Slides 4b.docx - Origin of Magma 1 Role of heat 1 • Temperature increase in the upper crust(geothermal gradient averages between 20 oC to 30oC per

Slides 4b.docx - Origin of Magma 1 Role of heat 1 •...

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Origin of Magma Role of heat: • Temperature increase in the upper crust (geothermal gradient) averages between 20 o C to 30 o C per kilometer depth • Rocks in the lower crust and upper mantle are near their melting points • Any additional heat may induce melting Melting temperatures of dry peridotite are everywhere higher than geothermal gradient Origin of Magma Role of volatiles: • Volatiles (primarily water) cause rocks to melt at lower temperatures • Important factor where oceanic lithosphere descends into the mantle Wet peridotite partially melts at depths below ~60 km (lower crust and upper mantle) to produce basaltic magma The geothermal gradient shows how temperature increases with depth in the Earth The geothermal gradient shows how temperature increases with depth in the Earth 10/5/2016 1 10/5/2016 1 10/5/2016 1 10/5/2016 1 10/5/2016 1 10/5/2016 1 10/5/2016 1 10/5/2016 1 10/5/2016 1 10/5/2016 1 10/5/2016 1 Mantle Melting Through The Release of Volatiles O Role of pressure: • An increase in confining pressure with depth increases a rock’s melting temperature (e.g. higher temperatures are required to melt deeper rocks) • Conversely, mantle surface experiences a • When confining pre decompression meltin 10/5/2016 1 10/5/2016 1 10/5/2016 1 A plume of solid peridotite rising at constant temperature encounters lower pressures at shallower depths. Plume eventually enters field of partial melting. Partial Melting • Magma forms as a hot liquid when pre-existing rock melts • When a rock is heated, certain minerals may begin to melt while other minerals remain solid: – This process is termed partial melting – The resulting liquid is called a partial melt Decompression Melting Partial Melting • The partial melt formed through the melting of certain minerals takes on the composition of the melted minerals • The solid minerals remaining behind define the composition of the residual solid The Order In Which Minerals Melt At Increasing Temperatures Can Be Predicted Using Bowen’s Reaction Series Decompression Melting 10/5/2016 2
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Decompression Melting of Mantle Peridotite Beneath Mid-ocean Ridges Produce Magma Basaltic Basaltic Melting Of Mantle Plumes Produce Basaltic Magma Mantle peridotite Melting of Mantle Above Subduction Zone Can Also Form Basaltic Magma At Depth Formation of Basaltic Magma • Partial melting of mantle peridotite produces a basaltic liquid • The melts begin to rise through the surrounding rock towards the surface • The melts eventually collect and pool in a magma chamber • Eruption of this magma onto the surface produces a basaltic lava flow Formation of Basaltic Magma • Most basaltic magmas originate from partial melting of an ultramafic rock called peridotite • Where formed: – Beneath mid-ocean ridges – Melting of mantle above subduction zone – Mantle plumes 10/5/2016 3 Early Crystallization of Basaltic Magma • The magma chamber slowly cools and eventually the first minerals begin to crystallize from the liquid: – Types of minerals that first
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  • Spring '09
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