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Plan Chem > U-Th-Pb Decay Systems > Flashcards

U-Th-Pb Decay Systems Flashcards

1
Q

Why is the simple isochron approach not particularly useful to the U-Th-Pb long-lived radioactive decay systems?

A

due to the mobility of Th, Pb and particularly U - they are very mobile even at low-grade metamorphic conditions and during weathering. this open system behaviour is enhanced by the radiation damage to the crystals that results from alpha-decay

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2
Q

Why is the U-Pb decay systems exploited to do long-lived radioactive dating?

A

we have two decay systems where 238U-206Pb, 235U-207Pb, where both the parent and the daughter nuclides are from the same element, meaning the parent isotopes and daughter nuclides are essentially identical in their chemical behaviour/reactivity.

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3
Q

what are the two U-Pb geochemical dating techniques called?

A
  1. the common Pb methods
  2. the zircon dating method
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4
Q

what things does the common Pb equation slope depend on

A

the time, and three constants (half-life 235, half-life 238 and 235U/238U)

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5
Q

if the slope on a 207Pb/204Pb vs. 206Pb/204Pb plot depends on time, what can it be treated as?

A

an isochron

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6
Q

the other common Pb equation cannot solve directly for time. how do we solve for time then?

A
  1. using simple iterative techniques for t until the slope matches; or
  2. by using tables
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7
Q

how does the common Pb equation differ from conventional equations?

A

it does not use the initial isotope composition or the parent-daughter ratio

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8
Q

in the common Pb method, what two things do we need to know to calculate sample age?

A
  1. samples shared the same initial Pb isotope composition at some time in the past
  2. have all remained closed to U & Pb loss/gain since then
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9
Q

classify U, Pb, and Th as either lithophile, siderophile, atmophile or chalcophile (can be more than 1)

A

U and Th are lithophile, hence they are concentrated in the crust within zircon and monazite
Pb is lithophile, siderophile and chalcophile (metal and sulfur and crust) - dispersed throughout crust and mantle

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10
Q

were the common Pb isochrons steeper or shallower in the past?

A

steeper - they are curved evolutions with time

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11
Q

why are the common Pb evolution curves curved?

A

because the parent isotope (235U) of 207Pb has a much shorter half life (700Ma) than the parent isotope (238U) of 206Pb (4.5Ga). this means the early earth produced much more 207Pb compared to now i..e production rates of 207Pb are almost zero, but there is large production of 238U to 206Pb.

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12
Q

what is the half life of the parent isotope 235U?

A

700Ma

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13
Q

what is the half life of the parent isotope 238U?

A

4.5Ga

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14
Q

what is the daughter nuclide of 235U?

A

207Pb

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15
Q

what is the daughter nuclide of 238U?

A

206Pb

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16
Q

all U-Pb systems that began with the same initial composition in the past can be plotted along a ______ line at any time, regardless of if this is today.

A

straight line, which were less steep in the past. these are the isochrons

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17
Q

what is the line with a slope corresponding to the age of the solar system on a Pb-Pb plot called?

A

the geochron

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18
Q

what are the two cases in which a sample may not lie on the geochron?

A

1) if the sample did not follow a single stage of isotope evolution
2) if the sample is younger than the age of the earth

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19
Q

what does the stacey and kramers growth curve aim to explain?

A

the evolution model for Pb in the continental crust as a two stage evolution

20
Q

what parent daughter ratio does troilite (FeS) have?

A

zero, because they have essentially zero U and high Pb (zero/anything = zero)

21
Q

does troilite (FeS) have high or low Pb concentrations?

A

high

22
Q

how many different mu (µ) values does the stacey and kramers growth curve have? between what ages is each curve used?

A

there are two µ values

between 4.57Ga and 3.7Ga
and between 3.7Ga and the present day

23
Q

in the Stacey and Kramers growth model, how many Ga ago do they put the continental crust differentiation event?

A

3.7Ga

24
Q

does the µ value increase or decrease following the single stage of CC differentiation in the Stacey and Kramers growth model?

A

increase

25
Q

which mineral is used to describe the Pb isotope evolution of the CC to the present day?

A

Galena (PbS)

26
Q

the 3.7Ga age of the CC differentiation event is somewhat arbitrary. what is the only justification of this age? what is a more realistic interpretation?

A

only justification - CC formation occurred at 3.7Ga in Greenland
more realistic interpretation - the 3.7Ga event just approximates gradual changes that occurred in the Archaean

27
Q

Th and U are refractory and lithophile elements. The Th and U concentrations of the BSE are higher than the chondritic value (by about 2.7x). Why?

A

bc voltile constituents were lost from earth, and lost during core formation.

28
Q

why is the BSE Pb concentration low?

A

-Pb is volatile, and was depleted in the Bulk Earth to begin with
- Pb was depleted from the BSE due to core formation (Pb is slightly siderophile/chalcophile)

29
Q

why is the BSE U/Pb ratio 50x that compared to U/Pb of CI-Chondrites

A

Pb is lost to the core ad is volatile so was depleted in the BE, whereas CI-chondrites have a much lower U/Pb

30
Q

Th and U are slightly more incompatible than Pb during mantle melting. Therefore would you expect:

U/Pb (continental crust) > U/Pb (mantle/BSE)
or
U/Pb (continental crust) < U/Pb (mantle/BSE)

A

U/Pb (continental crust) > U/Pb (mantle)

31
Q

despite Th and U being more incompatible than Pb during mantle melting, U/Pb (continental crust) > U/Pb (mantle/BSE) is not supported by observations. Why?

A
32
Q

individual reservoirs (crust, depleted mantle, upper mantle) do not fall on the Geochron, why?

A

because they are younger and have undergone a multi-stage evolution

33
Q

given that U is more incompatible than Pb during mantle melting, where should the following plot relative to the Geochron?

a younger enriched reservoir (high U/Pb, and high µ)
a younger depleted reservoir (low U/Pb, and low µ)

A

enriched reservoir below/RHS of Geochron
depleted reservoir above/LHS of Geochron

34
Q

should the continental crust plot below or above the Geochron?

A

below (high U/Pb)

35
Q

should the depleted mantle plot below or above the Geochron?

A

above (low U/Pb)

36
Q

are OIB more enriched than MORB? why?

A

yes. OIB derived from lower mantle (CMB hotspots = enriched). MORB derived from depleted upper mantle.

37
Q

despite being derived from the depleted upper mantle, with a low U/Pb, MORB also plots to the right of the Geochron i.e. is enriched. Why?

A
38
Q

what is the only reservoir that plots to the left (the depleted side) of the Geochron?

A

the lower continental crust

39
Q

what is the Pb paradox?

A

the DMM, which sources MORB, is apparently enriched, as it plots to the right of the Geochron. however, other radioactive decay systems (Rb/Sr, Sm/Nd, Lu-Hftell us that the DMM is a depleted system.

40
Q

why does the BSE not fall directly on the 4.55Ga (meteorite) Geochron?

A

the Pb isotope composition of the BSE might reflect either:
1. late core formation or late addition
2. late addition of volatile Pb to the Earth

further info:

both processes change the U/Pb ratio - Pb is volatile whilst U is refractory; U is lithophile, while Pb is chalcophile/weakly siderophile.
At this time, the Earth was still forming and undergoing significant changes. The process of core formation was likely ongoing, and it is possible that the late stage of core formation may have influenced the Pb isotope composition of the BSE - Pb is chalcophile and moderately siderophile. Additionally, during this period, the Earth was bombarded by asteroids and comets, and volatile elements like Pb may have been added to the Earth’s mantle through these processes.

41
Q

what age is the isochron which accommodates the Pb isotope composition of the BSE?

A

~4.50 to 4.45Ga (50-100Ma younger than Geochron)

42
Q

which of volatile addition or late core formation is the most likely explanation for Pb addition to the BSE.

A

late core formation

not late volatile addition bc this requires a v.large late veneer with a mass of 1-2% of the BSE mass. such a large late veneer addition is not in accord with the BSE abundances of the highly siderophile elements.

43
Q

which decay system can be coupled with the U-Pb decay system to give a better estimate on the growth of the core?

A

Hf-W

44
Q

what is the mostly likely explanation for the second Pb paradox issue (that MORB source mantle has enriched U/Pb)

A

that the MORB-source mantle (DMM) has an anomalously high (enriched) U/Pb

45
Q

what are the two main pathways of anomalous non-magmatic transfer of Pb from mantle to CC that might explain the Pb paradox?

A
  1. hot black smoker fluids emitted at MOR - black smokers leach Pb sulfides from basaltic oceanic crust and transfer this Pb into seawater, this Pb is then deposited in marine metalliferous sediments. such sediments are then accreted onto the continental crust at subduction zones.
  2. alternatively or additionally, the sediment hosted Pb might also subduct into mantle at subduction zones. these wet slabs are slowly heated on descent, dehydrated and these fluids transfer Pb into the mantle wedge, where it is incorporated into the arc magmas that ultimately form new continental crust.

Plan Chem (14 decks)

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