Mid-Ocean Ridge Basalts (MORB), oceanic crust as well as ophiolites

1- Oceanic crust as well as ophiolites

  • The oceanic crust (made of basalts) is formed on mid-ocean ridges. MORB = Mid-Ocean Ridge Basalt.
  • This is the principal site of volcanic activity on Earth, as well as the nearly abundant igneous rock! But nearly of it is underwater…
  • Occasionally, slivers of oceanic crust are tectonically emplaced on continents (in convergence zone): ophiolites. They locomote on inward early on convergence stages (“obduction”, ex. inward Oman), as well as are after deformed as well as metamorphosed (typically to blueschists) inward collision wedges (peri-Tethyan orogeny, from the Alps to Himalaya).
  • Most of the ophiolites belong to the peri-tethyan system.

2-  Structure of the oceanic crust

Influenza A virus subtype H5N1 “typical” oceanic crust (or ophiolitic sequence) is made of iv layers:
  • Layer 1 is pelagic sediments
  • Layer 2 is made of basaltic pillows, as well as their feeder dykes (“sheeted complex”).
  • Layer three is made of gabbros, approximately of them amongst a cumulative texture. Occasional “plagiogranites” (diorites to trondjhemites) are found, evidencing differenciation (fractional crystallization ?).
  • Layer 4 is the peridotitic mantle, the overstep of it existence peridotite cumulates (dunite), higher upwards “normal” drape lherzolites or harzburgites, perhaps amongst evidences for melting.

3-     Petrology

Mostly tholeitic basalts. Although they are chemically slightly oversaturated (quartz-normative), they typically are olivine-bearing (evidence for crystallization as well as incomplete crystals separation!).
Differenciation (FC?) is demonstrated past times major elements trends, consistent amongst the formation of Ol, Pg as well as Px.
This corresponds to a greater extent than or less to the observed petrologic sequence (dunites – gabbros), as well as to the predicted development on a Di-An-Fo diagram (more or less – the diagram predicts Ol-anorthosites which are non observed. Remember, however: it is a simplified organization for Mg-rich halt members…).

4-      Origin of MOR magmas

Influenza A virus subtype H5N1 molten zone is (seismically) observed nether the ridges.
Petrology, geochemistry, champaign evidences (melt veinelets inward peridotites) advise melting of the mantle, at nowadays below the ridge (shallow melting of a Pg- or Sp-peridotite).
Source as well as multifariousness of sources inward the oceanic crust
Several types of MORB are observed: Normal, as well as enriched (N-MORB, E-MORB). E-MORB are richer inward LREE as well as incompatible elements inward general.
This suggests several distinct sources; the deeper mantle, regarded every bit to a greater extent than enriched, is a skillful candidate for the root of E-MORBs.
Probably tiddler influx of E-MORB type inward a dominantly N type magma sleeping room at the ridge. Mixing betwixt the 2 to a greater extent than or less efficient (the to a greater extent than melt, the to a greater extent than efficient – then fast ridges are to a greater extent than mixed, come across below).
NB- See newspaper past times Schilling (to last presented past times John-Michael. Also come across the drape melting newspaper (Pearce), presented past times Jaco.

5-  Fast as well as tiresome spreading ridges, HOT as well as LOT types

Fast Slow
No axial valley, high “domes”, overlapping spreading centers, smooth.
Deep axial valley, irregular topography, narrow ridge.
Limited extension, lateral grabens
Important extension, axial graben
Oceanic crust thickness
Complete, basalts-gabbros-dunites-harzburgites
Incomplete, discontinous gabbro intrusions or basaltic pillows on lherzolite substratum
Peridotite type
Melt fraction
Fast spreading correlates amongst high flat of melting, yielding lots of basalts as well as strongly depleting the balance mantle, which becomes harzburgitic. Slow spreading .. is the contrary province of affairs !
Two extreme cases, Harburgite Ophiolite Type as well as Lherzolite Ophiolite Type.

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