Michael
Peter J.
Michael
Peter J.
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ArticleLinks from mantle to microbe at the Lau Integrated Study Site : insights from a back-arc spreading center(The Oceanography Society, 2012-03) Tivey, Margaret K. ; Becker, Erin ; Beinart, Roxanne A. ; Fisher, Charles R. ; Girguis, Peter R. ; Langmuir, Charles H. ; Michael, Peter J. ; Reysenbach, Anna-LouiseThe Lau Integrated Study Site (ISS) has provided unique opportunities for study of ridge processes because of its back-arc setting in the southwestern Pacific. Its location allows study of a biogeographical province distinct from those of eastern Pacific and mid-Atlantic ridges, and crustal compositions along the ridge lie outside the range of mid-ocean ridge crustal compositions. The Lau ISS is located above a subduction zone, at an oblique angle. The underlying mantle receives water and other elements derived from the downgoing lithospheric slab, with an increase in slab influence from north to south. Water lowers the mantle melting temperature and leads to greater melt production where the water flux is greater, and to distinctive regional-scale gradients along the ridge. There are deeper faulted axial valleys with basaltic volcanism in the north and inflated axial highs with andesites in the south. Differences in igneous rock composition and release of magmatic volatiles affect compositions of vent fluids and deposits. Differences in vent fluid compositions and small-scale diffuse-flow regimes correlate with regional-scale patterns in microbial and megafaunal distributions. The interdisciplinary research effort at the Lau ISS has successfully identified linkages between subsurface processes and deep-sea biological communities, from mantle to microbe to megafauna.
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ArticleHydrothermal venting in magma deserts : the ultraslow-spreading Gakkel and Southwest Indian Ridges(American Geophysical Union, 2004-08-18) Baker, Edward T. ; Edmonds, Henrietta N. ; Michael, Peter J. ; Bach, Wolfgang ; Dick, Henry J. B. ; Snow, Jonathan E. ; Walker, Sharon L. ; Banerjee, Neil R. ; Langmuir, Charles H.Detailed hydrothermal surveys over ridges with spreading rates of 50–150 mm/yr have found a linear relation between spreading rate and the spatial frequency of hydrothermal venting, but the validity of this relation at slow and ultraslow ridges is unproved. Here we compare hydrothermal plume surveys along three sections of the Gakkel Ridge (Arctic Ocean) and the Southwest Indian Ridge (SWIR) to determine if hydrothermal activity is similarly distributed among these ultraslow ridge sections and if these distributions follow the hypothesized linear trend derived from surveys along fast ridges. Along the Gakkel Ridge, most apparent vent sites occur on volcanic highs, and the extraordinarily weak vertical density gradient of the deep Arctic permits plumes to rise above the axial bathymetry. Individual plumes can thus be extensively dispersed along axis, to distances >200 km, and ∼75% of the total axial length surveyed is overlain by plumes. Detailed mapping of these plumes points to only 9–10 active sites in 850 km, however, yielding a site frequency F s , sites/100 km of ridge length, of 1.1–1.2. Plumes detected along the SWIR are considerably less extensive for two reasons: an apparent paucity of active vent fields on volcanic highs and a normal deep-ocean density gradient that prevents extended plume rise. Along a western SWIR section (10°–23°E) we identify 3–8 sites, so F s = 0.3–0.8; along a previously surveyed 440 km section of the eastern SWIR (58°–66°E), 6 sites yield F s = 1.3. Plotting spreading rate (us) versus F s, the ultraslow ridges and eight other ridge sections, spanning the global range of spreading rate, establish a robust linear trend (F s = 0.98 + 0.015us), implying that the long-term heat supply is the first-order control on the global distribution of hydrothermal activity. Normalizing F s to the delivery rate of basaltic magma suggests that ultraslow ridges are several times more efficient than faster-spreading ridges in supporting active vent fields. This increased efficiency could derive from some combination of three-dimensional magma focusing at volcanic centers, deep mining of heat from gabbroic intrusions and direct cooling of the upper mantle, and nonmagmatic heat supplied by exothermic serpentinization.
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PreprintChemistry of hot springs along the Eastern Lau Spreading Center( 2010-12-08) Mottl, Michael J. ; Seewald, Jeffrey S. ; Wheat, C. Geoffrey ; Tivey, Margaret K. ; Michael, Peter J. ; Proskurowski, Giora ; McCollom, Thomas M. ; Reeves, Eoghan P. ; Sharkey, Jessica ; You, Chen-Feng ; Chan, Lui-Heung ; Pichler, ThomasThe Eastern Lau Spreading Center (ELSC) is the southernmost part of the back-arc spreading axis in the Lau Basin, west of the Tonga trench and the active Tofua volcanic arc. Over its 397-km length it exhibits large and systematic changes in spreading rate, magmatic/tectonic processes, and proximity to the volcanic arc. In 2005 we collected 81 samples of vent water from six hydrothermal fields along the ELSC. The chemistry of these waters varies both within and between vent fields, in response to changes in substrate composition, temperature and pressure, pH, water/rock ratio, and input from magmatic gases and subducted sediment. Hot-spring temperatures range from 229º to 363ºC at the five northernmost fields, with a general decrease to the south that is reversed at the Mariner field. The southernmost field, Vai Lili, emitted water at up to 334°C in 1989 but had a maximum venting temperature of only 121ºC in 2005, due to waning activity and admixture of bottom seawater into the subseafloor plumbing system. Chloride varies both within fields and from one field to another, from a low of 528 mmol/kg to a high of 656 mmol/kg, and may be enriched by phase separation and/or leaching of Cl from the rock. Concentrations of the soluble elements K, Rb, Cs, and B likewise increase southward as the volcanic substrate becomes more silica-rich, especially on the Valu Fa Ridge. Iodine and δ7Li increase southward, and δ11B decreases as B increases, apparently in response to increased input from subducted sediment as the arc is approached. Species that decrease southward as temperature falls are Si, H2S, Li, Na/Cl, Fe, Mn, and 87Sr/86Sr, whereas pH, alkalinity, Ca, and Sr increase. Oxygen isotopes indicate a higher water/rock ratio in the three systems on Valu Fa Ridge, consistent with higher porosity in more felsic volcanic rocks. Vent waters at the Mariner vent field on the Valu Fa Ridge are significantly hotter, more acid and metal-rich, less saline, and richer in dissolved gases and other volatiles, including H2S, CO2, and F, than the other vent fields, consistent with input of magmatic gases. The large variations in geologic and geophysical parameters produced by back-arc spreading along the ELSC, which exceed those along mid-ocean ridge spreading axes, produce similar large variations in the composition of vent waters, and thus provide new insights into the processes that control the chemistry of submarine hot springs.
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ArticleMORB generation beneath the ultraslow spreading Southwest Indian Ridge (9–25°E) : major element chemistry and the importance of process versus source(American Geophysical Union, 2008-05-15) Standish, Jared J. ; Dick, Henry J. B. ; Michael, Peter J. ; Melson, William G. ; O'Hearn, TimothyWe report highly variable mid-ocean ridge basalt (MORB) major element and water concentrations from a single 1050-km first-order spreading segment on the ultraslow spreading Southwest Indian Ridge, consisting of two supersegments with strikingly different spreading geometry and ridge morphology. To the east, the 630 km long orthogonal supersegment (<10° obliquity) dominantly erupts normal MORB with progressive K/Ti enrichment from east to west. To the west is the 400 km long oblique supersegment (up to 56° obliquity) with two robust volcanic centers erupting enriched MORB and three intervening amagmatic accretionary segments erupting both N-MORB and E-MORB. The systematic nature of the orthogonal supersegments' ridge morphology and MORB composition ends at 16°E, where ridge physiography, lithologic abundance, crustal structure, and basalt chemistry all change dramatically. We attribute this discontinuity and the contrasting characteristics of the supersegments to localized differences in the upper mantle thermal structure brought on by variable spreading geometry. The influence of these differences on the erupted composition of MORB appears to be more significant at ultraslow spreading rates where the overall degree of melting is lower. In contrast to the moderate and rather constant degrees of partial melting along the orthogonal supersegment, suppression of mantle melting on the oblique supersegment due to thickened lithosphere means that the bulk source is not uniformly sampled, as is the former. On the oblique supersegment, more abundant mafic lithologies melt deeper thereby dominating the more enriched aggregate melt composition. While much of the local major element heterogeneity can be explained by polybaric fractional crystallization with variable H2O contents, elevated K2O and K/Ti cannot. On the basis of the chemical and tectonic relationship of these enriched and depleted basalts, their occurrence requires a multilithology mantle source. The diversity and distribution of MORB compositions, especially here at ultraslow spreading rates, is controlled not only by the heterogeneity of the underlying mantle, but also more directly by the local thermal structure of the lithosphere (i.e., spreading geometry) and its influence on melting processes. Thus at ultraslow spreading rates, process rather than source may be the principle determiner of MORB composition.