Download Petrogenesis of intermediate magmas at Mount Baker volcano

Petrogenesis of intermediate magmas at Mount Baker volcano, northern Cascade arc
Ricardo Escobar, WWU Geology
Mount Baker is an andesitic stratovolcano in the northern Cascade volcanic arc. The most
primitive (basalt/high-Mg andesite) and most differentiated (dacitic) magma types have been analyzed
to elucidate magma sources, but no studies have thus far been aimed at understanding the origin of the
most voluminous component of this volcano, the andesites. I propose to examine the geochemistry of
intermediate magmas of Mount Baker in the attempt to determine if the andesites are derived from the
three parental magma types at Mount Baker identified by Moore and DeBari (2012): typical calc-alkaline
basalt, high-Mg andesite and MORB-like low K tholeiite (LKT). I will also assess the role magma mixing
and fractionation have in generating andesitic magmas. Furthermore, I propose to constrain the
“parent-less” Nooksack Falls dacite with an andesitic magma source.
Hildreth et al. (2003) published the foundational study of Mount Baker’s eruptive chronology
along with major element data for most flows. Despite this, the voluminous andesites erupted at
Mount Baker have not been geochemically characterized and their origin remains unknown. It is
important to study the intermediate magmas of Mount Baker considering andesitic volcanism is
synonymous with arc volcanism and is the dominant magma type erupted in the Cascade volcanic arc
(Carmichael, 2002). Commonly, intermediate magmas erupt as a result of felsic and mafic magmas
mixing, and the diversity of lavas erupted at Mount Baker further suggests that recharge filtering (Kent
et al., 2010) could exert an important control. However, other hypotheses for andesite generation can
be simple fractionation from basalt with or without crustal contamination (Grove and Kinzler, 1986) or
primary melting of subducted plates (Yogodzinski and Kelemen, 1998). In their study of Mount Baker
lavas, Baggerman and DeBari (2011) state that intermediate magmas are not derived from a single
source, and open-system processes are dominant during petrogenesis. However, compositions were
limited to basaltic andesites and very felsic andesites to dacites, not intermediate compositions. Further
setting the stage, Moore and DeBari (2012) showed that three mafic magma types come from distinct
mantle sources beneath Mount Baker. One of the magma types, high-Mg andesites, have been shown to
produce some dacitic compositions by fractional crystallization, and these dacites clearly undergo
recharge mixing with more primitive compositions to provide complex differentiation histories (Gross,
2012; Baggerman and DeBari, 2011). However, this fractionation history and recharge mixing has only
been quantitatively constrained for basaltic andesites and dacites. A missing link in these studies is the
voluminous intermediate compositional range magmas, which may serve as parental magmas for
dacites. These results will tie into the larger more global question about andesite genesis in arcs. Do
andesites represent an intermediate liquid step along a fractionation sequence from basalt to dacite, or
as suggested by Reubi and Blundy (2009), do they represent mixtures of crystals andliquid from distinct
sources? And in either case, can they further fractionate to produce more felsic compositions.
This study will focus on the petrography and whole rock and mineral chemistry of the
intermediate range magmas to determine their role as derivatives of basalts and parents of dacites by
fractional crystallization or the result of mixing between mafic and felsic magmas. I have chosen four
flows mapped by Hildreth (2003) ranging in 54 - 62 silica weight %, in an effort to collect a
representative range of intermediate samples: Dobbs Creek, Dobbs Cleaver, Swift Creek and Coleman
Pinnacle. In addition, Coleman Pinnacle is the only hornblende-bearing andesite at Mount Baker. I will
discern if this hydrous magma series comes from a distinct mantle source, and if Coleman Pinnacle
andesite is parental to the hornblende-bearing Nooksack Falls dacite (using fractional modeling). At least
30 samples from these four flows will be collected in summer 2013.
I will make detailed petrographic analyses of thin sections from these flows to determine the
mineralogy and diversity of phenocrysts populations and to preselect phenocrysts for further
analysis by microprobe and LA-ICPMS. Observations of disequilibrium textures and complex
zoning of phenocrysts will hint as to whether recharge mixing is occurring (Kent, 2010).
Mineral chemistry by microprobe (major element) and LAICPMS (trace element) will be used to
establish crystallization conditions and compositions for fractionation modeling. Microprobe
analyses will be done at UW and LA-ICPMS analyses at WWU during the fall of 2013.
Whole rock major and trace element data will provide the geochemistry of the intermediate
magmas, allowing for modeling calculations to determine relationship with the other mafic (Moore and
DeBari, 2012) and felsic (Gross, 2012) magmas. These analyses will be done by ICPMS and XRF at WSU
during the fall of 2013 due to the lack of facilities at WWU. Samples for whole rock analyses will be
prepared at WWU in the summer of 2013. The end results will then be compared to the data of Gross
(2012) and Moore and DeBari (2012) to evaluate the relationship of the andesites to the dacitic and
mafic magmas of Mount Baker. As proposed in the hypothesis, characterizing andesite petrogenesis at
Mount Baker, involving modeling techniques utilizing mineral and whole rock compositions, will provide
a detailed big picture understanding of the diverse magmatic relationships identified at Mount Baker.
References
1. Baggerman, T.D. and DeBari, S.M., 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 to Mineralogy and Petrology, v. 161, p. 75-99.
2. Borg, L.E. and Clynne, M.A., 1998, The petrogenesis of felsic calc-alkaline magmas
from the southernmost Cascades, California: Origin by partial melting of basaltic lower
crust: Journal of Petrology, vol. 39, p. 1197-1222.
3. Carmichael, I.S.E., 2002, The andesite aqueduct: perspectives on the evolution of
intermediate magmatism in west-central (105-99°W) Mexico: Contributions to
Mineralogy and Petrology, v. 143, p. 641-663.
4. Gross, J., 2012, Felsic magmas from Mount Baker in the northern Cascade arc: origin and
role in andesite production: WWU Master’s thesis.
5. Grove, T.L. and Kinzler, R.J., 1986, Petrogenesis of andesites: Annual Reviews Earth
Planetary Science, vol. 14, p. 417-454.
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7. Heaton, T.H. and Kanamori, H., 1984, Seismic potential associated with subduction in
the northwestern United States: Bulletin of Seismological Society of America, vol. 74, p.
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8. Hildreth, W., Fierstein, J. and Lanphere, M., 2003, Eruptive history and geochronology
of the Mount Baker volcanic field, Washington: GSA Bulletin, v. 115, p. 729-764.
9. Kent, A.J.R., Darr, C., Koleszar, A.M., Salisbury, M.J. and Cooper, K.M., 2010,
Preferential eruption of andesitic magmas through recharge filtering: Nature Geoscience,
vol. 3, p. 631- 636.
10. Moore, N.E. and DeBari, S.M., 2012, Mafic magmas from Mount Baker in the northern
Cascade arc, Washington: probes into mantle and crustal processes: Contributions to
Mineralogy and Petrology, vol. 163, p. 521-546.
11. Reubi, O. and Blundy, J., 2009, A dearth of intermediate melts at subduction zone
volcanoes and the petrogenesis of arc andesites: Nature Letters, v. 461, p. 1269-1274.
12. Yogodzinski, G.M. and Kelemen, P.B., 1998, Slab melting in the Aleutians: implications
of an ion probe study of clinopyroxene in primitive adakite and basalt: Earth and
Planetary Science Letters, vol. 158, p. 53-65.