scholarly journals Geothermal heating and episodic cold-seawater intrusions into an isolated ridge-flank basin near the Mid-Atlantic Ridge

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Keir Becker ◽  
Richard E. Thomson ◽  
Earl E. Davis ◽  
Heinrich Villinger ◽  
C. Geoffrey Wheat
Keyword(s):  
Bottom Water ◽  
Ocean Bottom ◽  
Geothermal Heat ◽  
Bottom Water Temperature ◽  
Geothermal Heating ◽  
Mid Ocean Ridge ◽  
Before And After ◽  
Bottom Waters ◽  
Mid Atlantic Ridge ◽  
Ocean Ridge

AbstractSix-year records of ocean bottom water temperatures at two locations in an isolated, sedimented deep-water (∼4500 m) basin on the western flank of the mid-Atlantic Ridge reveal long periods (months to >1 year) of slow temperature rises punctuated by more rapid (∼1 month) cooling events. The temperature rises are consistent with a combination of gradual heating by the geothermal flux through the basin and by diapycnal mixing, while the sharper cooling events indicate displacement of heated bottom waters by incursions of cold, dense bottom water over the deepest part of the sill bounding the basin. Profiles of bottom water temperature, salinity, and oxygen content collected just before and after a cooling event show a distinct change in the water mass suggestive of an incursion of diluted Antarctic Bottom Water from the west. Our results reveal details of a mechanism for the transfer of geothermal heat and bottom water renewal that may be common on mid-ocean ridge flanks.

scholarly journals Sr–Nd–Pb–Hf Isotopic Constraints on the Mantle Heterogeneities beneath the South Mid-Atlantic Ridge at 18–21°S

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1010
Author(s):  
Yun Zhong ◽  
Xu Zhang ◽  
Zhilei Sun ◽  
Jinnan Liu ◽  
Wei Li ◽  
...  
Keyword(s):  
Isotopic Data ◽  
The South ◽  
Mantle Heterogeneities ◽  
Depleted Mantle ◽  
Mid Ocean Ridge ◽  
Trace Elemental ◽  
Heterogeneous Mantle ◽  
Mid Atlantic Ridge ◽  
St Helena ◽  
Ocean Ridge

In an attempt to investigate the nature and origin of mantle heterogeneities beneath the South Mid-Atlantic Ridge (SMAR), we report new whole-rock Sr, Nd, Pb, and Hf isotopic data from eight basalt samples at four dredge stations along the SMAR between 18°S and 21°S. Sr, Nd, and Pb isotopic data from SMAR mid-ocean ridge basalts (MORBs) at 18–21°S published by other researchers were also utilized in this study. The SMAR MORBs at 18–21°S feature the following ratio ranges: 87Sr/86Sr = 0.70212 to 0.70410, 143Nd/144Nd = 0.512893 to 0.513177, 206Pb/204Pb = 18.05 to 19.50, 207Pb/204Pb = 15.47 to 15.71, 208Pb/204Pb = 37.87 to 38.64, and 176Hf/177Hf = 0.283001 to 0.283175. The 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 176Hf/177Hf ratios of these MORBs varied considerably along the SMAR axis. The variable compositions of the Sr–Nd–Pb–Hf isotopes, combined with the corresponding whole-rock major and trace elemental abundances reported in previous studies, suggest that the SMAR MORBs at 18–21°S were probably derived from a heterogeneous mantle substrate related to a mixture of depleted mantle (DM) materials with a small amount (but variable input) of HIMU (high-μ, where μ = 238U/204Pb)- and enriched (EMII)-type materials. The HIMU-type materials likely originated from the proximal St. Helena plume and may have been transported through “pipe-like inclined sublithospheric channels” into the SMAR axial zone. The EMII-type materials possibly originated from a recycled metasomatized oceanic crust that may have been derived from the early dispersion of other plume heads into the subcontinental asthenosphere prior to the opening of the South Atlantic Ocean. In addition, the contributions of subducted sediments, continental crust, and subcontinental lithospheric mantle components to the formation of the SMAR MORBs at 18–21°S may be nonexistent or negligible.

The origin of compositional variation in basalts recovered by submersible drill from Mount Glooscap, Mid-Atlantic Ridge at 36°25′N

1984 ◽  
Vol 21 (8) ◽  
pp. 934-948 ◽  
Author(s):  
James A. Walker ◽  
Patrick J. C. Ryall ◽  
Marcos Zentilli ◽  
Ian L. Gibson ◽  
Jarda Dostal
Keyword(s):  
Trace Elements ◽  
Low Temperature ◽  
Carbonate Rocks ◽  
Compositional Data ◽  
Major And Trace Elements ◽  
Compositional Variation ◽  
Mid Ocean Ridge ◽  
Mid Atlantic Ridge ◽  
Large Peak ◽  
Ocean Ridge

A large peak in the crestal mountains of the Mid-Atlantic Ridge, about 16 km west of the AMAR rift valley at 36°25′N, was sampled for basalt with a submersible electric rock core drill on a comparable surficial scale as the FAMOUS area. Twenty-eight basalt samples from seven drilling stations have been analyzed for major and trace elements. Many of the samples come from flows lying under a cover of carbonate rocks and therefore could not have been sampled by a submersible or a dredge.Through comparisons with published compositional data, it appears that, unlike "FAMOUS-generated" basalts, "AMAR-generated" basalts are, on average, more evolved and are always LREE enriched. Most of the in- and between-hole compositional variation can be accounted for by low-temperature alteration, accumulation of phenocrysts, and low-pressure, relatively low-temperature fractional crystallization. A source heterogeneous in trace elements or undergoing variable degrees of partial melting is necessary to explain the remaining compositional variation. If the large peak can be interpreted as a single volcano, it may be that lavas become progressively more differentiated with time at mid-ocean ridge volcanoes as they commonly do at subduction zone volcanoes.

scholarly journals Global variation of seismic energy release with oceanic lithosphere age

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Nicolás Pinzón ◽  
Carlos A. Vargas
Keyword(s):  
Cross Sections ◽  
Thermal Evolution ◽  
Seismic Energy ◽  
Oceanic Lithosphere ◽  
Physical Factors ◽  
Additional Tool ◽  
Mid Ocean Ridge ◽  
Mid Atlantic Ridge ◽  
Spreading Rates ◽  
Ocean Ridge

AbstractVariations in Mid Ocean Ridge seismicity with age provide a new tool to understand the thermal evolution of the oceanic lithosphere. The sum of seismic energy released by earthquakes during a time, and for an area, is proportional to its lithospheric age. Asthenospheric temperatures emerge on ridge centers with new crust resulting in high seismic activity; thus, the energy released sum is highest on the young lithosphere and decreases with age. We propose a general model that relates the systematic variation of seismic energy released with the lithospheric age. Our analysis evaluates the main physical factors involved in the changes of energy released sum with the oceanic lithosphere age in MOR systems of different spreading rates. These observations are substantiated based on three cross-sections of the East Pacific Rise, six sections in the Mid Atlantic Ridge, and three profiles in the Central Indian Ridge. Our global model provides an additional tool for understanding tectonic processes, including the effects of seismicity and mid-plate volcanism, and a better understanding of the thermal evolution for the young oceanic lithosphere.

scholarly journals Southern Ocean bottom water cooling and ice sheet expansion during the middle Miocene climate transition

2020 ◽  
Author(s):  
Thomas J. Leutert ◽  
Sevasti Modestou ◽  
Stefano M. Bernasconi ◽  
A. Nele Meckler
Keyword(s):  
Southern Ocean ◽  
Bottom Water ◽  
Middle Miocene ◽  
Ocean Bottom ◽  
Ocean Drilling Program ◽  
Bottom Water Temperature ◽  
Ice Volume ◽  
Global Ice Volume ◽  
Clumped Isotope ◽  
Climate Transition

Abstract. The middle Miocene climate transition (MMCT, ~14.5–13.0 Ma) was associated with a significant expansion of Antarctic ice, but the mechanisms triggering the event remain enigmatic. We present a new clumped isotope (∆47) bottom water temperature (BWT) record from 16.0 Ma to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean, and compare it to existing BWT records. We show that BWTs in the Southern Ocean were ~8–10 °C during the middle Miocene greenhouse, and thus considerably warmer than today. Nonetheless, bottom water δ18O (calculated from foraminiferal δ18O and ∆47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset demonstrates that BWTs at Site 747 decreased by ~3–5 °C across the MMCT. This cooling preceded the stepped main increase in global ice volume, and appears to have been followed by a transient bottom water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Additional processes and feedbacks required for substantial ice growth may have contributed to the observed decoupling of Southern Ocean BWT and global ice volume.

scholarly journals Geochemistry and Mineralogy of Basalts from the South Mid-Atlantic Ridge (18.0°–20.6°S): Evidence of a Heterogeneous Mantle Source

Minerals ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 659 ◽  
Author(s):  
Yun Zhong ◽  
Weiliang Liu ◽  
Zhilei Sun ◽  
Chris Yakymchuk ◽  
Kefa Ren ◽  
...  
Keyword(s):  
Magma Mixing ◽  
Primitive Mantle ◽  
Spreading Ridge ◽  
Spinel Lherzolite ◽  
The South ◽  
Depleted Mantle ◽  
Mid Ocean Ridge ◽  
Heterogeneous Mantle ◽  
Mid Atlantic Ridge ◽  
Ocean Ridge

The South Mid-Atlantic Ridge is a typical slow-spreading ridge that represents a modern example to understand mantle composition and the evolution of mid-ocean ridge magmatism. In this paper, we investigate basalt samples dredged from four locations along the South Mid-Atlantic Ridge ranging from 18.0° to 20.6°S. The basalts belong to the tholeiitic series and exhibit normal mid-ocean ridge basalt (N-MORB) geochemical features with variable enrichments of Rb, Th, U, and Pb and depletions of Ba and Sr relative to primitive mantle. Some samples have negative Nb–Ta anomalies whereas others have positive Na–Ta anomalies to average N-MORBs. Plagioclase phenocrysts, microphenocrysts, and microlites occur in the in the matrix; phenocrysts and microphenocrysts are bytownite and labradorite in composition. Olivine phenocrysts are forsterite (Fo87 to Fo96). Chemical zoning in phenocrysts are interpreted to record crystal fractionation and magma mixing. Cores of plagioclase phenocrysts have higher anorthite values (An72–83) and estimated crystallization temperatures (~1180–1240 °C) that may suggest a xenocrystic origin. The lower anorthite proportions of rims of plagioclase phenocrysts (An65–71) and microphenocrysts (An54–72) yield lower estimated crystallization temperatures of ~1090–1120 °C and ~980–1060 °C, respectively. Rims of plagioclase phenocrysts and microphenocrysts may be generated in different environments such as magma chambers or magma channels, respectively. The basalt samples probably originated from partial melting of a depleted mantle spinel lherzolite source with a minor contribution of enriched materials possibly derived from the Saint Helena plume and subcontinental lithospheric mantle in the asthenosphere. Variable compositions of the basalt samples suggest heterogeneous mantle that includes depleted and enriched components at the South Mid-Atlantic Ridge between 18.0°–20.6°S.

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