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:

1) Investigate melt generation, storage and extraction through thick lithosphere

2) Evaluate global geochemical volatile fluxes

Müller et al. (2008)

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)

Rubin and Sinton, 2007Dick et al., 2003

Analogy 1: melt transfer through thick lithosphere

• 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)

Analogy 2: Focused volcanism

Mantle Bouguer anomaly along SWIRBull’s eye pattern of seismic velocities in the Gulf of California

Dick et al., 2003

Wang et al., 2009

• 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)

• 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?

• Major elements by electron probe (MIT)

• Trace elements by ion probe (DTM 6f)

• Volatiles by ion probe (WHOI 1280)

Major elements

Melt inclusions are more primitive than glasses and represent relatively low degree melts, as expected

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

10 kbar3 kbar0.4 kbar

MgO (wt %)

• 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

Volatiles in Gakkel Ridge melt inclusions

Glasses are in equilibrium with eruption depth, melt inclusions show diverse compositions and are trapped at up to 9km depth below the seafloor

Olivine Mg# = 89

• 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

Melt Migration and Aggregation beneath the Gakkel Ridge

• 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

MORB mantle source CO2 content

Saal et al., 2002

Saal et al., 2002

Photo by M. Moreira

• 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

Global fluxes: Ridge vs. Subduction

Wallace, 2005

MORB flux estimatesShaw et al., EPSL, 2010

Conclusions

1) 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

2) Melt inclusions are more primitive than the glasses, and can be described by a liquid line of descent originating from a single pooled melt

3) 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.

The fate of MARGINS:

Zelda the fortune teller, Buckhorn Saloon

• metamorphic studies• Combine experimental,

geochemical, geophysical and modeling approaches

• Timescales of melt and fluid transfer

• global geochemical fluxes

Make progress towards understanding melt and fluid transfer to the surface at all margins: