Metamorphic age

Because Sm and Nd are both rare Earth elements and have similar chemical properties, and because they often occupy crystalline sites that are not easily altered, this system is resistant to alteration by later events (such as a later metamorphic event). Hence, this system is often applied to determine old and formation ages, whereas other systems (such as K-Ar) may be applied to obtain metamorphic ages. 

When the date of interest refers to the crystallization of rocks or minerals from a melt and all of the above criteria are met, the interpreted age is best referred to as an igneous age. When minerals in a single rock are the products of solid-state recrystallization during metamorphism and all the above criteria are met, then the calculated date may be referred to as a metamorphic age. 

Many Archaean cratons show marked lateral heterogeneity. This heterogeneity can be expressed by many different attributes ages and distribution of greenstone belts, plutonic and (or) metamorphic age domains, contrasting struc-... 

Figure 1.62. Location of epithermal-type deposits in Japan (Shikazono and Shimizu, 1988a). 1 Green tuff and subaerial volcanic region of Tertiary/Quaternary ages, 2 Main Paleozoic/Mesozoic sedimentary terranes, 3 Main metamorphic terranes. TTL Tanakura tectonic line, ISTL Itoigawa-Shizuoka tectonic line, MTL Median tectonic line. Open circle epithermal Au-Ag vein-type deposits, solid circle epithermal base metal vein-type deposits, open triangle epithermal Au disseminated-type deposits.

Nicolaysen NO (1961) Graphic interpretation of discordant age measrrrements of metamorphic rocks. Arm NY Acad Sci 91 198-206... 

The application of the laser probe to meteorite chronology is illustrated by a study of Ca-Al-rich inclusions from the Allende meteorite [7]. This study was able to show that the K in the inclusions studied mainly concentrated in veins and rims with very little, if any, K in the major minerals. The limit obtained is something of the order of 10 ppm. On the other hand, the major minerals do contain appreciable 40Ar. Individual chondrules and the matrix were also studied in the Allende meteorite from places adjacent to the Ca-Al-rich inclusions. For these samples the ages varied from 3.3 to 4.4 G.y. There appears to be evidence that the Allende meteorite has been subjected to numerous metamorphic events, presumably of a collisional origin. 

Mid- to lower-crustal granulite facies rocks. Markl et al. (1997) reported 8 Cl values of mid- to lower-crustal Precambrian aged granulite and retrograde amphibolite facies metamorphic rocks from the Lofoten Islands, Norway (Fig. 1). In part, their study shed... 

Figure 11.14 (A) Internal Rb-Sr isochron for a system composed of three crystalline phases of initial compositions Aq, Bq, and Q formed at time t = 0 and thereafter closed to isotopic exchanges up to time of measurement t, when they acquired compositions A, and C. (B) Effects of geochronological resetting resulting from metamorphism or interaction with fluids. X, X2, and X3 bulk isotopic compositions of the three rock assemblages. In cases of short-range isotopic reequilibration, the three assemblages define crystallization age and original ( Sr/ Sr)o of the system the three internal isochrons (concordant in this example) define resetting age.

Figure 11.18 Apparent K-Ar ages of minerals from Idaho Springs Formation (Front Range, Colorado, 1350-1400 Ma) in zone subjected to contact metamorphism by intrusion of a quartz monzonite (Eldora stock, 55 Ma). Reprinted from S. R. Hart, Journal of Geology, (1964), 72, 493-525, copyright 1964 by The University of Chicago, with permission of The University of Chicago Press.

Harrison T. M. and McDougall I. (1981). Excess " °Ar in metamorphic rocks from Broken Hill, New South Wales Implications for " °Ar/ Ar age spectra and the thermal history of the region. Earth Planet. Sci Letters, 55 123-149. 

The Kokchetav Massif of northern Kazakhstan is a very large, fault-bounded metamorphic complex of Late Proterozoic-Paleozoic protolith age, surrounded by the Caledonian rocks of the Ural-Mongolian fold belt. The Kokchetav UHP and HP belt runs NW-SE extending at least 150 km long and 17 km wide. This massif has attracted much interest since the discovery of metamorphic diamonds. It is the first locality where microdiamonds were found within metamorphic rocks derived from crustal material. 

Therefore, to kineticists and informed geochronologists, the age obtain from an isochron equation or from Example 1-5 is an apparent age, and is called the closure age (Dodson, 1973) because it means the age since the closure of the mineral, not necessarily since the formation of the mineral. The closure age may differ from the true age or formation age because of diffusive loss (or exchange) of the daughter nuclide. For the closure age to be the same as the formation age, the mineral must have cooled down rapidly (for volcanic rocks) or formed at not-so-high a temperature (for metamorphic rocks) so that diffusive loss from the mineral is negligible. 

A monazite crystal in a metamorphic rock is about 100 pm across. It is estimated that the peak metamorphic temperature was 700°C and monazite formed near peak metamorphism. The metamorphic event lasted for about 20 Myr. Find the diffusion distance and evaluate whether the monazite grain lost a significant amount of Pb. That is, evaluate whether monazite grain can be used to determine the age of peak metamorphism. 

In fact, even if the grain were held at 700°C for the duration of the age of the Earth, the diffusion distance would still be negligible. Hence, loss of Pb from monazite is negligible, and the grain can be used to determine the age of peak metamorphism. 

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