Source components and intensive parameters of magma genesis in the CentAm and North IBM arcs: analyses using Arc Basalt Simulator model
We have developed a general mass balance model nfor magma genesis in subduction zones and applied it to IBM and Central American arcs. The Arc Basalt Simulator (ABS) model includes: 1) P-T dependent compositional variations of fluids from subducted altered oceanic crust (AOC) and sediment (SED); 2) zone refining chemical modification of slab-derived fluid by interaction with mantle peridotite; and 3) metasomatism and fluid flux melting of mantle peridotite caused by the modified fluid. Application of the model to the northern Izu arc (N-Izu) and Central America arc (CentAm) highlights differences between the two arc systems. The N-Izu basalts are modeled to have derived from a common SED/AOC = 10/90 slab composite with slab fluid dehydration at 880C/4GPa (VF: volcanic front) and 980C/6GPa (RA: rear arc). Mantle wedge peridotite (PERID) composition is assumed to be a common 8% MORB extracted primitive mantle (DMM) with depleted DM isotopic composition for both VF and RA. Melting conditions of the mantle are estimated to be F=24% with 0.4-0.5% fluid flux rate at 1.0 GPa (VF) and 2-4%F with 0.07% fluid flux rate at 2.3 GPa (RA). The CentAm arc basalts require at least three AOC components:Cocos-Nazca Spreading Center (CNS), Northern Galapagos domain (NGD), and Southern Galapagos domain (SGD), with increasing HIMU component in that order. SED component is a unique mixture between hemipelagic and carlcareous sediments. The PERID component ranges from undepleted PM to 4% MORB-depleted PM (DPM) with isotopic composition represented by Utila OIB in the RA of Honduras, which is unaffected by Cocos Plate components. The Guatemala-El Salvador VF basalt requires CNS-AOC, SED, and DPM with SED/AOC = 1/99 slab, dehydrated at 950C/4GPa, and mantle melting at 2%F/2.4GPa with flux rate at 2%. RA alkali basalt of the same segment (Yojoa volcano) requires same AOC and SED but needs undepleted PM melted with fluid flux from SED/AOC = 3/97 slab dehydrated at 1000C/6GPa, and mantle melting at 3%F/2.3GPa with a flux rate 0.05%. Costa Rica segment VF basalts (strongest HIMU signature) require NGD- AOC, SED, and DPM peridotite for Arenal and PM for Platanar basalts. Slab dehydration conditions are SED/AOC = 2/98 for Arenal and 0.7/99.3 for Platanar at 1000C/6GPa and 1100C/6GPa, respectively. Melting conditions are 2%F/2.3GPa with flux rate 0.2% for Arenal and 5%/2.3GPa with flux rate 2% for Platanar. The most rear arc OIB La Providencia basalt needs the strongest HIMU SGD-AOC with SED/AOC = 10/90 slab dehydrated at 1000/6GPa fluxed to melt PM at 1%F/2.3GPa with 0.01% flux rate. Slab dehydration temperature is generally higher beneath CentAm (950-1100C) than beneath N-Izu (880-980C), especially for Costa Rica (1000-1100), reflecting the much younger age of the CentAm slab. Mantle melting degree is high for N-Izu VF (24%F) but low for CentAm and N-Izu RA (2-5%F). Fluid flux rate is low beneath RAs of both arcs (0.01-0.05%) and larger beneath both volcanic fronts (0.2-2%). VF sub-alkaline basalts have relatively low fluid flux (0.05-0.5%) compared to larger flux for the alkali basalt (2%) in VF (Platanar) and behind VF (Yojoa). Intensive parameters derived from the calculations appear to be consistent with the two different tectonic settings between N-Izu and CentAm.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1009 Geochemical modeling (3610;
- 1031 Subduction zone processes (3060;