Chondrite volatile content

Chondrites are subdivided into carbonaceous (C), ordinary (O), and enstatite (E) varieties (Fig. 2.8). Carbonaceous chondrites are volatile rich and contain abundant carbon in their matrix. Because they have a high volatile content they are thought to be the most primitive of all chondrites. Within this group there are a number of varieties named after type specimens designated Cl, CM, CV, etc. An earlier classification used Cl to C3. Cl chondrites are the most primitive meteorites within the carbonaceous chondrite groups and the most primitive of all meteorite types. They are the least chemically fractionated and have the highest volatile content. Ordinary chondrites, as their name implies are the most abundant... 

The most primitive of the chondritic meteorites are thought to represent the "raw material" from which the Earth and other terrestrial planets were accreted. These most primitive chondrites, the Cl - carbonaceous chondrites (see Section 2.3.3.1), are identified by their high volatile content (they may contain up to 30 wt% of HaO, S, and C). They lack evidence of thermal processing after their accretion and so can be treated as the least altered condensates of the solar nebula. This view is supported by a plot of element concentrations in Cl chondrites against concentrations in the solar photosphere. There is a strong 1 1 correlation for all elements except the gaseous elements (H, He, N, O, and the... 

The temperature of 50% condensation of a given element in the Solar Nebula defined by Wasson (1985) is 1037 K for Cu and 660 K for Zn. The much more volatile character of Zn with respect to Cu conditions the relative abundances of the two elements among the dififerent classes of chondrites. Copper concentrations vary from 80 to 120 ppm in carbonaceous and ordinary chondrites (Newsom 1995). In contrast, Zn concentrations decrease from 310 ppm in the volatile-rich Cl to 100 ppm in CO and CV, and to 50 ppm in ordinary chondrites. McDonough and Sun (1995) estimate the Cu and Zn content of the Bulk Silicate Earth to be 30 and 55 ppm, respectively. 

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. 

The refractory condensate model has fallen out of favor, including with Lewis (1988). Nevertheless, it is a useful end-member case. Goettel (1988) calculated the composition of the silicate portion of an ultrarefractory Mercury (Table 2, column 2). This model composition contains no FeO or volatiles, and has large concentrations of the refractory elements—aluminum, calcium, and magnesium. We calculated the thorium and uranium contents of such refractory condensates by assuming chondritic Al/Th and Al/U ratios. A surface of this composition will contain many of the phases in calcium-aluminum-rich inclusions (CAls), such as forsterite, anorthite, spinel, perovskite, hibonite, and melilite. 

Figure 13 Major and moderately volatile elements in carbonaceous chondrites and in the Earth s mantle. All data are normalized to the RLE Ti. There is a single trend for RLE, Mg-Si, and moderately volatile elements. The Earth may be viewed as an extension of the carbonaceous chondrite trend. The low Cr content in the present mantle (full symbol) is the result of Cr partitioning into the core. The open symbol is plotted at the extension of the carbonaceous chondrite trend. Data for ordinary chondrites are plotted for comparison. Similar chemical trends in carbonaceous chondrites and the Earth are evident. H-chondrites are very different (sources Wolf and Palme, 2001 Wasson and...

Figure 16 Mn/Na versus Mn/Al in chondritic meteorites. The two moderately volatile elements Na and Mn have the same ratio in all chondritic meteorites and in the primitive Earth s mantle, here designated as BSE. The low Mn/Al content of the Earth s mantle reflects enrichment of A1 and depletion of Mn. Because of the chondritic Mn/Na ratio of the Earth s mantle, it is unlikely that a significant fraction of the Earth s inventory of Mn is in the core (source...

Figure 4 The relative abundances of the lithophile elements in the primitive mantle (or silicate Earth) plotted versus the log of the 50% condensation temperature (K) at 10 atm pressure. The relative abundances of the lithophile elements are reported as normalized to Cl carbonaceous chondrite on an equal basis of Mg content. The planetary volatility trend (negative sloping shaded region enclosing the lower temperature elements) establishes integrated flux of volatile elements at 1 AU. Data for condensation temperatures are from Wasson (1985) chemical data for the chondrites are from Wasson and Kelleme3m (1988) and for the Earth are from Table 2.

Water cycling in the Archaean More difficult to predict is how water was distributed in the Archaean mantle. It will be shown later (see Section 5.2) that the Earth was initially volatile-rich, when it accreted and it subsequently lost water and other volatiles. Indicative here is the comparison between the water content of carbonaceous chondrites, the likely primitive material of Earth accretion (up to ca. 10 wt%), and the estimated water content of the present-day silicate Earth and hydrosphere (0.19-0.24 wt%, see Table 5.2). 

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