Magmagenesis

See http://plate-tectonic.narod.ru/petrographyigneouslinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L13-Mantle.html

I. Introduction

A. Variability

-Many magmas at surface. How to produce them? Many sources; Many processes?

-Most voluminous magmas are basalts

B. Source composition

-Mantle samples vary mineralogically: ophiolites / samples dredged from oceanic fracture zones; ultramafic nodules in basalts; xenoliths in kimberlites

-Of these, spinel and garnet lherzolite are candidates for original mantle material: correct chemistry to produce basaltic melts; correct density and seismic velocities

C. Lherzolite (ol + opx + cpx)

-lherzolite plots between tholeiite and ultramafics:partially melt lherzolite to get tholeiitic liquid + ultramafic residual solid

- composition of lherzolites - chemically the same, mineralogically different

- phase relations in aluminous lherzolite

D. How to make the mantle melt

-Raise the temperature: hot spots - good for local magma production, but can''t account for volume of MORB

-Lower the pressure : divergent boundaries - upwelling under mid-ocean ridge - decompression melting

-Chamge the composition: specifically, add volatiles = H2O, CO2 - either would lower melting temperature; hydrous and carbonate minerals exist as accessory phases in mantle: ~0.1 wt% water, < for C; this could trigger <1% melting at depths of 100-200 km

II. Melting of a chemically homogeneous mantle


A. projection to base of basalt tetrahedron

-Recall for melting, first melt occurs at eutectic T and composition

-For low P (shallow), eutectic is in silica-saturated part of diagram => quartz tholeiite

-For higher P (deeper), eutectic is in silica-undersaturated part => alkaline basalt

B. Pyrolite experiments

-Def: synthetic mantle-type material (Green and Ringwood)

-melting results at different T and P: right side of diagram shows various results of fractional crystallization as the different magmas ascend toward the surface; note variety possible: silica over-saturated basalts (even granophyres) to silica under-saturated (nephelinite)

C. Summary of these and other experiments

-Composition of primary magma depends on depth of melting and separation, degree of melting, any volatiles present

-Composition of basalt at the surface also depends on any F.C. that occurs.

-Tholeiitic magma => shallow melting, OR, higher degree of melting, OR, fractional crystallization during ascent of deeper magma, OR, presence of water

-Alkaline magma => deeper melting, OR, lower degree of melting, OR, presence of CO2

III. Mantle heterogeneity

A. Consider xenoliths again

-Fertile have more incompatible elements (lherzolites)

-Depleted (dunites) have lower incompatibles: these are interpreted as already being removed by a partial melting event.

-So enriched can produce basaltic magma, and in the process it becomes depleted.

B. Trace elements

OIB have negative slope - enriched in incompatible elements; MORB have positive slope - depleted in incompatible elements (This can''t be developed by any process of partial melting or fractional crystallization of assumed chondritic mantle/To get + slope, must start with material that is already depleted in incompatible elements).

-This is important: most voluminous magma type is apparently derived from a source that has experienced a prior melting event

B. Isotopes

- plot of Sr isotopes vs. Nd isotopes: recall Rb is more incompatible than Sr, so a depleted source evolves to lower 87Sr/86Sr; recall Nd is more incompatible than Sm, so a depleted source evolves higher 143Nd/144Nd

-MORB plots at upper left => depleted source

-Various oceanic islands plot toward lower right => less depleted source

-Star = isotopic composition of chondrites : most samples are depleted to different degrees; some OIB may actually be slightly enriched

D. Mantle convection

-If whole mantle convects, this should erase variability.

-If mantle convects in upper (depleted) and lower (fertile to enriched) layers, heterogeneity could be preserved