Melting Weathering Deposition & Lithification Burial, Heat, Pressure = Metamorphism Crystallization

Bowen’s Reaction Series Pluton Formation

Part 1 - See Pluton Diagram on next page 1) Magma is molten or partially melted rock from the asthenosphere (weak layer) of the upper mantle) or from magma bodies in the lower crust.

Pluton Formation Plutons form and move upwards much like the blobs of wax in a lava lamp…

Pluton Formation 3) If magma cools and hardens underground, it forms large plutonic formations called batholiths. These often form the core of mountain ranges (Ex: Sierra Nevadas).

Pluton Formation 4)When magma is forced upward into cracks in overlying rocks it forms intrusive rock formations such as dikes and sills.

Extrusive Rocks Intrusive Rocks (Plutons) Intrusive Rocks (Plutons) Extrusive Rocks

Fractional Crystallization 5) Cooling magmas crystallize (turn solid) between 1200 o C and 600 o C. 6a) Mafic minerals have a high melting point (M.P.) tend to crystallize first. Olivine Pyroxene Ca-Spar

Fractional Crystallization 5) Cooling magmas crystallize (turn solid) between 1200 o C and 600 o C. 6a) Ultra-Mafic and Mafic minerals have a high melting point (M.P.) tend to crystallize first.

Fractional Crystallization 6b) Felsic minerals have a low M.P. tend to crystallize last (as the magma cools). Quartz K-Spar Muscovite Mica

Fractional Crystallization 6b) Felsic minerals have a low M.P. tend to crystallize last (as the magma cools).

Fractional Crystallization 7) As minerals crystallize and fall out, the magma’s composition changes. (Mafic magma becomes more felsic as more and more mafic minerals drop out.) Felsic minerals are still molten and continue to rise Mafic minerals crystallize and fall out

Fractional Crystallization Mafic Magma Felsic Magma High temp mafic minerals crystallize and settle out of magma. Mafic Minerals Remaining magma becomes more and more felsic as mafic minerals crystallize and settle out.

Fractional Crystallization 8)Additionally, felsic minerals from the solid overlying rock mix in with the pluton’s magma, which also tends to make the magma more felsic as it rises.

Fractional Crystallization & Pluton Formation Andesite Rhyolite Ultra-Mafic Mafic Felsic Pluton Composition Intrusive Rock Type Intermediate Peridotite Gabbro Diorite Granite Plutons Rising Masses of Magma 9)As a result, plutons found near the surface tend to be more felsic in composition, while plutons which harden deeper down tend to be more mafic : Basalt

Felsic

Ultra Mafic Magma 10)The very deepest plutons are very low in silica and are called ultra-mafic plutons. Olivine Peridotite

Intermediate Magma 11)When a mafic pluton has become somewhat more sialic, but still contains a significant quantity of mafic minerals, it is described as being intermediate in composition. Diorite

“Short Cuts” 12) Sometimes, magma can find a pipe (conduit) which carries the magma rapidly to the surface (before it can harden). 12) Sometimes, magma can find a volcanic pipe (conduit) which carries the magma rapidly to the surface (before it can harden). This allows magma which normally cools in the crust to reach the surface while it is still molten. This allows magma which normally cools deep in the crust to reach the surface while it is still molten.

“Short Cuts” lava is an example of a deep-crust mafic magma that cools and hardens at or near the surface. Basalt lava is an example of a deep-crust mafic magma that cools and hardens at or near the surface. (ie. Lava can (ie. Lava can be felsic, be felsic, mafic or mafic or intermediate) [No ultra-mafic lava for over 2½ billion years!!]

Bowen’s Reaction Series 1)Bowen’s Reaction Series lists the order in which minerals crystallize out of a cooling magma as the pluton rises. 2)The mafic minerals have a high M.P. (1200 o C) and turn solid deep in the crust when the magma body first started to rise. 3) The felsic minerals have a low M.P. (600 o C) and turn solid near the surface as the magma body reaches the end of its upward rise. [See Diagram on the next slide.]

Bowen’s Reaction Series

4)The minerals at the bottom of the Bowen’s Reaction Series Chart are the first to turn solid, because they have the highest melting point (also the freezing point!). OlivineAugite (Pyroxene) Ca-Spar

Bowen’s Reaction Series

5)The minerals at the top of the chart are the last to turn solid, because they have the lowest melting points and the magma has to really cool off before they turn solid. Quartz Muscovite Mica K-Spar

Bowen’s Reaction Series

Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K - Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Felsic Intermediate Mafic Ultra-Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca Continuous Discontinuous

Bowen’s Reaction Series 6)As a result, any given rock will usually be made up of minerals which: a)crystallize at about the same temperature b)form at approximately the same depth in the earth’s crust. c)are closest together on the B.R.S. chart

Bowen’s Reaction Series 7)Geologists can identify a mineral in a given rock by seeing what other minerals are present and looking at the B.R.S. chart to see what minerals are most commonly found together in the same rock. (“I.D. the mineral by the company it keeps.”) Granite Gabbro

Bowen’s Reaction Series Side Bar Question: Granite contains: 1)Pink K-Spar Crystals2) Clear Quartz Crystals 3)Black Mica Flakes4) White Na-Spar Xtls …and some little black chunks that look like someone drew dots on it with a black sharpie!! Go to next page… Granite

Bowen’s Reaction Series Which of these chunky black minerals is found in granite? (Which mineral belongs more with the other 5 above?) a) Hornblende or b) Augite?? Felsic

Bowen’s Reaction Series How do we know that the little black chunks are the black chunky amphibole hornblende and not the black chunky pyroxene augite??? Augite forms at much higher temperatures and much deeper in the crust than K-Spar, Quartz, Na-Spar and Muscovite & Biotite mica.

Bowen’s Reaction Series Hornblende is closer to the other minerals on the Bowen’s R.S. Chart, which means it forms at closer to the same temperature and is more likely to be found at the same depth in the crust as the other minerals in granite. (See # 6 in Part 2 of your notes.)

Bowen’s Reaction Series Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K - Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Sialic Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca Silicate Structure Felsic

Silicate Structures

Bowen’s Reaction Series

Side Bar Question 2: Describe the trend in the degree of sharing of oxygen atoms in the minerals of the Discontinuous Series as you go from Independent Tetrahedra at the bottom of the chart to 3-D Framework at the top of the chart: Answer: The degree of sharing increases consistently increases going up the chart: Ind. Tetrahedra = 0 shared Chain Structure = 2 shared Sheet Structure = 3 shared 3-D Framework = all 4 shared

Bowen’s Reaction Series The minerals that form at the top of the B.R.S. chart are much more stable at or near the surface, while the minerals at the bottom of the B.R.S. are more stable in the deep crust and mantle. Indeed, Quartz and K-Spar are not able to form at great depths – it’s too hot. Olivine can be brought to the surface, but the “cold, wet nasty” conditions found near the surface cause olivine to weather badly.

Bowen’s Reaction Series Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K - Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Sialic Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca Silicate Structure Granite contains these minerals:

Bowen’s Reaction Series

Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K - Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Sialic Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca Silicate Structure Diorite contains these minerals:

Bowen’s Reaction Series

Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K - Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Sialic Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca Silicate Structure Gabbro contains these minerals:

Bowen’s Reaction Series

Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K - Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Sialic Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca Silicate Structure Peridotite contains these 3 minerals:

Bowen’s Reaction Series

Follow these trends on the right side of the BRS chart: When Formed: First to form  Last to Form Resistance to Weathering: Least stable  Most Stable Where Formed: Deep in Crust  Near Surface

Bowen’s Reaction Series