IT'S ABOUT TIME

The geologist, like any person, uses TIME in two distinct ways. These ways are:

< 1. RELATIVE TIME in which two or more events are compared without any numerical reference. For example ­ I will see you in the cafeteria during lunch; the Beatles first performed in the United States during the presidency of Lyndon B. Johnson; or January precedes February.

< 2. ABSOLUTE TIME in which time is measured in some unit (seconds, minutes, hours or years) from some starting point ­ going either forward or backwards. Examples: this is 1996 A.D.; Herodotus studied deltas in Egypt about 420 B.C.; this lecture started l5 minutes ago.

The difference between geologic time and the examples given from every day life is the length of time involved. Geologic time is vast, with immense and variable blocks in each relative unit, and very large numbers of years in absolute measurements.

Let us first take a look at relative time in geology. This was the first type of time used in the profession, and to this day is the framework of the way time is handled.

RELATIVE TIME

The geologic equivalent of the calender is the Geologic Time Scale, which is the standard relative method of time usage in the younger portion of the Earths history. It is based on sedimentary rocks and fossils. In the older portion of the Time Scale, known as the Precambrian, only a simplified version is used, because of the types of rocks involved (largely igneous and metamorphic), the sporadic nature of their outcrop, and the dearth of fossils. The definition of the Time Scale, and its world­wide application are governed by a few relatively simple postulates:

1. An arbitrary choice of rock outcrops somewhere, to make the type section to which all other exposures are correlated.

2. The three rules of StNno [1638-1686] for sedimentary rocks:

a. original horizontality

b. lateral continuity or extension

c. superposition (oldest on the bottom of a stack of beds).

3. Huttons' [1726-1797] observations ­ the law of cross­cutting relations, and its corollary ­ the rule of inclusions.

4. The principle of faunal succession, and its corollary, the use of fossils for correlation; based on the work of Cuvier [1769-1832] and Smith [1769-1839].

REMEMBER: a fossil is any evidence of previous life. Faunal refers to animals and can be contrasted to floral which refers to plants. An inclusion is a bit of one rock incorporated into another.

ABSOLUTE TIME

Determining absolute time requires two basic factors:

1. some kind of a rate function, or a process occurring at a fixed rate.

2. a quantitative measurement allowing determination of the length of time that the rate function has gone on. For example ­ one old method of absolute dating involved measuring the amount of salt in the ocean. The rate step was the yearly net input of salt into the ocean, and the amount was a determination of the salt content of the ocean.

While there are a variety of absolute dating methods in use, most of the commonly used ones are based on natural radioactivity. In radioactive decay an atom spontaneously changes to another atom as its nucleus emits (or rarely captures) a subatomic sized particle. The starting atom is called a PARENT and the new one formed is the DAUGHTER. The rate of daughter production is constant and unique for each parent 6 daughter system. For a statistically large number of parent atoms, as is usual in nature, the rate function, called the decay constant (8), is absolutely valid, and constant.

The basic equations for determining dates are: 1. P = Poe-8t

where: Po is the number of parent atoms originally,

P is the number present today

8 is the decay constant

D* ­the radiogenic daughter present now, is measured:

2. D*=Po(1 - e-8t)

The date needed is the time "t" elapsed since the system (rock or mineral being dated) formed [ln is the natural logarithm]:

3. t=l/8 ln(D*/P +l)

The useful concept of HALF LIFE [tl/2 ]­ the time in which one­half of the parent decays away is: tl/2 = ln 2/8

SYSTEMS USED IN DATING: A) Both P and D measured:

PARENT DAUGHTER MODE HALF LIFE in 109 yrs

40K 6 40Ar electron capture 1.25

87Rb 6 87Sr beta 48.8

238U 6 206Pb 8 alpha + 6 beta 4.47

235U 6 207Pb 7 alpha + 4 beta 0.704

232Th 6 208Pb 6 alpha + 4 beta 14.0

147Sm 6 143Nd alpha 106.

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B) Only P measured: HALF LIFE

14C 6 14N beta 5730 years

< FACTORS REQUIRED FOR A RELIABLE DATE:

1. Knowledge of the half life.

2. Rock or mineral must be a closed system ­ [no gain or loss of P or D to or from outside by any means.]

3. No initial D in sample ­ or some way to account for initial D.

4. Adequate sample in terms of amount and purity.

5. Good geological control - what does a date mean geologically?.

< MATERIALS COMMONLY USED FOR DATING:

K­Ar method: minerals: micas [biotite, muscovite], sanidine (KAlSi308), hornblende; rocks: basalt, obsidian.

Rb­Sr method: mostly rocks such as granite, diorite or gneiss; sometimes K­spars and micas.

U­Pb method: usually the mineral zircon.

Th­Pb method: usually the mineral monazite.

Sm­Nd method: usually the mineral garnet or mafic rocks [basalt, gabbro].

In all of these methods the isotopic analyses are done by mass spectrometry, and chemical analyses are done by X-Ray fluorescence, inductively coupled plasma spectrophotometry; or other analytical methods such as atomic absorption or flame photometry, or, rarely, by isotope dilution mass spectrometry.

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Radiocarbon method: plant or animal remains, carbonate rocks.

Radiocarbon dating is usually done by anti-coincidence Geiger or proportional counting of beta decays. Rarely is an accelerator mass spectrometer used.

NOTE: The radiocarbon method is used primarily in archeology, its half life is too short for most geologic problems

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Prepared by: Michael Bikerman 1996