Whole-rock isochrons

Ma (Fig. 9.16). The 40Ar-jyAr ages appear to be considerably older, but are compromised by the presence of trapped 40Ar from the Martian atmosphere and possibly inherited radio-genic Ar as well. In contrast to the mineral (internal) isochrons, whole-rock Rb- Sr and Pb-Pb isochrons for shergottites suggest a much older age of-4.1 Ga (Bouvier et al., 2008). Most workers interpret this age to reflect planetary differentiation that established mantle... 

Watanabe et al. (1993) have determined a whole rock isochron age of 107 15 Ma (yi b = 1.42 X 10 Vy) for well-preserved pillowed basalts in Western Shikoku with their basalt initial ratio of 0.70401. 

In their study of the Laacher See eruption. Bourdon et al. (1994) obtained an isochron from their data on phonolitic pumice glasses (probably with U/Th ratios similar to those of the whole rocks) and glasses from cumulate nodules. The fractionation of U/Th ratios is attributed in that case to the crystallization of accessory U and Th-enriched phases such as sphene and apatite. The age of 14.3 6.5 ka is similar to the ages deduced from mineral isochrons (see section 3.5) and to the eruption age of 12.5 0.5 ka (Fig. 7b). Thus differentiation within the phonolitic magma occurred shortly before eruption. 

Figure 7. Two examples of whole-rock isochrons, (a) Isochron obtained on a lava nnit of the Vico volcano (Central Italy) (Villemant and Flehoc 1989). (b) Isochron defined by glass and groundmass analyses of the Laacher See phonolite (Bonidon et al. 1994). In both examples, differentiation occurred shortly before eraption since calculated ages are indistinguishable from eraption ages.

Schaeffer et al. (1993) analyzed mineral phases extracted from the 1985 dacitic pumice erupted by Nevado del Ruiz (Colombia) pyroxene, plagioclase, magnetite, glass and whole rock define an U-Th isochron with a near-zero age (7 6 ka, 2g Fig. 10b). 

It should be noted that, even when good U-Th mineral isochrons indicate old crystallization ages, Ra- Th disequilibria can be found in minerals, groundmass and whole rocks. It is the case for Mt. St Helens and Mt. Shasta (Volpe et al. 1991, 1992) and Vesuvius (Black et al. 1998) (see Fig. 10c,d) this requires either a selective Ra enrichment in the magma shortly before eruption accompanied by further crystallization... 

Whole rock U-Th isochrons can also provide an age for the differentiation process, but they are rare because of the difficulty of fractionating the U/Th ratio through crystallization of major minerals. In the two reported examples (Laacher See phonolites, and Vico ultrapotassic volcanics), whole-rock isochrons show that differentiation occurred shortly before eruption. A similar conclusion was reached in the case of the Longonot volcano (Kenya), whose trachytes define the only reported ( Ra)/Ba-(230Th)/Ba whole-rock isochron. 

Figure 11,17 Whole rock K-Ar isochron of Tuff IB strata from Olduvai Gorge, Tanzania. From G. Faure (1986), Principles of Isotope Geology, 2nd edition, copyright 1986 by John Wiley and Sons. Reprinted by permission of John Wiley Sons.

Figure 11.23 shows the isochron obtained by Marshall and De Paolo (1982) for the granite batholith of Pikes Peak (Colorado). The effectiveness of the double-spike technique is evident, especially when we see that aliquot-spiked samples do not fall on the best-fit interpolant (York s algorithm York, 1969). The obtained age (1041 32 Ma) is consistent with that previously obtained with Rb-Sr whole rock analyses (1008 13 Ma see Marshall and De Paolo, 1982, for references). The initial ratio ( Ca/ Ca)o of 151.0 is identical, within the range of uncertainty, to upper mantle values, indicating negligible contamination by old crust components the relative K/Ca abundance in the earth s mantle is about 0.01, a value too low to alter the primordial (" Ca/" Ca)o composition. 

Figure 11,25 Th-Pb and U-Pb isotopic compositions of whole rocks from Granite Mountains Formation (Wyoming) based on data of Rosholt et al. (1973). 2.79 Ga isochrons based on Pb-Pb age are superimposed, to highlight effect of differential elemental remobilization.

The Lac Turgeon Granite was previously dated at >980 Ma, based on a U-Pb Concordia intercept (Rimsaite 1982), and 948 23 Ma from a whole-rock Rb-Sr isochron. It has an initial Sr/ Sr of 0.7090 0.0043 (Fowler Doig 1983) it is relatively young suggesting it has been partially reset or remained open. 

After formation, the parent and daughter should have not been lost from or gained by the system to be dated. That is, the system must be a closed system after the event. In some ideal cases, for a system disturbed by later events, there can be two different isochrons, a mineral isochron, from which the age of the later disturbance (e.g., metamorphic event) can be calculated, and a whole rock isochron, from which the age of the original igneous event can be calculated. That is, the ages of both events may be inferred. 

Sm/Nd weight ratio by the factor 0.6046. Plot the isochron and find the age of the rocks (using only the whole rock data) and the initial ratio. 

Sm-143Nd evolution diagram for nine mineral fractions and a whole-rock sample of lunar norite 78238. The inset shows the deviations from the best-fit line in parts in 104. In spite of a small amount of scatter attributed to the inclusion of tiny fragments of other rock types into the measured sample, the isochron is very well defined. After Edmunson et al. (2009). 

The Re- Os method was first applied to extraterrestrial samples in the early 1960s when Hirt et al. (1963) reported a whole-rock isochron for 14 iron meteorites that gave an age of 4 Ga. Further development of this system was hindered by several technical difficulties. Rhenium and osmium each exist in multiple oxidation states and can form a variety of chemical species, so complete digestion of the samples, which is required to chemically separate rhenium and osmium for mass spectrometry, is difficult. In addition, accurate determination of rhenium abundance and osmium isotopic composition requires spiking the samples with isotopically labeled rhenium and osmium, and equilibration of spikes and samples is challenging. A third problem is that osmium and, particularly, rhenium are very difficult to ionize as positive ions for mass spectrometry. These problems were only gradually overcome. 

Re-1870s evolution diagram for the St. Severin LL6 chondrite. The isochron is generated from three whole-rock fragments and two metal separates. Deviations from the best-fit line are shown in part in 103 in the inset. Modified from Chen etal. (1998) using X = 1.666 x 10-11 yr ... 

The Re- Os system does not lend itself to the determination of internal isochrons for most meteorites. Chen et al. (1998) produced an internal isochron for the St. Severin (LL6) chondrite using metal separates and whole-rock samples (Fig. 8.21). The slope of the isochron gives a date of 4.60-0.15 Ga (X = 1.666 x 10 11 yr 1), consistent with expectations based on other chronometers, but not precise enough to improve upon other techniques. 

Attempts to generate whole-rock isochrons for chondrites have proved less successful, primarily because of difficulties with the chemistry. As chemical procedures improved, the spread around isochrons determined for H chondrites decreased significantly (Meisel et al., 1996 Chen et al., 1998). The most recent data exhibit relatively little variation in 187Re/1880s ratios and the data cluster closely around the IIAB isochron. 

Often, however, the sample does not lend itself to measurements of multiple phases. In these cases, one can create a model isochron in which the initial isotopic ratio of the daughter element (D(/Dref) is assumed and is used to anchor the -intercept. With this assumption, a single measurement can provide the initial ratio [(NR/Ns)o of an object. A third type of isochron diagram makes the assumption that a group of bodies all formed from the same homogeneous isotopic reservoir at the same time. This whole-rock isochron is constructed from bulk measurements of each sample, and the resulting slope and initial ratio describe the reservoir from which all of the samples formed. 

Noncumulate eucrites whole-rock isochron (Rb-Sr) --------------------------- — Eucrites internal isochrons (Rb-Sr) -... 

Chondrules from CV3 chondrites give a confusing picture. A high-precision Pb-Pb whole-rock isochron for Allende chondrules gives an age of 4565.32 0.81 Ma, almost 3 Myr after CAIs (Connelly and Bizzarre, 2009). The I- Xe system in enstatite should have closed... 

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