Geothermal & Hydrothermal Models

1. Geothermal & Hydrothermal Models OCEA/ERTH 4110/5110 Introduction to Marine Geology

2. 1. Geothermal Models of the Lithosphere

3. A. Heat Flow Basics Definition of 1-D conductive heat flow For conduction, heat flows from hotter to colder region by transfer of molecular kinetic energy. The material itself remains stationary. This contrast with convection, where heat is transferred by motion of the material. Generally conduction is less efficient than convection. q =    -k∙dT/dz heat flow = -    thermal           ∙           gradient                                                 conductivity units:cgs                   cal/(cm2-s)        cal/(cm-s-oC)               oC/cm mks                  W/m2               W/(m-oC)                     oC/m typical values: q:         1-2 μcal/(cm2-s)  =  42-84 mW/m2 k:         2-3 ∙ 10-3 cal/(cm-s-oC)  =  0.84-1.26 W/(m-oC)   (sediment) dT/dz:  5 ∙ 10-4oC/cm  =  5 ∙ 10-2oC/m

4. Total energy output compared to other processes • Geothermal Heat and Earth Processes • thermal power builds mountains and solar energy destroys them • temperature state determines earth structure - e.g. phases, composition, magnetic, electrical, rheological properties • variations in distribution due to plate tectonics

5. B. Measurement • Need to reduce environmental variations on surface • Deep boreholes (land and sea)  Expensive, compensate for heat produced +/- fluid circulation during drilling • Use short marine probes in deep water where temperature is stable Fig. 1 Dalhousie heat flow probe on deck of CCGS HUDSON Fig. 2 Results of temperature, conductivity and heat flow versus depth

6. Ocean Heat Flow Averages • Basin values more uniform than ridge crests • Ridge crests have large range  Poisson distribution Fig. 3 Histograms of heat flow values for various tectonic regions of oceanic crust

7. Plate Model Half-Space Model Narrow Intrusion Thickening Plate Ridge Ridge T=0 T=0 Broad Upwelling Thickening Plate Ta = constant C. Theoretical models for thermal cooling of lithosphere Half-space vs. Plate models v v h(t) = sqrt(t) h(t) = constant Ta = constant Ta = constant -> heat added to base of plate Fig. 4 Boundary conditions for thermal models of the oceanic upper mantle. All models have temperature T=0 at the surface. The plate models require a constant T at depth h. These boundary conditions require the mantle heat flow beneath the lithosphere to increase with increasing age of seafloor. h(t) = constant Extra heat added to base of plate

8. Results of Thermal Model Calculations Fig. 5 Isotherms in the plate model are given by dashed lines for a plate thickness of 100 km. For the half-space model, isotherms are shown by solid lines. Initial mantle temperature is 1325 oC. • Heat flow determined from q=k∙dT/dz at z=0 • Depths determined from isostatic balance assuming equal pressure at some depth below surface

9. Isostasy Between Spreading Centre and Ocean Basin ρa = ρo (1 – α T); α = 3 ∙ 10-5oC-1 and ρo = 3.34 g/cm3 Isostatic Balance ∑ ρ z = constant. Equating this balance between the two sides of the figure dr ρw + (L + d – dr) ρa = d ρw + L ρL dr (ρw – ρa) + L ρa + d ρa = d ρw + L ρL dr (ρw – ρa) = d (ρw – ρa) + L (ρL – ρa) solve for d  using values in figure  d ≈ 2.5 + 3.73 = 6.23 km. This is water depth for the old ocean basins.

10. Older heatflow values fit can not discriminate between models • Shallower depths in old ocean favor plate model • Recent observations of higher than expected values of heat flow in old oceans suggest thinner plate (L= 95 km) Comparison of data to theory Heat Flow Plate 2 (95 km; 1450 C) 250 Plate 1 (125 km; 1350 C) HS 200 150 Heat Flow (mW/m²) 100 50 0 0 50 Age (Ma) 100 150 2 Subsidence Plate 2 Plate 1 HS 4 Fig. 6 Heat flow and basement depth (subsidence) versus age for all ocean basins compared to half-space (HS) and plate models (Plate 1 and Plate 2). The data are bounded by +/- 1 standard deviation, showing the scatter in the observations. Note that when all data are combined, they are fit better by the thinner and hotter (plate 2) model. Depth (km) 6 8 0 50 100 150

11. D. Physical models to maintain lithospheric thickness in older regions • Small scale convection just below plate • Theoretically predicted by calculation and lab experiment • Possibly seen in gravity field in some locations Fig. 7 Illustration of the large-scale flow in the vicinity of a ridge and the superimposed small-scale longitudinal rolls which form away from the ridge. The small-scale convection does not help move the plates but does bring heat from lower in the mantle to help heat the bottom of the plate.

12. 2. Heat Flow and Hydrothermal Circulation in Young Oceanic Crust

13. (a) (b) (c) A. Heat Flow in young crust • Observations more scattered for young oceanic regions < 30 Ma • Average values for ages < 15 Ma less than predicted by conductive thermal models Fig. 9 Model and observed heat flow vs. age for fast (a) and slow (b) spreading ridges determined near the ridge crest Fig. 8 Observed and model heat flow vs. age for various ocean basins. PSM and GDH1 are both plate models but with different assumed asthenospheric temperature (Ta) and plate thickness (L).

14. Dependence on sediment thickness • higher values closer to theoretical if only include observations in areas with sediment cover and no basement outcrops within 10-20 km • lower values extend to larger ages (up to 80 Ma) for other regions (e.g. Indian, Atlantic Oceans) due to less sediment accumulation Fig. 10 Heat flow measurements located w/r/t magnetic isochrons near the Juan da Fuca ridge offshore Vancouver Is. (left) plotted as a function of distance away from the ridge crest (right). The observed values increase and begin to agree with model values once the sediment cover becomes continuous. Fig. 11 Plot of heat flow averages vs sqrt(age) for regions of continuous sediment cover compared to model predictions Fig. 12 Comparison of how heat flow values approach theoretical predictions at different ages for different ocean spreading centres depending on the rate of sediment deposition.

15. B. Hydrothermal Circulation • Explanation of low values from circulation of water within crust driven by heat at MOR • Basalt is more porous (permeable) than sediment so sediment cover shuts off supply of seawater. Hydrothermal circulation still continues but effects are masked by sediment cover • Most high temperature activity is at or near ridge crests. • Observations of both high (300 oC) and low (50 oC) outward flow in localized mounds. • Produces unusual biological communities (clams, crabs, tube worms, etc.) which feed on sulfur eating bacteria. • High food concentrations allow these animals to attain much larger than normal size. • Most activity does not last longer than ~20 yrs in one place as indicated by greatest ages of clams http://youtu.be/J_jubgEzG9A Fig. 14Black smoke spews from the 45m tall Godzilla spire, while tubeworm colonies thrive on a neighbouring hot vent in the submarine rift valley of Endeavour Ridge. This video was gathered during a NEPTUNE Canada installation and maintenance cruise aboard the R/V Thompson by the deep-sea ROV ROPOS, operated by the Canadian Scientific Submersible Facility. Fig. 13 Schematic representation of hydrothermal circulation in young oceanic crust in areas having (a) sparse, (b) in-complete and (c) extensive sediment cover.

16. Images from the VENTS Program & NeMO Project - a multi-year observatory on Axial Volcano in order to examine the relationships between volcanic events, the chemistry of seafloor hot springs, and the biologic communities that depend on them. Location of Neptune CANADA Observatory Network

17. Mineral exchange between water and hot rock • Rocks gain H2O, Na, Cl, Mg from water • Water gains Si, K, Ni, Cu, Ca from rock Fig. 15 Concentrations of various dissolved ions in standard seawater during interaction with powdered basalt at 200oC and 500 bars.

18. Rates of Seawater Flow in Rocks • Can be determined best by pore water chemistry in sediment cores • Example for Ca of influence of hydrothermal processes on water chemistry F= H / {cw(T2-T1)} where: T1=3 oC; T2=50-300 oC; cw=4*103 J/kg-oC; H=170*1018 J/yr therefore, F=(1.4-9)*1014 kg/yr total mass of oceans = 1370*1018 kg so residence time t= 1370/(1.4-9) = (1.5-9.8)*106 yr Ocean basins are 100-200 Myr old so water can cycle through many times in that period. Rate of ocean floor generation = 3 km2/yr * 5 km = 15 km3/yr Volume of seawater = (1.4-9)*1014 kg/yr / 1012 kg/km3 = 140-900 km3/yr Thus water/rock ration is 9-60:1 Exchange of Ca minerals Mass of Ca gained from experimental data = 500 ppm/yr 500*10-6 * 1.4-9*1014 = 7-45*1010 kg/yr Total Ca in seawater is 400 ppm in 1370*1018 kg = 5.5*1017 kg. So, Ca can be completely filled by this process after 5.5*1017 kg / 2.5*1011 kg/yr = 2.2 Myr This is an important contributing factor to ocean water chemistry along with influx from stream runoff and deposition of sediments.

19. General model of circulation • Controls on depth of circulation • Depths of earthquakes at ridge crest • Metamorphism of crust • Measurements of permeability vs. depth (crack closure under pressure) seawater Fig. 16 Schematic cross-section normal to a spreading axis, illustrating hydrothermal convection in oceanic crust. Lines and arrows indicate downward and lateral flow of seawater and its expulsion in a narrow zone of heated upflow. This leads to the deposition of ore minerals on the surface and within the crust, leading to high concentrations of metal-rich sediments on the ridge crest.

20. World-wide Distribution of Hydrothermal Vents depends on spreading rate Fig. 17 Locations of hydrothermal vent fields as of the mid-1990’s. Fig. 18 Incidence of hydrothermal plumes vs. spreading rate for the mid-Atlantic ridge (RR, MAR), North and South East Pacific Rise (NEPR, SEPR), and Juan da Fuca ridge (JDFR).

21. Web Sites: Reading List: Davis.pdf, Thermal aging of oceanic lithosphere, in: Handbook of Seafloor Heatflow, CRC Press ocean_basins_ch5.pdf ... The Ocean Basins ch. 5 • NOAA VENTS Program • Neptune CANADA • Nova Online: Life in the Abyss. Deep-sea chemo-synthetic biology