. Differences in age values between different regions may provide a basis for following total current movements, or average long-term wind directions.

Liberian Age Province (about 2,700 m.y.)and Adjacent Provinces in Liberia and Sierra Leone

Whole-rock Rb-Sr dating of rocks from the crystalline basement has disclosed an age province in Liberia and Sierra Leone of about 2,700 m.y. The approximate eastern boundary of this province with the adjoining Eburnean age province of about 2,000 m.y. has been found in eastern Liberia. Much younger rocks of PanAfrican age (about 550 m.y.) bound the ancient province in Sierra Leone and western Liberia in a belt adjacent to the coast. Groups of infolded metasedimentary and metavolcanic rocks have been tentatively identified as associated with two of these ages. The Kambui Schists of Sierra Leone show whole-rock Rb-Sr ages of about 2,700 m.y. The pelitic and iron-bearing metamorphic rocks in the Marampa Formation of Sierra Leone, and similar sections in the Nimba Range, Liberia, appear to be about 2,200 m.y. old, and may fall within the typical Eburnean age range. The age values are scattered, however, and the Nimba rocks may be equivalent t o the Kambui. The Kasila Group of Sierra Leone, and a coastal belt in Liberia believed to be a continuation of the Kasila, yield typical PanAfrican ages of about 550 m.y.

Geology and age of the Parguaza rapakivi granite, Venezuela

The 1,550-m.y.-old Paraguaza granite of the northwestern Guayana Shield in Venezuela represents one of the larger, apparently anorogenic rapakivi intrusive rocks of the world. The massive rapakivi granite intrudes foliated granitic rocks and associated volcanic rocks of trans-Amazonian age in a structural setting transcurrent to the general northeast-southwest trend of the older basement rocks of the Guayana Shield. Age relations and the geochemistry of the rapakivi suggest an anatectic origin from tensional effects developed by internal distortions within a continental mass. The extensive 1,550-m.y.-old Parguaza intrusion in Venezuela is correlated with 1,550-m.y.-old basement rocks underlying the Amazon Basin in Brazil and suggests a widespread “Parguazan” event 1,500 to 1,600 m.y. ago which affected a large part of the northwestern and southern Guayana Shield, extending as far south as the Guapore craton of Brazil. The Parguazan event therefore marks an important Proterozoic episode in the tectonic evolution of the widespread Precambrian Shield area of northern South America.

Radiogenic argon and strontium diffusion parameters in biotite at low temperatures obtained from Alpine Fault uplift in New Zealand

Abstract Rapid uplift along the Alpine Fault zone in New Zealand has exposed ancient mica schists which are estimated to have been at depths up to 9000 ft prior to the fault displacement, on the assumption that the smooth surface represented by the present, flat topped, peak elevations is truly representative of the mature Pliocene surface prior to uplift and glaciation. K-Ar age measurements on the micas show losses of radiogenic argon that vary with depth of burial up to almost complete loss from biotite at a depth of 9000 ft. The mean value of the diffusion parameter ( D a 2 ) calculated is 6 × 10−16 sec−1 at an estimated temperature of 110°C. This value is much higher than expected but is compatible with effectively complete retention of argon at surface temperatures, and with measured diffusion losses at high temperatures, if in the relationship D a 2 = ( D 0 a 2 ) exp (- E RT ) D 0 a 2 = 1 sec −1 and E = 27 Kcals/mole. A single Rb-Sr analysis on one of the low argon biotite samples showed an almost equal loss of radiogenic Sr87.

RbSr isotopic evidence in the origin of potash-rich lavas of Western Italy

Abstract The 87 Sr/ 86 Sr ratio in the potassic lavas of the Roman province has been found to vary from 0.706 to 0.711, with the ratio rather constant for each volcano, but generally increasing from south to north. The increase in 87 Sr/ 86 Sr is found to be correlated roughly with total Rb and Sr, both of which reach unusually high levels of concentration. It is concluded that the continental potassic volcanic association in general (leucite-basalt to potash-trachyte), by analogy with this volcanic province, could not have derived its unusually high Sr from carbonatites or the normal types of basalt, or from the assimilation of limestone. The Sr isotopic evidence suggests that these unusual magmas were derived from the refusion or anatexis of ancient continental sialic rocks. The concentrations of Sr and other trace elements suggest that the sialic rocks were deeply depressed into the high pressure regime where garnet and clinopyroxene are dominant residual phases.

The dependence of K-Ar age on the mineralogy of various particle size ranges in a shale

Abstract The mineralogy and K-Ar age of the whole rock and six size fractions, ranging from Whatever the cause of the low age of the IMd material, it is obvious that the whole rock age is not directly indicative of the age of sedimentation, as it is composed of a mixture of ages in the different size fractions of the shale.

RADIATION DAMAGE IN ZIRCON: A POSSIBLE AGE METHOD

The relationship has been studied between crystal structure damage and total alpha particle irradiation in zircon. The total irradiation was based on estimated or measured age times alpha activity, and the damage was determined by the change of the x-ray diffraction angle from the (112) plane as measured to .01° by x-ray spectrometer. The angle was found to be 35.635° (2θ) for completely undamaged material and it approached 35.1° asymptotically as the zircon had suffered increasing dosages of alpha irradiation and become metamict. The difference between the measured angle and 35.635° was found to equal 0.535 exp [− 2.31 × 10 −16 α ] degrees, where α = total irradiation in alphas/mg. The results have been interpreted to indicate that 4.5 × 10 8 atoms are displaced per alpha disintegration and that the annealing rate is essentially zero at earth9s surface temperature. The process appears to be usable for age measurement but generally limited to zircons of less than 500 alphas/mg·/hr· activity. It was further found that the zircon became “half-metamict” after a radioactive energy release of 685,000 cal./gm., indicating that only a fraction of a per cent of the energy can be stored in the structure as disordered atoms, the remainder being lost as heat. Observations on the high annealing rate of common rock minerals shows that the amount of energy capable of being stored in a rock by alpha damage is negligible in questions relating to petrogenesis and volcanism.

Rb-Sr age of stony meteorites

Abstract A suite of samples of stony meteorites with a spread in Rb 87 Sr 86 ratios from about 0.02 to 3.0 has been analysed for Rb and Sr contents and Sr isotopic compositions. These whole-meteorite analyses all coincide within experimental error with a single isochron. The slope of this isochron yields an age of (4.52 ± 0.12) × 109yr using a decay constant of 1.39 × 10−11yr−1forRb87. The meteorites that form the isochron are all observed falls, and consequently are comparatively fresh (unweathered) samples. Each point on the isochron represents several analyses. On the same basis, the intercept of the isochron yields an initial Sr 87 Sr 86 ratio of 0.698 ± 0.001. This value is based on a normalization to 0.1194 of the meteoritic Sr 86 Sr 88 ratio, and is relative to a value of 0.7085 for the Sr 87 Sr 86 ratio in a standard sample of SrCO3 (Eimer and Amend, lot No. 492327) which was analysed alternately and repeatedly with the meteorite Sr samples during this investigation. It is inferred from this investigation that the source materials of the stony meteorites contained Sr whose Sr 87 Sr 86 ratio was 0.698, and that this source material was chemically differentiated into a variety of Rb-Sr systems. The close fit of the samples to a single isochron indicates that this chemical differentiation must have occurred within a relatively short interval of time, and that the individual meteorites are samples of systems that have remained closed ever since. This investigation favors a decay constant of about 1.38 × 10−11yr−1forRb87, in so far as this value is required to make the Rb-Sr age of the meteorites coincide with Pattersons (1956) Pb-isotope age for the meteorites and Earth of 4.55 × 109 years.

Investigation of Initial Sr87 / Sr86 Ratios in the Sierra Nevada Plutonic Province

One to three whole-rock samples from each of more than a dozen discrete plutonic intrusions in the east-central Sierra Nevada batholith have been analyzed for Sr 87 /Sr 86 and Rb/Sr ratios to obtain information on initial Sr 87 abundances. The initial Sr 87 /Sr 86 ratios in the rock magmas forming this province appear to have been in the range 0.7073 ± .0010 in the majority of cases. This range is definitely higher than that found for modern alkali-type and tholeiite-type basalt magmas of oceanic regions, which commonly range between 0.703 and 0.705. However, it is much lower than the average Sr 87 /Sr 86 ratios found in Precambrian sialic regions which range from 0.71 to 0.73. It seems clear therefore that the Sierra Nevada magmas were not derived solely either from the typical source regions of oceanic basalt or from the melting of ancient crustal sial. It is possible that these magmas represent a mixture of oceanic basalt and crustal sial, as would be the case of anatexis in a geosyncline containing much volcanic material of fairly recent origin and some terrigenous sialic detritus. They may instead be of mantle derivation with admixtures of crustal material assimilated during their rise. The whole-rock Rb-Sr age results derivec from the study indicate that the Lamarck and Mount Givens Granodiorites and the alaskite of Evolution Basin and porphyritic biotite granite of Dinkey Lakes form a younger group of intrusive rocks of 90 ± 10 m.y. Although the sampling was not designed for isochron age studies, it appears that most of the remaining rock units are considerably older.

K-AR DATING OF SEDIMENTARY ILLITE POLYTYPES

The illite in several cyclothemic Pennsylvanian shales and clays has been separated on the basis of particle size into 2M 1 and IMd polytypes. K-Ar dates show the 2M 1 component to be considerably older than the Pennsylvanian. The K-Ar age of the IMd component is less than half the age of Pennsylvanian sedimentation. The low age may be due to preferential Ar loss because of the small particle sizes involved or to reorganization and K-fixation in montmorillonite and degraded micas in post-Pennsylvanian time.