Petrology/ Hardrock Geochemistry

Disciplinary Groups: Petrology/ Hardrock Geochemistry

Moderator: Mike Perfit; Co-moderator: Dennis Geist



Participants (list your name here):
    •    Dan Fornari
    •    Chris Russo
    •    Ken Rubin
    •    Jeff Standish
    •    David Graham
    •    Adam Goss
    •    Ian Ridley

Discussion topics (list ideas and suggestions here):
    •    relationship between eruptive fissures and lava geochemistry
    •    relationship between lava compositional heterogeneity and pre-eruptive processes
    •    what we do and don't know about subannual to centurial magmatic timescales
    •    differences in magma production and modification through out the global MOR and their manifestations within the ISSs
    •    what we do and don't know about how the mantle melts, and advances made and yet to be achieved from the ISS perspective
    •    what we do and don't know about primary magma interactions with the crust from the ISS perspective
    •    the primary zone of melt accumulation and differentiation

Breakout group discussion summary:
What We Know
There are no microbes in magma but….
Magmatism drives the hydrothermal systems which are related to biologic communities therefore: Mantle to Microbe
Range of compositions varies along strike, and compositional discontinuities often exist at structural/tectonic discontinuities at various scales. First order (All 3 ISS), second and third order (EPR).

Endeavor
The magnitude of eruptions?
Dominantly EMORB (very different from S. JdF)
More evolved compositions off-axis (similar to EPR)
Evidence for off-axis eruptions
Diking without eruption of lava
Diking frequency
Slower magma ascent rate than EPR
Tectonomagmatic units are defined by chemical types
Near-axis seamounts are more mafic than the axial MORB

EPR
We have very high sampling density - best spatial resolution
Shows the small scale of change/heterogeneity
The magnitude of eruptions – 2 events mapped and can be compared
Dominantly NMORB on axis – EMORB minor exclusively off-axis
Documented young lavas off-axis (both by off-axis flow and eruptive sites)
Off-axis evolved basalts – related to off-axis melt lenses?
Diking events related to hydrothermal activity
We think most of the 2006 magma passed through the AMC
Extreme range of lava compositions at 9°N OSC (basalt to dacite)- likely crustal melting/assimilation
Ascent rate from AMC is higher than Endeavor
Assimilation of crust is stronger than Endeavor
Relationship between structural segmentation and petrology

Near axis seamounts and Siqueiros Transform are more primitive and sample more depleted MORB mantle.

Lau
We know more about the petrology of Lau than that of any other back arc ridge
Wide range of compositions: Basalt to dacite
Petrogenesis of evolved lavas – similar to EPR or not?
Similarity to NMorb at north end (no subduction component)
Multiple flavors of arc enrichment, and correspondence between arc and back-arc
Near-axis seamounts are more mafic but sample weird mantle (some are more depleted in HFSE and LIL but not REE)

What Do We Need to Know?
Why there are no microbes in magma
All 3 ISSs
What a dike does to the thermal and mechanical history of the crust (observational evidence).
What’s the proportion of intrusive rock deposited, on an episodic basis
What is the distribution of magma through the crust?
What sort of chemical processing of magma occurs in the lower crust
How does the lower crust form?
How much crustal contamination is there? Is it pervasive? Where does it occur?
What is the crystal/liquid beneath the roof of the chamber?
What is the extent of reaction between magma and crustal mush? Can we tell mush reaction from AMC processing?
Relationships of magma composition and evolution to sulfide deposits- how do MOR or BArc systems relate to terrestrial massive sulfide deposits and their environs.

Endeavor
Distribution of off-axis lavas and their composition (quite diverse)
Why all the EMORB (particularly since SJDF is NMORB)
Low-pressure multi-saturation (Fractional crystallization shallower than EPR) – but AMC is deeper.
Why is there less contamination in lavas (lower Cl/K)

EPR
What is the extent of the neovolcanic zone?
How much of the young off-axis lava erupted from the axis?
Tie petrogenesis of evolved rocks to Lau
Need reflection interpretations

Lau
Why are the hydrothermal systems so small compared to terrestrial volcanic massive sulfide deposits?
What is the longevity of the hydrothermal system, and how does that interface with the magmatic plumbing?
Are the hydrothermal systems different in the south, where there are siliceous magma chambers?
How do the differentiated rocks compare to those at the 9°N OSC?
What is the extent of crustal interaction by the magma (can’t use halogens).
Sampling density is too low to deduce the km-scale segmentation, as at other ISSs