Isochron diagram

Figure 2. ( °Th/ Th) vs. isochron diagram (parentheses denote activity ratios). In this...

Figure 10 (on facing page). Several examples of mineral isochrons obtained on near zero age volcanic rocks. The figure compares the isochron diagrams obtained from both and data. 

Figure 11. isochron diagram for mid-ocean ridge, ocean island and eontinental basalts compiled... 

Figure 13. Model curves for continuous fluid addition to a MORB-type mantle source in (a) a °Th-isochron diagram, and (b) a ( Ra/ °Th) versus diagram. The time constant for...

Figure 6. Rosholt (A) and Osmond (B) isochron diagrams for the same, synthetic AS data, shown as their lo error ellipses. Apparent age is 115.7 9.7 ka (MSWD = 1.7), calculated from either the slopes of the Rosholt isochrons or the intercepts of the Osmond isochrons.

Figure 7. Identical data plotted on isochron diagrams with different choices of axes. Note the...

The 11 points of the isochron diagram are drawn in Figure 5.13. Their respective error ellipse (1 a error) could be drawn using the method described in Section 2.4, but errors are too small for the ellipses to be clearly seen. 

The duration of testing is not specified, but ISO 11403-1 [36] proposes that the loads should be 20%, 40%, 60% and 80% of the maximum load for the respective temperature and that strains should be tabulated after 1,10,100,1,000 and 10,000 h (10,000 h equals 13.7 months). This data will enable creep strain curves and an isochronous diagram to be prepared (load plotted against strain for each duration) with sufficient accuracy for design. 

Three types of isochron diagrams are used in discussions of short-lived radionuclides. An internal isochron is one based entirely on measurements from the object being dated. By measuring several minerals with different parent/daughter elemental ratios, one can obtain an array of data that gives both the initial ratio (NR/Ns)a and the initial isotopic ratio of the daughter element Figure 8.25 shows an example of an internal isochron for an... 

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. 

Figure 28 Al- Mg isochron diagram for the Allende type B CAI 3529-Z. (source Podosek et al, 1996).

Figure 29 isochron diagram for a large number of CAIs. Data taken from numerous sources listed in... 

Figure 1 Mn-Cr isochron diagram for carbonate fragments from the Cl chondrites, Orgueil, and Ivuna. Cr excesses are found in all the fragments analyzed by ion microprobe, indicating the presence of live Mn at the time of carbonate crystallization (after Endress et aL, 1996).

Figure 2. A two-dimensional isochron diagram for stalagmite SVC-98-3.B from Spring Valley Caverns, Minnesota (the third dimension, not shown here, would be The three sub-samples, labeled left,...

BOX 1.1 FIGURE 1 Isochron diagram for the Sm-Nd system showing a suite of 13 samples which formed at the same time, but which have slightly different Sm/Nd ratios. The calculated age for the 13 samples is 3,540 30 Ma. 

Common techniques found to yield useful solidification ages include the Pb-Pb method Sm (t,/2 = 106 Ga) - Nd "Rb (t,/2 = 48 Ga) - Sr and Re (ti/2 = 41 Ga) Os (cf, 1, 2, II, IS). Generally, methods used to determine solidification ages depend upon data depicted in isochron diagrams, for example, in which enrichment of radiogenic Sr is proportional to the amount of Rb, with Sr being taken for normalization. The slope of such a line yields an "internal isochron" for a meteorite or a single inclusion of a... 

Roddick JC (1978) The application of isochron diagrams in " °Ar/ Ar dating A discussion. Earth Planet Sci Lett 41 233-244... 

The foundations of modern geochronology were laid at the turn of the century in the work of Rutherford and Soddy (1903) on natural radioactivity. They showed that the process of radioactive decay is exponential and independent of chemical or physical conditions. Thus rates of radioactive decay may be used for measuring geological time. Isotopic systems used in age calculations are listed in Table 6.1 and Box 6.1. In this section we discuss two of the most common techniques used in geochronological calculations — isochron diagrams and model age calculations. This is followed by a discussion of the significance of the calculated ages. 

Technique Decay scheme. Decay constant, X (yr. ) Ratios ploned x-axis on the isochron diagram jf-axis... 

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