Basalt: pl, augite, hypersthene, olivine, spinel (45-52 SiO 2 ) 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.OlivineClinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites Dunite (c)

spinifex texture Komatiite sample displays " spinifex texture " defined by extremely acicular olivine phenocrysts(blue colored )--probably a sign of rapid crystallization from a significantly-undercooled magma. olivine 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

AFM diagram: can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline series Figure 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,

Ocean islands and seamounts Commonly associated with hot spots Ocean Intraplate Volcanism Figure After Crough (1983) Ann. Rev. Earth Planet. Sci., 11,

Currently there are 3 Hawaiian volcanoes 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 is building for a new eruption in the next few years Mauna Loa, which last erupted in 1984 and is building for a new eruption in the next few years Loihi, which erupted in 1996Loihi, which erupted in 1996 All three of these active Hawaiian volcanoes share the Hawaiian hot spot, but retain unique volcanic histories and compositions. 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 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

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

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 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 km 3 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 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,

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 of differentiation

Layering in igneous rocks

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

These plumes can also explain the LIPs Models show that plume heads should be created as a plume initiates and rises Mantle Plumes