The composition of the fine granite corresponds almost to a natural minimum in the ~-Of-Ab water system. One essential result of the analytical work was a determination of the approximate age of the rock. Patterson, Silver and McKinney (5) measured the isotopic composition of lead from a whole-rock sample from the coarse.
Relationships of this type were not found on the convex side of the central dike.
GEOLOGIC MAP OF
MOUNT RUBIDOUX
RIVERSIDE, CALIFORNIA Legend
The coarse granite has a striking blue-green color, enhanced by an unusual degree of transparency of the feldspar crystals. The accumulated data on optical properties of the minerals in the granites are shown in table 2. Chemical analyzes of the coarse- and fine-grained granites, and the norms calculated from these, are shown in.
The crystallization temperature of the Rubidoux granites must have been very close to 700oC.
Iron
PRELIMINARY CONSIDERATIONS Introduction
It is recognized that some of the sources of uranium (and lead) in the rock, such as inter-. Samples of the coarse granite were selected to be as free as possible of inclusions. From the lower part of the profile, granular separated C-zone material was collected and marked.
The sample was collected from a rock located on the inside of the uphill portion of the Mt.
RADIOACTIVE ACCESSORY MINERALS IN THE
The results of the isotopic studies have been interpreted under the assumption that the Rubidoux granites crystallized and their isotopic systems closed, essentially simultaneously and over a time interval too short to be discriminated by current methods. The composition of lead in the potash feldspar of the coarse granite (next chapter) was used to correct the analyzes of allanite and uranotorite. The intersection of the two lines is the corrected mixture of radiogenic sample lead and spiked lead, and can be easily read to the nearest atomic abundance of 0.05%.
In some cases, the calculated lead concentration had to be corrected for a shift in the peak concentration. Twelve zircon fractions from fresh and weathered samples of coarse- and fine-grained Mt. All zircon fractions were found to have essentially the same physical properties... colorless to pale brown, with the color of the entire sample being a pale yellowish brown.
Many crystals show very poor development of the interfacial edges giving the grains a rounded appearance. A number of tests were made to determine reproducibility and are shown in Table 6. Three variables were considered: 1) the reproducibility of the bulk processing procedure, 2) the effect of the size of the .. sample brought into solution , and 3) the effect of adding spike to the sample before or after fusion. samples of the fresh coarse granite, SCB-36, were processed and splits of the R200 zircon fractions were analyzed. Pb206/U238 ratio is not correct, partly due to additive errors from the calibration of the 207 point and a correction needed for drift in the concentration of the uranium peak.
Other sources of error and the precision of mass spectrometry are discussed in Chapter "6 and in Appendix A. The age of the Rubidoux granites is such that only the Pb206;U238 ratio can be used as an accurate measure of. the purpose of this chapter is the most important aspect of the data in Table 7, that the different samples.
RAY DIFFRACTION AND ZIRCONIUM/HAFNIUM MEASUREMENTS
It is possible that an external episodic event could have caused the apparent losses of lead. If it is accepted that zircon and biotite became closed isotopic systems essentially simultaneously, this result seems to support the hypothesis of a younger episodic event. The late Mesozoic and Tertiary history of the general area of Southern California has been quite complex in terms of plutonic, volcanic, and tectonic activity, but it is difficult to associate any other specific event with the Mt.
Therefore, there is no compelling evidence to choose between external or internal causes of the discordance, although the available data seem to strongly favor an episodic event of variable local intensity. The isotopic results indicate that the actual age of the Rubidoux granites must be at least 116 million years. An extrapolation to zero concentration based on the data from SCB-36 and SCB-I02 suggests an upper limit of 130 million years for the true age of the rocks (Fig. 5).
The extrapolated age is considered a maximum because the sensitivity to perturbation is believed to decrease faster than decreasing radioactivity as zero concentration is approached. Therefore, it is thought that the true age of the Rubidoux granites is probably in the order of 120 million years. In order for the time being some of the geo-chronological problems caused by the disagreement in the.
Silver and the author visited the Commercial Quarry at Crestmore and collected a sample of quartz monzonite porphyry from a large block of fresh rock lying at the base of the upper face of the quarry. Therefore, it is interpreted that the apparent age of the rocks is very close to the true age.
Uranium Concentration (ppm)
COMMON LEAD AND THE TOTAL ROCK SYSTEM Introduction
The isotopic composition of this original or common lead must be known in order to estimate the total isotopic balance. In the present study it is assumed that there is only one common lead in each of the mountains. The petrological homogeneity of the granites and the absence of any apparent secondary alteration support this assumption.
The composition of the common lead was determined by various analyzes of lead isolated from potash feldspar. After this, the isotopic composition of lead and the concentration of lead and uranium in whole rock samples were measured. Feldspar exudates were obtained from two different hand samples of the fresh coarse granite, SCB-36, and from one hand sample of the fine granite, SCB-102.
Aliquots for alumina analyzes were passed through nylon cloth sieves and desired size fractions were placed in a mixture of tetrabromoethane and acetone adjusted to a specific gravity of approximately 2.60. From one hand sample of coarse granite, a P200-R400 reticulated potassium aluminate concentrate was divided into two parts, one of which was placed directly into solution for one day and the other was acid-washed in hot concentrated HNO for one hour. From a hand sample of fine granite, P200-R400 mesh concentrate was placed directly into the solution.
Five feldspar and nine total rock lead composition analyzes were statistically evaluated to determine the accuracy of the mass spectrometry. Furthermore, the relatively low intensity of the mass 204 signal allows for a consistent bias when reading the chart.
Mt. Rubidoux Granites, Feldspar Lead Isotopic Compositions
Mt. Rubidoux Granites, Feldspar Lead
The most likely source in the feldspar analyzes is the ammonium citrate. However, the lead composition of SCB-36 Fslle is significantly more radiogenic than the other two samples in this set. SCB-36 FsIB is virtually the true common line of the coarse granite, and this is the interpretation proposed here.
However, the consistency of the data (Fig. 10) suggests that the fine granite may have a common lead that is naturally richer in uranium-derived lead than the common lead of the coarse granite. Thus, the common lead of the coarse granite may be slightly more primitive because it has assimilated an older lead from the wall rock. The lead concentrations in the different feldspar samples show a correlation with the size fraction. The deposits of the coarse granite samples were examined.
If so, it probably reflects contamination of the coarse granite system with a more primitive lead rather than enrichment of the fine granite system by the decay of uranium. Whole rock samples were obtained as splits of the powders from which potassium feldspar concentrates were. The aggregate rocks of the two-hand samples of the fresh coarse granite are labeled SCB-36 TRI and TRII.
Since it was not desirable to wash the samples with acid, an investigation was carried out into the possible isotopic composition of foreign lead that could possibly have been introduced into the natural environment, especially from the air since mountain The composition of the coarse granite feldspar lead, SCB -36 FsIB, is shown for reference, and the solid lines in Fig.
Mt. Rubidoux Granites, Total Rock Lead Isotopic Compositions
Mt. Rubidoux Granites, Total Rock Lead Isotopic Compositions
Y' Hence
Before considering specific forms of interim subsidiary losses, it is good to examine their validity. If the loss mechanism is the same throughout the life of the mineral, the assumption seems plausible. Similarly, the current information indicates that the value of the constant k is 1 or less.
Three situations will be considered that demonstrate some of the limiting features of intermediate daughter loss according to the model framework developed above. Taking emanation loss as an example, assume that the loss is directly proportional to the total concentration of the daughter element in the crystal. However, complete loss is not allowed, so the lower cord extension is prohibited.
Figure 18 shows that the apparent lower intercept increases with sample age. Again, the endpoints of the line can be determined by assigning the limit values ° and 1 to p. Some efficiency characteristics of this system have been discussed by Silver and Deutsch (52).
The effluent is not collected until the color change reaches the bottom of the resin column. The effluent is not collected until the color change reaches the bottom of the column.
