Timescales of Magmatic Processes

From Core to Atmosphere

Anthony Dosseto, Simon P. Turner, James A. Van-Orman, Anthony Dosseto, Simon P. Turner, James A. Van-Orman

• Editore: Wiley-Blackwell
• Anno: 2011
• ISBN:
  9781444348262
• Edizione: 1
• Argomento: Sciences physiques
• Categoria: Sismologie et volcanisme

Quantifying the timescales of current geological processes is critical for constraining the physical mechanisms operating on the Earth today. Since the Earth's origin 4.55 billion years ago magmatic processes have continued to shape the Earth, producing the major reservoirs that exist today (core, mantle, crust, oceans and atmosphere) and promoting their continued evolution. But key questions remain. When did the core form and how quickly? How are magmas produced in the mantle, and how rapidly do they travel towards the surface? How long do magmas reside in the crust, differentiating and interacting with the host rocks to yield the diverse set of igneous rocks we see today? How fast are volcanic gases such as carbon dioxide released into the atmosphere?

This book addresses these and other questions by reviewing the latest advances in a wide range of Earth Science disciplines: from the measurement of short-lived radionuclides to the study of element diffusion in crystals and numerical modelling of magma behaviour. It will be invaluable reading for advanced undergraduate and graduate students, as well asigneous petrologists, mineralogists and geochemists involved in the study of igneous rocks and processes.

1

Extinct Radionuclides and the Earliest Differentiation of the Earth and Moon

G. CARO¹ AND T. KLEINE²

¹CRPG-CNRS, Nancy Université, Vandoeuvre-les-Nancy, France ²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany

SUMMARY

The extinct ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd systems provide key chronological constraints on the major episodes of planetary differentiation. Both chronometers can be considered extinct after approximately 5–6 half-lives (i.e., after 50 Myr and 400 Myr respectively) and are therefore selectively sensitive to early events. Application of ¹⁸²Hf-¹⁸²W chronometry shows that segregation of the Earth’s core may have been complete no earlier than 30 Myr after formation of the solar system and probably involved at least partial re-equilibration of newly accreted metallic cores within the terrestrial magma ocean. The current best estimate for the termination of the major stage of Earth’s accretion and segregation of its core is provided by the age of the Moon, which formed as a result of a giant collision of a Mars-sized embryo with the proto-Earth. According to ¹⁸²Hf-¹⁸²W systematics this event occurred 50–150 Myr after CAI formation. As the Earth cooled down following the giant impact, crystallization of the magma ocean resulted in the formation of the earliest terrestrial crust. While virtually no remnant of this protocrust survived in the present-day rock record, the isotopic fingerprint of this early event is recorded in the form of small ¹⁴²Nd anomalies in early Archean rocks. These anomalies show that magma ocean solidification must have taken place 30–100 Myr after formation of the solar system. In contrast, ¹⁴⁶Sm-¹⁴²Nd systematics of lunar samples show that the lunar mantle may have remained partially molten until 300 Myr after CAI formation. Therefore, extinct chronometers indicate that accretion and differentiation of the Earth proceeded rapidly. The core, mantle and crust were completely differentiated less than 100 Myr after formation of the solar system.

INTRODUCTION

The accretion and earliest history of the Earth was an episode of major differentiation of a magnitude that probably will never be repeated. Frequent and highly energetic impacts during the Earth’s growth caused widespread melting, permitting separation of a metallic core from a silicate mantle in a magma ocean. Soon after the major stages of the Earth’s growth were complete, the magma ocean solidified and a first protocrust formed. Earth is an active planet, however, concealing most of the evidence of its earliest evolutionary history by frequent rejuvenation of its crust. Consequently, there is no direct rock record of the Earth’s origin and earliest evolution, but fortunately witnesses of the Earth’s earliest evolution have been preserved as small isotope anomalies in the terrestrial rock record. The short-lived ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd isotope systems provide key constraints for understanding the Earth’s accretion and earliest differentiation, and in this chapter, the basic theory of these isotope systems and their application to models of the Earth’s formation and differentiation will be discussed.

The starting place for the accretion of the Earth is the solar nebula, a flattened disk of gas and dust orbiting the young Sun. Within the inner regions of the solar nebula, dust grains collided and stuck together to form a large population of meter- to kilometer-sized objects. Gravity and gas drag caused these planetesimals to collide and form increasingly larger bodies in a period of runaway growth, the products of which include numerous Moon- to Mars-sized planetary embryos. Collisions among these bodies mark the late stages of accretion, culminating in the formation of a few terrestrial planets that sweep up all the other bodies. The Moon probably formed during this period and involved a ‘giant impact’ of a Mars-sized body with Earth at the very end of Earth’s accretion (Chambers, 2004).

Planetary accretion is intimately linked with heating and subsequent melting of the planetary interiors. The decay of short-lived radioactive isotopes, especially ²⁶Al (t_½ = 0.73 Myr), and collisions among the planetary embryos, caused the planetary interiors to heat up. At some critical size, melting will have started within the planetary bodies, causing separation and segregation of a metallic core (Stevenson, 1990; Rubie et al., 2007). As a consequence, all major bodies of the inner solar system and also many smaller bodies, are chemically differentiated into a metallic core and a silicate mantle. However, some planetary bodies, the chondrite parent bodies, escaped differentiation. They are too small for impact heating to be significant and formed after most ²⁶Al had decayed away. Chondrites thus provide information on what planetary bodies looked like before differentiation began. As such, the chondrites are invaluable archives for investigating planetary differentiation.

Melting in the interior of a planetary object permits the denser components to migrate towards the center, thereby forming a core. Metallic iron melts at lower temperatures than silicates, such that core formation can occur either by migration of molten metal through solid silicate matrix or by separation of metal droplets from molten silicate. The latter process is probably appropriate for core formation in the Earth, where giant impacts caused the formation of widespread magma oceans. Once differentiation began, it proceeded rapidly. The downward motion of dense metal melts result in the release of potential energy and hence further heating, which further triggers differentiation (Stevenson, 1990; Rubie et al., 2007).

As the Earth’s mantle cooled following the last giant impact, the terrestrial magma ocean started to crystallize, ultimately resulting in the formation of the earliest terrestrial crust by migration and crystallization of residual melts near the surface. This process probably took place over timescales of the order of 10,000 to 100,000 years, depending on the blanketing effect of the early atmosphere (Abe, 1997; Solomatov, 2000). As demonstrated by the presence of detrital zircon in an Archean sedimentary formation from Western Australia (Wilde et al., 2001), the earliest terrestrial crust must have solidified <150 Myr after formation of the solar system. Little is known, however, about this ancient protocrust, as subsequent mantle-crust exchanges led to a complete rejuvenation of the Earth’s surface, leaving virtually no remnant older than 3.8 Gyr. Clues on the age, l ifetime and composition of the Earth’s crust can thus only be obtained through the study of early Archean rocks, which sampled the mantle at a time when chemical and isotopic fingerprints of the earliest differentiation processes had not been completely erased by mantle mixing.

This chapter will be divided into three sections. The first section will introduce the main concepts and the reference parameters used for constraining the chronology of core-mantle and mantle-crust differentiation using the ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd chronometers, respectively. The second section will be dedicated to the ¹⁸²Hf-¹⁸²W chronology of the Earth’s accretion and core formation on the Earth and the Moon. The last section will examine the chronology and mechanisms of mantle-crust differentiation on the Earth, Mars and the Moon, as obtained from application of the ¹⁴⁶Sm-¹⁴²Nd system to meteorites and planetary material.

SYSTEMATICS AND REFERENCE PARAMETERS FOR SHORT-LIVED RADIONUCLIDES

The two-stage model for short-lived nuclide systems: ¹⁸²Hf-¹⁸²W

Tungsten has five stable isotopes, all non-radiogenic with the exception of ¹⁸²W, which was produced by β^--decay of the short-lived isotope ¹⁸²Hf (^t_½ =9 Myr). Because W is a moderately siderophile (iron-loving) element, whilst Hf is lithophile (rock-loving), the Hf/W ratio is fractionated by processes involving segregation of metal from silicates during formation of planetary cores. The ¹⁸²Hf-¹⁸²W system has thus been extensively studied in order to derive chronological constraints on terrestrial accretion and core formation. The development of the Hf-W system as a chronometer of core formation goes back to the pioneering work of Lee & Halliday (1995) and Harper & Jacobsen (1996).

The siderophile behavior of W depends on several parameters including pressure, temperature and in particular oxygen fugacity. As a consequence, partitioning of W in planetary cores varies drastically among the terrestrial planets. The Hf/W ratio of the bulk silicate Earth is estimated to be ~17, which is significantly higher than the chondritic Hf/W ratio of ~1. As Hf and W are both refractory elements, this difference cannot be accounted for by cosmochemical f...