Mantle keels in the Hadean

Continental crust is typically underlain by low-density regions (or keels) in the mantle (see Section 10.5.1). If extensive areas of continental crust had existed in the Hadean, one might predict that such keels could have survived even if the overlying crust had vanished by various recycling processes. However there is no sign of them, although such keels underlie most Archean stable continental blocks (or cratons). The age of formation of the peridotites that dominate such lithospheric keels...

Delamination and its problems

An apparent paradox disturbs this model of an andesitic bulk crustal composition. It has become an article of faith among petrologists that the bulk composition of subduction-zone volcanics, as for any magmas derived ultimately from the mantle, should be basaltic 10 . But if island arcs are the source of continental material, how does one produce siliceous continents from basalt The problem has led to the popularity of delamination models. In these, basalts are the fundamental building blocks...

The Post Archean upper crust

What is the composition of the present (Post-Archean) upper crust and how might it be determined One approach has involved widespread sampling programs of surface rocks such as those carried out over the Canadian shield and across the USSR. These estimates generally indicate that the major-element composition of the Post-Archean upper crust is mostly similar to that of granodiorite 12 . Because of the extreme variability in the exposed upper crust, obvious to the most casual observer, such...

Sedimentary rocks on Mars

The existence of ancient layered deposits has been known since Mariner 9. However, recent imaging shows the remarkable extent of such deposits, especially in Noachian and Hesperian terrains and geomorphological evidence points to many of these deposits being sedimentary rocks 69 . Most sedimentary deposits are relatively flat-lying with layer cake stratigraphy or in the case of impact-related and pyroclastic deposits may be draped over pre-existing topography. Depositional environments are...

Primary and secondary crusts on Mercury

Can we distinguish between primary or secondary crusts on Mercury There are two options from the currently limited amount of data. The apparent resemblance between the composition of the mercurian and the lunar highlands crust suggests that the entire crust might be primary. This interprets the various plains units to have originated as debris sheets from basin-forming impacts as we concluded above and that no subsequent melting occurred in the mercurian mantle. The alternative view is that the...

Akilia island southwest Greenland

The reason for discussing this small island on the southwest coast of Greenland is that it provides a good example of the problems and controversies encountered in dealing with these oldest surviving rocks. It contains banded quartz-pyroxene rocks that have been claimed to have originally been a sedimentary banded iron formation, older than 3.85 Gyr. Apatite crystals from this sequence have also been claimed to contain graphite with low C C ratios. This isotopically light carbon was heralded as...

Mantle structure

This seems to be an appropriate place in which to comment briefly on the nature of the terrestrial mantle, from which both the oceanic and the continental crusts are ultimately derived. However, we resist the temptation to draw conclusions applicable to other rocky planets from the silicate mantle of the Earth. Even though Venus may possess similar abundances of the major elements, the lack of water and the absence of subduction and recycling will make for a distinctive mantle on our twin...

The composition of the Moon

The mean lunar radius is 1737.1 km, which is intermediate between that of the two jovian satellites of Jupiter, Europa (r 1561 km) and Io (r 1818 km). The Moon is much smaller than the jovian satellite Ganymede (r 2634 km), which in turn is the largest satellite in the Solar System and like the saturnian satellite Titan, is larger than Mercury. Although the jovian satellites and also Titan are comparable in mass, the Moon Earth ratio is the largest satellite-to-parent ratio in the planetary...

Notes and references

Hutton, James (1788) The Theory of the Earth, T. Cadell. 2. The term Hadean Eon is due to Preston Cloud (see Chapter 2, Oasis in Space). An excellent review is given by Kramers, J. (2007) Hierarchical Earth accretion and the Hadean Eon. J. Geol. Soc. London 164, 3-17. The Hadean extends from the end of the accretion of the Earth around 4500-4450 Myr ago to the formation of the oldest recognized rocks that have ages somewhere between 4030 and 3850 Myr. The Nectarian (3850- 4200 Myr) and...

The magma ocean

The geochemical evidence is decisive that at least half and probably the whole Moon was molten at or shortly following its accretion. This stupendous mass of molten rock is referred to as the ''magma ocean'' and a very energetic mode of origin of the Moon, such as delivered by the giant impact hypothesis, is required to account for it. The concept of the magma ocean has proven robust. Several decisive pieces of evidence require that the Moon was mostly melted at or shortly following accretion....

Hypsometry

One manifestation of the terrestrial continental-oceanic crust dichotomy is bimodal hypsometry that corresponds to the crustal type i.e., oceanic vs. continental . Martian hypsometry is also bimodal, broadly similar to Earth. The martian bimodal distribution corresponds largely to the hemispheric dichotomy separating ancient southern highlands from younger low-standing smooth plains to the north. On Earth, the bimodal distribution in crustal elevation reflects isostatic response to differing...

Europium as a universal tracer

In many respects, the rare earth element REE europium, an element whose abundance rarely exceeds a few parts per million in crustal rocks of any planet, turns out to be one of the most useful elements in geochemistry and cosmochem-istry. By its enrichment or depletion relative to the other REE, one can trace much of the history of processes in the Solar System and the Earth. But the element is also useful to astrophysicists. Europium is formed in stars almost entirely by the r-process of...

A primary crust the highland crust of the Moon

Comparisons with the Earth's geologic style, though inevitable, have proved to be treacherous guides to the Moon. Every school child is aware that the Moon is not a planet. So why begin this discussion on planetary crusts with examples from a planetary satellite The reason is that the two types of crusts on the Moon, that form the lighter highlands and the darker maria, are among our best examples of primary and secondary crusts. Their origin and evolution are better understood than those of...

Meteorites and planetary composition

The most reasonable internal structures for the terrestrial planets involve metallic iron cores overlain by silicate mantles, and it was Victor Goldschmidt who pointed out that the metallic, sulfide and silicate phases in meteorites were analogues 27 . Although this generalization still holds, it has not proven possible to correlate specific classes of meteorites, either alone or in combination, with the composition of the bulk Earth. Neither K U ratios, volatile element compositions, nor...

Crustal composition

Venera Venera Images

Although it might appear difficult to establish an overall composition for the crust of Venus, there are many clues. Much information is provided by the geomorphology of the surface as revealed by the Magellan radar. The smooth plains that dominate the surface appear to be volcanic. This is reinforced by the presence of a million basaltic-looking volcanoes. But the lessons of comparative planetology must engender caution and portraits may be deceptive. Fortunately the extraordinary USSR Venera...

The solar nebula and the giant planets

The solar nebula from which the Sun and planets formed had three basic constituents loosely gases, ices and rock. The dominant component was gas 98 hydrogen and helium . The heavier elements metals to the astronomers that amounted to about 2 by mass, had accumulated in the interstellar medium from 10 billion years of nucleosynthesis in previous generations of stars. Abundant elements such as carbon, oxygen and nitrogen were present in the nebula as ices e.g. as water, methane, carbon monoxide,...

Interaction with seawater

There is considerable interaction between the ocean and the upper portions of the underlying crust 18 . Studies of ophiolite complexes on land have revealed that Fig. 8.2 The wide variation of TiO2 versus Mg in lunar mare basalts compared to MORB. Data for MORB from 23 and for lunar basalts from Shearer, C. K. et al. 2006 Thermal and magmatic evolution of the Moon, in New Views of the Moon eds. B. L. Jolliff et al. , Reviews in Mineralogy and Geochemistry 60, pp. 365-518. most of the process...

The depletion of the volatile elements in the inner nebula

Volatile Refractory Elements

One might have expected that the composition of the Earth and the other inner planets would mirror that of the original rock component represented by the CI chondrites of the solar nebula. However the moderately volatile elements that have condensation temperatures in the range 400-1100 K are strongly depleted both in the Earth, Venus and Mars as well as in many classes of meteorites Fig. 1.2 . 1 The planets their formation and differentiation 100000 TTTT1 I I I I I I I I I I I I I I I I Fig....

Mare basalt ages

The mare basalts are very old by terrestrial standards. The oldest ages for returned lunar mare basalts come from Apollo 14 breccias. These aluminous low-Ti basaltic clasts found in the breccias range in age from 3.9 to 4.3 Gyr. The oldest basalt sample returned from a maria is Apollo sample number 10003, one of the very first samples collected by the Apollo 11 crew. It is a low-K basalt from Mare Tranquilitatis with an age of 3.86 0.03 Gyr. This gives a younger limit for the age of the Imbrium...