Chemical Structure of The Galapagos Lavas
Analysis of the radioactive isotopes of the lavas on the islands of the Galapagos archipelago and on the Carnegie Ridge shows that there are four major reservoirs of magma that mix in varying combinations to form the volcanic province.
The four types are: PLUME – this is magma associated with the plume itself and is similar to magmas from other ocean islands within the Pacific. It has the characteristics of intermediate Strontium (Sr), Neodymium (Nd) and Lead (Pb) ratios. The PLUME lavas are found predominately in the west of the islands, around Ferdinandina and Isabela Islands, which is near to the current position of the hotspot. The PLUME lavas erupted on Fernandina and Isabela are relatively cool. Analysis shows that they are as much as 100 degrees Celsius cooler than those in Hawaii. The cause of this is not fully understood but may be due to cooling in the lithosphere or being relatively cool at formation in the mantle. They are then found in lower quantities in a horseshoe pattern north and south of the central islands mixing with the other reservoirs as it progresses east. PLUME lavas are also found in the lavas from the Galapagos Spreading Centre due to convection and mixing of all of these lavas. In the upper mantle convection currents bring in mantle material at shallow angles from the south of the Galapagos Spreading Centre. These convection current will draw in some PLUME type magma to the spreading centre where it is then erupted.
DGM – (Depleted Galapagos Mantle), this has similar characteristics to ocean ridge basalts throughout the Pacific and the Galapagos Spreading Centre. Partial melting of the upper mantle as a result of the spreading centre will leave mantle material depleted in some compounds. It has low Sr and Pb isotope ratios and high Nd ratios. DGM is found in the central islands of the Galapagos such as Santiago, Santa Cruz, San Cristobal and Santa Fe. It fills in the centre of the horseshoe formed by the PLUME lavas to the west, north and south.
FLO – (Floreana), characteristic of that islands lavas. It is thought that this reservoir came from subducted ocean crust that has been entrained by the mantle plume. It has enriched Sr and Pb ratios and is enriched with trace elements. FLO is associated principally with the island of Floreana and shows up on the mixing of lavas within the Galapagos along the southern side archipelago and is diluted to the east and north of there.
WD – (Wolf Darwin) is unique in the Pacific and resembles material from an Indian Ocean Ridge system. It is found on the Wolf and Darwin Islands and the seamounts that connect them along the Wolf Darwin Lineament. It has a unique Pb ratio. WD is located along the northern side of the archipelago and dilutes to the east and south.
Read more about this topic: Galápagos Hotspot
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