Search

New U-Pb zircon ages are reported for granulite facies crustal xenoliths brought to the surface by mafic lavas in the Snake River Plain. All samples yield Meso-to-Neoarchean ages (2.4–3.6 Ga) that significantly expand the known extent of the Archean Wyoming Craton at least as far west as the west-central Snake River Plain. Most zircon populations indicate multiple growth episodes with complexity increasing eastward, but they bear no record of major Phanerozoic magmatic episodes in the region. To extrapolate this work further west to the inferred craton boundary, zircons from southwestern Idaho batholith granodiorites were also analyzed. Although most batholith zircons record Cretaceous formation ages, all samples have zircons with inherited cores—with some recording Proterozoic ages (approaching 2 Ga). These data enhance our perspectives regarding lithosphere architecture beneath southern Idaho and adjacent areas and its possible influence on Cenozoic magmatism associated with the Snake River Plain–Yellowstone "melting anomaly". [ABSTRACT FROM AUTHOR]

This article, titled "Becoming FDR: The Personal Crisis That Made a President," explores the impact of President Franklin D. Roosevelt's struggle with polio on his life and presidency. The author, Jonathan Darman, argues that FDR's hardships and struggles after contracting polio shaped him into the great president who led the country through the Great Depression and World War II. The article also discusses how FDR's polio diagnosis transformed the life of his wife, Eleanor, and propelled her into a leadership role in politics and social justice. The book provides a detailed study of FDR's transformation and the ways in which his personal crisis affected the United States during his presidency. [Extracted from the article]

Jeremi Suri's book, "Civil War by Other Means: America's Long and Unfinished Fight for Democracy," explores the ongoing conflict over race and democracy in America that began with the Civil War and continues into the present day. Suri argues that the events following the Civil War, including voter suppression, segregation, and political violence, set the stage for the exclusion and political violence witnessed in the insurrection at the United States Capitol on January 6, 2021. The book examines key historical moments, such as the rise of the Ku Klux Klan and the presidency of Ulysses S. Grant, to demonstrate how the Republican Party shifted away from advocating for civil rights and racial justice. Suri concludes by offering suggestions for resolving the conflict over American democracy, including political reforms and a greater understanding of history. Overall, the book provides critical insight into contemporary political and social issues. [Extracted from the article]

Boy Soldiers of the American Revolution. By Caroline Cox. (Chapel Hill, NC: University of North Carolina Press, 2016. Pp. xvi, 211. $29.95.) Shedding light on the previously overlooked experiences of [...]

Provides a list of U.S. history Internet sites for secondary school social studies teachers that are suitable for students. Explains that the websites are categorized by time period and includes brief descriptions of their features. Gives a general list of U.S. history websites and a list of questions for website evaluation. (CMK)

This document is a correction notice for an article titled "U-Pb LA-ICP-MS Zircon Dating of Crustal Xenoliths: Evidence of the Archean Lithosphere Beneath the Snake River Plain." The correction addresses three errors in the original publication. The errors include the insertion of the letters 'ITT' in the geologic background section, the modification of the first sentences in the results section, and a small edit in the discussion section. The authors state that these errors do not affect the scientific conclusions of the study. [Extracted from the article]

Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle (1-4). As a consequence, the sub-arc mantle wedge is widely believed to be oxidized (3,5). The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source (6,7). Here we show that Zn/[Fe.sub.T] (where [Fe.sub.T] = [Fe.sup.2+] + [Fe.sup.3+]) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, [Fe.sup.2+] and Zn behave similarly, but because [Fe.sup.3+] is more incompatible than [Fe.sup.2+], melts generated in oxidized environments have low Zn/[Fe.sub.T]. Primitive arc magmas have identical Zn/[Fe.sub.T] to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/[Fe.sub.T] during early differentiation involving olivine requires that [Fe.sup.3+]/[Fe.sub.T] remains low in the magma. Only after progressive fractionation does [Fe.sub.3+]/[Fe.sub.T] increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood., Oxygen fugacity [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is an intensive parameter describing the chemical activity of [O.sub.2] in a given system (fugacity represents a corrected partial pressure due to [...]

Supereruptive volumes of low-[delta][sup.18]O rhyolites define the Snake River Plain--Yellowstone Plateau volcanic province, begging the question of the sources and the processes by which [sup.18]O-depleted magmas are generated. New analyses show that Archean crustal xenoliths from the central and eastern Snake River Plain have normal-[delta][sup.18]O signatures of 6[per thousand]-9[per thousand], precluding them as a low-[delta][sup.18]O source in the genesis of >10,000 [km.sup.3] of low-[delta][sup.18]O ([delta][sup.18]O < 6 [per thousand]) Snake River Plain rhyolites. Synthesis of O-Sr-Nd-Pb isotope data for Archean xenoliths and Snake River Plain magmas defines separate compositional fields of rhyolites and their crustal and mantle sources. Sr-O and Nd-O isotopic mixing models for the most recent volcanic fields in the eastern Snake River Plain, Yellowstone Plateau and Heise, show that normal-[delta][sup.18]O rhyolites have variable crust (~30%-50%) and mantle (~50%-70%) proportions that are specific for each eruption. Low-[delta][sup.18]O rhyolites can be traced along a genetic array of mixing lines from normal-[delta][sup.18]O rhyolite end members to a low-[delta][sup.18]O (~-1 [per thousand]) source. The data support a two-stage magma genesis process, in which normal-[delta][sup.18]O rhyolites are generated by partial melting and hybridization of the crust by mantle-derived basalt, and low-[delta][sup.18]O rhyolites tap ~20%-80% of hydrothermally altered portions of normal-[delta][sup.18]O rhyolitic rocks. This two-stage magma genesis process may be applicable to other caldera systems around the world for which the characteristic O isotope depletions are either less pronounced or undiscovered. doi: 10.1130/G30735.1

Author(s): Simon Turner (corresponding author) [1]; Sonia Tonarini [2]; Ilya Bindeman [3, 6]; William P. Leeman [4]; Bruce F. Schaefer [5] Evidence for the deep recycling of surficial materials through [...]

It has been proposed that high-Mg andesites (HMAs) from the Mount Shasta area may represent near-primary mantle melts, carrying signatures of slab melt interaction with the Cascadia mantle wedge. We present strong evidence that their formation involved mixing of dacitic and basaltic magmas and entrainment of ultramafic crystal material, and thus they cannot represent primitive magmas. The rocks contain (1) low-Mg# (65-72) clinopyroxene (cpx) and orthopyroxene (npx) phenocryst cores containing dacitic melt inclusions, and (2) high-Mg# opx and olivine xenocrysts, all of which are rimmed by euhedral overgrowths of cpx or opx similar in Mg# (87) to skeletal olivine phenocrysts. Textural relations indicate that ultramafic xenocrysts reacted with dacitic liquid, after which the contaminated magma mixed with basaltic liquid to produce a hybrid HMA bulk composition. High Mg, Cr, and Ni derive from the latter inputs, whereas high Sr/Y and overall adakite affinity is inherited from the dacite end member, which is arguably crustal in origin. We suggest that open system processes may be more important in the petrogenesis of HMAs than generally recognized, and that their magnesian compositions do not necessarily imply that they are primitive mantle melts. Keywords: high-magnesian andesite, primitive magnesian andesite, Mount Shasta, mixing, adakite, are volcanism.