Partition coefficients (Kd) are critical for quantitative modeling of the evolution of magmatic systems. Partition coefficients (Kd) are widely used in quantitative modeling of the evolution of magmatic systems (e.g.
Sample Used
In addition, Colombini et al. 2011) report separate compositions of zircon, titanite and glass (accessory minerals also measured by SHRIMP-RG; glass by LA-ICP-MS) from the same tuff sample (KPST01) and discuss the derived partition coefficients for zircon and titanite . We compare our Kd results with those of Colombini et al. 2011), and expand that data set by including Kd for all phases (except Fe-Ti oxides and allanite, as we were unable to find allanite in our crystal sections) measured by LA-ICP-MS and EDS.
Sample preparation
Estimates of the systemic conditions of the pre-eruptive Kingman distal outflow magma are about 220 MPa pressure (full range 185–230 MPa; thoroughly discussed in Pamukcu et al., 2015) and Zr- and -temperatures of the titanite range from 760 °C to 779 °C C (see Pamukcu et al., 2013, for a detailed discussion). As much as possible, we chose crystals with glass attached to their edges to better ensure that these are phases growing in the melt and not as inclusions in larger crystals, which is especially important in the case of minor accessory phases.
SEM analysis
Using a stereomicroscope, we hand-picked individual crystals for each mineral phase found in the crystal-rich fractions (both heavy and light fractions of MEI separation), as well as PST glass pieces from the glass-rich fraction. 2 Note: essential structural elements (ESCs) for each phase are indicated by #; values limited by stoichiometry are indicated by &; ESCs in biotite are omitted from apatite Kd, denoted by *, as these are likely to be problematic due to their enriched concentrations in biotite and our non-conventional method of obtaining apatite data.
LA-ICP-MS analysis
We analyzed all mineral phases at or as close as possible to the edge of the grains. Our apatite data are only reliable for elements with high concentration in apatite and low concentration in biotite, due to the memory effect resulting from long washout times in the instrument used.
Data reduction
Essential structural components (ESC) for each mineral are indicated by circular symbols, while diamonds represent scattered elements. a) Larger minerals (amphibole, biotite, sanidine, plagioclase). The partition coefficients (Kd) for each element are then calculated by dividing the average element concentration in each mineral phase by the average concentration of that element in the glass.
Titanite
Chevkinite
Data symbols correspond to the studied rock type as follows: square = high-silica rhyolite; diamond = rhyolite. Data symbols correspond to the studied rock type as follows: square = high-silica rhyolite; diamond = rhyolite.
Zircon
Apatite
Data symbols correspond to the studied rock type as follows: square = high-silica rhyolite; diamond = rhyolite; triangle = granite; circle = dacite. Data symbols correspond to the studied rock type as follows: square = high-silica rhyolite; triangle = granite; circle = dacite.
Amphibole (Hornblende)
Biotite
Sanidine
Plagioclase
Data symbols correspond to the rock type studied as follows: square = high quartz rhyolite; diamond = rhyolite; triangle = granite.
Controls on REE distribution
While zircon is a ubiquitous phase, it is also not very abundant; in the PST case, titanite, which is by far the most abundant REE-rich accessory, controls the behavior of the HREE.
Controls on large ion lithophile element (LILE) distribution (Rb, Sr, Ba)
Crystallographic controls on partitioning
For sets of cations that do not match a curve of best fit, an assumed fit is calculated (see section 4.4 for details) and plotted as a dashed curve (eg HFSE). Onuma plot (Ln(Kd) vs. ionic radius) for putative HFSE fits to zircon, chevkinite, and amphibole (see Section 4.4 for details).
The Austurhorn Intrusive Complex, SE Iceland
We present clear geochemical evidence and supporting field observations for open-system processes in the Austurhorn magmatic system, including the physical exchange of zircons between magmas of contrasting compositions, consistent with previous interpretations based on field relationships (e.g. Furman et al., 1992b; Mattson et al., 1986). Regional map of southeastern Iceland showing the four main composite-gravel intrusions exposed in the area (black shaded areas; adapted from Gale et al., 1966). Inset: Overview map of Iceland and its main tectonovolcanic zones (adapted from Carley et al., 2011): Northern (NVZ), Eastern (EVZ) and Western Volcanic Zones (WVZ), Snæfellsness Volcanic Belt (SVB), Reykjanes Ridge (RR) , the Reykjanes Volcanic Belt (RVB), the South Iceland Seismic Zone (SISZ), the Öræfi Volcanic Belt (ÖVB), the Tjornes Fault Zone (TFZ), the Kolbeinsey Ridge (KR) and the Central Belt -Iceland (MIB).
The Mafic-Silicic Composite Zone (MSCZ)
Current Views on Silicic Magma Petrogenesis at Austurhorn
Sigmarsson (2010) presents oxygen isotope evidence for extensive crustal recycling from intermediate silicic acid samples from the AIC. They argue for partial melting of low-δ18O metabasaltic crust as the dominant source of low whole-rock δ18O values (-2.1 ‰ for a 53.6 wt% SiO2 sample; +0. 5 ‰ for a 64.8 wt% SiO2 sample, confirmed by the presence of low δ18O rhyolitic glass and clinopyroxene phenocrysts in Austurhorn samples. With this study we build on previous work by others and expand the geochemical dataset available for the Austurhorn system.
Fieldwork
Whole-Rock Analyses
Based on whole-rock major element compositions and Zr concentrations, we calculated model zircon saturation temperatures (ZSTs) for all samples using the formulation of Watson and Harrison (1983), as revised by Boehnke et al. most siliceous rocks, the calculated ZSTs are likely to be modestly low, while for whole-rock mafic compositions the ZSTs tend to seriously underestimate the temperatures at which zircon actually saturates (e.g. Miller et al., 2003; Harrison et al., 2007; Boehnke et al., 2013; Moecher et al., 2014; McDowell et al., 2014). We then removed Ti from the Hf fraction in a second stage of chemistry (a crucial step, as excess Ti has been shown to alter the measured Hf isotopic composition; Blichert-Toft et al., 1997).
Zircon and Imaging
Using these constraints, we assume uniform values of aTiO2 = 0.5 and aSiO2 = 1.0 in our model estimates of zircon crystallization temperatures (e.g. McDowell et al., 2014). Keck Foundation Center for Isotope Geochemistry (Los Angeles, CA), following the methods described by Trail et al. We track instrument configuration, operating parameters and data. reduction methods described by Fisher et al. 2014), except that U-Pb ages were not determined simultaneously.
Field Observations
Silica rocks in the MSCZ are commonly granular and may be aggregated. In some places, the MSCZ is also in contact with the coarse-grained Hvalnesfjall gabbro (sample IA-G-3; see Fig. 2; Furman et al., 1992a). We refer to all other gabbroic units within the MSCZ as “sheet” gabbro (e.g. sample IA-G-5).
Petrography
A zone of homogeneous fine-grained granophyre, without mafic enclaves and clastics, surrounds the MSCZ. A small body of gabbro, referred to as the "coastal" gabbro of the MSCZ, is exposed within the MSCZ at the Hvalneskrókur point and, unlike the mafic sheets elsewhere in the MSCZ, is coarse-grained and gradational with the surrounding siliceous units (Sample location IA -G-1). The Hvalnesfjall gabbro (IA-G-3) is distinctly coarser than all MSCZ gabbros and shows the least alteration.
Whole-Rock Geochemistry
The chemical properties of Hvalnesfjall and foliated gabbro are more cumulative compared to coastal gabbro. The Hvalnesfjall gabbro (IA-G-3) has lower REE concentrations than the MSCZ gabbro and a more pronounced positive Eu anomaly. Silica samples not processed for zircon extraction (i.e. without Hvalnesfjall gabbro) are shown as the “Other MSCZ” group. a) SiO2 vs
Zircon Results
Zircons from the onshore MSCZ gabbro (IA-G-1) are larger than those from silicic samples, and they display diverse internal patterns, including oscillatory, sectored and patchy zoning (characterized by irregular and discontinuous patches of high CL intensity) and largely unzoned homogeneous interiors (Fig. 7). A few zircon analyzes of the heterogeneous granophyre (IA-NS-2) and the diorite (IA-NS-6) show. Chondrite-normalized zircon rare earth element (REE) abundances in samples from the Austurhorn Intrusive Complex.
Field Interpretations
In some areas of the MSCZ, large sheet-like mafic bodies are preserved, and their margins are commonly broken up into enclaves and groups within the surrounding silicic rocks (see Fig. 3h,i), suggesting a common form of mafic recharge. may be in the system by intrusion of mafic sheets that dissolve into the host magma (e.g. We interpret the field relationships to indicate a mushy environment for the AIC in which the partially solidified silicic host material is continuously reheated, reanimated, and. The range in average individual unit ages indicate a time scale of peak magmatic construction of the AIC of ∼300 kyr.
Zircon Abundance, Size, and Morphology
Zircon Elemental & Isotopic Compositions
Maximum Ti-in-zircon temperatures in the most silicic sample for which zircons were analyzed (granophyre IA-NS-2) roughly equal zircon saturation temperatures (ZSTs) for their host rock (∼850ºC; Fig. 13 ) . Estimated whole-rock zircon saturation temperatures (ZST), calculated using the formula derived by Watson & Harrison (1983) as revised by Boehnke et al. 2013 ), plotted against model zircon crystallization temperatures, calculated using the Ti-in-zircon thermometer of Ferry & Watson (2007) , for silicic units of the Austurhorn Intrusive Complex. These values fall within the lower, less depleted part of the Hf isotopic range of Icelandic basalts, including the detached end and the lower end of the rifted range ( Peate et al., 2010 ; see Fig. 10b and Table 5 ).
U-Pb Geocrhonology
The elemental compositions of the first group (comprising ∼60% of the HSG zircon population) are similar to those of zircons from all other MSCZ rocks as well as other Icelandic siliceous rocks (Padilla et al., in review; Carley et al. al. Well-developed (oscillatory) growth zoning is one of the most typical features of magmatic zircon (Corfu et al., 2003; Hanchar and Miller, 1993; Hoskin and Schaltegger, 2003). Individual in situ δ18O values of zircon in most samples suggest significant contributions from altered crust in the genesis of their parental silicic magmas (cf. Bindeman et al., 2012).
Austurhorn Intrusive Complex
In addition, I investigate temporal differences in magmatism by comparing the SIIS with spatially unrelated intrusions elsewhere in Iceland: the Lýsuskarð intrusion, a younger intrusion in western Iceland, and the Sandfell laccolith, an older intrusion in eastern Iceland.
Reyðarártindur Intrusive Complex
Due to its proximity and similarity to other intrusions, Reyðarártindur has been assumed to be contemporaneous with the rest of the SIIS (e.g. Moorbath et al., 1968), although it has not been previously dated and no data has been published since it was first mapped by Cargill et al. Although smaller in size, it has similar features, especially in the MFCZ, and voluminous granophyres that make Reyðarártindur well suited for geochemistry.
Slaufrudalur Intrusive Stock
Inset: Overview map showing the location of two other silicic intrusions as well as the main tectonovolcanic zones in Iceland (modified from Carley et al., 2011): Northern (NVZ), Eastern (EVZ) and Western Volcanic Zones (WVZ), Snæfellsness Volcanic Belt (SVB), Reykjanes Ridge (RR), Reykjanes Volcanic Belt (RVB), South Island Seismic Zone (SISZ), Öræfi Volcanic Belt (ÖVB), Tjornes Fracture Zone (TFZ), Kolbeinsey Ridge (KR), and Central Iceland Belt (MIB). The coordinates listed are all in the WGS84 UTM grid. a) Austurhorn Intrusive Complex (modified from Furman et al., 1992b); b) Reyðarártindur Intrusive Complex (modified after Gale et al., 1966);. The nature of interactions varies greatly and is very different from intrusion to intrusion. a) Austurhorn Intrusive Complex (southeast Iceland);.
Vesturhorn Intrusive Complex
Lýsuskarð Intrusive Complex
These rocks and their associations bear a close resemblance to those of the SIIS, in particular the Austurhorn and Vesturhorn intrusion units. A careful study of the geochemical characteristics of Lýsuskarð allows comparison between similar silicic magmatic systems from different tectonic regimes in Iceland and provides a new perspective for addressing temporal variations in the dynamics of silicic magmatism.
Sandfell Laccolith
Whole-Rock Geochemistry
Zircon Geochemistry
Based on the elemental and isotopic compositions of zircons from the Austurhorn Intrusive Complex, Padilla et al. for review; see Chapter III) establish general compositional fields for intrusive zircons from the Austurhorn. Instrument reproducibility (relative accuracy and error) for secondary geochemical glass standard NIST-612 (Pearce et al., 1997), analyzed as unknown at 40 µm. Instrument reproducibility (relative accuracy and error) for secondary geochemical glass standard NIST-612 (Pearce et al., 1997), analyzed as unknown at 80 µm.
