Lecture 8: 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.

• Trace elements and isotopes in MORB/OIB
• Large low shear velocity provinces (LLSVPs)
• Current models of the mantle

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.

Na2O data from MORB

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

0 Depth to the ridge axis (km) 6

Potential temperatures beneath ridges and oceanic islands

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

Strong geochemical evidence for the existence of thermally driven mantle plumes

Trace elements in MORB and OIB (review compatibility trends)

Tristan, South Atlantic

Tubuai, French Polynesia

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?
  • sketch: $\frac{^{87}Sr}{^{86}Sr}$ (x) vs $\frac{^{143}Nd}{^{144}Nd}$ (y)

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?
  • sketch: $\frac{^{87}Sr}{^{86}Sr}$ (x) vs $\frac{^{143}Nd}{^{144}Nd}$ (y)

³He/⁴He

• ³He is primordial, and ⁴He is generated by alpha decays
• atmospheric ³He/⁴He ratio = 1.4x10^-6
• continental crust has low ratios ³He/⁴He = 0.01 R/R$_A$
• MORB have rather uniform values of 8 $\pm$ 1 R/R$_A$
• OIB range from 5 to 30 R/R$_A$

³He/⁴He

• ³He is primordial, and ⁴He is generated by alpha decays
• 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

Models of the mantle

Large low shear velocity provinces (Tuzo and Jason)

Large low shear velocity provinces (Tuzo and Jason)

The fate of slabs

The fate of slabs

Models of the mantle

What do you think the source of plumes is? Where do plumes come from?

plumes come from LLSVPs and LLSVPs are the fate of subducted slabs

"It is well-agreed that these are likely subducted slabs, remnants of slabs, and/or mantle cooled by slabs. Two, nearly antipodal regions of the lowermost mantle, beneath Africa and the Pacific, are characterized by lower-than-average shear wave speeds, and it is these anomalously slow regions that are referred to as the Large Low Shear Velocity Provinces (LLSVPs)."