Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons

Continental crust forms from, and thus chemically depletes, the Earth's mantle. Evidence that the Earth's mantle was already chemically depleted by melting before the formation of today's oldest surviving crust has been presented in the form of Sm-Nd isotope studies of 3.8-4.0 billion years old rocks from Greenland(1-5) and Canada(5-7). But this interpretation has been questioned because of the possibility that subsequent perturbations may have re-equilibrated the neodymium-isotope compositions of these rocks(8). Independent and more robust evidence for the origin of the earliest crust and depletion of the Archaean mantle can potentially be provided by hafnium-isotope compositions of zircon, a mineral whose age can be precisely determined by U-Pb dating, and which can survive metamorphisms(4). But the amounts of hafnium in single zircon grains are too small for the isotopic composition to be precisely analysed by conventional methods. Here we report hafnium-isotope data, obtained using the new technique of multiple-collector plasma-source mass spectrometry(9), for 37 individual grains of the oldest known terrestrial zircons (from the Narryer Gneiss Complex, Australia, with U-Pb ages of up to 4.14 Gyr (refs 10-13)). We find that none of the grains has a depleted mantle signature, but that many were derived from a source with a hafnium-isotope composition similar to that of chondritic meteorites. Furthermore, more than half of the analysed grains seem to have formed by remelting of significantly older crust, indicating that crustal preservation and subsequent reworking might have been important processes from earliest times.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62681/1/399252a0.pd

First Steps in Eukaryogenesis: Physical phenomena in the origin and evolution of chromosome structure

Our present understanding of the origin and evolution of chromosomes differs considerably from current understanding of the origin and evolution of the cell itself. Chromosome origins have been less prominent in research, as the emphasis has not shifted so far appreciably from the phenomenon of primeval nucleic acid encapsulation to that of the origin of gene organization, expression, and regulation. In this work we discuss some reasons why preliminary steps in this direction are being taken. We have been led to examine properties that have contributed to raise the ancestral prokaryotic programmes to a level where we can appreciate in eukaryotes a clear departure from earlier themes in the evolution of the cell from the last common ancestor. We shift our point of view from the evolution of cell morphology to the point of view of the genes. In particular, we focus attention on possible physical bases for the way transmission of information has evolved in eukaryotes, namely, the inactivation of whole chromosomes. The special case of the inactivation of the X chromosome in mammals is discussed, paying particular attention to the physical process of the spread of X inactivation in monotremes (platypus and echidna). When experimental data is unavailable some theoretical analysis is possible based on the idea that in certain cases collective phenomena in genetics, rather than chemical detail, are better correlates of complex chemical processes

The Nevados de Payachata volcanic region (18°S/69°W, N. Chile)

Subduction-related volcanism in the Nevados de Payachata region of the Central Andes at 18°S comprises two temporally and geochemically distinct phases. An older period of magmatism is represented by glaciated stratocones and ignimbrite sheets of late Miocene age. The Pleistocene to Recent phase (≤0.3 Ma) includes the twin stratovolcanoes Volcan Pomerape and Volcan Parinacota (the Nevados de Payachata volcanic group) and two small centers to the west (i. e., Caquena and Vilacollo). Both stratovolcanoes consist of an older dome-and-flow series capped by an andesitic cone. The younger cone, i. e., V. Parinacota, suffered a postglacial cone collapse producing a widespread debris-avalanche deposit. Subsequently, the cone reformed during a brief, second volcanic episode. A number of small, relatively mafic, satellitic cinder cones and associated flows were produced during the most recent activity at V. Parinacota. At the older cone, i. e., V. Pomerape, an early dome sequence with an overlying isolated mafic spatter cone and the cone-forming andesitic-dacitic phase (mostly flows) have been recognized. The two Nevados de Payachata stratovolcanoes display continuous major- and trace-element trends from high-K 2 O basaltic andesites through rhyolites (53%–76% SiO 2 ) that are well defined and distinct from those of the older volcanic centers. Petrography, chemical composition, and eruptive styles at V. Parinacota differ between pre- and post-debris-avalanche lavas. Precollapse flows have abundant amphibole (at SiO 2 > 59 wt%) and lower Mg numbers than postcollapse lavas, which are generally less silicic and more restricted in composition. Compositional variations indicate that the magmas of the Nevados de Payachata volcanic group evolved through a combination of fractional crystallization, crustal assimilation, and intratrend magma mixing. Isotope compositions exhibit only minor variations. Pb-isotope ratios are relatively low ( 206 Pb/ 204 Pb = 17.95–18.20 and 208 Pb/ 204 Pb = 38.2–38.5); 87 Sr/ 86 Sr ratios range 0.70612–0.70707, 143 Nd/ 144 Nd ratios range 0.51238–0.51230, and γ 18 O SMOW values range from + 6.8% o to + 7.6% o SMOW. A comparison with other Central Volcanic Zone centers shows that the Nevados de Payachata magmas are unusually rich in Ba (up to 1800 ppm) and Sr (up to 1700 ppm) and thus represent an unusual chemical signature in the Andean arc. These chemical and isotope variations suggest a complex petrogenetic evolution involving at least three distinct components. Primary mantle-derived melts, which are similar to those generated by subduction processes throughout the Andean arc, are modified by deep crustal interactions to produce magmas that are parental to those erupted at the surface. These magmas subsequently evolve at shallower levels through assimilation-crystallization processes involving upper crust and intratrend magma mixing which in both cases were restricted to end members of low isotopic contrast.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47805/1/445_2005_Article_BF01073587.pd