Aula_basalts

Prévia do material em texto

Basalt: pl, augite, hypersthene, olivine, spinel (45-52 SiO2)
	Ultramafic volcanics : 
		komatiite : olivine, high Mg, low Ti 
		kimberlite : olivine, phlogopite, matrix 
 (diatreme, brecciated)
Basalts and Ultramafic Volcanic Rocks
Classification of Igneous Rocks
Figure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS.
(c)
Olivine
Clinopyroxene
Orthopyroxene
Lherzolite
Harzburgite
Wehrlite
Websterite
Orthopyroxenite
Clinopyroxenite
Olivine Websterite
Peridotites
Pyroxenites
90
40
10
10
Dunite
Komatiite sample displays "spinifex texture" defined by extremely acicular olivine phenocrysts(blue colored )--probably a sign of rapid crystallization from a significantly-undercooled magma. 
rare ultramafic volcanic rocks(>18 wt.% MgO ). 
olivine
Komatiite
Occurrences
	Rift volcanism: tensional forces (MORB) tholeiitic, tholeiitic-rhyolitic, andesite (rare)
	Subduction zone volcanism: compressional setting (andesite, dacite, rhyolite) composite volcano. diverse basalt (calc-alkaline ~ tholeiitic)
	Intraplate volcanism: hot spot environment- OIB, LIPs(Large Igneous Proveince)
Ophiolite development
Alkali vs. Silica diagram for Hawaiian volcanics:
	Seems to be two distinct groupings: alkaline and subalkaline
12
10
8
6
4
2
35
40
45
50
55
60
65
%SiO2
%Na2O + K2O
Alkaline
Subalkaline
AFM diagram: can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline series
Figure 8-14. AFM diagram showing the distinction between selected tholeiitic rocks from Iceland, the Mid-Atlantic Ridge, the Columbia River Basalts, and Hawaii (solid circles) plus the calc-alkaline rocks of the Cascade volcanics (open circles). From Irving and Baragar (1971). After Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548.
F
A
M
Calc-alkaline
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
T
h
o
l
e
i
i
t
i
c
Ocean islands and seamounts
Commonly associated with hot spots
 
Ocean Intraplate Volcanism
Figure 14-1. After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165-193.
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More enigmatic processes and less voluminous than activity at plate margins
No obvious mechanisms that we can tie to the plate tectonic paradigm
As with MORB, the dominant magma type for oceanic intraplate volcanism is basalt, which is commonly called ocean island basalt or OIB
41 well-established hot spots Estimates range from 16 to 122
Currently there are 3 Hawaiian volcanoes that we can easily classify as active: 
	 Kilauea, actively erupting since 1983 
	 Mauna Loa, which last erupted in 1984 and is building for a new eruption in the next few years 
	Loihi, which erupted in 1996 
	  All three of these active Hawaiian volcanoes share the Hawaiian hot spot, but retain unique volcanic histories and compositions. 
  Mauna Loa, or "Long Mountain" in Hawaiian, is located on the island of Hawaii. It is pictured above rising 13,680 ft. (4,170 m) above sea level (this photo was taken from over Loihi seamount, some 30 km or so to the south). Since 1832, Mauna Loa has erupted 39 times; its last eruption was in 1984. 
Hawaiian Scenario
Cyclic, pattern to the eruptive history
1. Pre-shield-building stage somewhat alkaline and variable (alkali olivine basalt)
2. Shield-building stage begins with tremendous outpourings of tholeiitic basalts 
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Early, pre-shield-building stage that is more alkaline and variable, but quickly covered by the massive tholeiitic shields
Recent studies of the Loihi Seamount encountered a surprising assortment of lava types from tholeiite to highly alkaline basanites. 
Shield-building: Kilauea and Mauna Loa (the two nearest the hot spot in the southern and southeastern part of the island) are presently in this stage of development
This stage produces 98-99% of the total lava in Hawaii
Hawaiian Scenario
3. Postshield Stage Waning activity more alkaline, episodic, and violent (Mauna Kea, Hualalai, and Kohala). Lavas are also more diverse, with a larger proportion of differentiated liquids
4. Rejuvenated Stage A long period of dormancy, followed by a late, post-erosional stage. Characterized by highly alkaline and silica-undersaturated magmas, including alkali basalts, nephelinites, melilite basalts, and basanites
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The two late alkaline stages represent 1-2% of the total lava output
Note all three OIB series are represented in Hawaii
Is this representative of all islands? Probably not
Mantle origin
	Generation depth >40km (Seismic data)
	Phase equilibria > 80km
	Mantle xenolith
These pictures show an example of an olivine-clinopyroxene bearing mantle xenolith from the 1800-1801 lava flow of Hualalai.
Mantle plume
	Age progression of volcanism
	Trends of volcanic chain
	Plume hypothesis
	Geochemistry (He-isotope, eNd,…)
	But plate tectonics can not easily explain volcanism in the interiors of plates
	Because of the presumed excess heat responsible for volcanism, such features are called hotspots
Hotspots
Iceland
Hawaii
Galapagos
Reunion
Yellowstone
Afar
Azores
Easter
Tristan
	Where these hotspots occur in the ocean basins they generally occur at the tips of “aseismic ridges” or island and seamount chains
	Tracks on the same plate are generally parallel
Hotspots
Intraplate Volcanism
	Columbia River Basalt (Flood Basalt)
	No petrographic, chemical variation
	Qtz-tholeiite, olivine tholeiite, tholeiitic andesite
Global distribution of flood basalt provinces. 
2,000,000 km3 of lava 
There are no "volcanoes" as such found 
in these provinces !!
LIPs(Large Igneous Proveince)
Map showing the present extent of the Columbia River flood basalts (gray area on the map). 
A sequence of about 20 Columbia River basalt lava flows in the canyon of the 
Grande Ronde River, Washington state. Each flow is 15 to 20 meters thick. 
Magmatic history
	Matle melting, ascent of magma
	Pl, ol, px fractionation
	Magma mixing, assimilation of crust
Enriched mantle-plume component (Ba, Th, Nb)
Fractionation(Eu-anomaly)
Nd, Sr
Figure 15-4. Present setting of the Columbia River Basalt Group in the Northwestern United States. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Also shown is the Snake River Plain (SRP) basalt-rhyolite province and proposed trace of the Snake River-Yellowstone hot spot by Geist and Richards (1993) Geology, 21, 789-792. 
Model for the CRFB Province
	Melting within a plume head (initial stages of the Yellowstone hot spot). 
	The plume head contains stringers of recycled oceanic crust that melts before the peridotite,yielding silica-rich basaltic magma equivalent to the main Grande Ronde basalts. 
	The large plume head stalls and spreads out at the base of the lithosphere and the basaltic magma underplates the base of the crust, where it melts some crust to create rhyolite. 
	Basalt escapes along a northward trending rift system to feed the CRBG.
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	Melting within a heterogeneous plume head (initial stages of the Yellowstone hot spot). 
	The plume head contains recycled stringers of recycled oceanic crust that melts before the peridotite, yielding a silica-rich basaltic magma equivalent to the main Grande Ronde basalts and leaves a garnet-clinopyroxene residue. 
	The large plume head stalls and spreads out at the base of the resistant lithosphere and the basaltic magma ponds (underplates) at the base of the crust, where it melts some crust to create rhyolite. 
	Basalt escapes along a northward trending rift system to feed the CRBG.
Diagrammatic cross section illustrating possible models for the development of continental flood basalts. DM is the depleted mantle (MORB source reservoir), and the area below 660 km depth is the less depleted, or enriched OIB source reservoir. Winter (2001) An Introduction to Igneous and Metamorphic Petrology.
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The Muskox Intrusion
Mechanisms of
differentiation
Layering in igneous rocks
Large Igneous Provinces
Large Igneous Provinces
	Many hotspots can be traced back to massive volcanic eruptions of flood basalts, creating LIPs
Columbia River Basalts16.5 Ma
Washington, USA
Deccan Traps 66 Ma India
	Morgan deduced that these thermal plumes must rise from a thermal boundary layer and proposed that they originate at the core-mantle boundary (~2900 km)
Mantle Plumes
Plume
Plume
	These plumes can also explain the LIPs
	Models show that plume heads should be created as a plume initiates and rises
Mantle Plumes
Plagioclase
Olivine
Pyroxene
Gabbro
Troctolite
Olivine
 gabbro
Plagioclase-bearing ultramafic rocks
90
(b)
Anorthosite
Plagioclase
Olivine
Pyroxene
Gabbro
Troctolite
Olivine
 gabbro
Plagioclase-bearing ultramafic rocks
90
(b)
Anorthosite