1. Petrogenesis of antecryst-bearing Mexican arc basalts: Insights into along-arc variations in magma ponding depths, H 2 O contents, and surface heat flux Georg F. Zellmer (g.f.zellmer@massey.ac.nz) Mattia Pistone, Yoshiyuki Iizuka, Benjamin Andrews, Arturo Gomez-Tuena, Susanne Straub & Elizabeth Cottrell

2. Motivation: global arc surface heat flux variations Western TMVB Eastern TMVB (±1 stdev)

3. Global arc surface heat flux variations

4. Sample locations in geodynamic context

5. Bulk compositions

6. Representative petrography (BSE)

8. But: TMVB rocks are wet!  Plag is antecrystic, formed from more evolved low-An# liquids Experimental fields from Waters & Lange, 2016 Phenocryst ranges

9. Combine mineral chemistry with MELTS modeling  A new pressure sensor: Example: Western TMVB First plag compositions formed during MELTS isobaric cooling, H 2 O saturated microlites phenocrysts 1. tag to MELTS 2. read off P

10. Example: Central TMVB Example: Eastern TMVB

11. Do this for all samples… - striking along-arc P trend

12. Moho H 2 O variations if stalling triggered by degassing Data from MIs and hygrometry in grey

13. Mush zone depth affects surface heat flux:

14. Mush zone depth affects advected surface heat flux:

15. Conclusions 1.Mafic eruptives sampled along the entire TMVB display petrographic and mineral chemical features consistent with antecryst uptake from evolved mush zones into aphyric mafic melts. 2.We have developed a pressure sensor to determine the depth of crystal mush formation/remobilization, based on a combination of plagioclase antecryst chemistry and MELTS modelling. 3.Magma/mush ponding depths increase eastward along the TMVB. 4.If ponding is induced by degassing (i.e. P-sensor = degassing sensor), magmatic H 2 O contents increase eastward, consistent with a eastward increasing age of the subducting slab. 5.Surface heat flux in the TMVB is directly influenced by the observed magma/mush ponding depth variations, and on average is higher than globally due to low primary H 2 O contents in the western and central arc.