1. Melting processes and volatile fluxes at the Gakkel Ridge – do ultra-slow spreading systems reveal insights to Rift evolution?
Alison Shaw, Mark Behn, Susan Humphris, Rob Sohn (WHOI) Patricia M. Gregg (LDEO)
Goals:
Investigate melt generation, storage and extraction through thick lithosphere
Evaluate global geochemical volatile fluxes

2. Müller et al. (2008)

3. Observed variability at mid-ocean ridges
Crustal thickness is relatively constant at spreading rates >20mm/yr, but becomes highly variable at ultra-slow rates
Slower spreading ridges tend to show greater geochemical variability – due to lower F and/or lack of steady state magma chambers
spreading rate (cm/yr)
Dick et al., 2003
Rubin and Sinton, 2007

4. Analogy 1: melt transfer through thick lithosphere
Several models are possible of how and where the melt is extracted and what happens to it during transport
Analagous to non-volcanic rifting
Ultra-slow spreading ridges on average have thicker lithosphere as compared to faster-spreading ridges
melting is terminated at greater depth – thus, the mantle melts to a lower degree (lower mean F)

5. Analogy 2: Focused volcanism
Focused volcanism at rifted continental margins (e.g., Gulf of California, Red Sea)
Punctuated volcanism along ultra-slow spreading ridges (e.g., Gakkel Ridge, SWIR)
Dick et al., 2003
attribute these low-velocity anomalies to dynamic, buoyancy-driven upwelling and melting initially triggered by extension that began in the Gulf region about six million years ago.
Volcanic rifts vs. non volcanic rifts (i.e., Iberia margin (Newfoundland))
Wang et al., 2009
Bull’s eye pattern of seismic velocities in the Gulf of California
Mantle Bouguer anomaly along SWIR

6. How do global volatile fluxes at ridges compare to subduction zones?
Major elements by electron probe (MIT)
Trace elements by ion probe (DTM 6f)
Volatiles by ion probe (WHOI 1280)
Do the melt inclusions record deep melts? If so, what can be learned about melt aggregration and melt migration processes through thick lithosphere?
Is our data consistent with melting models specific to the Gakkel Ridge?
How do global volatile fluxes at ridges compare to subduction zones?
discrete volcanic centers separated by amagmatic basins

7. Major elements
Low degree melting as expected for US spreading ridges
Melt inclusions are more primitive than glasses and represent relatively low degree melts, as expected

8. Trace elements
Melt inclusions from Global MORBs typically show geochemically diverse compositions – potentially representing melts trapped prior to homogenization in a magma chamber or due to melt/rock reactions
Argues for significant pooling and aggregation

9. 10 kbar
3 kbar
0.4 kbar
MgO (wt %)
Volatile data can potentially be used to assess the depth at which pooling occurs.
Gakkel-specific thermal model combined with the melting model of Kinzler & Grove predicts a pooled melt composition and LLD that fits the data remarkably well
These results combined with the trace element data imply efficient pooling of melts beneath the Gakkel Ridge

10. Volatiles in Gakkel Ridge melt inclusions
Olivine Mg# = 89
Glasses are in equilibrium with eruption depth, melt inclusions show diverse compositions and are trapped at up to 9km depth below the seafloor

11. Melt Migration and Aggregation beneath the Gakkel Ridge
Crystallization begins at ≥9 km depth and continues to seafloor
Data suggest multiple crystallization paths, consistent with a model where rising melts begin to crystallize within the thick thermal boundary layer
Melting model shows that CO2 contents represent primary undegassed melts

12. MORB mantle source CO2 content
Popping rock: up to 1000 ppm (Javoy & Pineau, 1991), CO2/Nb= 530 – predicts ~160 ppm (Cartigny et al., 2008)
Siqueiros intra-transform fault (EPR): CO2/Nb= 239 – predicts ~73 ppm
Gakkel CO2/Nb (=443) predicts 134 ppm CO2
Saal et al., 2002
Saal et al., 2002
Photo by M. Moreira

13. Global fluxes: Ridge vs. Subduction
Water and CO2 fluxes from subduction zones and ridges are very similar
S from arcs is an order of magnitude lower than ridges and Cl is an order of magnitude higher
Wallace, 2005
MORB flux estimates
Shaw et al., EPSL, 2010

14. Conclusions
Gakkel Ridge melt inclusions and glasses record remarkably homogeneous compositions, consistent with deep, low degree melts which would be expected for melting in thick lithosphere
Melt inclusions are more primitive than the glasses, and can be described by a liquid line of descent originating from a single pooled melt
The data support a model in which melts are efficiently pooled in a magma chamber located in the upper mantle (~9 km depth) – significantly deeper than previously imaged magma chambers
Thanks to: Funding from NSF, NASA, and Woods Hole Oceanographic Institution. Andrey Gurenko (WHOI), Nilanjan Chatterjee (MIT), Erik Hauri (DTM) for assistance with data collection.

15. The fate of MARGINS:
Make progress towards understanding melt and fluid transfer to the surface at all margins:
metamorphic studies
Combine experimental, geochemical, geophysical and modeling approaches
Timescales of melt and fluid transfer
global geochemical fluxes
Zelda the fortune teller, Buckhorn Saloon