Solar System Chronology - Data

Question 1

which isotope dating method was used first to obtain a reasonable estimate for the age of the solar system and the Earth?

Answer 1

the Pb-Pb isotope dating system

Question 2

what is the geochron?

Answer 2

the isochron with a slope that corresponds to an age for the solar system of 4.55 +- 0.07Ga calculated using meteorites

Question 3

how was it found realised that the Earth and meteorites have similar ages?

Answer 3

Pb/Pb isotope data for earth pelagic sediment plotted on the meteorite isochron (geochron) within error

Question 4

what is one negative of using long-lived decay systems?

Answer 4

their large half-lives results in large uncertainties of 50Ma -100Ma in some cases, meaning we are unable to resolve Ma-scale differences in ages e.g. unsure whether CAIs or chondrules form first

Question 5

what is the BABI useful for?

Answer 5

the basaltic achondrite best initial is useful for constraining the initial Sr isotope composition of the solar system

Question 6

where are basaltic achondrites from?

Answer 6

a differentiated parent body, similar to the Earth but much smaller

Question 7

The 187Re/187Os decay system is useful for dating iron meteorites because iron meteorites have relatively high concentration of one classification of elements?

Answer 7

siderophile elements

Question 8

which chronometer provides the most precise absolute age data

Answer 8

U-Pb (238U-206Pb, 235U-207Pb)

Question 8

why are the U-Pb chronometers so powerful?

Answer 8

1. they can be combined to obtain Pb-Pb ages requiring no information on the parent daughter (U/Pb) ratio of a sample
2. 235U short half life (~700Ma) provides excellent age resolution for early solar system processes

Question 9

which solid material is taken to be the formation age of the solar system?

Answer 9

refractory CAI inclusions from carbonaceous chondrites

Question 10

data suggests that chondrules and CAIs started to form at approximately the same time, but which formed over a shorter time span?

Answer 10

CAI formation occurred over ~50,000 years whereas chondrule formation was over Ma.

Question 11

how can relative ages from Hf-W chronometer be converted into absolute ages?

Answer 11

the solar systems initial 182Hf is known from the Hf-W isochron for CAIs, which have an absolute age from Pb-Pb

Question 12

there is a 12Ma age difference between the silicate and phosphateP Pb-Pb ages for the solar system. Why?

Answer 12

This age difference reflects slow cooling of the parent body and late isotopic closure of the phosphates (cooling age). i.e. the phosphates continued diffusing isotopes in and out for 12Ma after the silicates.

Question 13

what are 2 reasons for the large spread of Pb-Pb chondrite ages?

Answer 13

1. late isotopic closure of labile minerals e.g. phosphates vs silicates (long cooling ages)
2. resetting by shock heating, opening the system and resetting age

Question 14

why are variable ages obtained for the H chondrites?

Answer 14

they have different petrological grades, higher petrological grades are younger as they experienced more metamorphism

Question 15

analysis of different types of meteorites e.g. achondrites (silicate mantle and crusts) vs magmatic iron meteorites, yields age information on what process?

Answer 15

planetary differentiation

Question 16

which two extinct radioactive nuclide systems are used to date differentiation?

Answer 16

53Mn-53Cr
182Hf-182W

Question 17

to date differentiation, what is required for both nuclides?

Answer 17

they have different fractionation/partition coefficients into e.g. silicates/metals

Question 18

chondritic meteorites have a Hf/W ratio of ?

Answer 18

1.3

Question 19

Hf is lithophile, whilst W is moderately siderophile, therefore the mantle will have a Hf/W ratio of >1.3 or <1.3?

Answer 19

> 1.3

Question 20

chondritic meteorites have a Hf/W ratio of 1.3, producing more 182W over time. due to their short half-life (~9Ma) after ~50Ma no more Hf -> W. therefore if we look at the the Hf/W ratio of an iron meteorite, if it has lower 182W compared to chondritic meteorite, you can say the core differentiation happened early. whereas if the 182W content of the iron meteorite is very high >1.3

Answer 20

:-)

Question 21

many iron meteorites have epsilonW values less radiogenic than the solar system indicating they are older than CAIs. why?

Answer 21

low eW values are most prominent for irons that had a long exposure history in space, where galactic cosmic rays destroyed some of the 182W. this makes it seem the iron meteorites have higher Hf/W ratios than they do

Question 22

what is the cosmogenic isotope effect (Hf-W)?

Answer 22

Hf-W is used to date the age of core segregation as, Hf and W are both refractory, but Hf is lithophile and concentrated in silicates whilst W is moderately siderophile and concentrated in the metallic core. differences in the epsilonW values of iron meteorites reflects differences in the timing of metal silicate segregation.

many iron meteorites have eW values less radiogenic than the initial value of the solar system indicating they are older than the solar system. however this is only the case for iron meteorites that had a long exposure time in space. high-energy cosmic ray interacts with the nucleus of an atom (e.g. W), causing nucleons (protons and neutrons) to be expelled from the atom, destroying the W, leading to a higher Hf/W ratio (less W)

Question 23

why does the amount of sulfur in a metal yield different epsilonW values?

Answer 23

sulfur lowers the melting temperature of a metal, so more sulfur allows melting and silicate-metal differentiation to occur earlier, yielding lower epsilonW values

Question 24

if the epsilonW offset between chondritic meteorite and the silicate mantle of mars is large, did mars core differentiate early or late?

Answer 24

early

Question 25

if the epsilonW offset between chondritic meteorite and the silicate mantle of mars is small, did mars core differentiate early or late?

Answer 25

late

Question 26

how is the epsilonW of mars bulk silicate mantle determined?

Answer 26

through the intersection of the chondritic meteorite epsilonW trend with the epsilon142Nd value

Question 27

what are epsilon values

Answer 27

ε = Rsample - Rstandard/ Rstandard x 10,000
i.e. values in a sample compared to a standard per ten thousand

Question 28

what are delta values (isotope notation)?

Answer 28

δ = Rsample - Rstandard/Rstandard x 1000 = ‰ difference
values in a sample compared to a standard per thousand