182 research outputs found
Picture Gorge Basalt: Internal stratigraphy, eruptive patterns, and its importance for understanding Columbia River Basalt Group magmatism
The Picture Gorge Basalt (PGB) of the Columbia River Basalt Group (CRBG) has been previously thought to be limited in its eruptive volume (\u3c3000 \u3ekm3) and thought to not extend far from its type locality. At present, PGB represents only 1.1 vol% of the CRBG with a relatively limited spatial distribution of ~10,000 km2. New age data illustrate that the PGB is the earliest and longest eruptive unit compared to other main-phase CRBG formations and that some dated basaltic flows reach far (~100 km) beyond the previously mapped extent. This study focuses on extensive outcrops of basaltic lavas and dikes south of the type locality at Picture Gorge, in order to reassess the spatial distribution and eruptive volume of the PGB. Field observations coupled with geochemical data indicate that PGB lava flows and mafic dikes covered a significantly greater area than shown on the published geologic maps. We find that additional mafic dikes located farther south of the original mapped distribution have geochemical compositions and northwest-trending orientations comparable to the dikes of the Monument dike swarm. We also identify new lava flows that can be correlated where stratigraphic control is well defined toward the original mapped PGB distribution. Our analyses and correlations are facilitated by comparison of 20 major- and trace-element abundances via a principal component analysis. This statistical comparison provides a new detailed distribution of PGB with stratigraphic significance that more than doubles the total distribution of PGB lavas and dikes and brings the eruptive volume to a new minimum of at least ~4200 km3. Geochemically correlated basaltic lavas and dikes in the extended distribution of PGB represent the earlier and later sections of the internal PGB stratigraphy. This is an intriguing observation as new geochronological data suggest an eruptive hiatus of ~400 k.y. during PGB volcanic activity, which occurred from 17.23 Ma to 15.76 Ma.
The geochemical identifiers used to differentiate PGB from other main-phase CRBG formations include lower TiO2 (\u3c2 \u3ewt%) concentrations, lower incompatible trace-element (i.e., La, Th, and Y) abundances, and a more pronounced enrichment in large- ion- lithophile elements (LILEs) on a primitive mantleânormalized trace-element diagram (Sun and McDonough, 1989). Geochemical characteristics of PGB are interpreted to represent a magmatic source component distinct from the other main-phase CRBG units, possibly a localized backarc-sourced mantle melt. However, this source cannot be spatially restricted as there are observed PGB lava flows and dikes extending as far east as Lake Owyhee and as far south as Hart Mountain, covering at least 15,000 km2. In context with the existing stratigraphy and the new extent of PGB lavas and dikes, these ages and coupled geochemical signatures demonstrate this mantle component was not spatially localized but rather tapped across a wide region
Paleomagnetism and Paleosecular Variations From the Plio-Pleistocene Golan Heights Volcanic Plateau, Israel
Recommended from our members
Intraplate seamounts as a window into deep earth processes
Seamounts are windows into the deep Earth that are helping to
elucidate various deep Earth processes. For example, thermal and mechanical
properties of oceanic lithosphere can be determined from the flexing of oceanic
crust caused by the growth of seamounts on top of it. Seamount trails also are
excellent recorders of absolute plate tectonic motions and provide key insights into
the relationships among plate motion, plume motion, whole-Earth motion, and
mantle convection. And, because seamounts are created from the partial melts of
deep mantle sources, they offer unique glimpses into the chemical development and
heterogeneity of Earthâs deepest regions. Current research efforts focus on resolving
the fundamental differences between magmas generated by passive upwelling
from upper mantle regions and deep mantle plumes rising from the core-mantle
boundary, mapping the different modes of mantle plumes and mantle convection,
reconciling fixed and nonfixed mantle plumes, and understanding the prolonged
volcanic evolution of seamounts. The role of intraplate seamounts is pivotal to this
research, and we must collect vast amounts more geochemical and geophysical
data to advance our knowledge. These data needs leave the ocean wide open for
future seamount exploration
Dating Clinopyroxene Phenocrysts in Submarine Basalts Using ^(40)Ar/^(39)Ar Geochronology
Dating submarine basalts using ^(40)Ar/^(39)Ar geochronology is often hindered by a lack of potassiumâbearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to lowâtemperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for ^(40)Ar/^(39)Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112 Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002â0.4, suggesting that other Kâbearing phases hosted within the clinopyroxene are likely degassing to yield the ^(40)Ar/^(39)Ar age information. There are three possible origins for the K and corresponding ^(40)Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (>60% ^(39)Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values >0.05. We conclude that clinopyroxene dating by the ^(40)Ar/^(39)Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents
Defining the word âseamountâ
Author Posting. © Oceanography Society, 2010. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 23, 1 (2010): 20-21.The term seamount has been
defined many times (e.g., Menard, 1964; Wessel, 2001; Schmidt and
Schmincke, 2000; Pitcher et al., 2007; International Hydrographic
Organization, 2008; Wessel et al., 2010) but there is no âgenerally
acceptedâ definition. Instead, most definitions serve the particular
needs of a discipline or a specific paper
Vailuluâu Seamount
Author Posting. © Oceanography Society, 2010. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 23, 1 (2010): 164-165.Vailuluâu seamount is an active underwater
volcano that marks the end of
the Samoan hotspot trail
Seamount sciences : quo vadis?
Author Posting. © Oceanography Society, 2010. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 23, 1 (2010): 212-213.Seamounts are fascinating natural
ocean laboratories that inform us about
fundamental planetary and ocean
processes, ocean ecology and fisheries,
and hazards and metal resources. The
more than 100,000 large seamounts
are a defining structure of global
ocean topography and biogeography,
and hundreds of thousands of smaller
ones are distributed throughout every
ocean on Earth
Recommended from our members
Seamount subduction and earthquakes
Seamounts are ubiquitous features of the seafloor that form part of the fabric of oceanic crust. When a seamount enters a subduction zone, it has a major affect on forearc morphology, the uplift history of the island arc, and the structure of the downgoing slab. It is not known, however, what controls whether a seamount is accreted to the forearc or carried down into the subduction zone and recycled into the deep mantle. Of societal interest is the role seamounts play in geohazards, in particular, the generation of large earthquakes
Millennial-Scale Instability in the Geomagnetic Field Prior to the Matuyama-Brunhes Reversal
Changes in the Earth's magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere. Paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes (polarity reversals), but dating uncertainties have hindered progress in this understanding. Here, we report a paleomagnetic record from multiple lava flows on Tahiti that bracket the MatuyamaâBrunhes (MâB) polarity reversal âŒ771,000 years ago. Our highâprecision ^(40)Ar/^(39)Ar ages constrain several rapid and shortâlived changes in field orientation up to 33,000 years prior to the MâB reversal. These changes are similar to ones identified in other less wellâdated lava flows in Maui, Chile, and La Palma that occurred during an extended period of reduced field strength recorded in sediments. We use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates. This prolonged 33,000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennialâtoâmillennialâscale polarity changes in the presence of a strongly weakened dipole field
- âŠ