Temporal variations in the influence of the subducting slab on Central Andean arc magmas: Evidence from boron isotope systematics

Abstract

Editor: B. Marty Keywords: boron isotopes melt inclusions subduction zone geometry southern Central Andes slab-derived fluids Cenozoic arc magmatismThe Pampean flat-slab segment in the southern Central Andes represents an ideal setting at which to investigate how changes in the tectonic configuration of a subduction zone (convergence angles and rates, seamount subduction and shallowing slab angle) affects the recycling of subducted components to arc magmas. To constrain sources, particularly of slab-derived fluids and their contribution to arc magmatism, boron isotope and select major and trace element compositions were determined for pyroxene-and zircon-hosted melt inclusions obtained from a suite of Paleocene to Miocene arc magmatic rocks, from the southern Central Andes. Considerable changes in δ 11 B values and boron concentrations are observed with time. Significantly lower δ 11 B values (average = −1.9 ± 2.2h (1σ )) and B/Nb ratios (average = 3.3 ± 1.3 (1σ )) were obtained for melt inclusions from Oligocene arc rocks (∼ 24 Ma) compared to those from the Paleocene (∼ 61 Ma) (averages = +1.6 ± 0.8h and 17.8 ± 1.4 (1σ ), respectively) and the Miocene (∼18 Ma) (averages = +4.7 ± 1.9h and 11.9 ± 5.5 (1σ ), respectively). A slab-derived fluid with a δ 11 B composition of +1.5h, primarily derived from altered oceanic crust on the down-going slab, affected the source of the Paleocene arc magma. The source of the Oligocene arc magmas received less boron derived from the subducting slab ( 1% fluid addition) than the Paleocene and Miocene arc magmas (up to 3.5% fluid addition). This is consistent with a greater depth to the slabmantle interface and is potentially related to the widening of the volcanic arc and more distal position of these samples relative to the trench during this time period. The higher δ 11 B values (up to ∼9h) obtained for the Miocene melt inclusions record an increase in the influence of serpentinite-derived fluids on the source of arc magmas after ∼19.5 Ma. This is approximately coeval with the subduction of the Juan Fernandez Ridge (JFR), suggesting that the oceanic lithosphere associated with the subducting JFR in the Early Miocene was hydrated and serpentinised, similar to the present day ridge. As serpentinisation increases the buoyancy of the slab this finding supports the link between the intersection of the JFR with the Andean margin and the onset of flat-slab subduction.

Geodynamic controls on the contamination of Cenozoic arc magmas in the southern Central Andes: Insights from the O and Hf isotopic composition of zircon

“…This segment of the Andean margin has been volcanically inactive since the Late Miocene ($6 Ma) (e.g., Kay et al, 1999;Bissig et al, 2003;Litvak et al, 2007) due to a decrease in the angle (<10°a t 100 km depth) at which the oceanic Nazca plate subducts beneath the South American continent (e.g., Pilger, 1981Pilger, , 1984Yañ ez et al, 2001). The present day, low angle of subduction has been attributed to the subduction of the Juan Fernandez Ridge (JFR) which began intersecting the Andean margin during the early Miocene ($18 Ma) (e.g., Yañ ez et al, 2001;Jones et al, 2014). The shallowing of the Nazca plate caused the position of the volcanic arc to expand and migrate to the east (e.g., Kay et al, 1987;Kay and Mpodozis, 2002).…”

Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid–rock interaction in white mica (Lago di Cignana, Western Alps)

“…The high B contents are more problematic to explain, but high B contents (250 mg/l) were reported from fluids interpreted to reflect slab dehydration (Boschetti et al 2017) and determined in melt inclusions (up to ca. 200 μg/g) thought to reflect a slab fluid influence (Jones et al 2014). Serpentine minerals often have high B concentrations, typically around 10-100 μg/g (Benton et al 2001;Vils et al 2008), which makes them a suitable source for high-B fluids.…”

Halogen (F, Cl) Concentrations and Sr‐Nd‐Pb‐B Isotopes of the Basaltic Andesites From the Southern Okinawa Trough: Implications for the Recycling of Subducted Serpentinites

“…(2011) and Jones et al. (2014), in which the release of B from the slab is coupled with water loss (Marschall et al., 2006). The details about the modeling are given in the supporting information (Figure S7).…”

“…Thus, the fluids released from the slab (AOC + sediment) at Benioff zone depth (∼90 km) beneath the Irabu Knoll are predicted to have even lower δ 11 B values (∼+3.1‰ for AOC fluids and ∼−8.5‰ for sediment fluids) according to a slab dehydration model (AOC:Sediments = 9:1) ( Figures S7). The model follows the methods of Tonarini et al (2011) and Jones et al (2014), in which the release of B from the slab is coupled with water loss (Marschall et al, 2006). The details about the modeling are given in the supporting information ( Figure S7).…”