Chemical Indicators of Earth's Core Formation from the Mantle, Slides of Geochemistry

Download Chemical Indicators of Earth's Core Formation from the Mantle and more Slides Geochemistry in PDF only on Docsity! 1 GG325 L35, F2012 Lecture 35 Growth and Differentiation of Planet Earth – Formation of the Core and Moon Today 1. Core Formation imprint on the mantle 2. origin of the moon 3. Earth Accretion summary GG325 L35, F2012 Core Formation continued… Chemical indicators from the Mantle Recalling the discussion at the end of last lecture… Two things are apparent from the tables below: a) Sideophile elements are not as low in the mantle as would be expected from pure metal-silicate equilibration. They are 5-350 times more enriched than expected for complete equilibrium between silicate and molten iron. Volatile siderophiles appear to be even more enriched than non-volatile ones. These could argue for incomplete equilibration (e.g., a kinetic or spatial effect), an impure Fe phase (e.g., a chemical effect) and/or addition of a volatile rich component after core formation. b) Chalcophile elements are depleted in the silicate Earth relative to chondrites, but not as depleted as many of the siderophiles are. This could argue against much S in the core. Docsity.com 2 GG325 L35, F2012 Element abundances of the Earth’s mantle normalised to CI chondrite and Ti (data from Palme and O’Neill, 2003). Siderophile elements have metal–silicate partition coefficients that are >1 and were therefore depleted from the mantle relative to CI during core formation. They can be divided into three basic groups, weakly siderophile (grey symbols), siderophile (unfilled symbols) and highly siderophile (black squares). Lithophile elements are not depleted from the mantle as a result of core formation (all other black symbols). An additional depletion trend affecting both lithophile and siderophile elements results from volatile behavior (circles) which is considered to be a broad function of the elements’ 50% condensation temperatures from the solar nebula at 10−4 bar (data from Wasson, 1985). GG325 L35, F2012 Chemical indicators of core formation from the mantle Before we interpret the data further, let's look at how we might estimate the siderophile content of the early mantle. Direct Method: 1. analyze actual samples of mantle (e.g., xenoliths in lava flow) Indirect Methods: 1. analyze a siderophile element in a mantle-derived lava, and then apply a melting model to estimate the element’s concentration in the unmelted mantle source of that lava. 2. Analyze a “melting invariant” trace-element ratio of one siderophile element and one non-siderophile, refractory element in a mantle- derived lava and use some fancy footwork to estimate the concentration of the refractory element using the chondritic primitive mantle model. Docsity.com 5 GG325 L35, F2012 Chemical indicators of core formation from the mantle Indirect # 2, continued. 3. Absolute Siderophile Concentration Calculation We can use the following equation to calculate the absolute concentration of our siderophile element (Mo) in the mantle: (Mo/Nd)old basalt x (Nd/Al)chondrites x Almantle = Momantle Note that this final equation utilizes three well-known facts to determine a fourth: U similar behavior of Mo and Nd during formation of a basalt from the mantle U similar volatility of Nd and Al during condensation and accretion. U the absolute concentration of Al in the mantle. The same procedure can be applied to other siderophile elements too. For instance, for W, one would use the invariant W/U ratio and the fact that Uvolatility . Alvolatility to calculate Wmantle from Almantle GG325 L35, F2012 Chemical indicators of core formation from the mantle Some Results of Indirect # 2 Both Mo and W are considered "moderately" siderophile. Applying the above procedure, we find that: Momantle and Wmantle are somewhere in between what would be expected from complete equilibrium with molten Fe and no equilibrium with molten Fe. Docsity.com 6 GG325 L35, F2012 Chemical indicators of core formation from the mantle Some Results of Indirect # 2 On the other hand, the highly siderophile elements (e.g., platinum group elements, denoted below as “PGE") appear to have PGEmantle all in equilibrium with molten Fe . PGEmantle Predicted observed And among equally siderophilic elements, the more-volatile ones (“Y”) are enriched relative to the more-refractory ones (“X”) in the mantle. Xmantle all in equilibrium with molten Fe . Ymantle all in equilibrium with molten Fe but Ymantle > Ymantle all in equilibrium with molten Fe so that Yearly mantle > Xearly mantle more-volatile more-refractory GG325 L35, F2012 Core Formation Chemistry Summary Interpretation: Some combination of 3 conditions governed core formation: 1) incomplete equilibrium of the whole mantle with molten Fe 2) some siderophile elements were added after the core formed 3) the molten Fe wasn't pure, thus changing the affinity of various elements for the "polluted" molten Fe phase. To distinguish 1 and 2, we compare the relative abundances of more and less volatile elements of the same siderophilicity with the relative abundances of same volatility and differing siderophilicity elements. To distinguish 1 and 3, we compare the relative abundances of various siderophile elements between silicate and pure Fe versus silicate with of impure Fe. Docsity.com 7 GG325 L35, F2012 For instance, Recent experimental results demonstrate the influence of S on the exchange coefficients (KD) of V, Nb, Mn, Ga, In, Zn, Cr and Ta into the core. They are displayed here as a function of mol fraction of S in the metal (XS). Clearly, some elements are sensitive to S (such as V and Cr), whereas others are not (such as In and Nb) Filled symbols are results from this study (2 GPa, 2023 K; 6 GPa, 2373 K; 9 GPa, 2373 K, 18 GPa, 2573 K); others from the literature. Mann et al., 2009. GG325 L35, F2012 Core Formation Chemistry and Homogeneous Accretion This topic is very much an area of ongoing research, but it looks like at least some volatile rich chondritic material was added to the mantle after the core was formed to explain the moderately siderophile/very volatile element abundances. This means that core formation occurred before accretion was complete. In general, the mantle can be considered to have been ~90% equilibrated with pure molten Fe, which implies the Earth had built up to at least 90% of it’s current size when core formation occurred. Other evidence for this late chondritic veneer being added material to Earth after core formation comes from estimates of Fe+3/Fe+2, CO2/CO and H2O/H2 in the mantle. All are slightly higher than expected for homogenous accretion yet lower than expected for heterogeneous accretion. Docsity.com 10 GG325 L35, F2012 The formation of our Moon Lunar sampling expeditions provide invaluable samples of the lunar crust. GG325 L35, F2012 The formation of our Moon The moon is compositionally very similar to Earth in some aspects. ‚ oxygen isotopic composition ‚ bulk major element composition, but in the moon’s case there’s much less of: Tmetallic Fe (overall), although the silicate portion of the moon has more Fe2+ than the silicate Earth Tsiderophile elements (e.g., Ga, Ge) Tmoderately volatile elements (e.g., Na, K, Rb, Cs) Thighly volatile elements (e.g., Bi, Pb, As). And there’s significantly more of Trefractory elements Al, Ca, Ti Docsity.com 11 GG325 L35, F2012 S iderophile and volatile elem ents GG325 L35, F2012 The formation of our Moon Of the four likely scenarios for the formation of the moon: j capture of an extra terrestrial body. j ”Auto” fission from the Earth j Separate condensation near Earth i ”Forced” fission from Earth, via impact (“collision”) the latter is the most commonly accepted, because it explains most of the observed facts. Docsity.com 12 GG325 L35, F2012 Source: Taylor, 1979 0.50-0.678001200-1600Na ppm --0.00750.031Rb/Sr 1.9-2.54016-21Sr ppm 0.45-0.60.300.50-0.66Rb ppm 2.0-2.715257-76Th ppb 0.25250010,000K/U 2.0-2.74015-20U ppm 0.50-0.67100150-200K ppm Moon/EarthMoonEarth Abundances of refractory and volatile elements in the Earth and Moon The formation of our Moon The giant impact scenario is supported by:
the relative proportions of refractory and volatile elements on the Earth and Moon
the E-M homogeneity of refractory-element ratios
and similar E-M oxygen-isotope ratios GG325 L35, F2012 Formation of our moon The giant impact hypothesis: First proposed by R. Daly in the 1940s, and later by Hartmann and Davis (1974). Current theory is that the Moon formed when the Earth (at >0.5 its present size) was struck at low angle by a slow-moving (~5 km/s) body slightly larger than Mars during the latest stages of accretion. The impact would have occurred after core formation on both the Earth and the impacting body, which is sometimes called “Theia.” Theia is postulated to have accreted from material in roughly the same annulus of the solar nebula as the Earth (supported by the O-isotope data) and presumably was similarly depleted in highly and moderately volatile elements relative to the chondrites. The collision partially disrupted/melted the Earth’s mantle and completely disrupted/melted Theia. Docsity.com