Lecture 6: Hot spots, seamounts, and ridges

The topography and geology of the seafloor offers some of the only clues we have to understanding the hidden workings of the mantle below. We have discussed some of the largest features of ocean basins, and today we consider the small seamounts and their critical role in this story.

• Plumes, stationary or not?
• Axial depth vs melt chemistry
• Trace elements and isotopes in MORB vs OIB

We acknowledge and respect the lək̓ʷəŋən peoples on whose traditional territory the university stands and the Songhees, Esquimalt and W̱SÁNEĆ peoples whose historical relationships with the land continue to this day.

Features of the seafloor

The source of lavas must be deep

Swells and depth anomalies

Transects across Hawaii (Crough 1983)

Gravity higher than expected.. more mass! so the broad swell and peaks show volcanoes and the swell, the lows show the flexural load from the volcanoes. what models are consistent with broad high topography?

from below more mass everywhere and not compensated, so anomaly is same as top layer 2) thick crust is compensated but not the volcano, so anomaly at the volcano only 3) plate compensated mostly, but positive and negative anomaly near the volcano not compensated 4) compensated by low density litho root, but not at volcano 5) swell not compensated and volcano not compensated?

How are swells supported?

Constant crustal thickness

Dunn et. al. 2024

Are swells actually supported by thinner lithosphere?

Lee et. al. 2004

Are swells actually supported by thinner lithosphere?

Lee et. al. 2004

Are swells actually supported by thinner lithosphere?

Lee et. al. 2004

The PLUME experiment

Seismic station network locations (Wolfe et. al. 2009)

The PLUME experiment

S-wave anomalies (Wolfe et. al. 2009)

The PLUME experiment

S-wave anomalies (Wolfe et. al. 2009)

Thermal Plume simulations

Breaking the mold

• Ages:
  • Bioko: active
  • Pricipe: ~6 Ma
  • Sao Tome: ~1-3 Ma
  • Continental volcanos: ~1-3 Ma

Thermochemical plumes

Are plumes stationary?

Tarduno et. al. 2003

Are plumes stationary?

Tarduno et. al. 2003

Large igneous provinces and flood basalts

mckenzie and Bickle

1. define potential temperature in terms of thermodynamics. illustrate usefulness eg: The interior of the upper mantle is likely to have a temperature gradient which differs

little from the adiabatic gradient and hence material will increase in temperature by 200 °C on sinking 300 km (equivalent to a change in pressure of about lOGPa). Hence, if substantial vertical movements occur, the temperature differences are not a good guide to differences in heat content. Such differences are, however, clearly reflected in differences of the potential temperatures which are therefore used throughout this paper.

"The resulting freedom to move ridges, irrespective of convective geometries in the mantle below, removed one of the major difficulties faced by the early concepts of sea floor spreading. From the point of view of magma generation on ridges these results are of great importance because they lead to a natural explanation of why the oceanic crust is of such a uniform thickness (see section 3)."

What can hotspots tell us about the mantle?

Klein and Langmuir 1987 The temperature and flow regime of the mantle should, in part, control the extent of partial melting that the mantle undergoes as it ascends beneath ocean ridges. The extent of melting should, in turn, govern both the chemistry of ocean ridge basalts and the thickness of the oceanic crust. Crustal thickness, to first order, should be related through isostatic compensation to the zero-age depth of ocean ridges. Thus, variations in ocean ridge basalt chemistry, axial depth, and crustal thickness should correlate with each other and with mantle temperature variations

Are ridges passive or active?

• Ridges move with respect to a reference frame (what reference frame??)
• What happens when a ridge is offset by a transform fault?
• Challenges posed by these questions are solved if the ridge system is the result of spreading plates
• Melting is a passive process driven by spreading, not driven by hot sheets of rising mantle

Basalt chemistry tells us about process at depth: example from MORB

"The temperature and flow regime of the mantle should, in part, control the extent of partial melting that the mantle undergoes as it ascends beneath ocean ridges. The extent of melting should, in turn, govern both the chemistry of ocean ridge basalts and the thickness of the oceanic crust. Crustal thickness, to first order, should be related through isostatic compensation to the zero-age depth of ocean ridges. Thus, variations in ocean ridge basalt chemistry, axial depth, and crustal thickness should correlate with each other and with mantle temperature variations." Klein and Langmuir, Global Correlations of Ocean Ridge Basalt Chemistry with Axial Depth and Crustal Thickness, 1987

• Why would zero-age depth and chemistry correlate?
• Geochemistry review:
  • What trends in MgO content of melt do you expect with increasing melt fraction? (think about minerals in peridotite)
  • What trends in NaO content of melt do you expect with increasing melt fraction? (sodium behaves like an incompatible trace element)

Potential temperatures

In our daily lives we have an intuitive understanding that a hot object generally has more heat content (enthalpy) than the same object after it cools. This intuition is less useful when considering materials on Earth (rocks, air, water) that are moving quickly across pressure gradients.

The first law of thermodynamics can be stated as:

\begin{equation} \begin{gathered} & dQ & = &~dU~& + &~PdV & \\ & change~in~heat~ & = & ~change~in~internal~energy~& + &~work~done~on~the~environment & \end{gathered} \end{equation}

where $dV$ is the change in volume, and $P$ is the pressure. When considering adiabatic processes, where there is no change in heat, $dQ=0$, we find that the temperature of a material changes due to work done by the system.

\begin{equation} \begin{gathered} & dQ & = &~dU~& + &~PdV & \\ & 0 & = &~C_vdT~& + &~PdV & \\ & & &~C_vdT~& = &~-PdV & \\ \end{gathered} \end{equation}

$C_v$ = heat capacity at constant volume

Potential temperatures

\begin{equation} \begin{gathered} & dQ & = &~dU~& + &~PdV & \\ & 0 & = &~C_vdT~& + &~PdV & \\ & & &~C_vdT~& = &~-PdV & \\ \end{gathered} \end{equation}

So when the volume change is positive (expansion), dT must be negative (cooling). Alternatively if we considered the case of constant volume, using $PdV = - \frac{VdP}{\gamma}$, then decreases in pressure lead to decreases in temperature ($\gamma$ is a positive ratio of the specific heat for the material at constant pressure and constant volume).

The potential temperature, $T_p$, is the temperature defined at a reference pressure, and it allows us to use our daily intuition about temperature when considering the energy (heat content) in a parcel of rock, water, or air. Potential temperatures of the mantle control the starting point for melting during adiabatic decompression.

Axial depth, crustal thickness, and melting

Melting peridotite

Peridotite melting experiments

(-0.5 is the solidus, 0.5 is the liquidus)

Peridotite melting experiments

(-0.5 is the solidus, 0.5 is the liquidus)