Lectures 4-6: Hot spots and seamounts

• Early ideas:
  • Origin of the Hawaiian Islands
  • Convective models of the mantle
  • Swells and depth anomalies
• Geochemistry weighs in:
  • Seafloor Basalt Geochemistry and Potential Temperature
  • Compatibility and decay systems
  • The fate of slabs
  • Large igneous Provinces and Plumes

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

A possible origin of the Hawaiian Islands (Tuzo Wilson, 1963)

"It is less obvious why a stream of volcanoes should arise like a series of bubbles from a point beneath the island of Hawaii which is far from the rising current." Tuzo Wilson, A possible origin of the Hawaiian Islands, 1963

Possible convective models

The volcanic consequences of each convective model

The volcanic consequences of each convective model

The source of lavas must be deep

The plume

"In my model there are about twenty deep mantle plumes bringing heat and relatively primordial material up to the asthenosphere and horizontal currents in the asthenosphere flow radially away from each of these plumes..... This model is compatible with the observation that there is a difference between oceanic island and oceanic ridge basalts." Jason Morgan, Convection Plumes in the Lower Mantle, 1971

"In my model there are about twenty deep mantle plumes bringing heat and relatively primordial material up to the asthenosphere and horizontal currents in the asthenosphere flow radially away from each of these plumes."

This model is compatible with the observation that there is a difference between oceanic island and oceanic ridge basalts

A recent world gravity map10 computed for spherical harmonics up to order 16 shows isolated gravity highs over Iceland, Hawaii, and most of the other hotspots. Such gravity highs are symptomatic of rising currents in the mantle. Even if the gravity measurements are inaccurate (different authors have very different gravity maps), the fact remains that the hot- spots are associated with abnormally shallow parts of the oceans. For example, note the depth of the million square kilometres surrounding the Iceland, Juan de Fua, Galapagos, and Prince Edward hotspots. The magnitude of the gravity and topographic effect should measure the size of the mantle flow at each hotspot.

Swells and depth anomalies

Supporting swells

Supporting swells

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)."

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

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}

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 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.

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 FeO 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)

Axial depth, crustal thickness, and melting

Peridotite melting experiments

Peridotite melting experiments

Na2O data from MORB

What else could be controlling the Na to Mg ratio?

Normalized (MgO = 8%) Na$_2$O

0 Depth to the ridge axis (km) 6

Variations in spreading rate?

Potential temperatures beneath ridges and oceanic islands

The partitioning of Mg in Olivine is sensitive to temperature while the Fe/Mg partitioning is not

However, basalts represent a homogenized account of the mantle melting process, and are unlikely to yield the maximum temperatures experienced during melting. And estimates from ocean crust thickness rely on knowledge of melt productivity

This paper presents an alternative method for estimating T, using the forsterite (Fo) contents of olivine phenocrysts and olivine-liquid equilibria. These techniques are used to test the mantle plume hypothesis by determining the thermal anomalies that drive volcanism at Hawaii and Iceland.

Olivine-liquid equilibria are particularly useful for T estimation because (1) the ratio [X Fe /X Mg ] ol / [X Fe /X Mg ] liq (or K D (Fe-Mg) ol-liq ) is nearly constant over a wide range of temperatures, bulk composi- tions and fO 2 conditions [Roeder and Emslie, 1970] (except for a slight increase at high P and T [Herzberg and O’Hara, 1998]) and (2) the ratio liq X ol Mg /X Mg (K d (Mg)) is highly sensitive to temper- ature (X Fe and X Mg are cation fractions of Fe and Mg, respectively; see section 2.2.3).

Trace elements in MORB and OIB (review compatibility trends)

Models of the mantle

Sm and Rb

• $^{147}$Sm$\rightarrow^{143}$Nd
  • Sm more compatible than Nd
• $^{87}$Rb$\rightarrow^{87}$Sr
  • Rb highly incompatible (and more incompatible than Sr)

• What trends do you expect between primitive mantle, MORB, and continental crust?
  • If plumes sample primitive mantle, what should their radiogenic Nd and Sr look like?

Sm and Rb

• $^{147}$Sm$\rightarrow^{143}$Nd
  • Sm more compatible than Nd
• $^{87}$Rb$\rightarrow^{87}$Sr
  • Rb highly incompatible (and more incompatible than Sr)

• What trends do you expect between primitive mantle, MORB, and continental crust?
  • If plumes sample primitive mantle, what should their radiogenic Nd and Sr look like?

³He/⁴He

• atmospheric ³He/⁴He ratio = 1.4x10^-6
• continental crust has low ratios ³He/⁴He = 0.01 RA
• MORB have rather uniform values of 8 $\pm$ 1 RA
• OIB range from 5 to 30

³He/⁴He

• atmospheric ³He/⁴He ratio = 1.4x10^-6
• continental crust has low ratios ³He/⁴He = 0.01 RA
• MORB have rather uniform values of 8 $\pm$ 1 RA
• OIB range from 5 to 30

Nb/U should trace continental material

• OIBs have MORB-like Nb/U ratios, suggesting that they sample the same recycled source material
• $^{3}$He/$^{4}$He ratios can be high in some ocean island basalts, hard to reconcile with a common source for MORB and OIB

Models of the mantle

The fate of slabs

The fate of slabs

Thermal Plume simulations

Large igneous provinces and flood basalts

Thermochemical plumes

Breaking the mold

Breaking the mold

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

Are swells actually supported by thinner lithosphere?

Are swells actually supported by thinner lithosphere?

Are swells actually supported by thinner lithosphere?

"hot lines" like cameroon - no age progression Halliday 1990/Fitton and dunlop other non-hot volcanoes in the ocean - Batiza 1982

What about resurgence in hawaii and the shallowing of the LAB?

Published: 27 April 1973 Iceland Mantle Plume: Geochemical Study of Reykjanes Ridge J.-G. SCHILLING

https://www.sciencedirect.com/science/article/pii/0012821X82901613?via%3Dihub

1982 Mantle plumes from ancient oceanic crust Author links open overlay panelAlbrecht W. Hofmann , William M. White

Hotspot melting generates both hotspot volcanism and a hotspot swell? Jason Phipps Morgan, W. Jason Morgan, Evelyn Price First published: 10 May 1995

Science . 1989 Oct 6;246(4926):103-7. doi: 10.1126/science.246.4926.103. Flood basalts and hot-spot tracks: plume heads and tails M A Richards, R A Duncan, V E Courtillot PMID: 17837768 DOI: 10.1126/science.246.4926.103

Thermal origin of mid-plate hot-spot swells S. Thomas Crough Geophysical Journal International, Volume 55, Issue 2, November 1978, Pages 451–469, https://doi.org/10.1111/j.1365-246X.1978.tb04282.x

Volume: 56 (1972)

Issue: 2. (February)

First Page: 203

Last Page: 213

Title: Deep Mantle Convection Plumes and Plate Motions

Author(s): W. Jason Morgan (2)