Posted by: magmatist | December 3, 2012

MBVRC research grant report- Julie Gross, dacite magma generation at MBVF

MBVRC provided funds to three WWU  students in 2012. This was made possible by support from t-shirt sales, field trips, and donations. Thanks to all who contributed.

Julie Gross

Julie Gross

Here is the first report from one of these students. JULIE GROSS has just completed her Masters in Science in the Geology Department. Julie, who lives in Kendall, investigated the geochemistry of three dacite lavas at the Mount Baker volcanic field. She hypothesized possible pathways for the generation of dacitic magma from parental basaltic melts. We will publish reports from the other students as we receive them. MBVRC provided funds to obtain isotope chemical analyses on three of her samples. Here is Julie’s report to MBVRC’s supporters.

Cartoon illustrating generalized path of mafic (basaltic) magma moving out of the mantle into the crust, where it might become dacitic (felsic) in composition. Image courtesy Sue DeBari, WWU geology.

Cartoon illustrating generalized path of mafic (basaltic) magma moving out of the mantle into the crust, where it might become dacitic (felsic) in composition. Image courtesy Sue DeBari, WWU geology. Click to ENLARGE any image.

The formation of felsic (SiO2 > 60 wt %) magmas in subduction zones is a subject of active research due to their potential contributions to the continental crust. Felsic rocks resulting from the cooling of these magmas include dacite and rhyolite lavas and tephras, and plutonic equivalents: granodiorite, tonalite, and true granites. Magmatic arcs, such as the Cascade Range where Mount Baker resides, provide an excellent laboratory to address pertinent questions such as: “What kinds of magma are forming in subduction zones?”, “How are they evolving?”, and “How do they contribute to crustal growth?” One of the primary research goals of this study is to determine if felsic magmas in the Mount Baker volcanic field (MBVF) represent new additions to the crust rising from the mantle, or are they derived from older magmas that had already crystallized in the crust and then re-melted by later heating. In addition, felsic magmas are often important contributors to magma mixing processes that generate less felsic andesitic compositions. These andesitic compositions are ubiquitous in magmatic arcs and, in the case of Mount Baker, quite voluminous: andesite makes up nearly the entire erupted volume of Mount Baker, and roughly half of the erupted volume within the MBVF. The second part of this study focuses on what kind of role these felsic magmas played in producing the voluminous andesites that make up the bulk of Mount Baker.

There are two primary pathways that produce felsic magmas in subduction zone complexes. 1) Mantle-derived basalts can undergo “crystal fractionation”. This is the removal of early-forming crystals such as olivine and pyroxene (rich in magnesium and iron, poor in silica) as they form in a cooling basalt magma; this gradually evolves a more Si-rich, Mg-poor composition of the remaining magma. 2) Older basaltic rocks deep in the continental crust may partially melt to make a new generation of more felsic magma. In addition, since the subducting Juan de Fuca plate is young and hot, the “slab melt” concept (partial melting of the subducted basaltic oceanic plate) as a source of felsic melt was also investigated. This study focuses mainly on the fractionation and slab melt models because these models add new Si-rich material to the pre-existing continental crust. While partially melting basaltic lower crust does produce felsic magma, it does not add new material but rather “recycles’ preexisting crust.

Three dacite lavas in the northern portin of the MBVF were sampled for this study.

Three dacite lavas in the northern portion of the MBVF were sampled for this study.

Three dacitic lava units from the MBVF were sampled for this study: the dacite of Nooksack Falls (149 ± 5 thousand years [ka], and the only hornblende-bearing dacite in this study); the dacite of Cougar Divide (a hybrid unit with both dacitic and andesitic compositions, 613±8 ka); and the dacite of Mazama Lake (undated but older than Cougar Divide, and younger than the Kulshan caldera forming event (1.15 million years [Ma]).  (Use this link to find an on-line field trip to the Cougar Divide dacite on the Northwest Geology Field Trips blog.) All of these lava flows are small erosional scraps that erupted from volcanoes much older than the young Mount Baker andesite edifice. Major and trace element mineral chemistries were obtained from these samples; isotopic composition analyses are still in process at the University of Washington. Mineral chemistry and petrography were used to determine which units represented original compositions (i.e. representative of the original igneous process that produced that lava). Nooksack Falls and Mazama Lake units were determined to be close representatives of an original composition, with minor additions of foreign material (assimilation of adjacent crustal rocks into the magma).  The dacite of Cougar Divide exhibited evidence for mixing of two or more different magma intrusions, and is not representative of a single igneous process (such as fractionation). Grant funds from MBVRC will go towards obtaining isotope data (Sr, Nd, and Pb systems are in the process of being analyzed) for each sampled unit to help better constrain the role played by contamination of magma from surrounding crustal rocks, and to provide further rigorous testing of any modeling (i.e. crystal fractionation or magma mixing modeling).

Sample location maps. Top to bottom: Dacite of Cougar Divide, dacite of Nooksack Falls, and dacite of Mazama Lake.

Sample location maps. Top to bottom: Dacite of Cougar Divide, dacite of Nooksack Falls, and dacite of Mazama Lake.

How about partial melting of the oceanic plate as a source of felsic magma at the MBVF? Melted subducted oceanic plates have a characteristic geochemical signature, characterized by particular ratios of rare earth elements1 such as Lanthanum (La), Ytterbium (Yb), and Yttrium (Y), the element Strontium (Sr), and others.  None of the units sampled bore the distinct geochemical characteristics of slab melts, such as high La/Yb ratios and high Sr/Y ratios coupled with low Y concentrations, effectively ruling that hypothesis out. Mathematical models that attempt to reproduce possible fractionation of major and trace elements could reproduce compositions observed in the dacite of Mazama Lake from the high-Mg basaltic andesite of Tarn Plateau (one of the primitive2 magma types erupted at Mount Baker and studied by WWU graduate student Nicole Moore (Moore and Debari, 20123), but could not reproduce the dacite of Nooksack Falls from any known Mount Baker parental magma.  The addition of the isotope data partly funded by MBVRC may require adjustments to the simple fractionation model currently in place for the dacite of Mazama Lake to accommodate an assimilation component as well.

columns in Nooksack Falls dacite. A steep cross-country descent is required to reach this lava flow.

columns in Nooksack Falls dacite. A steep cross-country descent is required to reach this lava flow.

The presence of hornblende in Nooksack Falls precludes a lower crustal melt origin for Nooksack Falls (hornblende is a water-rich mineral and crustal melts are typically dry). One possibility is that Nooksack Falls dacite evolved from a parental magma composition that has yet to be recognized in any MBVF lavas. Mixing models were largely successful; mixing between high-Mg basaltic andesite of Tarn Plateau compositions and the dacite of Mazama Lake could reproduce major and trace element compositions observed in the hybrid Cougar Divide unit from this study, as well as the hybrid Boulder Glacier unit from a previous study (Baggerman and DeBari, 20114).

Dacite of Cougar Divide.

Dacite of Cougar Divide.

The results from this study start to piece together a role for felsic magmas at Mount Baker. There is evidence for the addition of new felsic crust evolving out of mantle magma moving upward, as well as evidence that these felsic magmas are involved in mixing processes that produce andesitic compositions. Now that  data is available for the felsic compositions on Mount Baker, several new research paths have opened up. Closer analysis of the main cone building andesites may allow us to determine the extent of the magma mixing process at Mount Baker. In turn, this may allow us to make better estimates of how much felsic material is actually present at depth, which further addresses the primary question of this study; how are felsic magmas contributing to the formation of continental crust?

Dacite of Mazama lake protrudes into the small lake, one of the Chain Lakes on the west side of Table Mountain.

Dacite of Mazama Lake protrudes into Iceberg Lake, one of the Chain Lakes on the west side of Table Mountain.

1REE: http://en.wikipedia.org/wiki/Rare_earth_element

2 a primitive magma is a basaltic magma that has gone through minimal differentiation (or changes in crystal or chemical composition) from the original (‘primary’) magma in the mantle.

3Moore, N. and DeBari, S., 2012, Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes: Contributions in Mineralogy and Petrology v. 163, p. 521–546: http://link.springer.com/article/10.1007%2Fs00410-011-0686-4?LI=true#page-1

4Baggerman, T. and DeBari, S., 2011, The generation of a diverse suite of Late Pleistocene and Holocene basalt through dacite lavas from the northern Cascade arc at Mount Baker, Washington: Contributions in Mineralogy and Petrology. v.161, p.75–99: http://faculty.wwu.edu/~debari/web/pubs_files/Baggerman&DeBari11.pdf

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