Volatile elements abundances

Volatile element abundances in CV chondrites (normalized to Cl chondrites and silicon) lie along a linear array on semi-log plots versus their 50% condensation temperatures. This depletion pattern persists, whether the elements are siderophile, lithophile, or chalcophile. 

Mars bulk silicate (mantle + crust) composition, estimated from Martian meteorites by Wanke and Dreibus (1988). This composition differs from the bulk silicate of Earth, because of differences in volatile element abundances and core differentiation. 

Constraints on the origin of the Earth s volatiles from volatile element abundance patterns... 

Kramers (2003) calculated major and minor (noble gas) volatile element abundance patterns in the Outer Earth Reservoir (the atmosphere, hydrosphere, oceanic and continental crust, and recycled components in MORB-source mantle). These are presented, normalized to solar abundances, together with data for chondrites in Fig. 5.6. The following observations can be made ... 

Kramers, J.D., 2003. Volatile element abundance patterns and an early liquid water ocean on Earth. Precambrian Res., 126, 379-94. 

Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

Chondrite classes are also distinguished by their abundances of both volatile and refractory elements (3). For volatile elements the variation among groups results from incomplete condensation of these elements into soHd grains that accrete to form meteorite parent bodies. Volatile elements such as C,... 

In Chapter 1 and again above, we introduced the cosmochemical classification of elements based on their relative volatilities in a system of cosmic (solar) composition. In a cooling solar gas, elements condense in a certain order, depending on their volatility (Table 7.1). Condensation and evaporation partition elements between coexisting gas and solid (or liquid) phases, and the removal of one or the other of these phases can fractionate element abundances of the system as a whole from their original cosmic relative proportions. 

Elemental abundances in CR2 chondrites normalized to the Cl composition and plotted in order of decreasing volatility from left to right. Lithophile elements are shown with open circles, siderophile elements with black circles, and chalcophile elements with gray circles. CR2 data from Kallemeyn etal. (1994). 

Wai, . M. and Wasson, J. T. (1977) Nebular condensation of moderately volatile elements and their abundances in ordinary chondrites. Earth and Planetary Science Letters, 36, 1-13. 

As implied by their names, chondritic IDPs have roughly cosmic bulk compositions. Element ratios for hundreds of analyzed particles are roughly chondritic (data for CP IDPs are shown in Fig. 12.7) (Schramm et al., 1989). An exception, though, is carbon, which is significantly more abundant in IDPs. The mean carbon content of I DPs is 10 wt.%, relative to 3.2 wt.% for Cl chondrites (Bradley, 2004). The abundances of trace elements in bulk IDPs scatter from 0.3 to 3 times Cl, and volatile elements especially tend to be enriched (Flynn and Sutton, 1992). Higher abundances of carbon and of volatile elements, relative to the most solar-like carbonaceous chondrites, support the contention that IDPs are among the most primitive materials known. 

Figure 12.17a shows lithophile element abundances, and Figure 12.17b shows sid-erophile and chalcophile element abundances in CM chondrites, normalized to Cl chondrites. Illustrated for comparison are the abundances in CO chondrites, which are the anhydrous carbonaceous chondrite group most closely allied to CM chondrites. As in other chondrites, the greatest differences are in volatile elements. The volatile and moderately volatile elements in CM chondrites are present at 50-60% of the abundances of the refractory elements. The volatile elements are primarily located in the matrix, and the matrix comprises 50-60% of CM chondrites. This implies that the matrix has essentially Cl abundances of all elements, while the chondrules and refractory inclusions have Cl relative abundances of refractory elements but are highly depleted in the volatile elements. The sloping transition in the region of moderately volatile elements indicates... 

In most respects, asteroid 4 Vesta is geochemically similar to the Moon. As judged from howardite-eucrite-diogenite (HED) meteorites (see Chapter 6), Vesta is an ancient, basalt-covered world (Keil, 2002). Its rocks are highly reduced, and its depletions in volatile and siderophile element abundances resemble those of lunar basalts. And like the Moon, Vesta is hypothesized to have had an early magma ocean. The exploration of Vesta is now in progress, and within a few years we may have enough data to discuss it in a similar way that we have considered the Moon. 

Chondrites. Over 909, of the meteorites lhat are observed lo fall out of the sky are classified as chondrites, samples lhat are distinguished from terrestrial rocks in many ways. One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Chondrites also have basically undifferentiated elemental compositions lor most nonvolatile elements and match solar abundances except for moderately volatile elements. The imtsl cunipositionally primitive chondrites are members for the type I carbonaceous f Cl I class. 

The fascination with the abundances of the atomic nuclei is that they inform of ancient events. The events that are recorded in their populations depend upon the material sample in question. In the crust of the Earth, they record its geologic evolution. Silicon in that crust is much more abundant than iron, for example, because the Earth s crust is sandy, whereas its iron sank to the Earth s core during its early molten state. In the Earth s oceans the elemental abundances reflect their solubilities in water. In the Earth s atmosphere, their numbers reflect their volatilities. And so it goes. Such abundance-sets reflect and record the geophysical history of the Earth and the chemical properties of the chemical elements. Atmospheric carbon dioxide (C02) and methane (CH4) record an extra wrinkle, the impact of human beings on the Earth s atmosphere. 

---