Kevin Practicals Flashcards
182Hf-182W chronometry of core formation
Bulk planet = chondrites = 1 Hf/W
Core = low Hf/w
Mantle = high Hf/W
Simplest interpretation is that the Earths core has a mean age about 30 Myr after the start of the solar system
Outline the principal limitations of using the Hf-W isotope system
Earth grew by accretion of smaller planets and planetesimals, so the W isotope composition of the silicate Earth is dependent upon the (1) history and nature of accretion (that is, how long accretion occurred and the composition of planetesimals accreted to the Earth) (2) the degree of metal-silicate equilibration with Earth’s mantle-core.
In the abstract of their paper Kruijer et. (2015) argue that tungsten isotopes are useful for testing the ‘late veneer’ hypothesis,
(i) Outline the two reasons that they give in the abstract.
If the ‘late veneer’ did occur,
(ii) How would this have affected the 182W isotope composition of Earths mantle.
(iii) what 182W would you expect the Moon to possess relative to the Earth
(i) First, the late veneer material would have had a different W isotope composition to the bulk silicate Earth. Second, proportionally more material was added to the Earth than to the Moon.
(ii) addition of chondritic W with a relatively unradiogenic 182W composition would lower the isotope composition of the silicate mantle to less radiogenic values.
(iii) If the Moon received proportionally less late veneer than Earth then it should possess a slightly more radogenic 182W isotope composition.
(i) What is the problem with 182W isotope data for the Moon that existed prior to the study of Kruijer et al. (2015) and (ii) why is measuring the lunar 182W isotope composition more complicated than for rocks on Earth?
(i) Data for lunar rocks prior to the study of Kruijer et al (2015) is not precise enough to identify a signature for the late veneer, in particular, not precise enough to distinguish a difference between the Earth and the Moon.
(ii) Measuring the 182W isotope composition of lunar rocks is complicated by cosmic-ray induced secondary neutron capture reactions. These include 182W production via neutron capture by 181Ta, and neutron capture induced burnout of 182W, which lowers 182W.
How do the authors use (i) KREEP-rich basalts, and (ii) 182W vs 180Hf data to determine which lunar rocks give a robust 182W value?
(i) KREEP-rich basalts all have the same Ta/W ratio, so any variations in 182W due to neutron capture by 181Ta, are due to differences in exposure to cosmic rays, rather than differences in the Ta/W ratio.
(ii) 180Hf is only produced by cosmic-ray induced neutron capture, so any positive covaraition of 182W with 180Hf must be due to the same process (Figure 1 in the paper). And samples with an 180Hf = 0 must be little affected by neutron capture
What is the 182W value determined for lunar rocks, and how does it differ from that of the bulk silicate Earth.
The 182W value of the Moon is +0.27±0.04, and this is the difference between the Moon and the silicate Earth (Figure 2 in the paper)
Is the difference between the 182W value of the Earth and Moon consistent with the addition of a ‘late veneer’
The difference between the Earth and the Moon is consistent with the addition of 182W- depleted material (chondritic) to the Earth, with a total mass consistent with that derived from the HSE abundances in Earths mantle (Figure 3 in the paper). (so, prior to addition of the late veneer the Earth and Moon had an indistigusihable 182W isotope composition).
The recent study of Thiemens et al. (2019) suggests that the Earth and Moon have different Hf/W ratios and therefore that the difference in 182W, simply indicates that the Moon was formed before 182Hf became extinct. How do they explain the difference in Hf/W between the Earth and the Moon?
Either (i) The Moon-forming event occurred while Earth’s core was still forming and 182Hf is still decaying. Where increasingly oxidizing conditions later lower the BSE Hf/W, because W becomes less siderophile, or (ii) the formation of a small lunar core scavenged W from the BSM, which then increased its Hf/W.
(a) What is the unusual chemical characteristic of komatiites ?
(b) What does such chemical composition tell us about melting degree and how is this degree of melting usually explained ?
(c) How might komatiites provide reliable information on the chemistry of the deep mantle?
(a) High MgO content (and textures indicative of rapid cooling)
(b) High melting degrees, up to 40% (is this right?). Usually explained as the result of melting at high temperatures (or high water content)
- Earth was hotter so intercepted the solidus earlier
(c) High degrees of melting so reliable information on mantle compositions, and sample large volumes of the upper mantle
- shows mixing trend in the mantle
Using Figure 2a from the study of Maier et al (2009) (shown below) explain what they discovered that had not been observed in previous work.
Pt (at 25% MgO) versus age of komatiites
This study shows that younger komatiites are progressively more enriched in Pt at a given MgO content, and that the most signficiant HSE depletion is found in the oldest rocks, and concentrations similar to present-day were attained by 2.9 Gyr.
Addition of platinum maybe from late veneer and then mixing later = systematic increase
Outline the model favoured by the authors to explain the HSE depletion of early Archean komatiites ?
The preferred model assumes that the Earth’s mantle was initially depleted of HSEs by core formation, within the first 100 Myrs after its formation, followed by progressive re-enrichment with HSE in response to late addition of meteoritic material.
If komatiites originate from melting in the deep mantle then this signal reflects that of the deep melting, indicating that the progressive increase is due to mixing-in of the late veneer into the upper mantle and transport to the deeper mantle.
Highly siderophile elements and tungsten isotopes in 3.9 billion year old rocks
The study of Dale et al. (2017) presents highly siderophile element and tungsten isotope data for 3.9 billion year old volcanic rocks from Isua, Greenland.
(a) What do they observe for the measured tungsten isotope ratio 182W compared to that of modern terrestrial samples (their Figure 4, shown below) (b) When in Earth’s history must the isotope composition of source of the Isua volcanics have been generated ?
(a) The measured 182W/184W for the Isua volcanics is about 13 parts per million higher than the modern terrestrial samples.
(b) Because the half-life of 182Hf is about 9 Myr then this difference must have been generated in the first 50 Myr of Earth’s history. Which implies that the part of the mantle which melted to produce these volcanics must have survived 500-600 Myr. That is, from the first 50 Myr of Earth’s formation to 3.9 Million years (when the volcanic rocks were erupted at the Earth’s surface_
(a) Explain their preferred interpretation of the difference in 182W/184W between the Isua volcanics and the modern mantle.
(b) Explain whether this model fits with the HSE data from Maier et al. (2009) ?
(a) In their model Dale et al. (2017) consider that the difference relates to the late veneer. The mantle source sampled by the Isua volcanics represents a part of the mantle that has been relatively unaffected by the late veneer, whereas addition of the late veneer (with a chondritic 182W/184W isotope composition) has lowered the value of the modern mantle by 13 ppm.
(b) Tungsten is a moderately siderophile element and so will only be affected to a very small extent by addition of the late veneer. In contrast, HSEs are highly siderophile and should be srongly depleted by core formation, and subsequently enriched by the late veneer. The data of Maier et al. (2009) suggests that the earliest mantle was depleted in HSEs, consistent with the model proposed by Dale et al. (2017).
The figure below (their Figure 6) shows the e182W data and HSE abundance data for Isua volcanics and the Moon.
Are these results consistent with the interpretations of Kruijer et al. (2015) from last weeks practical ?
Broadly speaking yes, the value e182W value for the Moon could be taken to represent the composition of the mantle prior to the addition of the late veneer. The mantle source of the Isua volcanics is a little heavier, reflecting the addition of some proportion of the late veneer. While the modern mantle has the full complement of HSEs.
OIB plumes and H
Ocean island basalts (OIB) derived from mantle ‘plumes’ sometimes have 3He/4He ratios that are much higher than mid-ocean ridge basalts (MORB). High 3He/4He and solar-like Ne isotope ratios reflect a higher proportion of primordial volatiles in the source region, and indicate that ‘plumes’ tap a deep mantle reservoir that is significantly less degassed than the asthenospheric mantle.
