The Earth II: The Core; Mantle Reservoirs Lecture 46 Composition of the Core • In the case of the Earth’s core, we have only two types of constraints: • Geophysical: o density and seismic velocity derived from seismology and moment of inertia. Also must generate a geomagnetic field. o Density suggests a material 5-10% less dense than Fe. • Cosmochemical: o What materials of appropriate density are available in sufficient abundance to constitute 1/3 the mass of the Earth? o Iron meteorites provide a compositional model of the core. o Again we turn to a chondritic model: we infer that siderophile elements missing from the silicate Earth are in the core. o For refractory siderophile elements, they should be in chondritic relative proportions. o For non-refractory siderophiles, the volatility trend provides a means of estimating composition. Volatility Trend Composition of the Core Understanding Core Formation • Metal/silicate partition coefficients depend on pressure and oxygen fugacity. • Today, the core–mantle boundary, is at 135 GPa and 3000–4000 K. • Experiments suggest metal silicate equilibration took place at lower pressure (as in planetesimals, there were the building blocks of Earth). Mantle Geochemical Reservoirs & Evolution Mantle Reservoirs • • • We previously looked at the composition of the silicate Earth (BSE). This composition is also known as ‘primitive mantle’ (mantle after core segregation, but before crust formation). In reality, the mantle is processed, heterogeneous, and no known sample of mantle matches exactly the ‘primitive mantle’ composition. Isotope ratios of basalts (particularly oceanic ones) provide views of the time-integrated composition of their sources. o • Basalts are useful because they are common and because their production involved larger regions (>100 km3) of mantle. Elemental compositions are changed during melting, but isotope ratios are not. Isotope ratios shows a fundamental two-fold division of basalts: MORB and OIB. Sr-Nd Mantle Array Nd-Pb Isotope Systematics MORB & the Depleted Mantle • • • • • Seafloor spreading creates 3 km2 new area of ocean floor each year (an equal area is subducted) and ~20 km3 of mid-ocean ridge basalt (MORB) forms to fill the gap. They are the most voluminous magmas on the planet. Compare to others, they have uniform tholeiitic (richer in Si, poorer in alkalis than alkali basalt) and are relatively poor in compatible elements. They have low 87Sr/86Sr and 206Pb/204Pb and high εNd and εHf ratios implying low time-integrated Rb/Sr, U/Pb, Nd/Sm, and Hf/Lu ratios - that is low values of ratios of more-to-less incompatible element ratios. They provide evidence of a voluminous (incompatible element-) depleted upper mantle (DUM) or DMM (depleted MORB mantle). The origin of the this DUM is most readily explained by removal of an (incompatible element-rich) melt that has formed the continental crust. How Much DUM? • • • • • • Suppose we consider the Earth as consisting of three reservoirs: o o o Primitive mantle Continental crust Depleted Mantle o We don’t necessarily know the εNd of continental crust, but we do know its Sm/Nd ratio and can guess at its age. We write a series of mass balance equations that allow us to solve for the fractional mass of depleted mantle, assuming we know the εNd and Nd concentrations of the other 2 reservoirs and their masses. We can solve for the relative mass of depleted mantle. Likely answer is ~30% if BSE εNd = 0 (chondritic) but 40-100% if the Earth has εNd = 3-7, consistent with collisional erosion. Bottom line: at a minimum, melt has been extracted from a lot (~30%) or perhaps most of the mantle to form the continental crust. If substantial volumes of primitive mantle remain, we see little direct evidence of it. OIB Reservoirs • The OIB are more diverse. • They can be divided into 4 main groups: o o o o St. Helena (HIMU) Kerguelen (EM I) Society (EM II) Hawaii (PREMA) • This suggests several distinct (chemical) evolutionary pathways. Primitive Mantle • Convergence of OIB arrays at Zindler & Hart’s PREMA (prevalent mantle) together with the observations that the highest 3He/4He ratios occur in basalts with εNd of 3-7 suggests primitive mantle might be a background component of many OIB sources, provided primitive mantle has εNd of 3-7 as collisional erosion (or the EER hypothesis) predicts. Mantle Plumes • • • Oceanic island volcanoes (e.g., Hawaii, Iceland, Azores) are widely (but not universally) thought to be products of mantle plumes columns of hot (but solid) rock rising convectively from the deep mantle (perhaps from the core-mantle boundary driven by heat from the core). Although still a bit controversial, seismic evidence is increasingly consistent with this. Thus OIB sample deep mantle reservoirs (reservoir(s) could be small: D’’ a candidate). Evolution of OIB reservoirs • • • • • Many OIB have 87Sr/86Sr greater and εNd lower than BSE. This requires something other than melt extraction. Incompatible element pattern consistent with melt enrichment. Although plumes come from the deep mantle, incompatible element patterns suggest upper mantle processes (deep mantle melts have very different incompatible element patterns). Thus although they come from the deep mantle, their chemistry bears the signature of upper mantle processing. Slope on 207Pb/204Pb-206Pb/204Pb plots suggest heterogeneity is ancient, but not as old as the Earth itself. Extended rare earth or “spider” diagram, in which the BSE-normalized abundances are plots and elements are ordered by incompatiblility. Mantle Plumes from Ancient Oceanic Crust • Hofmann and White (1982) proposed that the distinct composition of OIB sources (mantle plumes) comes from oceanic crust (+continent derived sediment) subducted into the deep mantle. • This material is heated (by the core) and eventually rises to the surface as mantle plumes (finally melting in the upper most 100-200 km).