Karl R. Wirth

Northern Galapagos Province: Hotspot-induced, Near-ridge Volcanism at Genovesa Island.

Genovesa Island is a small volcano located between the Galapagos hotspot and the Galapagos spreading center. Several observations suggest that Genovesa may not have originated as a direct product of the Galapagos plume, but instead by anomalous activation of the upper mantle by the plume. Genovesas lavas exhibit remarkably homogeneous, depleted compositions with no detectable plume contribution; the volcanos trace element contents indicate a deeper origin than either pristine spreading center lavas or most lavas from the Galapagos Archipelago. Genovesa is virtually indistinguishable from the Lamont Seamounts (near the East Pacific Rise) in composition, volume, height, and distance from the ridge; and Genovesa formed close to its current near-ridge location, more recently than previously assumed (age younger than 350 ka). Numerous similar volcanoes populate the northern perimeter of the Galapagos Archipelago. We propose that the northern volcanic province is the result of the serendipitous combination of excess temperatures, weak lithosphere, and regional stresses from interaction between the plume and the ridge, yielding volcanism where none would be observed otherwise. The Galapagos system may define an eruptive process at hotspots, distinct from the Hawaiian model, in which plume-related volcanism can be regionally diffuse, coeval, and compositionally variable. Such a mechanism has profound implications for our understanding of plume-ridge interactions, as well as for island ages and adaptive radiation in the Galapagos.

Detrital Zircon Provenance of the Mesoproterozoic Midcontinent Rift, Lake Superior Region, U.S.A.

We report the ages of detrital zircons (207Pb/206Pb LA-ICPMS; ) from 12 clastic units that are representative of the Mesoproterozoic Midcontinent Rift System (MRS) of North America. Magmatism of the MRS spanned 1115–1085 Ma. The Puckwunge, Nopeming, and Bessemer basal sandstones and the Oronto and Bayfield Group sandstones received a predominance of Keweenaw Rift–Grenville (1.3–1.0 Ga) zircons but also contributions of Wolf River (1.45 Ga), Yavapai (1.75 Ga), Penokean (1.8 Ga), and Archean (2.6–3.6 Ga) ages. Interflow sedimentary rocks in the North Shore Volcanic Group received only rift-aged zircons. The maximum depositional ages derived from detrital zircons also better constrain the age-stratigraphic relationship of the unfossiliferous rift section, in comparison to the overlying fossiliferous Franconia ( zircons), Mt. Simon (), and Munising () late Cambrian transgressive sands. We also report the date of a granite xenolith (∼1 km3) in the Beaver Bay Complex, which has a rift age (1091 Ma; ) and is therefore not a rafted inclusion of Archean basement as previously suspected.

Provenance of Quartz Arenites of the Early Paleozoic Midcontinent Region, USA

Quartz arenites characterize much of the early Paleozoic sedimentary history of the midcontinent region. Despite numerous studies, the century-long debate on how these arenites formed is still unresolved, primarily because of the compositional and textural purity of the deposits. In this study, we present an extensive data set of detrital zircon geochronology from the early Paleozoic supermature arenites of the midcontinent region, and we offer new constraints about their origin. Our results coupled with compiled provenance information from older basins and orogens may indicate that the Cambrian and Ordovician arenites represent sediment reworking primarily of two different older basins. The Cambro-Ordovician sediment was transported to the midcontinent region by two early Paleozoic river systems that sourced from the paleo-east (Huron basin) and paleo-northeast (midcontinent rift region).

Geochemical and Mineralogic Characterization of Middle Stone Age Tools of Laetoli, Tanzania, and Comparisons with Possible Source Materials

Laetoli preserves a rich Middle Stone Age (MSA) artifact assemblage. We examine basaltic MSA materials collected through surface survey at a number of Laetoli localities, as well as representative samples of nearby volcanic rock exposures (i.e., Ogol Lavas, Lemagurut), using petrographic (mineralogic, textural) and geochemical (major and trace element composition) techniques. Petrographic analysis allows for the informal discrimination of three groups of artifacts based on lithologic criteria (Types A, B, and C). The petrographic basis for these groupings is supported by whole-rock geochemical characteristics. Type C artifacts are readily distinguished from Type A and B artifacts, as well as from local volcanic sources, using both major and trace element data. Type A and B artifacts are more similar in character, and geochemical data suggest that Type A artifacts were likely manufactured from the Ogol Lavas. Type B artifacts share some petrographic and geochemical characteristics with volcanic lavas from Ogol, Lemagurut, and Olmoti, but are not a straightforward match for any single source location. The compositions of Type C artifacts do not match those of any nearby volcanic sources used for comparison in this study, nor do they match published data from other volcanic centers in the region. These data demonstrate that multiple raw material sources were utilized in the manufacture of basaltic MSA artifacts at Laetoli. In addition, the dissimilar petrographic and geochemical features of the MSA artifacts and nearby bedrock exposures indicate more distal sources for the artifacts, despite the local availability of basalt. Raw materials for tool-making might have been transported to the site by streams or by Middle Stone Age hominins.

Rooted Brooks Range ophiolite: Implications for Cordilleran terranes: Comment and Reply

[Saltus et al. (2001)][1] present a two-dimensional model for rooted versus ophiolitic mafic and ultramafic rocks throughout the Cordillera based on a local geophysical anomaly in part of the southwest Brooks Range. Their model estimates a mafic and ultramafic rock body thickness of ∼8 km.

Earth Science Instrumentation and Facilities Program Review

The Instrumentation and Facilities (IF) program of the U.S. National Science Foundations Division of Earth Sciences (NSF/EAR) supports a remarkably broad span of research, both in the science that is addressed and in the nature of awards provided, according to a review by a Committee of Visitors on 22–24 August 2007. The present article, written by committee members, provides a synopsis of our findings to promote community-wide discussion of the size, scope, and responsibilities of the IF program. The IF program funds infrastructure ranging from an individual investigators purchase of laboratory equipment to support for major, multiuser facilities requiring millions of dollars per year (Figure 1). Partial support is also provided for technicians (for up to 5 years, with a decreasing level of funding each year).