• 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.

Plagioclase

OlivinePyroxene

Olivine gabbro

Plagioclase-bearing ultramafic rocks

90

(b)

Anorthosite

OlivineOlivine

ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene

LherzoliteLherzoliteH

arzb

urgi

te

Wehrlite

Websterite

OrthopyroxeniteOrthopyroxenite

ClinopyroxeniteClinopyroxenite

Olivine Websterite

PeridotitesPeridotites

PyroxenitesPyroxenites

90

40

10

10

DuniteDunite

(c)

Komatiite sample displays "spinifex texturespinifex 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

12

10

8

6

4

2

35 40 45 50 55 60 65

%SiO2

%N

a2O

+ K

2O

Alkaline

Subalkaline

Alkali vs. Silica diagram for Hawaiian volcanics:Alkali vs. Silica diagram for Hawaiian volcanics:Seems to be two distinct groupings: Seems to be two distinct groupings: alkalinealkaline and and subalkalinesubalkaline

F

A M

Calc-alkaline

T

ho leiitic

AFM diagram:AFM diagram: can further subdivide the subalkaline can further subdivide the subalkaline magma series into a magma series into a tholeiitictholeiitic and a and a calc-alkalinecalc-alkaline series 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.

Ocean islands and seamountsCommonly associated with hot spots

Ocean Intraplate Volcanism

Figure 14-1. After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165-193.

Currently there are 3 Hawaiian volcanoes Currently there are 3 Hawaiian volcanoes that we can easily classify as active:that we can easily classify as active:

• Kilauea, actively erupting since 1983 Kilauea, actively erupting since 1983 • Mauna Loa, which last erupted in 1984 and Mauna Loa, which last erupted in 1984 and

is building for a new eruption in the next is building for a new eruption in the next few years few years

• Loihi, which erupted in 1996 Loihi, which erupted in 1996 •     All three of these active Hawaiian All three of these active Hawaiian

volcanoes share the volcanoes share the Hawaiian hot spotHawaiian hot spot, but , but retain unique volcanic histories and retain unique volcanic histories and compositions. compositions.

    Mauna Loa, or "Long Mountain" in Hawaiian, is located Mauna Loa, or "Long Mountain" in Hawaiian, is located on the island of Hawaii. It is pictured above rising 13,680 on the island of Hawaii. It is pictured above rising 13,680 ft. (4,170 m) above sea level (this photo was taken from ft. (4,170 m) above sea level (this photo was taken from over Loihi seamount, some 30 km or so to the south). over Loihi seamount, some 30 km or so to the south). Since 1832, Mauna Loa has erupted 39 times; its last Since 1832, Mauna Loa has erupted 39 times; its last eruption was in 1984. eruption was in 1984.

Hawaiian ScenarioCyclic, pattern to the eruptive history1. Pre-shield-building stage somewhat

alkaline and variable (alkali olivine basalt)

2. Shield-building stage begins with tremendous outpourings of tholeiitic basalts

Hawaiian Scenario3. 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

Mantle origin

• Generation depth >40km (Seismic data)

• Phase equilibria > 80km

• Mantle xenolith

These pictures show an example These pictures show an example of an olivine-clinopyroxene of an olivine-clinopyroxene bearing mantle xenolith from bearing mantle xenolith from the 1800-1801 lava flow of the 1800-1801 lava flow of Hualalai.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

EasterTristan

• 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 !!

LIPLIPs(s(LLarge arge IIgneous gneous PProveince)roveince)

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.

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.

The Muskox Intrusion

Mechanisms ofdifferentiation

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 MaWashington, 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