Zircon is an extremely durable mineral 28often found as a heavy mineral in stream sediments, that, after transport over great distances, shows limited chemical alteration or physical abrasion 23The widespread distribution of zircon in the continental crust, its tendency to concentrate trace elements lanthanides and actinidesits use in age dating, and its resistance to chemical and physical degradation 30 в 33 have made zircon probably the most useful accessory mineral in geologic studies.
Zircon has been identified as an actinide-bearing phase in polyphase ceramic waste forms Zircon also occurs in the Chernobyl lavas as an online dating india sites actinide-bearing phase 6в12 atomic percent uranium The propensity to incorporate actinides and its durability have lead to the suggestion that zircon be used to immobilize actinides 36 в Based on the ability of natural zircon to retain Pb, Gentry et al.
This early suggestion was prescient but did not evaluate the extent of solid-solution of actinides in zircon and did not consider the much greater radiation damage that would occur in such a radioactive waste form The zircon structure consists of triangular dodecahedral ZrO 8 groups that form edge-sharing chains parallel to the a axis and SiO 4 tetrahedral monomers that form edge-sharing chains with alternating ZrO 8 groups parallel to the c axis 25The regular increase in the unit cell volume with the increasing ionic radius of the A-site cation confirms the homologous topologies of these structures Fig.
That a pure, endmember composition, PuSiO 4has been synthesized 27 suggests extensive substitution of Pu for Zr is possible Data are from Keller 27 after Speer The zircon structure is part of a larger class of ABO 4 structure types silicates and phosphates and is closely related to the structure of monazite, CePO 4another durable phase commonly used in geologic age-dating.
These synthesized materials have provided the basis for detailed optical-absorption spectroscopy and electronic paramagnetic resonance studies of U, Pu, Cm, Np, and Gd incorporation into the zircon-structure type 45Detrital zircons in a quartzite at Mount Narryer, Western Australia have been dated at 4,в4, million years ago Mathe oldest terrestrial minerals yet found The oldest so-called intact crust is found in the early Archean 3,в3, Ma granitoids in northwestern Canada 50 and western Greenland The zircons formed at the same time as the first crustal rocks on Earth.
The oldest zircons in the solar system are found as rare inclusions in meteorites and were dated at 4, Ma There are many hundreds of papers that can be cited to illustrate the use of zircon in dating very old rocks, but of greater importance are that these field studies establish the effects of geologic processes on the durability of zircon either under surface conditions of sedimentary transport 29 or at much higher temperatures, e.
The most recent and dramatic example of the extraordinary ability of zircon to retain its U-Pb systematics is from studies of zircons from the Chicxulub impact structure of the Yucatan Peninsula 58 в The shocked zircons were exhumed from the Chicxulub basement rock during meteorite impact and dispersed in the fine dust of the impact cloud.
Discordancies in the U-Pb systematics e.
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The attack on Pearl Harbor was that mechanism; merely a means to an end. By destroying its Pacific Fleet, Japan expected to remove America from the Pacific equation for long enough to allow it to secure the resources it needed so desperately and hoped to crush American morale sufficiently to prompt Roosevelt to sue for peace.
President Roosevelt declares war on Japan following the attack at Pearl Harbor. Rumours that began in the war are still hanging around, well past their sell-by date, fuelled only by revisionist historians and conspiracy cranks.