Insights into mid-‐ocean ridge magma chambers from volcanic erup7ons •
Composi7onal varia7ons in sub-‐axial magma reservoirs and the processes that are responsible
•
The geometry of magma7c plumbing beneath and within magma reservoirs
•
How reservoirs are tapped during erup7ons
Mid-‐Ocean Ridge Magma Chambers 70’s-‐80’s view of MOR magma chambers Reservoir about as wide as the Mid-‐ Atlan7c Ridge inner riM valley Different parts of the reservoir tapped during different erup7ons
Hekinian et al. (1976)
Seismic imaging and petrological study required a revision to size, longevity and internal cons7tu7on of MOR magma chambers fast-‐spreading ridge
slow-‐spreading ridge
IntermiUent, deeper magma chamber, mostly Quasi-‐steady state shallow magma chamber with thin, erup7ble melt lens overlying a much smaller, deeper or less persistent than seismic resolu7on. Less erup7ble magma thicker region of par7ally crystalline mush
Most mid-‐Ocean ridge magma chambers are mostly small and mostly mush (lots of crystals), most of the 7me. Sinton & Detrick, 1992
Along-‐axis; fast-‐spreading ridge Along-‐axis
Sinton & Detrick, 1992
How much mixing along axis? What is geometry of injec7on (recharge) to the shallow crust? How are magma reservoirs tapped during erup7ons? -‐over what along-‐axis distances?
Mid-‐Ocean Ridge Erup7ons in North Iceland 1725-‐1729 Mývatn Fires near Krafla caldera
1725-‐1729 Mývatn lava Late 1975 a new earthquake swarm caused riMing extending ~20 km south and 60 km north of the caldera
Followed by a brief (20 min) outbreak of lava inside Krafla caldera
Dec. 1975
April 1977
Sept. 1977
July 1980
Oct. 1980
Jan. 1981
Nov. 1981
Sept. 1984
Krafla Lava
20 events; 9 eruptions Total erupted volume ~0.25 km2 from reservoir ~2.5 km below surface Erupted volume increased in each successive eruption; last one by far the largest Erupted lava volume ~1/4 that determined to have moved through the magma system based on deformation modeling (~1 km3)
Krafla and Mývatn are the latest two of five major riVing events in the last 2800 years Krafla Fires: 1975-‐1984 Mývatn Fires: 1725-‐1729 Dalseldar : ~900 A.D. Hólseldar: ~2300-‐2500 yrs. B.P. Hverfell : ~2600-‐2800 yrs B.P.
Lúdunt, NámaGall, Kröfluháls 7500 -‐11,000 yrs B.P.
HverGall Period
Lüdent Period
ErupZons grouped into episodes Episodes grouped into periods
Lessons from Krafla 1. RiVing events and associated erupZons in Iceland are episodic 2. Major riVing episodes comprise several events, some of which have associated erupZons. 3. ErupZons can re-‐occupy vents from earlier events and episodes 4. Lava can flow back into pre-‐exisZng fissures
Contemporaneous sampling during Krafla eruptions
March, 1980
October, 1980
Sample maps and chemical data from Karl Grönvold and Sæmundur Halldórsson November, 1981
September, 1984
More Lessons from Krafla 1. The combined episode flow field can be readily distinguished from the products of earlier episodes (flow fields); individual lava flows are not easily discernible once the episode is completed. 2. Two chemically distinct magma reservoirs tapped in single eruptions Caldera lava basically the same as the Mývatn lava erupted 250 yrs earlier
Krafla lava 1975-‐1984 Krafla caldera Mývatn lava 1725-‐1729 1977 Photo by S. Thorarinsson
Afar RiMing: 2005 -‐ present also is episodic
Seismo-‐acousZcally detected erupZon on the Gorda Ridge -‐ 1996
Chadwick et al., 1998
1996 North Gorda Ridge
TiO2 wt %
1.40
Chemically variable lava erup7on
1.30
1.20
1.10 61
63
65
Mg # 65.0
Systema7c varia7on with sample (erup7on?) loca7on
Mg #
64.0
63.0
62.0
42.675
42.680
42.685
Degrees N. Latitude
42.690
1993 Co-‐Axial
TiO2 wt %
1.75
1.70
1.65
1.60 47
48
49
50
Mg # 50.0
49.5
Mg #
49.0
48.5
48.0
47.5 46.51
46.52
46.53
Degrees N. Latitude
46.54
46.55
Is verZcal diking necessary to explain the chemical variaZons along axis? What is the relaZonship between magma transport and seismic propagaZon? •
Seismic propaga7on indicates migra7on of stress release, which may or may not track dike propaga7on
•
Along-‐axis migra7on of epicenters is expected for “unzipping” during ver7cal rise of inclined dikes
•
Correla7ons between seismic migra7on and deforma7on are consistent with lateral dike migra7on
Migra7on rates for various seafloor seismic “events” in the NE Pacific and Krafla [aMer Dziak et al., Geology, 2007]
18$ 31' S 2650
Southern EPR
3348
265 0
Rela7vely “young” lava fills the floor of a narrow axial graben in the S. Hump
260
18$ 32'
2650
26 00
2600
265
0
0
280 0
0 275
2650
2700
2650
18$ 33' ND-11
18$ 34'
3347 18$ 35' 113$ 25.5’
113$ 25'
113$ 24.5’ W
-‐ First discovered in 1993 (ND-‐11) -‐ later revisited in 1999 (Alvin dives 3347 and 3348)
2650
18$ 31' S
2.4
3348
But the northern and southern parts of the flow field have very different composi7ons, which cannot be related by frac7ona7on
Alvin 3347 ND 11
265 0
TiO 2 wt %
2.2
260
18$ 32'
2.0 1.8
Alvin 3348
0
1.4 6.0
26 00
2600
6.5
7.0
7.5
8.0
7.5
8.0
MgO wt %
2650
265
0
1.6
18$ 33' ND-11
Na2O wt %
280 0
0 275
2650
2700
2650
3.2
3.1
fract
iona
tion
3.0
18$ 34'
6.0
6.5
7.0
MgO wt %
3347 18$ 35' 113$ 25.5’
113$ 25'
113$ 24.5’ W
South Hump, EPR 18.5° S 18°31'S 2650
And the two different lava composi7ons differ in age by ~150 yrs
265
0
3348
00 26
00
28
0
2 75
2700
2650
2650
ND-11
2650
18°33'S
260 0
265
0
2600
18°32'S
18°34'S
Lava of 1865 ± 50 AD Early 17th C. lava
Magne7c paleointensity data of Bowles (2006) 3347 18°35'S 113°25.5’W
113°25'W
113°24.5’W
Depth (m)
N 2600
Samples from graben floor
3000
Layer 2A
S
South Hump Lava 3348
ND-11
3347
“Boundary” between the two lava flows coincides with discon7nuity in underlying AMC reflector and layer 2A structure
3400 3800
AMC
4200 18°30’
18°31’
18°32’
18°33’
18°34’
18°35’
Degrees S. Latitude
The rela7onship between lava composi7on and the AMC discon7nuity requires (?) near-‐ ver7cal (<1 km lateral) transport of lava from the AMC to the surface, at least for the younger lava erup7on
33
47 18
2A
’ er °33 Lay 18
48
33
’
°32
18
2600 m
’ °31 18 C
3000 3400 3800 4200
C
AM
AM
C
AM
’
°34
1 -1 ND
9°56'
Dated Lavas of the 2005-6 Eruption
0
1
2 km
Composi7onally heterogeneous EPR erup7ons near 9°50’ N 1.60
9°54'
1991-2 2005-6
1.55
northern area
TiO2 wt %
1.50
9°52'
1.45 1.40 1.35
9°50'
Bullseye area
1.30 1.25
56
58
59
60
61
62
63
Mg #
9°48'
southern area
9°46'
57
-104°18'
-104°16'
Chemical data from Rubin et al. (2001) and Goss et al. (2009)
2005-‐6 flow field map from Soule et al. (2007)
9°56'
Dated Lavas of the 2005-6 Eruption
0
1
2 km
9°54'
northern area
9°52'
Note: • Significant along-‐axis composi7onal varia7on, and • change in Mg# across the 7ny 9°52.5’N axial discon7nuity for both erup7ons
9°52.5'N third-order discontinuity 63
9°50'
Bullseye area
62 61
Mg #
60 59
9°48'
southern area
58 57
9°46'
56 9°46'
-104°18'
-104°16'
9°48'
9°50'
9°52'
Degrees N. Latitude
9°54'
9°56'
EPR near 17°30’ S 4 (maybe 5) erup7ve episodes in last ~500 yrs -‐ Last one in late 80’s to early 90’s
17$ 20’ S
17$ 30’
17$ 40’
17$ 50’
18$ 00’
18$ 10’
18$ 20’
18$ 30’
18$ 40’
18$ 50’ 113$ 40’
113$ 30’
113$ 20’
113$ 10’ W
Bergmanis et al., 2007
Lo-‐T, diffuse vents Hi-‐T vents
For last two erup7ons: • Center of erup7ve ac7vity where the magma lens is shallow and rela7vely low temperature; not at the hoUest part of the magma lens • MgO and magma temperature generally correlate with AMC depth
AMer Bergmanis et al., 2007
Step in MgO content south of the long-‐lived small axial discon7nuity at 17°29’S. South of 17°29’S, erupted volume and hydrothermal ven7ng are less, and the AMC is deeper and narrower. -‐ This rela7onship strongly suggests ver7cal diking from a magma reservoir characterized by long-‐lived chemical heterogeneity
AMer Rubin et al., 2010
Chemical variations within the two youngest lavas are dominated by mixing
MgO wt %
9
8 ng
- a low-MgO, low-206Pb/204Pb magma residing in the magma reservoir, and
ixi
M
7
- magma with high MgO and 206Pb/ 204Pb that was injected within 20 years prior to 1993 (collection date).
2.7
2.10
ng
2.6
2.00
M
ix i
Th/U
2.8
2.5
1.90 1.80
secular equilibrium
1.0
old magma
20 to 40 yrs
ng
18.7
1.70 1.60 1.50 1.40 1.30
M ixi
0.9
TiO2 wt %
(210Pb/226Ra)
2.4
young magma
18.8 206Pb/204Pb
1.20 1.10 50
52
54
56
58
Mg #
60
62
64
66
See-‐saw paUern of isotopic varia7on along axis
-‐ Varia7on in amount of newly arrived, high 206Pb/204Pb magma that is progressively mixed with resident low-‐206Pb/204Pb magma in the shallow melt lens. -‐ in this interpreta7on, the high 206Pb/204Pb regions represent loca7ons (concentra7ons) of recent magma injec7on to the shallow lens
MgO wt %
9
8 ng
ixi
M
7
2.7 ix in g
2.6
M
Th/U
2.8
2.5
(210Pb/226Ra)
2.4 secular equilibrium
1.0
old magma
20 to 40 yrs M
ixi
0.9 18.7
ng
young magma
18.8 206Pb/204Pb
Lava flow fields (from isolated erup7ve episodes) can be chemically heterogeneous. -‐ Some are; some aren’t
-‐ Where present, heterogeneity unlikely to be produced during erup7ons; rather it likely reflects varia7on in underlying magma reservoirs. -‐ suggests limited mixing in subaxial magma reservoirs, either because of their shape (road-‐kill-‐cigar-‐shaped melt lens), inhibited mixing in crystal-‐rich mush zones, or because frequency of recharge exceeds the 7me scales for mixing
Evidence for Ver7cal Diking during Mid-‐Ocean Ridge Erup7ons 1. Preserva7on of crustal reservoir proper7es in surface lava flow fields Requires ver7cal rise of magma to the surface with limited along-‐axis mixing, either within dikes or in surface lava aMer erup7on Examples: EPR 17.5°S, S. Hump 2. Correla7ons of along-‐axis chemical varia7ons with ridge axial discon7nui7es or other structures Axial discon7nui7es reflect deeper level varia7ons in magma chamber or thermal structure Examples: EPR 17.5°S; 9°50’N; Krafla 3. Along-‐axis varia7ons in magma temperature and chemistry Most likely correspond to similar gradients in reservoir chemistry and temperature Examples: EPR 17.5°S; 9°52’N; N. Gorda, Co-‐Axial Erup7ons represent only one (last) part of a riMing event. Considerable lateral migra7on possible prior to erup7on Flow fields without chemical heterogeneity provide liUle informa7on about nature of subaxial reservoirs or magma transport processes
