First of all you don't need to have two loops if I understood correct what you want to do. Also 'meta_key' => '_thumbnail_id'
isn't necessary. I suppose is just the thumbnail id and not a meta key created by you.
You could achieve what you want like that:
<div id="masonry">
<?php
$args = array(
'posts_per_page' => 10,
'orderby' => 'post_date',
'order' => 'DESC',
'post_type' => 'post',
'post_status' => 'publish',
'suppress_filters' => false );
$recent_posts = get_posts( $args );
foreach ($recent_posts as $key=>$post): setup_postdata( $post ); {
?>
<div class="item normal" data-order='1'><!--BEGIN .item -->
<div <?php post_class(); ?> id="featured-<?php the_ID(); ?>">
<?php
if (has_post_thumbnail(get_the_ID())) {
echo '<a href="' . get_permalink(get_the_ID()) . '">';
echo get_the_post_thumbnail(get_the_ID(), 'archive_grid');
echo '</a>';
}
?>
</div>
</div>
<?php } ?> //end of foreach loop
<?php get_template_part('includes/index-loadmore'); ?>
</div><!--END #masonry -->
<div id="masonry-new"></div>
<div class="post-navigation clearfix"><!--BEGIN .post-navigation -->
<?php dt_pagination(); ?>
</div><!--END .post-navigation -->
But this way you have to fix the pagination your self.
UPDATE:
It seems that get_posts doesn't work with pagination.
<div id="masonry">
<?php
$paged = 1;
if ( get_query_var('paged') ) $paged = get_query_var('paged');
if ( get_query_var('page') ) $paged = get_query_var('page');
$args = array(
'posts_per_page' => 10,
'paged' => $paged,
'orderby' => 'post_date',
'order' => 'DESC',
'post_type' => 'post',
'post_status' => 'publish',
'suppress_filters' => false );
$wp_query = new WP_Query( $args );
while ( $wp_query->have_posts() ) : $wp_query->the_post();
?>
<div class="item normal" data-order='1'><!--BEGIN .item -->
<div <?php post_class(); ?> id="featured-<?php the_ID(); ?>">
<?php
if (has_post_thumbnail(get_the_ID())) {
echo '<a href="' . get_permalink(get_the_ID()) . '">';
echo get_the_post_thumbnail(get_the_ID(), 'archive_grid');
echo '</a>';
}
?>
</div>
</div>
<?php endwhile; ?> //end while loop
<?php get_template_part('includes/index-loadmore'); ?>
</div><!--END #masonry -->
<div id="masonry-new"></div>
<div class="post-navigation clearfix"><!--BEGIN .post-navigation -->
<?php dt_pagination(); ?>
</div><!--END .post-navigation -->
<?php wp_reset_postdata(); ?>
If the second solution doens't work you have to provide the dt_pagination() function to see how is working.
Hope it helps.