Project

Discipline

EarlyEarth; Chronology; Chondrules; Plate tectonics

Lead
Wendepunkte in der Erdgeschichte II
Lay summary
Die Entstehung der Erde und die daran anschliessende geologische Entwicklung über die Zeit sind gekennzeichnet durch Phasen mit graduellen und stetigen Veränderungen, die abrupt zu einem neuen Modus von Veränderungen übergehen. Anschliessend an diese stark dynamischen Ereignisse geht die Entwicklung stetig weiter, aber mit einem neuen Prozessverlauf.  In der Erdgeschichte gab es immer wieder solche ausgeprägte Unstetigkeiten, und davon sollen zwei besondere Ereignisse in diesem Projekt untersucht werden: 1. Der chondrenbildende Prozess, der unmittelbar der Bildung von Planeten vorausging und möglicherwiese erst ihre Bildung ermöglicht hat, und 2) die Bildung von felsicher kontinentalen Kruste, die die Dichotomie in der Entwicklung der Erdkruste heute kennzeichnet und sich im Archaikum herausgebildet hat. Das Zeitintervall der Chondrenbildung ist eine kritische Phase bei der Bildung von Planeten aus Staub und Gas im frühen Sonnensystem.   Es wird untersucht werden, wann sich die Chondren in unterschiedlichen Meteoritengruppen, die jeweils von unterschiedlichen Planetesimalen stammen, gebildet haben. Ein besonderer Fokus liegt auf der Untersuchung von Chondren die anscheinend 2 Ereignisse erlebt haben.  Es gibt Chondren, die Zeichen einer eines zweiten Schmelzprozess zeigen und solche, die eine niedertemperierte Alteration aufweisen.  Präzise Altersdaten für die Ereignisse werden es ermöglichen den Ablauf der Schmelz- und Alterationsprozesse in den Chondren zu bestimmen und so die Prozesse von der Chondrenbildung bist zur Akkretion in Planetesimalen zeitlich und räumlich einordnen zu können. Die Untersuchung von Isotopen und Spurenelementgehalten an Gesteinen aus gut erhaltenen archaischen Kratonen (3.6-2.5 Ga) soll Hinweise erbringen auf die Menge der kontinentalen Kruste im Archaikum geben.  Die Herausbildung von dicker, felsischer Kruste seit dem mittleren Archaikum ist möglicherweise das Schlüsselereignis, das dazu führte, dass die Erde die Platentektonik als wichtigsten dynamischen geologischen Antriebsmechanismus entwickelt hat. Die neuen Daten sollen Hinweise darauf erbringen, wann dieser neue dynamische Prozess.  Eine wesentlich Frage wird sein: Ist die Ausbildung einer kontinentalen Kruste die Ursache oder das Resultat plattentektonischer Prozesse.

Direct link to Lay Summary

Last update: 04.02.2020

Number	Title	Start	Funding scheme

The goal of this research project will be to study distinct key events and processes that led to the formation of planets including the Earth and the differentiation processes in the early Earth that produced Earth`s first extensive felsic continental crust, a unique feature among the terrestrial planets. These processes were fundamental in the formation of the Earth and its evolution into a chemically differentiated planet that is geologically active, supports plate tectonics, and made it a habitable planet that enabled the origin of live and ensured its long term evolution. The project builds partially on results of the previous SNF grant “Turning Points in Earth History”. A key process in the early solar system that may have set the stage for the formation of planets is the formation of the first solids in the solar nebula and the early processing of these solids. The most primitive rocky material we have from the solar system are chondrites and they are considered to be the major building blocks of rocky planets. Different chondrite classes have distinct isotope and elemental signatures indicating that differentiation and mixing processes were involved in their formation. The parameter “time” plays a key role in understanding the sequence and the extent of fractionation and modification processes in the early solar system. Of particular interest is the timing of formation of the most primitive components and their subsequent accretion into planetary bodies. These early processes set the stage for the composition and subsequent evolution of planets. Ages for chondrule formation and the relationship of these ages with the composition of individual chondrules will provide firm constraints on the earliest stages of planet formation. We will date the formation of different chondrule types using the short lived isotope 26Al to construct a time line for the formation and reprocessing of chondrules and their final incorporation into the respective chondrite parent bodies. The data will also be used to evaluate the degree of 26Al homogeneity in the early solar system, and its role as a major heat source during the first few Myr of the solar system. The homogenous distribution of 26Al in the solar system and its presence since the formation of the first solids are two unproven assumptions that are central to its use a chronometer. A decisive event in the evolution of the Earth is the emergence of extensive felsic continental crust, which is characteristic for the Earth and apparently missing on other planets. Crust formation is major process that shapes the surface of the Earth and has a dominant influence on the composition and dynamics of the atmo- and hydrosphere. The formation of felsic continental leaves behind a complementary incompatible element depleted residual mantle. The rate of continental crust formation since its first formation at ca. 4.0 Ga was quite variable over time with most of present-day preserved crust having formed between 3.5 and 2.7 Ga. The causes and processes that led to the formation of this old crust are the focus of active research and different models have been proposed. A major difference in the models is the question whether plate tectonics is the underlying process or totally different forces dominated the Earth`s early history. Key parameters that preserve information on the extent and timing of mantle depletion and crustal enrichment are radiogenic isotopes that show complementary evolution in the two reservoirs. However, early on in Earth’s history the differences in the isotope ratios between the two reservoirs were very small and thus precise and reliable initial isotope ratios have to be determined on rocks and minerals that show excellent preservation with minimal later overprint. To achieve this it is planned to obtain high precision initial isotope ratios for Sr, Nd, Hf and Pb. We found that it is necessary to select minerals that preserve their isotope composition since their formation to obtain the most accurate initial ratios: Pb in feldspars, 87Sr/86Sr in apatite and barite, and 176Hf/177Hf in single zircon grains. No minerals preserve initial 143Nd/144Nd, thus whole rock data will have to be used for this isotope system. The regions suitable for such a study are the ca. 3.5-3.2 Ga old Singhbhum Craton (India) that represents the beginning and the 2.7-2.5Ga Eastern Dharwar Craton (India) that represents the end of the phase of rapid crustal growth. Both units were almost unaffected by later overprint. A comparison of the results obtained on these relatively pristine Paleo- to Neo-Archean rock suites will provide insights into the differentiation of the crust-mantle system for the time interval of maximum growth and possibly production of continental crust.