OCGC Seminar - Hanika Rizo

Earth's early mantle evolution inferred from extinct isotope systematics
in Archean rocks
 

Dr. Hanika Rizo
Département des sciences de la Terre et de l’atmosphère, Université du Québec à Montréal

Archean rocks


Thursday, January 15th, 2015
11:30 a.m.  

3120 Herzberg Laboratories
Carleton University  

 

Abstract

A fundamental question in planetary sciences is how the planets formed and how they evolved through time. In the case of the Earth, its composition and evolution during the Hadean-Eoarchean is widely debated and largely unknown due to the limited geological record. In the past few years, analytical developments to study the short-lived chronometers 182Hf-182W and 146Sm-142Nd in terrestrial rocks have allowed the investigation of the earliest phases of Earth’s evolution. These isotope tools are of particular interest since they are only sensitive to early planetary processes, the parent isotopes of these systems decaying completely into the daughter isotopes during the first tens to hundreds of million years of Solar System history. Tungsten is siderophile while Hf is lithophile, so metal-silicate segregation greatly fractionates the Hf/W ratio of both planetary mantles and cores. Both daughter nuclides, W and Nd, are more incompatible than the parent nuclides Hf and Sm, so modification to Hf/W and Sm/Nd ratios in the silicate Earth can also be caused by crystal-liquid fractionation. For example, the first undisputable evidence for a very early differentiation of the Earth’s mantle as a result of magma ocean crystallization came from the extinct 146Sm-142Nd chronometer. The excesses and deficits of 142Nd measured in Eoarchean rocks from Isua (Southwest Greenland), relative to modern terrestrial samples, imply that these rocks were derived from mantle sources that were, respectively, depleted and enriched in incompatible elements during the Hadean.