Magmatism at an active rift
zone in Afar, Ethiopia
David Ferguson Lamont-­‐Doherty Earth Observatory, Columbia University, NY David Pyle (Oxford), Talfan Barnie (Cambridge), John Maclennan (Cambridge), Gezahegn Yirgu (Addis Ababa), Clive Oppenheimer (Cambridge), Jon Blundy (Bristol) Overview: source to surface magmaIsm at Quaternary riK zones ME
R
Latitude 12.5o
12o
a
Se
20 km
40.5o Longitude
Ebinger, 2005 asthenosphere
Links between melt produc;on/
thermomechanical state of the mantle and recent magma;c ri6 zones/ri6 development? G. Aden
xis
ta
magmatic
segments
Afar
Rif
Timescales, volumes and physical mechanisms of melt emplacement at the ri6 surface Dabbahu
R.
Dabbah-Manda Hararo
magmatic segment
Melt ascent processes / lithospheric plumbing systems 41o
ALI bands 10, 9, 8
(2.20, 1.65, 1.25 um)
Dabbah-Manda Hararo
magmatic segment
Latitude 12.5o
ALI bands 7, 5, 3
(0.87, 0.66, 0.48 um)
Dabbahu
xis
ta
12o
Rif
20 km
40.5o Longitude
Ferguson et al. 2010
Dark material: proximal fire fountain deposits Proximal Sheet flow Scoria cones constructed along vent/
fissure Shear zones evolve into channels, levees develop Toe emplaced as sheet, similar to proximal flow Ferguson et al. 2010
Central flow channel 41o
Barnie and Oppenheimer. in prep.
ALI bands 10, 9, 8
(2.20, 1.65, 1.25 um)
Toe emplaced as sheet, similar to proximal flow Ferguson et al. 2010
Effusion Rate (Qf)
ow
Shear zones evolve into channels, levees develop g fl
nin
Wa
Scoria cones constructed along vent/
fissure Wadge, 1981 Eruption rate
Time
Dark material: proximal fire fountain deposits Proximal Sheet flow Waxing flow
Radiance curve from SEVIRI data – proxy for effusion rate Mean radiance * pixels ALI bands 7, 5, 3
(0.87, 0.66, 0.48 um)
Days Central flow channel Dalla Filla (Erta Ale) Nov. 2008 Dabbahu ri6 June 2009 Dabbahu ri6 Aug. 2007 Dabbahu ri6 May 2010 Melt volumes during Dabbahu riKing phase Wright et al. 2012.
Ini;al part of ri6ing cycles are dominated by intrusion, volcanism becomes more likely later in cycle (relaxa;on of tectonic stress e.g. Buck et al. 2006). Dabbahu ri6ing phase has very high intruded to erupted melt ra;o – no ‘excess’ erup;ble melt? Ferguson et al. 2010.
Melt generaLon and ascent: Trace element composiLons Rock / PM
100
Ethiopian
flood basalts
10
Recent Afar/N.MER
1
Red Sea
axial trough
Rb Th Nb K Ce Sr Nd Zr Eu Tb Ho Er Yb
Ba U Ta La Pb Pr Sm Hf Gd Dy Y Tm Lu
•  Afar lavas enriched in incompa;ble trace element compared to MORB/Red Sea lavas, similar to pre-­‐ri6 flood lavas -­‐ Afar lavas are not MORBs! •  TE ra;os such Ba/Nb, Ce/Pb and La/Nb similar to OIB Melt ascent Frac;ona;on corrected composi;ons for Dabbahu ri6 lavas shows different mantle-­‐melt equilibrium P-­‐T condi;ons for melts erupted on-­‐ and off-­‐ axis. Variable petrogene;c processes for lavas erupted ~20 km apart. 8
P-­‐T’s calculated using Si and Mg thermobarometer: Lee et al. 2009 a
ME
R
Afar
ta
xis
40.5o Longitude
41o
l. 2011
y et a
Roone
Depth (km)
Se
Dabbahu
G. Aden
Rif
Latitude 12.5o
8.6
8.8
9
10
R.
Dabbah-Manda Hararo
magmatic segment
12o
latitude
8.4
0
White et al. 2008 20 km
8.2
MER lavas show different ranges in crustal frac;ona;on depths depending on distance from ri6 margin -­‐ ri6 centre lavas ascend more rapidly 20
30
pyroxene equilibraRon depths for MER lavas Wonji Fault Belt Rooney et al. 2007 Silti-Debre Zeyit F.Z
Melt generaLon and ascent at an acLve riK zone in central Afar Separate plumbing systems feeding ri6 axis and off-­‐axis volcanoes Vast majority of melt supplied to shallow crust is intruded as dykes Primary mel;ng occurs in hot mantle, significant upwelling and mel;ng limited to depths > 80 km Higher degree melts erupted at axis, suggests mantle upwelling is focused beneath axis of recent segment Future ques;ons…. •  How do mel;ng processes vary along strike between the MER and the Red Sea? •  Quan;fica;on of melt volumes and volcanic output •  Volcanic architecture and erup;ve history/forma;on of ac;ve ri6 segments •  Structure and composi;on of the lithospheric plate – xenoliths, melt-­‐rock interac;ons, thermal models. •  What links mantle processes to recent surface focusing of magma;c-­‐tectonic ac;vity? magmatic
segments
asthenosphere
Thank you…….