Relationship between seismic P-wave velocity and the composition of anhydrous igneous and meta-igneous rocks

Author Posting. © American Geophysical Union 2003. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 4 (2003): 1041, doi:10.1029/2002GC000393.This study presents a new approach to quantitatively assess the relationship between the composition and seismic P-wave velocity of anhydrous igneous and meta-igneous rocks. We perform thermodynamic calculations of the equilibrating phase assemblages predicted for all igneous composition space at various pressure and temperature conditions. Seismic velocities for each assemblage are then estimated from mixing theory using laboratory measurements of the elastic parameters for pure mineral phases. The resultant velocities are used to derive a direct relationship between Vp and major element composition valid to ±0.13 km/s for pressure and temperature conditions along a normal crustal geotherm in the depth range of 5–50 km and equilibration pressures ≤12 kbar. Finally, we use the calculated velocities to invert for major element chemistry as a function of P-wave velocity assuming only the in situ temperature and pressure conditions are known. Compiling typical velocity-depth profiles for the middle and lower continental and oceanic crust, we calculate compositional bounds for each of these geologic environments. We find that the acceptable compositional range for the middle (15–30 km) and lower continental (≥35 km) crust is broad, ranging from basaltic to dacitic compositions, and conclude that P-wave velocity measurements alone are insufficient to provide fundamental constraints on the composition of the middle and lower continental crust. However, because major oxides are correlated in igneous rocks, joint constraints on Vp and individual oxides can narrow the range of acceptable crustal compositions. In the case of the lower oceanic crust (≥2 km), observed velocities are 0.2–0.3 km/s lower than velocities calculated based on the average bulk composition of gabbros in drill cores and exposed ophiolite sequences. We attribute this discrepancy to a combination of residual porosity at crustal depths less than ∼10 km and hydrous alteration phases in the lower crust, and suggest caution when inferring mantle melting parameters from observed velocities in the lower oceanic crust.This research was supported by National Science Foundation Grants OCE- 9819666, EAR-9910899, and EAR-0087706 (P.B. Kelemen)

Highly depleted cratonic mantle in West Greenland extending into diamond stability field in the Proterozoic

This study presents electron microprobe data for dunite xenoliths from a lamprophyre dyke located on the island of Qeqertaa, West Greenland. The minimum age of this dyke is Palaeoproterozoic and it experienced amphibolite facies metamorphism and deformation during that era. The samples consist of nearly 200 xenoliths with a size range of 0.5-8 cm. These dunite xenoliths have olivine Mg#, that range from 80.3 to 94.6 (n = 579) with a mean of 92.6. Orthopyroxene is found in three xenoliths and garnet in five others. The latter suggests the depth of the Qeqertaa xenolith suite to be near the diamond stability-field, which is substantiated by the finding of diamonds in bulk samples of the Qeqertaa dyke. This further indicates the presence of a lithospheric mantle domain dominated by high-Mg# dunite to this depth in Palaeoproterozoic time. Cr-rich spinel, in the 0.1–0.2 mm size range, is found within and between olivine grains in individual xenoliths. These Cr-spinels yield Fe–Mg exchange temperatures of 400–600 °C. However, the presence of intermediate spinel compositions spanning the lower temperature solvus suggests that equilibration temperatures were > 550 °C. Fe3 +#, expressed as 100 × Fe3 +/(Fe3 + + Al + Cr), is shown to be a useful parameter in order to screen for altered spinel (Fe3 +# > 10) with disturbed Mg# and Cr#. The screened spinel data (Fe3 +# < 10) show a distinctly different trend in terms of spinel Cr# versus Mg#, compared to unmetamorphosed xenoliths in Tertiary lavas and dikes from Ubekendt Ejland and Wiedemann Fjord, respectively, also located within the North Atlantic craton. This difference likely reflects amphibolite facies metamorphic resetting of the Qeqertaa xenolith suite by Fe–Mg exchange. Given the similarity of the Qeqertaa xenolith suite with the Ubekendt and Wiedemann suites, in terms of their olivine Mg# and spinel Cr# distribution, high-Mg# dunite is likely to be an important component of the subcontinental lithospheric mantle beneath the North Atlantic craton and appears to have spanned a vertical distance of at least 150 km in this region, even during the Palaeoproterozoic

Peridotite enclaves hosted by Mesoarchaean TTG-suite orthogneisses in the Fiskefjord region of southern West Greenland

AbstractThis study presents bulk-rock major, trace, and platinum-group element data, as well as mineral chemistry for peridotites which form large enclaves (up to 500×1000m) within Mesoarchaean orthogneisses of the Akia terrane in the Fiskefjord region, southern West Greenland. The largest peridotite body, known as Seqi, contains highly fosteritic olivine with a median Mg# of 92.6 and hosts extensive layers of chromitite, which can be traced for tens of metres with thicknesses of up to 30cm. Thinner (<100m thick), but extensive (up to 2000m long) peridotite sheets are associated with coarse norite and orthopyroxenite with distinct cumulate textures in the Amikoq complex, located a few tens of kilometres south of Seqi. Intercalated amphibolites of tholeiitic basaltic composition show complementary geochemical evolution to the peridotites, consistent with igneous crystal fractionation trends. The U-shaped trace element patterns of the peridotites may either reflect the parental melt composition from which these olivine-rich rocks were derived, or alternatively this feature may be the result of melt-rock interaction. Overall, we interpret the Fiskefjord region peridotites to have formed as ultramafic cumulates derived from Archaean high-Mg, low Ca/Al magmas, although their geodynamic setting remains to be established

Coexisting serpentine and quartz from carbonate-bearing serpentinized peridotite in the Samail Ophiolite, Oman

Tectonically exposed mantle peridotite in the Oman Ophiolite is variably serpentinized and carbonated. Networks of young carbonate veins are prevalent in highly serpentinized peridotite, particularly near low-temperature alkaline springs emanating from the peridotite. An unusual feature in some samples is the coexistence of serpentine and quartz, which is not commonly observed in serpentinites. This assemblage is unstable with respect to serpentine + talc or talc + quartz under most conditions. Serpentine in the carbonated serpentinites in this study is more iron rich than in most serpentinites reported in previous studies, and samples with co-existing quartz contain the most iron-rich serpentines. Calculations of thermodynamic equilibria in the MgO–SiO2–H2O–CO2 system suggest that serpentine + quartz may be a stable assemblage at low temperatures (e.g., <~15–50 °C) and is stabilized to higher temperatures by preferential cation substitutions in serpentine over talc. Based on these calculations, serpentine + quartz assemblages could result from serpentinization at near-surface temperatures. Clumped isotope thermometry of carbonate veins yields temperatures within error of the observed temperatures in Oman groundwater for all samples analyzed, while the δ^18O of water calculated to be in equilibrium with carbonate precipitated at those temperatures is within error of the observed isotopic composition of Oman groundwater for the majority of samples analyzed. As groundwater geochemistry suggests that carbonate precipitation and serpentinization occur concomitantly, this indicates that both hydration and carbonation of peridotite are able to produce extensive alteration at the relatively low temperatures of the near-surface weathering environment

The Cow That Stole Christmas: Framing the First U.S. Mad Cow Crisis

The discovery of bovine spongiform encephalopathy (BSE) in the United States made an impact on the beef industry. Determining how the BSE outbreak was framed by the news media is significant because research indicates that media shape public perceptions. This study examined how several key newspapers framed the 2003 outbreak of BSE in the United States. Determining how the media framed this issue can help communicators ensure bias-free media coverage of similar issues in the future. The study followed established framing analysis categories identified from the literature. There were 149 articles identified in The Washington Post, The Seattle Times, and USA Today for investigation in this study. Findings showed that the BSE issue was framed as an industry crisis and that the tone of the articles and headlines portrayed the beef industry negatively. When compared to the other two newspapers, USA Today framed the issue differently, with economic calamity being the dominant frame. The most heavily cited sources in the articles were government officials. This study recommends that media professionals avoid framing an issue for the public, focusing instead on reporting news in an objective and unbiased manner. Further research is recommended to examine the impact of tone and frame on specific audiences

Service Learning: A Case Study in an Agricultural Communications Course

Academic service-learning can be an effective and successful educational tool across many disciplines. The benefits afforded students and the community they serve are reciprocal in nature, thereby providing service to the community and capitalizing on a real-world learning environment for the students. Agricultural communications programs can offer for service-learning opportunities within the academic arena. The Campaign Planning for Agricultural and Natural Resources course at Oklahoma State University captures the essence of service-learning and provides students with an opportunity to use knowledge gained in previous courses to develop usable communications campaigns for small businesses or organizations. This case study should serve as a starting point for service-learning research in agricultural communications